1970-petukhov- heat transfer and friction in turbulent pipe flow with variable physical properties

Heat Transfer and Friction in Turbulent Pipe Flow with Variable Physical Properties

B . S . PETUKHOV High Temperature Institrite. Acudcrr~y of Science of the USSR. Moscoqc.. U S S R

I . Introduction . . . . . . . . . . . . . . . . . . . . . . 504 I1 . Analytical Method . . . . . . . . . . . . . . . . . . . 507

507 B . Eddy Diffusivities of Heat and Momentum . . . . . . . 510 C . &4nalytical Expressions for lemperature and Velocity Profiles,

516 111 . Heat Transfer uith Constant Ph?sical Properties . . . . . . . 521

525

A . Basic Equations . . . . . . . . . . . . . . . . . . .

Heat Transfer. and Skin Friction . . . . . . . . . . . .

A . Analytical Results . . . . . . . . . . . . . . . . . . 521 B . Experimental Data . . . . . . . . . . . . . . . . . .

I\. Heat Transfer and Skin 1:riction for Liquids with Variablc Viscosity . . . . . . . . . . . . . . . . . . . . . . . . 528 A . Theoretical Results . . . . . . . . . . . . . . . . . . 528 B . Experimental Data and Ihpirical Equations . . . . . . .

V . Heat Transfer and Skin Friction for Gases with Variable Physical 530

Properties . . . . . . . . . . . . . . . . . . . . . . . 533 A . Analytical Results . . . . . . . . . . . . . . . . . . 533 B . Experimental Data and Ihpirical Equations . . . . . . .

VI . Heat Transfer and Skin 1;riction for Single-Phase Fluids a t 540

Subcritical States . . . . . . . . . . . . . . . . . . . . 543 A . Analytical Results . . . . . . . . . . . . . . . . . . 543 B . Experimental Data and I

504 B. S. PETUKHOV

I. Introduction

Heat transfer in turbulent pipe flow has been investigated for almost 60 years. Nusselts paper published in 1910 was probably the first one analyzing this problem on a scientific basis (2) . I n this paper devoted to the heat transfer of turbulent gas flow in tubes, the similarity method was originally used for the correlation of experimental data on heat transfer. This is the reason for the continued interest in Nusselts paper.

During subsequent years different investigators performed numerous experimental studies on heat transfer processes in turbulent pipe flow for various fluids including liquid metals. As a result they formulated relations for the Nusselt number versus the Reynolds and Prandtl numbers for a wide range of Re and Pr. Reynolds (2) was the first who theoretically studied heat transfer in turbulent pipe flow. The relationship obtained between heat flux and wall shear stress, known as the Reynolds analogy, is valid only for Pr = 1. Some investigators have improved upon Reynolds analysis. For example, Taylor (3) and Prandtl ( 4 , 5 ) took into account approximately the influence of fluid flow peculiarities at the wall on heat transfer, assuming the flow to consist of a turbulent core and viscous (laminar) sublayer. Karman (6) improved this model by the introduction of an intermediate layer between a laminar sublayer and a turbulent core. The expressions for heat transfer obtained by Karnian and Prandtl are true for constant physical properties over the range 0.7 to 10-20 for Pr. T h e last restriction concerns the fact that they neglected turbulent heat transfer in a viscous sublayer (this leads to essential errors when Pr is large) and heat transfer by conduction in the turbulent core (this is not true for low Pr numbers). Further development of analytical methods for heat transfer in a turbulent pipe flow with constant physical properties was achieved when investigators digressed from the above assumptions and began to use more accurate relationships for the distributions of velocity and eddy diffusivities of heat and momentum along the pipe cross section (7-24). For example, Lyon (8) obtained an expression for the Nusselt number in the case of constant heat flux at the wall. This expression predicts the heat transfer rate, if the distributions of velocity and turbulent diffusivity of heat are known. The use of more accurate relationships for distributions of velocity and eddy diffusivities of heat and momentum require the application of numerical methods. Numerical calculations of heat transfer in turbulent pipe flow for constant physical properties were carried out in (8-12, 13, 14). The results of these predictions covering a wide range of Re and Pr numbers as a rule are in good agreement with experimental data.

Thus, nowadays the problem of heat transfer in a turbulent quasi-

HEAT TRANSFER AND FRICTION IN TURBULENT PIPE FLOW 505

steady fluid flow with constant physical properties in circular tubes has been rather fully investigated.

In reality, fluid physical properties depend on temperature. That is why heat transfer relations obtained with the assumption of constant physical properties can only be used in practice either a t small temperature differences in a flow or with physical properties changing slightly in the temperature range considered. In this case the effect of changing physical properties can be approximately accounted for by choosing the properties at a certain average fluid temperature.

I n heat transfer systems used in different fields of engineering, large temperature drops and high heat fluxes are often realized. In this case the large temperature gradients occur in a fluid flow. For example, in nuclear reactors the heat flux (E; cal./m2 hr) may be as high as several millions. In cooling systems for jet propulsion engines it may rise to several score of millions while a heat flux of hundreds of millions may occur i n some special kinds of apparatus. Liquids and gases whose physical properties are very responsive to temperature changes are often used as heat transfer fluids. Gases flowing at large temperature differences or some liquids (single-phase fluid) at subcritical states serve as examples. In these cases it is impossible to consider physical properties constant, because great errors would otherwise result. Under such conditions the analysis of the flow and heat transfer should include the dependence of physical properties on temperature.

For various types of fluids and for a given fluid the variation of the physical properties with temperature and pressure is not the same over different ranges of the state parameters. For such a fluid under these varying conditions it is presently impossible to describe the fluid flow and heat transfer by a single relationship valid for all conditions. As a consequence, the problem of a fluid flow and heat transfer with variable physical properties divides into several problems, and each problem corresponds to a certain type of dependence of physical properties on temperature and pressure. Therefore, the analytical expressions for fluids with constant physical properties are not universal in the case of variable physical properties. Theoretical studies of flow and heat transfer in fluids n i th variable physical properties are hindered by different mathematical and physical difficulties. T h e mathematical difficulties can be explained by the fact that the momentum and energy equations in the case of variable physical properties are coupled and nonlinear. However, these difficulties can be overcome, e.g., by using numerical methods and with the help of computers. The difficulties of a physical nature are more serious. T h e y may be attributed to the inability to prescribe analytical expressions for the turbulent diffusivities of heat

506 B. S. PETUKHOV

and momentum for fluids possessing variable physical properties. These expressions have been more or less studied for fluids with constant physical properties only; consequently, in the case of variable properties we have only a few theories which have not been verified experimentally. That is why analytical solutions of heat transfer in a turbulent flow with variable physical properties are not so accurate, e.g., in the case of a laminar flow. They must be verified by comparison with experimental data.

Experimental study of fluid flow and heat transfer for variable physical properties is also a very difficult problem because experiments at high temperatures, large heat fluxes, and high pressures are not easy to perform. Other aspects of the problem are the difficulties of interpretation and correlation of experimental data, because heat transfer and skin friction at variable physical properties depend on many parameters. That is why the dynamics and the heat transfer of turbulent flow with variable physical properties have not been studied in full. Nevertheless during the last 10-15 years striking progress has been made in this field.

Several theoretical papers are devoted to the fluid mechanics and heat transfer of turbulent pipe flow with variable physical properties.

Kutateladze (15-17) studied a gas flow at large temperature differences and small subsonic velocities. He obtained a correlation between the friction factor and the temperature ratio parameter (the wall temperature to the bulk gas temperature ratio) for the limiting case of Re + 00. And for a gas Pr m 1, it was assumed that the same relationship was valid for the heat transfer coefficient. These relationships are also approximately valid for the finite values of Re.

Deissler (10, 11, 18) and Goldmann (19) have developed methods for the calculation of heat transfer and skin friction for an incompressible fluid with an arbitrary temperature dependence of its properties. The essence of their methods is in the simultaneous numerical integration of the energy and momentum equations formulated on the assumption that heat flux and shear stress are constant (or changing linearly) along the pipe radius. The methods of Deissler and Goldmann differ with respect to the calculation of the eddy diffusivities of heat and momentum for constant and variable fluid properties. Deissler performed calculations of heat transfer and skin friction for some gases and a liquid; Deissler and Goldmann performed these calculations for water above the critical point.

Petukhov and Popov (14) have developed another method of calculating the heat transfer and skin friction for an incompressible fluid with an arbitrary temperature dependence of its properties. Analytical expressions for both heat transfer and skin friction and also for velocity and temperature profiles obtained from the energy and momentum

HEAT TRANSFER AND FRICTION I N TURBULENT PIPE FLOW 507

equations are the basis of this method. With the help of these expressions we can calculate heat transfer and friction using the method of successive approximations. Later on this method was used for calculation of heat transfer and friction of some gases with and without dissociation, and also for carbon dioxide at supercritical state parameters.

T h e fluid mechanics and heat transfer of turbulent pipe flow with variable physical properties have been studied both theoretically and experimentally. Published papers are available which contain many experimental results. In particular they present data obtained on heat transfer and skin friction for liquids under the conditions of substantial changes in viscosity, for some gases at large temperature differences, and for water, carbon dioxide, and some other substances at supercritical states. Some of the papers contain empirical equations correlating the experimental results.

In this paper we shall consider heat transfer and skin friction in turbulent pipe flow with variable physical properties. T h e constant properties solution will be considered only so far as is necessary for the flow and heat transfer analysis with variable physical properties.

11. Analytical Method

A. BASIC EQUATIONS

Turbulent flow is, of its nature, transient. Velocity, temperature, and other properties change continuously in time at every point of a turbulent flow. These changes are irregular fluctuations with respect to some temporal mean. This behavior allows us to represent different turbulent flow properties as the sum of the mean value, in time, and a pulsation of this value. So we can describe the field of real (instantaneous) velocities as a field of averaged (in time) velocities and the superimposed field of velocity fluctuations. We can do the same with temperature, pressure, and density fields and with other dependent variables. With this approach, transfer processes in a turbulent flow are controlled by two mechanisms: molecular and convective (turbulent). T h e first mechanism results in the appearance of viscous stresses proportional to the gradients of the averaged velocity and heat fluxes due to heat conduction which are proportional to the averaged temperature gradients. T h e second mechanism gives rise to turbulent stresses caused by momentum transfer due to velocity fluctuations and turbulent heat fluxes caused by heat transfer resulting from velocity and temperature fluctuations. This approach suggested by Reynolds allows us to pass from energy, momentum, and continuity equations for instantaneous values to the

508 B. S. PETUKHOV

corresponding equations for the averaged values. Hence, the solution of turbulent flows is reduced to the analysis of the averaged equations in combination with analytical expressions for turbulent diffusivities arising from some physically motivated assumptions in accordance with experimental data.

Furthermore, we shall consider a quasi-steady1 axisymmetric turbulent flow of an incompressible fluid with variable physical properties. We shall restrict our problem to the analysis of the fluid flow and heat transfer in circular pipes far from the entrance, i.e., in that region where thermal and velocity boundary layers coincide. At present it is possible to analyze this problem only approximately. Therefore we shall make the following assumptions:

1. Flow velocities are not large, so the energy dissipation can be neglected.

2. The effect of body forces is small in comparison with that of viscosity and inertia forces.

3. Physical properties change weakly over the range of temperature fluctuations (i.e., from T to T + T',where T is the averaged temperature value, T' is the temperature fluctuation); therefore, the physical properties at a given point can be considered constant and equal to the physical properties at the averaged temperature for this point.

4. Change of heat flux along the axis caused by thermal conductivityand turbulent diffusivity is small compared with its change along the radius.

5 . Change of normal stresses (viscous and turbulent) along the coordinate axes is small in comparison with the change in the shear stresses.

The averaged energy, momentum, and continuity equations for the conditions we have formulated can be written in a cylindrical coordinate system:

A turbulent flow in which averaged properties do not change with time.


where x and r are the axial and radial coordinates (x coincides with the pipe axis); W, and W , are the averaged (in time) values of the axial and radial components of the velocity vector, respectively; p, h, T , and P are the averaged density, enthalpy, temperature, and pressure; WE, W,, h, and T are the fluctuations of velocity (in the axial and radial directions), enthalpy, and temperature, respectively; h and p are the thermal conductivity and dynamic viscosity at temperature T and pressure P.

Since fully developed flow is being considered, it can be assumed that the change in the axial component of the mass velocity along the axis x is small, ix. ,

tI(pwT)/aY w 0

Therefore, from Eq. (4), the radial velocity component IY, = 0. In addition, if we assume that the change of viscous and turbulent shear stresses along the x axis is small,

i: (. 2;

then it is seen from Eq. (3) that over the cross section.

i.e.,

- p ? W ) w 0

a/& = 0, i.e., pressure P is constant

With these assumptions the set of Eqs. (1)-(4) is reduced to the following two equations:

On the right-hand side of Eq. ( 5 ) , in parentheses, the expression for the heat flux is given by

q = A(;iT/ar) - pWTh (7) ~

T h e first term of this sum is the heat flux due to conductivity, while the second item is accounted for by the eddy diffusivity of heat.

On the right-hand side of Eq. (6), the expression for shear stress is given by

u = - [p( ii W,/ar) - p W, W,.] (8)

Here the first term is the viscous stress and the second term is the turbulent stress.

5 10 B. S. PETUKHOV

In theoretical investiagations t;.e assumption is often made that the heat flux and shear stress vary linearly with respect to r , i.e.,

q = qwR and u = uwR (9)

or it is supposed that these quantities are constant along the radius

q = qw and u = uW (10)

Here qw and ow are the heat flux and the shear stress at the pipe wall, respectively; R = r/ro is the dimensionless radius; ro is the pipe radius.

We can see that the first assumption from (9) is fulfilled only in the case of slug2 flow, and the second assumption only for fully developed flow with constant physical properties. As for assumption (lo), it is not fulfilled for pipe flow. However, in a number of cases assumption (9) and even (10) do not introduce great errors into our analysis. This may be attributed to the fact that in calculations of heat transfer and skin friction a correct description of the flow near the wall is the most essential, and in this region assumptions (9) and even (10) are fulfilled approximately. Quantitative error estimations are given in Section 111, p. 521 (see Table I, p. 522).

Assumptions (9) and (10) allow us to replace Eqs. ( 5 ) and (6) by simpler ones which can be obtained from Eqs. (7) and (8) as a result of the substitution of q and u from (9) and (10).

For calculations with the help of Eqs. (5) and (6) it is necessary to express the turbulent heat flux pW,h and the turbulent shear stress p Wz W, as functions of the independent variables and the averaged flow properties. Then it is convenient to introduce the coefficients of the eddy diffusivities of heat and momentum.

B. EDDY DIFFUSIVITIES OF HEAT AND MOMENTUM

By the definition, the eddy diffusivity of heat is ~

- W,h E q = ___

ahiar

and the eddy diffusivity of momc iltum is

- W, Wrt E, = a WJar

A slug flow is a flow ~ v i t l i il uniform (over the pipe cross section) velocity profile.

HEAT TRANSFER AND FRICTION IN TURBULENT PIPE FLOW 511 ~-

If in Eqs. (7) and (8) we express W,h and W,W, in terms of E~ and E, and consider that & / i r ~ C,, aT/& (as pressure is constant over the cross section), then the expressions for heat flux and shear stress become

q = (A + pCDcO) aT/ar (13)

Though we have introduced eddy diffusivities of heat and momentum it is still very difficult to determine pW,h and pWsWr. These difficulties remain in the determination of eq and E , which should be considered as unknown functions of the independent variables and the averaged flow properties.

We might detertnine the eddy diffusivities of heat and momentum from semi-empirical turbulence theories such as Prandtls mixing length theory or Karmans local similarity theory. But the relations arising from the semi-empirical turbulence theory do not give the correct description of the eddy diffusivities of heat and momentum near the wall and near the pipe axis. More reliable information on the eddy diffusivities of heat and momentum may be obtained on the basis of experimental data correlations in light of the semiempirical turbulence theory. I n the case of fully developed flow with constant physical properties, as is seen from Eq. (6), the shear stress changes linearly along the radius, i.e., u = a,R. Substituting this relation into (14) and transforming to universal coordinates while nondimensionalizing, we obtain

~-

where q = W,/Z,* is the dimensionless velocity, 1 = v*y/v is the universal independent variable, E * = ( ~ , / p ) l ~ is the so-called friction velocity,y = (yo - Y) is the coordinate reference point at the wall, and

Thus, for the experimental determination of the eddy ditfusivity of momentum E, we need to measure only the velocity distribution along the tube section.

Many investigators have measured velocity profiles in circular and plane tubes in a turbulent isothermal flow. The surveys of these investigations can be found i n :I number of books and articles (20-23). The results of measurement show that the dimensionless velocity p and the dimensionless eddy diffrisivity of momentum E J V are continuous

10 = 71*r,/v.

512 B. S. PETUKHOV

functions of 7, and near the axis they are also functions of the radius R (or of the Reynolds n ~ m b e r ) ~ :

v = drl, R) = d17, Re) EuiV = (Eo/V)(% R) = (~U/V)(% Re)

For 7 -0, y = 7 and ~ ~ - 0 . At small 7, E, - q m , where m 2 3. Within a range of 71 > 30 and R > 0.85, Prandtl's logarithmic law of velocity distribution is valid:

(16)

( K and A are constant) and consequently E, - 7. Finally, when 7 > 30 and R < 0.85, y and E , depend both on 7 and R; when R m 0.5, E, passes through a maximum and tends to some constant quantity while approaching the axis.

Some authors suggested empirical and semi-empirical relations for the eddy diffusivity of momentum for an isothermal pipe flow. These relationships take into account the above-mentioned aspects of the variation in E, . We shall discuss only those which will be of use in further investigation. In the range 0 < 77 < 26 Deissler (ZZ, 24) suggested the equation

v = ( l / ~ ) In 7 + A

E,/V = n 2 q y [ l - exp(--n2~7)] (17)

where n = 0.124. For 7 > 26 he recommends using Karman's equation obtained with the assumption of local similarity of the velocity field:

where K = 0.36 Deissler has noted that the velocity profile in a turbulent core

calculated from (18) for E

HEAT TRANSFER .4ND FRICTION I N TURBULENT PIPE FLOW 513

For 0 < q < 50:

For q > 50:

where K 1 0.4 and qr1 = 1 1 . With large Prandtl numbers the main temperature change occurs

directly in the vicinity of the wall. In this case, for the calculation of heat transfer, i t is very important to describe turbulent transfer processes near the wall correctly. shall analyze Eqs. (17) and (20) from this point of view. .At small q Eqs. (17) and (20) can be simplified by series expansions of the exponential function and hyperbolic tangent, and by considering only the first t \no terms of the series. In addition, we can take - 71 in (17) which results in

and

c, , 'u = c174

E , , / U = Cn'73

where ~ n4 = 2.365 )i and f 2 = K/3qn2 = 1.102 'I\ lo--3. Thus, a t small 7 1 , according to Deissler co - q4, and according to

Reichardt E , - qB; also Eq. (17a) gives lower E, values than those predicted by Eq. (20a).

I n the literature there is no consensus on the value of the exponent rn in the equation for the region close to the wall i, = cq"' (here i,, = E , , / u ) . We can only say from theoretical considerations that m :: 3.4 Being inaccurate, measurements near the wall do not produce a reliable value for m. \Ye can indirectly infer m values from comparisons of the predicted values of heat and mass transfer mith experimental data at large Pr or Sc. Such a comparison was made in (25) on the basis of a statistical analysis of the experimental data on heat and mass transfer which shows that m ranges from 3 to 3.2. Apparently, this is close to reality.

Thus, Eq. (20) which gives m -~ 3 is more likely to describe the mechanisms of turbulent transfer near the wall than Eq. (17). Equations (20) and (2 I ) have some other advantages over Eqs. ( 1 7)-( 19). The relationship for c , : ! ~ , suggestccl b y Reichardt, has no discontinuities in the range q -. 50 Lvhich is important for the calculation of heat

* Lye can show by thi. continuity c.cluation that the function

5 14 B. S. PETUKHOV

transfer and skin friction. Besides, Eq. (21) takes into account changes in EJv with R (and with Re) in the central part of the tube, and on the tube axis it gives a nonzero value of E, which varies with Re that is in full agreement with experimental data. This means that the velocity gradient calculated from Eq. (21) is zero along the tube axis. Therefore, we shall use mainly Eqs. (20) and (21). In Fig. 1, E,/V versus 7 is plotted for

FIG. 1. c,/u 1s 7 and Re according to Reichardts data.

various Re numbers as calculated from Eqs. (20) and (21). Nevertheless, it should be noted that, with small Pr numbers, calculations of heat transfer (in the case of constant physical properties) using Eqs. (17) and (18) and (20) and (21) give similar results (see Section 111). The problem of heat diffusivity in a turbulent flow has been investigated less than that of momentum diffusivity. If we proceed from Reynolds concept that the turbulent diffusivities of heat and momentum are identical, we should take the ratio of the eddy diffusivities p = e , / ~ , = 1. As a matter of fact, Prandtls mixing length theory gives the same result. Reichardt (7) made an assumption that the p value near the wall equals unity, and it increases as the distance from the wall increases.

Having measured velocity and temperature profiles in a flow, and using Eq. (5) and relation ( I l ) , we can determine the eddy diffusivity of heat cq from this experinwntal data. Unfortunately such measurements

HEAT TRANSFER AND FRICTION I N TURBULENT PIPE F ~ o n 515

are not numerous. Resides, the accuracy of experimental values of eq is rather poor (especially in the wall region). The majority of measurements was carried out with liquid nictals and seldom with air. In this paper we shall not discuss the problems of eddy diffusivity and heat transfer for liquid metals. The measurements with air (26-29) show that at some small distance from the wall /3 : 1.2-1.5, but it decreases when the distancc from the wall and Re increase. However, some other workers (30) report that ,B is approximately unity near the wall and it increases with the distance from the wall. Lack of experimental data has as yet prevented us for determining the general relations for eq or p over a wide range of Re and Pr.

Presently, in predicting heat transfer, ,B is usually taken as one (a t least for Pr 2 I ) due to the ambiguity of estimating 8. This is justified since the results at constant properties predicted with such a n assumption are in good agreement with experimental data.

If the physical properties change with temperature, i t becomes necessary to take into account the influence of variable physical properties on the turbulent diffusivity expressions. As is known, this problem has not yet been systematically investigated. Therefore, a solution is usually based on some assumptions, and their validity can be confirmed only indirectly by comparing predicted values of heat transfer and skin friction with experimental data.

Deissler (11, 24) assumed that, to calculate the eddy diffusivity of momentum with variable physical properties, relations (17) and ( 1 8) may be used although they were obtained at constant physical properties. I n these relations the kinematic viscosity is considered a variable. In accordance with such an assumption the equation for E,!V takes the form

where e7.r = H'3!zw*, 7 u z'w*y;vu., vw* = (aw'p,)' '"

Constants n and K have the samc values as in the case of constant physical properties. Equation (22) is recommended when vw < 26, and Eq. (23) is used when rlW ':. 26.

Goldmann (29) suggested a method of calculating E , , with variable physical properties using the hypothesis that local turbulence characteristics a t a given point depcncl on physical properties a t that point and do not depend on physical properties changing in the vicinity of that point. In light of this hypothesis, Goldmann has come to the conclusion

516 B. S . PETUKHOV

that the velocity distribution with variable physical properties is described by the same relations between the generalized variables y+ and T + as those between the variables cp and q with constant physical properties. The generalized variables are described by

If p and v are constants, then cp+ = y and q+ = 11. Transforming in Eqs. ( 1 5) , from variables y and q to y+ and q+ we can

see that for the determination of E,/U with variable properties we can use the same relations as in the case of constant properties provided y and q are substituted for y+ and q+, respectively.

The comparison of predicted heat transfer results for a gas at high T,/T, values with experimental data shows that the calculation method for c0 with variable properties suggested by Goldmann produces better agreement with experimental heat transfer values than Deissler's method.

The ratio of the diffusivities of heat and momentum at variable properties, as in the case of constant properties, is usually taken to be one (/3 = 1).

C. ANALYTICAL EXPRESSIONS FOR TEMPERATURE AND VELOCITY PROFILES,

Considcr the problem of turbulent fully developed quasi-steady flow and heat transfer in a circular tube, assuming that the fluid is incompressible and its physical properties display some arbitrary temperature dependence. The problem is analyzed for the case of constant heat flux which is prescribed at the wall (qw = const) (14).

If the assumptions of Section 11, A are applied, this problem may be described by the energy and momentum equations ( 5 ) and (6). The left-hand side of Eq. (6) can be written as

HEAT TRANSFER, AND SKIN FRICTION

As earlier, a@W,)/ax w 0 was assumed (see Section 11, A). Taking all this into account and also using relationships (13) and (14) we obtain the equations

ah 1 a pw,- = --( r!?) ax r ar

- a ( P + pW2) = - ax

l a - - (ra) r ar

HEAT TRANSFER A N D FRIC.TION I N TURBULENT PIPE FLOW 517

where

In order to transform Eqs. (25) a n d (26) to ordinary differential equations, we shall make the additional assumption that the derivatives with respect to .x' on the left-hand side of Eqs. (25) and (26) are constant across the tube cross section, i.e.,

?k?1 ,Y = f@) (27) ?(P 1 pIIl,Z) i'.v = f&) (28)

M'ith variable physical properties and especially with variable heat capacity and constant heat flus at the wall, assumption (27) holds to a greater degree than the usual assumptions of linear change of q along the radius o r of uniform (over the section) longitudinal temperature gradient

For liquids and gases flowing at sniall subsonic velocities, the pressure gradients due to longitudinal density changes a/ax(pW.,.P) are, by far, smaller than the total pressure gradient dP/dx. Since P does not change along the tube section assumption (28) is also well founded under these conditions.

First, analytical expressions must be found for the enthalpy and temperature fields and the Nusselt number.

Multiplying Eq. (25) by Y dr, taking into account assumption (27), and integrating with respect to the radius from 0 to ro , we obtain

[aTjax = f(.)].

- l l l / t , V 2qw/pW,ro (29)

where gvz is the (over the section) bulk velocity. Substituting (29) into (25) and integrating from 0 to Y ~ v e obtain the expression for the heat flux distribution along the radius

where R r / r , . Since the pressure P is uniform over the tube section, we have

dh/& = CD a T p r

Using this relationship, we can write Eq. (13) in the form

q = (A/C',])(l ! .,/a) ah/&

518 B. S. PETUKHOV

or

9 ~ (WXl + P Pr %/V) where /3 = en/c0 . Simultaneous solution of (30) and (32) gives

where d = 2r0 is the tube diameter.

equation Having integrated (33) from R to 1, we obtain an enthalpy distribution

where h, is the enthalpy at the wall.

we find the analogous temperature distribution equation Solving (30) and (13) simultaneously and integrating from R to 1,

Now we shall calculate the bulk enthalpy (A,,), or, to be more exact, the enthalpy difference h, - h, , By definition,

Substituting h, - h from (34) into (36) and integrating by parts, we obtain

Now let us introduce the heat transfer coefficient from the definition

and the Nusselt number

where T,. and T,, are the wall temperature and the bulk temperature,


respectively; A, is the value at temperature T, ; cp is the average specific heat of the fluid within the temperature range Tb to T, ,

Substituting h, - 12, from (37) into (39), we obtain the expression for the Nusselt number

For constant physical properties, wc can reduce Eq. (41) to the well- known Lyon integral (8)

Now we shall deduce analytical expressions for the velocity profile and the friction factor.

Having multiplied Eq. (26) by r dr and taking into account assumption (28), we integrate first with respect to r from 0 to r , and second from 0 to r,, . Dividing the first of the obtained expressions by the second we get

a = o,R

This result is the consequence of assumption (28) and it essentially means that we approximated the real shear stress distribution (along the radius) by a linear one. Having substituted u into (14) and integrated from R to 1 we get the velocity distribution equation

where pw is the dynamic viscosity at temperature T, . The bulk velocity over the section is

pwx - 2 1' pWrR dR - 0

After having substituted W, from (43) we obtain

where p,,. is the density at temperature T, .

520 B. S. PETUKHOV

By definition, the friction factor is

5fw = 8 ~ w P w / ( P w , ) 2 (45)

Substituting Fz from (44) into (45), we obtain the expression

where

Rew = p, d/pW If the physical properties of the fluid are constant, expression (46)

takes the form

where Re = P d / p

Using the equations derived and the successive approximation method, we can calculate the Nusselt number and friction factor for a fluid with variable physical properties. For convenience of calculation in Eqs. (35), (41), (43), and (46) we should transform from the dimensionless radius R to the universal coordinate qw . The terms R and qw are connected by the following relation:

where R = 1 - V W / V O W

T W = . w * y / v w , Tow = ~ W * ~ O / ~ W , v w * = (.w/f w)12

The procedure used to calculate is as follows:

1. Values of ?low, q,vd, T, , and P are prescribed. 2. Having chosen one of the relationships for E,/v, the first approxima-

tion of the velocity and tcmperature profiles is calculated from Eqs. (43) and (35). Physical properties of the fluid are assumed constant and equal to their values at the wall temperature.

3. The distribution of the physical properties over the pipe section is calculated from the obtained (in the first approximation) temperature profile. Then the same equations determine co/v and the velocity and temperature profiles for the second approximation, physical properties variation being taken into account. Then, in the same way, the temperature profile is again calculated for the third approximation and all


higher approximations. The calculation is performed till the difference in temperature distribution of the (n + 1)th and nth approximations becomes smaller than some prescribed value within the range of which we can neglect the change of the physical properties.

4. E,/Y and the velocity distribution are calculated from the physical properties distribution obtained in the last approximation. Then from Eq. (41) we can find the value

Since q\,,d and T, are prescribed, then from the last relation we can find the average bulk enthalpy h,) and the appropriate bulk temperature T,, . After that the number Nu, = qwd/X(Tw - Tb) is determined. Using Eq. (46) we can calculate the friction factor ew .

The procedure is simplified in the case of constant physical properties, where the distribution of E , / V and a velocity profile are calculated, and Nu and [ obtained from Eqs. (42) and (47).

Naturally, numerical calculation demands the use of computers.

111. Heat Transfer with Constant Physical Properties

A. ANALYTICAL RESULTS

Consider the heat transfer solution for the case of a fully developed flow with constant properties i n a circular tube with constant heat flux at the wall. T h e calculation has been done by Petukhov and Popov (14) by the method discussed in Section 11, C. The eddy diffusivity of momentum was calculated from Keichardts Eqs. (20) and (21); j? = E , J ~ ~ was taken to be one. T h e calculation was done for Re - 104-5 x lo6 and Pr - 0-2000. The method of calculation takes into account the variation of the heat flux q and shear stress u along the radius. In order to estimate the errors which could appear with the assumption of uniform q and u along the radius, several values of Nu and E were calculated for q = qw and u = u, . The ratios of thc corresponding values of N u and 5 for varying q and u to these values for q = qw and u = a, are tabulated and presented in Table I . As can he seen from the table, the assumption of uniform q and u produces noticeable errors in N u and [ values, especially for low Re and Pr.

Nu vs Pr for various lie, according to the predicted values, is plotted in Fig. 2 (for Pr 3, 0.5). The calculations over the range Re and

522 B. S. PETUKHOV

1 10

100 lo00

TABLE I

N ~ / N ~ g = g ~ . u = o ~

1.14 1.11 1.10 1.09 1.08 1.07 1.08 1.06 1.05 1.07 1.06 1.04

l/lu=uw 1.17 1.12 1.10

RATIO OF Nu AND ,$ CALCULATED TAKING INTO ACCOUNT CHANGES OF q AND o ALONG THE RADIUS TO THEIR VALUES WHEN Q = Qw AND O = O W .

105 106

FIG. 2. Nu vs Re and Pr from predictions by Petukhov and Popov (solid lines) and Deissler (dotted lines).

HEAT TRANSFER AND F R I ( T I O N IN TURBULENT PIPE FLOW 523

Pr - 104-5 x lo6 and 0.5-2000, respcctively, are described by the interpolation equation

where

5 = (1.82 log Re - 1.64)~ (49)

Kl( [ ) = 1 + 3.45, k,(Pr) = 11.7 + 1.8 Prr1I3 The disagreement of the predicted Nu with Eq. (48) is within 1 yo

except for the ranges 5 J lo5 ~:c Re < 5 x lo6 and 200 < Pr < 2000 where it is 1-2 yo.

If in Eq. (48) K, and K , are taken constant and equal to 1.07 and 12.7, respectively, the equation becomes simpler:

((it%) Re Pr 1.07 + 12.7(t/8)1/2 (Pr2I3 - 1) Nu =

This equation suggested in paper (13) describes the predicted results with an accuracy of 5-6% over a range of 104-5 x lo6 for Re and 0.5-200 for Pr, and with loo/;, accuracy for 0.5 < Pr < 2000 and the same range of Re.

With very large Pr number an analytical expression for Nu (or Sh)5 may be obtained. In this case the integrand in Eq. (42) decreases rapidly when the distance from the wall increases and it becomes negligibly small at low q = z*y/v. Expression (20a) for E,/V is true for low +I. Substituting E,/Y from (20a) into Eq. (42), assuming R w 1 in the integrand expression, and making a change of variables from R to q, we obtain

Performing the indicated integration yields

In calculation of mass transfer instcad o f Nu and Pr numbers. thett- ditriisional analogies Shcr\vood (Sh) and Schmidt (Sc) numbers are used.

524 B. S. PETUKHOV

where

4 = 0.0855 E ( = - 3 43 2n

At Pr 3 100, Eq. (51) agrees with the results of more accurate numerical calculations to within an error of 10% and within 2% for Pr 3 1000. For small Prandtl numbers Eq. (51) produces overestimated Nusselt numbers which increase with decreasing Pr and increasing Re.

It is of interest to compare the results of this analysis with similar calculations of other authors. Deissler (22, 24) calculated the heat transfer for constant properties over the ranges 4 x lo3 < Re < 2 x lo5 and 0.73 < Pr < 3000 by using Eqs. (1 3) and (14) and assuming q = pw and cr = a,. From Eqs. (17) and (18) he calculated an eddy diffusivity assuming p = I . Sparrow et al. (31) calculated the heat transfer over the range lo4 < Re < 5 x lo5 and 0.7 < Pr < 150 by directly solving the energy equation. Here they used Deisslers expression for the velocity profile and eddy diffusivities, Eqs. (17) and (19), assuming p = 1. Deissler and Sparrow et al. produced similar results (it is natural since actually they used the same expressions for E ~ / v ) . Therefore in Fig. 2 the results of the analysis discussed earlier are compared only with the results of Deissler obtained over a wider range of Prandtl numbers.

With Pr < 10-20 the results of the earlier analysis are in good agreement with the results of Deissler. However, for higher Prandl number the results disagree. When Pr = 100 the Nusselt number according to Deissler is 15 yo lower and when Pr = 1000 it is 25 yo lower its value calculated by Eq. (48). For very large Pr (or Sc) [Pr (or Sc) - 2001 Deissler recommends the equation where

Ku = KD(f/8)1/2 Re Pr1/4 (52)

K D = 2 42nI.rr = 0.112

was obtained with the assumption that the eddy diffusivity of momentum eo/v is described by Eq. (17a).

Unlike Eq. (51), the expression (52) gives a weaker dependence of Nu on Pr. That is why at very large Prandtl numbers (or Sc) Nu (or Sh) calculated from (52) appears to be lower than that calculated from Eq. (51) (at Sc = lo5 it is lower by approximately a factor of two). Only by comparing the predicted results with the experimental data can we solve the problem to the extent that the predicted results correspond to reality. Before the discussion of the experimental data we shall note a very important fact.


The predicted results listed in this paragraph refer to the case of heat transfer with constant heat flux at the wall (q , = const, Nu =- Nu,). If heat transfer occurs at a constant wall temperature (t,% - const, Nu = Nu,) the relation of Nu with Re and Pr differs from that for the case when qw = const. However, theoretical analysis carried out in (9 ,32 , 33) reveals the following: the difference in Nu, and Nu, (when Re and Pr are the same) takes place only when Pr numbers are small, i.e., mainly for liquid metals. When both Pr and Re increase this difference decreases rapidly. When Pr ~ 0.7 and Re = lo4, Nu, is already only 4% greater than Nu, . \Then lie increases to lo5 this difference decreases to ~ 2 ' ) ~ ) ) and at Pr ~ 10 and K 2 lo5 the difference is less than 1 o/o.6 Thus, when Pr 2 0.7 and Re 2 lo4 the results of the heat transfer calculations both at q,, ~ const and at t , = const are valid.

B. EXPERIMENTAL DATA

A great number of experimental papers on heat transfer in turbulent pipe flow have been published. LJnfortunately, in many cases measurement accuracy was not high; therefore, heat transfer coefficients obtained experimentally often contain substantial errors which are difficult to estimate. Little experimental data of rather high accuracy have been reported in recent years (34-37). Mainly heat transfer for air and water flow has been measured, i.e., approximately over a range of 0.7-10 for Prandtl numbers. Only a few authors have obtained heat transfer data at Pr from 10 to 100-150 and a little higher. Heat transfer measurements were not performed for Pr - 1000 because of the great experimental difficulties. Therefore mass transfer experimental data were used over the range of Pr (Sc to be more exact) for which the exact analogy between heat and mass transfer processes was valid.

For comparison with the predicted results the most reliable experimental data have been chosen. The main characteristics of the data are given in Table 11. The heat transfer experimental data were extrapolated to the zero wall and flow temperature difference to avoid the affect of the dependence of the fluid physical properties upon the temperature. In some cases such extrapolation was performed rather accurately, while in other cases only approximately. Naturally the mass transfer experimental data need not be extrapolated because all the measurements were performed under isothermal conditions.

In Fig. 3 the predicted results described by Eq. (48) are compared with

All the listed results are for the case of fully developed Nu number. The difference between Nu,, and Nu, can be larger at the thermal entrance region.

526 B. S. PETUKHOV

TABLE I1

THE MAIN CHARACTERISTICS OF HEAT AND MASS TRANSFER EXPERIMENTAL DATA

Ref. Symbols Fluid Io/d" I / d " R e . Pr or Sc

Volkov and Ivanova (38)

Petukhov and Roizen (39)

Sukomel and Tsvetkov

Allen and Eckert (34)

Sabersky (37) Dipprey and

Yakovlev (36) Malina and

Sterman and

Hamilton ( 4 f ) b

Sparrow (35)

Petukhov (40)

Air

Air

Air. helium

Water

Water

Water Water, oil

Monoisopro-

Water and pyldiphenyl

water solutions of glycerine and metaxyl

0

40

50

96

48.5

-4 96

0, 30

75

48-370

39

80

30

46.2

70-80 30

89-125

13

12.5-350 -0.7

15-280 -0.7

9-40 0.67-0.71

13-110 8

150 1.2-5.9

19-140 2-12 12-100 3,48, 75

22-260 12-35

10-100 430-105

a lo/d and l /d are relative lengths of calming nonheated and heated sections. In Hamilton (41) an experimental tube wall made of benzoic acid served as a solid

phase.

the experimental data. The ordinate represents the ratio of the experimental Nusselt numbers to Nu,, predicted by Eq. (48) both at the same Re and Pr. The abscissa represents Re or Pr. As seen in the figure, experimental and predicted data are in good agreement. The divergence

'.' Nu0 1.0 ( 0 )

ao 4 6 3 10' 2 4 R e

I 2 4 6 a 1 0 2

FIG. 3. Experimental Nusselt numbers (Nu), predicted Nusselt numbers (Nuo): (a) air, (b) liquids (for symbols see Table 11).

HEAT TRANSFER AND FRICTION I N TURBULENT PIPE F ~ o w 527

of the experimental data from predictions does not exceed 5-6(), (except for a few points), this being ivithin the range of accuracy for both predicted and experimental data.

In Fig. 4 the predicted results are compared with the mass transfer

FIG. 4. Comparison of niass transfer rxperimental data (circles) n i th the analytical results of Petukhov and Popuv (solid lines) and Deissler (dotted lines).

experimental data at large Sc. Ihe solid curve in the lower plot corresponds to Eq. (48) (when Sc . lo3) and to Eq. (51) (when Sc . > 10. I n the upper plot the solid curve corresponds to Eq. (51). The dotted lines show Deisslers calculations (when Sc is approximately lo3 and higher, the curves being drakvn according to Eq. (52)). From this figure, Eqs. (48) and (51) are in quite good agreement with the cxpcrimental data over a wide range of Sc (up to 10. In comparison with the experimental data Ileisslers calculations produce lower Sh numbers. Therefore, when Sc =- lo3, the difference is approximately 20,,, and when Sc = lo5, the differencc increases to 50%.

The analysis shows t h a t , for practical heat and mass transfer calculations over a range of Pr or Sc - 0.5-IO3, Eq. (48) should be used, but for calculations in the range of Pr or Sc - 103-10, Eq. (51) is valid. Equation (50) may also be used for Pr or Sc ranging from 0.5 to 200. As mentioned above, these equations describe the range of Reynolds numbers from lo4 to 5 x 106.7

With high Re and simultaneously hiah P r or Sc, the validity of Eqs. (48). (50). and (51) has not yet been verified experimentally due to the absence of experimental data. I-loweTw, from the theoretical considerations \ve can assume that with high Re they +re also in good agreement with expcrimental data.

528 B. S. PETUKHOV

Empirical equations of the following type are widely used in practice:

Nu = c Rem Prn (53) Comparing Eq. (53) with Eqs. (48) and (51) it is easy to see that with

Eq. (53) at constant c, m, and n it is impossible to describe to a reasonable accuracy the change of Nu number with Re and Pr over a wide range of these parameters. A direct comparison of Eq. (53) with experimental data leads to the same conclusion. Allen and Eckert (34) have shown that for Re - 1.3 x 104-11 x lo4 and Pr = 8, Eq. (53) (when c = 0.023, m = 0.8, and n = 0.4) produces an error of up to 20%. An equation of the type (53) can be used for Nu = Nu (Re, Pr) only assuming that c, m, and n are functions of Re and Pr. For Re - 104-5.106 and Pr (or Sc) -0.5-105, m changes from 0.79 to 0.92, while n varies from 0.33 to 0.6.

IV. Heat Transfer and Skin Friction for Liquids with Variable Viscosity

A. THEORETICAL RESULTS

For liquids (condensed medium) far from their critical point only dynamic viscosity varies greatly with temperature; all the other physical properties (p, C, , A ) depend on temperature rather weakly. Therefore while investigating nonisothermal liquid flow, a model with variable viscosity may be used as a good approximation, other physical properties being assumed constant.

Deisslers paper ( I I ) should be noted as one of the papers devoted to the analysis of flow and heat transfer for liquids with variable viscosity. His analysis reduces to the simultaneous solution of Eqs. (13) and (14) in dimensionless form by means of the successive approximations method. As in the case of constant properties, q = qw and u = uw are assumed. The eddy diffusivity of momentum is calculated from Eqs. (22) and (23), and /3 is taken to be one. The variation of viscosity with temperature is taken into account only near the wall (7 < 26). T h e temperature dependence of viscosity is formulated as

where K is a constant; K varies from - 1 to -4. As for the turbulent core viscosity, it is considered constant here.

Deissler has calculated heat transfer and skin friction over the range of 1-103 for Pr and 4 Y 103-2 x lo5 for Re. His results are given as Nu = Nu (Re,Pr,.) [where the subscript x means that the physical


properties are evaluated at the characteristic temperature as defined below] and 5 = ((Re,) for constant physical properties, if in calculating Re, and Pr,r the values of dynamic viscosity are taken at the reference temperature:

Pr vs x is plotted in Fig. 5 for tmth heating and cooling of the fluid. The upper plot may he used for the calculation of heat transfer, the plot

t , = . l ( t w ~ t b ) + tb (54)

0 I I I 1 I I I l l I x'

I I l l I I I l l loo /om /U

Pr

FIG. 5 . Reference temperature for heat transfer (a) and friction (b) calculations by Deissler's method for fluids with vari;lhlc viscosity. Key: solid line, heating of fluid; dashed line, cooling of Ruid.

below is for the calculation of skin friction. The values of .y: in Fig. 5 correspond to K ~ 1 .O to -4.0 and pl,,/po - 0.5-2.0 (approximately). The exponent K does not affect strongly the shape of the curves x(Pr); nevertheless, the curves appear to be quite different in the cases of liquid heating and cooling.

Thus, according to Deissler's theory the effect of variable viscosity on heat transfer and skin friction does not depend on Re and changes only with Pr. However, when Pr : . 10, the reference temperature for the calculation of heat transfer varies weakly with Pr and is close to the arithmetic mean of t,, and t II .

In the following paragraph we shall discuss the agreement between the predicted results and the experimental data.

530 B. S. PETUKHOV

B. EXPERIMENTAL DATA AND EMPIRICAL EQUATIONS

Heat transfer and skin friction experimental data obtained under the conditions of essentially varying viscosity are not numerous; with rare exception they are not accurate and often do not agree well with each other. Therefore we shall use only that small amount of data which may be considered the most reliable.

Analyzing the experimental data we assume that the relation of Nu with Re and Pr, and the relation between 5 and Re at variable physical properties (in this case at variable viscosity) is the same as in the case of constant properties. This assumption is confirmed by the experimental data for liquids with variable viscosity, gases with variable physical properties, and certain other cases (see the following sections).

Of course, assuming a similar variation of Nu with Re and Pr, and 8 with Re for both constant and variable physical properties is only approximate. For example, Allen and Eckert discovered experimentally (34) (see Fig. 6) that variable viscosity affected heat transfer to a greater

4.4

f.3

2 12 z \

z 1.i

LO 1.r 1.2 f.3 1.4 16 f.8 2.0 2.5 3.0

p b ' p w

FIG. 6. Variable viscosity influence on the heat transfer in heated water: 0, 0 , A , v , and are Allen and Ikkert's experimental data for Re = 13,000, 20,500, 35,500, 62,500, and 110,000 at I'r 8. I , Deissler's predictions; 11, Nu/Nu, = (pb/pW)O.'*; 111, Nu/Nu, (/Jb/pw)".".

or lesser degree depending on Re. This result is of great interest and deserves to be studicd in more detail. However, the error in Nu arising from not including the influence of Re on Nu and p,,/pw is several percent. Because of the absence of systematic data it is impossible at present to take into consideration such effects.

If we proceed from the given assumption, the effect of variable

HEAT TRANSFER AND FliICTIoN IN TURBULENT PIPE FLOW 531

viscosity on heat transfer and skin friction can be estimated from the following relationships:

Nu = fNu(Pw/Pbh 6/60 f&w/Pb) ( 5 5 )

where N u and [ are the Nusselt number and the friction factor at variable viscosity obtained experimentally; Nu,, and so are the same numbers calculated by assuming constant physical properties with Re and Pr the same as for the corresponding Nu and [; pw and pb are the dynamic viscosities at T , and T,, , respectively. All the physical properties (except pw) in the expressions for the dimensionless numbers are calculated at the bulk temperature t , , for the given tube section.

In Fig. 6, Allen and Eckerts experimental data (34) on heat transfer for the case of water heating are compared with Deisslers predicted results and some empirical relationships. As is readily seen, the predicted results describe qualitatively the effect of variable viscosity on heat transfer but produce quantitatively overestimated values.

The relative change of heat transfer due to viscosity dependence on temperature can be expressed by the equation

W N u , = (Pb/PWY (56)

where Nu, is calculated from Eq. (48) or (50) and the n expression is determined from the experimental data.

As we can see from Fig. 6, n = 0.14 as suggested by Sieder and Tate (42) is overstated; n = 0.1 1 as suggested in (13 ,43) corresponds to experimental data for liquid heating better than n = 0.14.

To choose the correct value of n in Eq. (56) the heat transfer experimental data corresponding to heating and cooling for several liquids over a wide range of values pw/pl, (the main characteristics of these data are presented in Table 111) where treated. The results of the treatment are

TABLE 111

THE M A I N CHARACTERISTICS OF HEAT TItANSFEH EXPERIMENTAL DATA FOR F L L I D S \VITH V A R I A R I . I I VISCOSITY

Ref. Symbols Fluid I/d R e . Pr Pw/W

Yakovlev (36) 1 Water 70-80 19-123 2-12 0.19-0.77

Kreith and Butyl alcohol 38 42-78 23-30 0.08-0.45 Summerfield (45)

Petukhov ( I 3) Iransformci- oil 88 5-44 39-61 I .2-8.6 Petukhov (13) u 0 1 1 M S 88 5-14 134-140 1.6-38

532 B. S . PETUKHOV

FIG. 7. Variable viscosity influence on the heat transfer in different fluids for heating and cooling (for symbols see Table 111).

given in Fig. 7. The averaging curves drawn through the experimental points correspond to n = 0.1 1 when the fluid is heated (pw/pb < 1) and n = 0.25 when the fluid is cooled (pw/&, > 1). The value n = 0.25 is in agreement with Mikheev's recommendation (44). He suggested that one should take into account the effect of variable physical properties by means of (Prb/Prw)0.25 which takes the form (pb/pw)0'25 in the case of varaible viscosity and constant C, and A.

Thus, to calculate heat transfer in a turbulent flow with variable viscosity we can use Eq. (56) with n = 0.1 1 for the case of heating and n = 0.25 for the case of cooling. Equation (56) is valid over a range of 0.0840 for pw/pb , 104-1 .25 x lo5 for Re, and 2-140 for Pr.

Figure 8 illustrates the effect of variable viscosity on the friction factor where the measured values of Allen and Eckert (34) and Rohonczy (46) are presented. The former are obtained with heated water and the latter with cooled water. Deissler's predicted results and some empirical relationships are given in this figure. Both the predicted results and the empirical equation suggested by Sieder and Tate (42)

5/60 = (CLW/P~)O"~

in comparison with experimental data produce a weaker dependence of the friction factor on pw/pb .

The experimental data plotted in Fig. 8 are well described by the following simple equations:

Under heating (pw/pb < 1):

5/50 = &(7 - Pb/Pw) (57)

Under cooling (pw/pb > 1):


p w p b

FIG. 8. Variable viscosity influence on friction in water for both heating and cooling: (0) Allen and Eckert experiments (Re : 13,000-110,000, P r = 8); 0 Rohonczys experiments (Re = 33 . 103-225 . l oJ , Pr = 1.3-5.8). I, Deisslers calculation (when Pr = 8 for heating and P r = 2.3 for cooling); 11, f/[ : ( f L ~ / p h ) ~ ~ ; 111, (/to = 1/6(7 - ph/pW); Iv, f / f o ( f L ~ / f L h ) ~ ~ ~ .

The friction factor in an isothermal flow to is calculated from Eq. (49). Equations (57) and (58) are true over a range of 0.35-2 for p w / p b ,

104-23 x 1 0 4 for Re, and 1.3-10 for Pr. They are probably true over an even wider range of these parameters. However, this should be verified experimentally.

V. Heat Transfer and Skin Friction for Gases with Variable Physical Properties


Consider the analysis of the heat transfer and skin friction for a turbulent gas flow in a circular tube, far from the entrance with constant heat flux at the wall. The solution was obtained by Petukhov and Popov (14) using the method described in Section 11, C. The physical gas properties p , C, , A, p were considered as given functions of temperature. The variation of density with pressure and the energy dissipation in the flow was neglected. Therefore the analysis is valid only for gas flows with small subsonic velocities. The eddy diffusivity of momentum was determined according to Eqs.(20)and(21) and was extended to the case of

534 B. S. PETUKHOV

variable properties by introducing Goldmann's variable (24). The eddy diffusivity of heat eg was taken equal to E, (i.e., p = 1).

The calculations were carried out for air and hydrogen over the following ranges of the characteristic parameters: 104-4.3 X lo6 for Re, and 0.37-3.1 for T,/T,, in air, lo4-5.8 X lo6 for Re, and 0.37-3.7 for T,,,/Tb in hydrogen (here Re, = p d / p b ) .

10

0 8

06

0 4

0 2

0 0 2 0.4 0.6 0.8 I 0

1.0

0.8

0.6

0.4

0.2

0 0.2 0.4 0.6 0.8 1.0

FIG. 9. Distribution (dong radius) of dimensionless flow variables at variable (solid lines) and constant (dotted lines) air properties for Rew N 43 . lo3 and Pr N 0.70-0.71: (1) T w = 1000"K, To == 154"K, Tw/Tb = 3.11; (2) T w = 300"K, To = 902"K, Tw/Tb = 0.383.

Figure 9 illustrates how variable physical properties affect the distribution of the dimensionless flow parameters along the radius: temperature, velocity, mass velocity, heat flux, and eddy diffusivity __ of momentum. All the curves correspond to Pr, and Re, = p W d i p , (these numbers have approximately the same values). Due to the variation of physical properties the temperature profile appears more concave for cooling than for heating. The property variation doesn't affect the velocity profile so strongly as it does the mass velocity profile. For the case of the fluid being heated, the mass velocity profile is flatter than for the cooled fluid. As is seen from Eq. (30) a change in the mass velocity profile results in a redistribution of the relative heat flux along the radius. For heating, when pW, decreases at the wall and, consequently, convective heat transfer along the axis decreases too, a maximum occurs


in the q/q, distribution. For cooling, when the mass velocitv profile is more full, the pip, distribution becomes nearly linear. The property variation also affects greatly the distribution (along the radius) of the relative eddy diffusivity of monientum.

FIG. 10. Heat transfer versus tcmperaturc ratio parameter according to the analys~s.

Figure 10 represents the results of the heat transfer calculations as NuI,/NuO1, vs 8, where Nul, and Nu,,, are the Nusselt numbers for variable and constant gas properties, respectively and the same Re,, and Pr, 8 ; t9 = Tw/Tl , is the temperature ratio parameter. From Fig. 10, the predicted points diverge to a degree depending on the type of gas, wall temperature, and Reynolds number Re,,* = Re, ~ , ~ / p , , . This divergence is not surprising because we cannot take into account the influence of variable physical properties on heat transfer by means of only one temperature ratio parameter. Even so, the divergence of the points is not great. Other things being equal, the error. in Nul, for air and hydrogen does not exceed 1 (I: ,; Nu,, for hydrogen at T,. = 2000K is only 3-54/, greater than at T,. = 10OO"I i ; the error in Nu,, for air and hydrogen when Re,* = 14 ,( lo3 and 10'; is 3 ( ' { , .

* From here on, the suhscripts 1) and w mcan that the physical gas properties are evaluated at temperatures Tt, and T,, \\lien calculating the corresponding dimensionless numbers.

536 B. S. PETUKHOV

If the above-mentioned small errors in Nu, are neglected, the analytical results can be correlated by the equation

NUb/NU,b = 8" (59) where

u = -(a log 8 + 0.36) For cooling, a = 0. For heating, a = 0.3, and consequently n decreases with increasing 6. With these values for n, Eq. (59) describes the solution for air and hydrogen with an accuracy of f4%. For simplicity we can take n to be constant for heating also. Then, when n = -0.47, Eq. (59) describes the analytical results within &6y0. In the case of heated air, n = -0.5 produces slightly better results.

FIG. 11. Friction versus temperature ratio parameter (dots denote Petukhov and Popov's predictions; for symbols see Fig. 10).

In Fig. 11 the predicted friction factor is plotted as tl) 'to,, vs 8. Here [,) = Suwpl,/(~~z)2 and to,) are the friction factors at variable and constant physical gas properties and at the same Re,, , respectively. I t should be noted that, contrary to the case of heat transfer, Re,* greatly affects the shape of thc curve f , , / fo, , vs 8, but as for the wall temperature and the type of gas, their influence is not very significant. The solutions for air and hydrogen are presented by the equation

t b / t " b = 8" where

I I = -0.6 -1- 5.6(Re,*)-0,3s


for heating and

n = -0.6 + 0.79(Rew*)-0.11 for cooling.

Equation (60) describes the calculated results within 2-3 010 over the range 0.37-3.7 for 0 and 14 x 10"-1O6 for Re,*. As Re,* varies over the indicated range, n goes from -0.44 to -0.58 for the case of heating and from -0.32 to -0.42 for the case of cooling. If in Eq. (60) n is taken as -0.52 for heating and -0.38 for cooling, this equation describes the calculated data to within 700 accuracy in the first case and 4", in the second.

Kutateladze and Leontiev (17) have obtained an analytical expression for the functions describing the influence of variable physical properties on heat transfer and skin friction in a turbulent gas flow when Re + 00. By making some assuniptions (u and q, vary identically with respect to the radius for both constant and variable properties, the velocity and temperature fields are similar, C,, = const) they obtained the following relationship:

The authors also recommend this expression for finite Re values, based on the empirical fact that the influence of varying physical properties on heat transfer and skin friction depends weakly on Re.

Equation (61) produces stronger dependence of heat transfer and skin friction on the temperature ratio parameter than Petukhov and Popov's analysis, but the difference is not more than 10% (see Figs. 10 and 11). At large Re Eq. (61) is in good agreement with Eq. (60).

Deissler and Presler ( 4 7 ) analyzed the heat transfer for a number of gases (argon, helium, air , hydrogen, and carbon dioxide), taking into account the temperature dependence of their physical properties. They used the same method (Deisslcr's method) as for fluids with variable viscosity (see Section I V , A), the only difference being in considering all the physical properties of the fluid ( p , C,, , p, A ) as functions of temperature. T h e results are given by the equation

N u , = Re;l4/31 (62)

The subscript x denotes that the physical properties in Xux and Re, are evaluated at the temperature

r . I , . = X(Tw - Tb) + Tb

vl w 00

TABLE IV

MAIN RESULTS OF HEAT TRANSFER EXPERIMENTAL INVESTIGATIONS FOR GAS WITH VARIABLE PHYSICAL PROPERTIES

Ref. Method of accounting variable

Gas lld Reb . B properties influence

Il'in (48) Air 59, 62

Humble, Lowdermilk, and Air

Desmon (49) 30-1 20

Bialokoz nnd Saunders (50) Air 29-72

Weight and Walters (51) Hydrogen

Taylor and Kirchgessner (52) Hclium 60,92

McCarthy and Wolf (53) Hydrogen 43, 67

McCarthy and Wolf (54) Hydrogen, 21-67

helium

7-60

7-300

124-435

3.2-60

7-1500

5-1 500

0. j 6-2.3

0.46-3.5

1.1-1.73

1-4

1.6-3.9

1.5-2.8

1.5-9.9

Nu,, c R c " . ~ 8"

B 0.5-0.9 0.9-1.2 1.2-2.3 W

cn

W 3

c 0.0218 0.0212 0.0223

n 0 - 0 . 2 7 -0.58

Nub = 0.023 R e t 8 P r t 4 B (for x /d > 60)

n = O a t 0 < 1

n = -0.55 at 0 > 1

Nub - 0.022 Pr;,' 0-".5

Nub = 0.021 Re:,8Prr 6-0.575

a t great I /d

Nu = 0.021

p~~~ - 0,023 K ~ o . . " pru.a 8-11.3

Pr?' at great Z/d

~ ) n

Nu = 0.045 P r t 4 - 0-O.' b ( X 0 ' 1 5

d

e

x 3: 0

(b) Local heat transfer:

Wieland (55)

Taylor (56)

McEligot, Magee, and

Leppert (57)

Kirillov and Malugin (58)

Lelchuk, Elphimov, and

Fedotov (59)

Perkins and

Worsoe-Schmidt (60)

Volkov and Ivanov (38)

Petukhov, Kirillov, and

Maidanic (61)

Helium,

hydrogen

Hydrogen,

helium

Air, helium,

nitrogen

Nitrogen

Air, carbon

dioxide, argon

Nitrogen

Air

Nitrogen

250

77

160

138

77-206

160

48-370

80-100

1 2 . 8

1.5-5.6

1.1-2.5

7- 1 60 I . 1-2.3

14-600 1.1-2.7

18-280 1.3-7.5

14-400 1.1-2.1

13-300 1-6

Nuf = 0.021 Prf0.4 3:

far from the entry m

Nuf = 0.021 Pr;.*

z U

far from the entry

Nu,, = 0.02 1 Re::.8 PrtS 0-O.' 2 0

0 z

at . x / d 3. 30 =!

b z b b Nu = 0.021 ReO.fl Pr0.4 e-(l.5 at x/d > 50 and M < 1 4

Nu = 0.024 ReO.fl c

c m m r b b

at x / d > 40

r r -Q

Nub = 0.0193 Re: f l Prl 8 - O 55 m

at r ' d ' 100

0 Nu = 0.021 Re".R Pr:.4 0" at x / d :> 80 <

TZ = -(0.9 log 0 + 0.205) ul w

540 B. S. PETUKHOV

where the parameter x depends on the type of gas, Re,, and T, . At Re, > lo5, x 'v 0.4 for all the gases.

From Fig. 10, Deisslcr's results for air and hydrogen are very close to each other. However, from these results we can see that the change in heat transfer with the temperature ratio parameter is less pronounced in comparison with the results of (14) and (17).

B. EXPERIMENTAL DATA AND EMPIRICAL EQUATIONS

A great number of experimental papers are devoted to the investigation of the heat transfer between the tube wall and the gas flow at large temperature differences when physical properties cannot be considered constant. Table IV presents a schematic summary of the main results. Heat transfer in the case of gas heating for constant heat flux at the wall (and sometimes for variable qw) is the object of the majority of the papers. In the case of gas cooling heat transfer, experimental data are neither numerous nor complete. In the earlier papers investigators measured the average (along tube) heat transfer coefficients. The data obtained from these investigations, especially in short tubes, do not reveal the real relation between the local Nu and 8. Therefore, in recent papers the local heat transfer is the prime subject of the invest igat i~n.~ Heat transfer measurements were performed mainly with diatomic gases (air, N, , H,) and to a lesser degree with monatomic (He, Ar) and triatomic (CO,) gases. The experiments cover the temperature ratio parameter range approximately from 0.4 to 4. In some papers the values of 8 approached 6 and even 10. But such high values of 0 were as a rule found in the entrance region of the tube.

The results of heat transfer measurements at large temperature differences between the wall and the gas flow are usually presented as

Nu b = c Re:* Pr t4 8" (63)

Sometimes 8 is not included in an equation of the type (63) because the influence of variable physical properties is sometimes accounted for by the appropriate choice of a reference temperature. For example, with physical property values chosen at the temperature T, = $(Tb + Tw), the experimental data can be correlated satisfactority.

In the entrance region of the tube the coefficient c and also the exponent n [from (61) and (62)] in Eq. (63) change with x/d. Far from the entrance, i.e., when xld > 40-100, c becomes constant and n becomes independent

Table I V does not provide heat transfer data in relatively short tubes, as heat transfer expressions in the entrance region have not been discussed in this paper.

HEAI TRANSFER AND FRICTION I N TURBULENT PIPE FLOW 541

10 I 2 14 1 6 18 2 0 2 5 3 0 3 5 4 0

FIG. I ? . Heat transfer versus temperature ratio parameter according to predictions and empirical equations.

of x/d. Table IV contains empirical equations obtained by different investigators for distances far from the entrance. In Fig. 12 some of these equations are compared with each other and with Eq. (59) which represents the analytical results. In Fig. 12 the abscissa is the ratio of Nu,, at variable properties to Nu,,,, at constant properties, calculated from Eq. (48).

For the case of gas heating Iable IV and Fig. 12 show t h a t c varies from 0.019 to 0.024, and n ranges from -0.3 to -0.7. This differences is most probably attributed to the fact that n is actually not constant. Its value decreases while B increases. Both theoretical analysis and experimental data (48, 61) confirm this. Therefore, using Eq. (63) as the mathematical representation of the experimental data when n = const, different investigators obtain different values for n depending on the range of 0. Extrapolation of these data (obtained when 0 2 I ) to 0 = 1 leads naturally to different values of c.

For the case of gas heating Fig. 12 shows good agreement of the analytical results Eq. (59) with the experimental data only if the empirical equations are not extrapolated past the limits imposed upon 0 by the experiments. Also, within this restriction a good degree of consistency is observed for all the experimental data considered. Neither the experimental nor the analytical results reveal a noticeable influence of the Reynolds number and the type of gas on the shape of the curve Nub/NuO,, vs 8. Apparently this influence, at least for the gases

542 B. S. PETUKHOV

investigated (see Table IV), is within the range of experimental error. Thus, for practical heat transfer calculations in the case of gas heating when 1 < 0 < 4, Eq. (59) may be used.

As was mentioned above, heat transfer for gas cooling has not been investigated extensively. In particular, there is no experimental data on local heat transfer. Therefore, the analytical results can only be compared with experimental data on average heat transfer. Empirical equations of Fig. 12 show that for gas cooling the heat transfer is larger than when 6' = 1. Over the range of the experiments the ratio Nu,/Nuob was found to be independent of 6'. Perhaps the scatter of points, found in the experiments, prevented the actual relation between Nu, and 0 from being observed. Nevertheless, in the case of gas cooling, even under the restriction stated, the relation between Nub/NuO,, and 0, from experimental data, appears to be significantly weaker than that obtained analytically. Further experiments and theoretical studies are necessary to explain these differences. To obtain new data for practical calculations of heat transfer under the conditions of gas cooling, the simple equation suggested by Ivashchenko (63) from the treatment of Ilyin's experimental data (48) may be used:

h'Ub/NU,b = 1.27 - 0.278 (64)

This equation is valid when 0.5 < 0 < 1 . The skin friction in the pipe flow of a gas at large differences between

the wall and flow temperatures has been studied (48, 57, 60, 62). Unfortunately, the data obtained in these papers are contradictory. This probably stems from the inaccuracy of the measurements (the experimental data differs from the recommended relations by 20-25 yo). Ilyin's experimental data (48) are given in Fig. 11. From these data the value of n in Eq. (60) is -0.58 for the case of gas heating and n = 0 for gas cooling, though here ~ l ~ / ~ o l , > 1. Thus, in the case of gas heating Ilyin's data are in good agreement with the analytical results, but they are lower than the analytically predicted data in the case of gas cooling.

Perkins and Worsoe-Schmidt (60) found that far from the tube entrance ( x / d > 55) whenlo 1 < 6' < 4 the following equation is valid:

where to, is the friction factor in an isothermal flow. In calculating Re,*, the physical gas properties in the expression are evaluated at the wall temperature, i.e., Re,* = (Pdd, 'pw)(pw/pb) . If we assume that

lo At the entrance region 0 values approached 7.5.

HEAT TRANSFER AND FRlc'TION IN TURBULENT P I P E I;I.OlV 543

tow - Re,,, *-0.2 and p - 7'" 6q, it can easily be shown that Eq. (65) 61, 5% = &0.264 (66)

takes the form

But, when the experimental data arc plotted as ( , , / f" , , vs H as indicated by (60), there is considerable sc'itter i n the results. According t o the data of Lel'chuk and Ilyadyakin, the exponent in Eq. (66) should be -0.16 (instead of -0.264), while AlcEligot's work (57) indicates a value of -0.1 for 6 - 1.0-2.4. 'Thus the evperiniental data from Rets. (57 ,60 , 62) differ significantly. T h e general trend of these papers is to suggest a weaker relationship between (,,/[,,,, and 6 than predicted analytically, thus indicating the need for further investigation.

VI. Heat Transfer and Skin Friction for Single-Phase Fluids at Subcritical States


By heat transfer in single-phase media at subcritical states (or, shortly, in the supercritical range) is meant that the heat transfer takes place at supercritical pressures and at subcritical or pseudocritical temperatures (i.e., at temperatures corresponding to the maximum heat capacity at constant pressure).

We can explain heat transfer anomalies in the supercritical region by the fact that physical properties in this region change considerably and in a special way.

Figure 13 shows the behavior of physical properties with temperature. This figure presents the data on carbon dioxide at P = 100 bar. T h e majority of the physical properties do not change monotonically: heat capacity a t constant pressure has characteristic maxima; heat conductivity and viscosity coefficients usually pass through a minimum.

Density changes considerahly but the volumetric coefficient of thermal expansion attains a maximum. 'I'herefore, even though the tcniperature difference in the flow is sniall (T,\ - T,, rn lO-2Oc'C), the physical properties change considerably across the tube. For instance, if T,, < Till < T, (here T,, , is a pseudocritical temperature) as the distance from the wall increases, C,, increases rapidly, goes through a maximum, and then decreases. I n the case of heating density increases rapidly from the wall to the tube asis. T h e variation of physical properties along the radius is especially significant a t high heat fluxes and at large temperature differences between the wall and the fluid, respectively.

If heat transfer takes place at rather small temperature differences then

544 B. S. PETUKHOV

FIG. 13. Carbon dioxide physical properties at P = 100 bar.

irrespective of the change of physical properties with temperature, the calculation of such a process may be carried out by assuming that the physical properties are constant. Therefore, Eq. (48) is also valid for turbulent liquid pipe flow when T, - T,, + O in the supercritical region. Figure 14 depicts the heat transfer coefficient 01 calculated from Eq. (48) for carbon dioxide vs TI, at different pressures P > P,, .

Of course, the behavior of 01 essentially depends on the specific variation of the carbon dioxide physical properties with T and P. American investigators Deissler and Goldmann (19, 64) were the first who theoretically analyzed heat transfer and skin friction at supercritical states, taking into account the variation of the physical properties of the fluid with temperature. In both papers the authors studied heat transfer in a fully developed turbiilcnt liquid flow in a circular tube, the heat flux at the wall being constant (qw = const). Having written Eqs. (13) and (14) in a dimensionless form, they solved these equations simultaneously by the successive approximation method. The distribution of q and along the radius was taken either constant (Deissler) or linear (Goldmann). Deissler and Goldmanns methods differ from one another because they used different methods of calculating the eddy diffusivity of momentum at variable fluid properties. Deissler used Eqs. (22) and (23) and Goldmann determined E , from the universal velocity profile (for constant properties the profile is calculated by Deisslers method) in terms of his own variables ~f and 711 [see relationships (24) ] . In both cases /3 was taken to be one.


FIG. 14. I-Ieat transfer coeficicnt for carhon dioxide in the supercritical region calcu- 100 kgihr). lated \vith the assumption of coristant physical properties (d = 6.7 mni, G

Deissler predicted heat transfer and skin friction for the case of heated water over a temperature range of 204-65OoC, pressure being equal to 344 bar. T h e results tvere correlated by using the reference temperature which proved to be a complicated and nonmonotonic function of T,,./T,, and T,, . lhis function is plotted in Fig. 18. Goldmann also carried out heat transfer and skin friction calculations for the case of heated water at a pressure of 344 bar, but over a wider temperature range of 260-840 C. Ihe analytical solution of the heat transfer is presented by the equation

which is based on the assumption that with both constant and variable

546 B. S. PETUKHOV

FIG. 15. Goldmanns analytical results for water at P = 344 bar (solid lines) in comparison with experimental data (dotted lines; circles represent averaged experimental data). Ranges of paratneters: d = 1.27-1.9 mm, qw = (0.32-9.5) . lo6 W/m*; ,,w = (2-4) lo3 kg/sec-m2. -

properties the heat transfer varies with FW and d in the same way. The functionf(Tw, Tb) is plotted in Fig. 15. If the physical properties were constant f would be a linear function of T,. . Thus, divergence from a linear relation characterizes the influence of variable physical properties.

Using Goldmanns method Tanaka et al. (65) performed heat transfer calculations for carbon dioxide at P = 78.5 bar for both a heated and a cooled fluid. Their results are given in Fig. 16. As is seen in the figure, the graph of the heat transfer coefficient a vs t,, at t,, w t,ll passes through a maximum, which may be attributed to the presence of the appropriate maxima of C J T ) and Pr( 7) (see Fig. 13). The maximum value of a in the case of cooling (qw < 0) is higher than that in the case of heating (qw > 0) and decreases when q,\ increases. This is because the conductivity in the viscous sublayer for cooling is almost twice that for heating, and it decreases when qw and T, increase. For cooling a decreases with increasing qIv at a smaller rate than for heating.

For the calculation of heat transfer and skin friction in a supercritical region, Popov (66) used the method given in Section 11, C. The eddy diffusivity of momentum was determined from Eqs. (20) and (21) extended to the case of variable physical properties by introducing


0 Q U (u

E

3 \

7 - 0

fU

8

6

4

2

FIG. 16. 01 vs t b for carbon dioxide both heated and cooled ( P = 78.5 bar, W/m2: ( 1 ) 0.70; (2) 1.4; (3) 2.8; G = 3.9 x kglsec, d = 10 nim) at qw x

(4) -0.70; ( 5 ) - 1.4; (6) -2.8.

Goldmanns variable (24). The value of f l was taken to be one. The calculation was done for the heating of carbon dioxide in a circular pipe far from the inlet at constant qw , Calculations were made for P/P,, = 1.33 (P = 98 bar).

0.94 < TblTm < 1.24, and 0.97 < TwITm < 1.24.

The results are presented as interpolation equations for heat transfer and skin friction.

Comparison of the analytical results with the experimental data shows that the relation of the Nu number to Re and Pr and to 5 and Re at variable properties is approximately the same as at constant physcial properties. Then the ratios Nu,,/Nu,, and [ , , / f o b depend only on the behavior of the physical properties with temperature. For the given liquid NuJNu,, is a function of T,, , T,,, , and P or T,/T,, , TWIT,,, , and P/P,, , which is more convenient for the comparison of the data obtained for different pressures and liquids.

In Figs. 15, 17, and 18, some predicted values are compared with experimental data and empirical equations. T h e behavior of Nu,,/Nu,, with T w / T m is clearly seen in Fig. 18. When Tb/Tlrl < 1 heat transfer increases with T w / T m increasing; when T w / T , 1, it reaches the maximum and then decreases. If T,, /Tnl 3 1 heat transfer decreases when Tw/T , increases (see Fig. 17).

FIG. 17. cy vs Tw for water at P = 345 bar, T w / T b = 1.25, = 2150kg/m2-sec, and d = 9.4 mm: ( 1 ) Deisslers predictions; (2) Goldmanns predictions; (3) Svenson et al.s empirical equation.

FIG. 18. Nub/Nuob vs Tw/Tm for carbon dioxide at P = 98 bar, Tb = 303K, G = 100 kg/hr, and d = 4.08 mm: ( I ) Popovs predictions; (2, 3, 4) empirical equations (69), (71), and (68), respectively; circles represent experimental data.


I t can be seen from Fig. 17, taken from (67), that Deisslers theoretical results produce lower heat transfer values in comparison with experimental data (approximately two times lower). Bringer and Smith (68), who calculated the heat transfer for heated carbon dioxide by Deisslers method, have found a disagreement of 2 5 O / , with their measured values. Having calculated the heat transfer by using Deisslers method but Prandtls expression for the mixing length in a turbulent core and approximating the influence of the density fluctuations, Melik- Pashaev (69) have obtained better agreement between calculated and experimental data. Goldmanns predicted results (see Figs. 15 and 17) are in good agreement with the experimental data when T, - T,, 5 100C. Disagreement increases with increasing T, - T,, , reaching 25 o/o when T, - T,, = 250-300C. Povovs theoretical results (66) are in agreement with the experimental data for carbon dioxide within 20,,. Figure 18 illustrates this comparison.

Hsu and Smith (70) made an attempt to take into account (approximately) the influence of density fluctuations on the turbulent momentum diffusivity. T h e calculations for carbon dioxide have shown that this correction improves the agreement between the predicted and experimental heat transfer data. However, the predictions for hydrogen (72) with a correction for density fluctuations displayed less favorable agreement with experimental data than the same predictions without the correction. Thus, the problem of accounting for fluctuations due to density and other physical properties has not as yet been satisfactorily solved.

One might be tempted to expect a substantial influence of free convection on heat transfer in a supercritical forced flow, because density changes considerably with temperature. Hsu and Smith (70) analyzed this problem theoretically for the case of a turbulent flow of heated carbon dioxide in a vertical tube. As should be expected, the effect of free convection on heat transfer becomes more pronounced with higher G r and lower Re. For example, when Gr = 108 and Re = lo4 the heat transfer rate increases approximately two times due to free convection; when Gr = 108 and Re = lo5 free convection does not affect the heat transfer.

The analysis of the predicted results reveals that when physical properties do not change considerably over the flow cross section it is possible to satisfactorily describe the heat transfer mechanism in the supercritical region (but not for all possible regimes of flow and heat transfer). The disagreement found between the predicted values and the experimental data can probably be attributed to the inexact analytical methods and to some uncertainity in the estimation of the physical

550 B. S. PETUKHOV

properties at subcritical states.ll In case of significant variations in physical properties over the tube cross section and for some specific flow and heat transfer regimes (the so-called regimes with diminished and enhanced heat transfer, see below) the present theoretical methods do not permit the satisfactory description of heat transfer at supercritical states. In this case considerable disagreement between the predicted results and the experimental data is observed. There is every reason to believe that this disagreement may be attributed to the unsatisfactory estimation of the variable physical properties effect (mainly of density) on turbulent diffusivity. So, for

1970-petukhov- heat transfer and friction in turbulent pipe flow with variable physical properties

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