Correlation for Prediction of Heat TransferCoefficient for Pool Boiling Using

Ti02- NanofluidSudev Dasl, Sagnik Paf, Harinder Singh' and Swapan Bhaumik"

I Mechanical Engineering Department, NIT Agartala, Tripura,India2 Department of Mechanical Engineering, Ill'I, Banipur; WB,India

J Department of Chemical Engineering, Naida Institute af Engg. & Tech. (NIET), Greater Naida/+ Mechanical Engineering Department, NIT Agartala, Tripura.India

E-mail address: sbhaumik65l9@gmail.com,chemicdev@gmail.com

Abstract- The present study represents the developmentof theoretical correlation for pool boiling of nanofluidhaving TiO, as nanoparticle and water as base fluid in amechanically polished stainless steel flat plate as a boilingsurface, where the concentration of the TiO, particles inwater increases from 0.32 wt. % to 0.72 wt. % to observe theeffects on heat transfer coefficient. The theoretical heat fluxcompared with the experimental heat flux which is within±30% error band agreement. Heat transfer coefficient ofnucleate pool boiling using nanofluid increases withnanoparticle concentration at high heat fluxes.Approximately 22% increase in heat transfer coefficient isobserved for 0.72 wt. % nanoparticle concentration.

Thus, newly developed generalized correlation (givenbelow) used knowing different thermal properties ofnanofluid and heating surface for prediction of heat transfercoefficient:

I

h = {(1 +2.Sq»Jj}hjg [g{O- q»P~+q>pp-pv}r-

[{(1- qJ)PfCf+ qJppcp}(Ts- TSQt)kl1jn I~T-T )

{(1 - qJ)Pj + qJPp}Csfhjgl1n/cnjl1 S SQt

I. INTRODUCTION

Conventional fluids, such as water, engine oil andethylene glycol are normally used as heat transfer fluids.Although various techniques are applied to enhance theheat transfer. The low heat transfer performance of theseconventional fluids obstructs the performanceenhancement and the compactness of heat exchangers.The use of solid particles as an additive suspended into thebase fluid is a technique for the heat transferenhancement. Improving the thermal conductivity is thekey idea to improve the heat transfer characteristics ofconventional fluids. Since a solid metal has a largerthermal conductivity than a base fluid, suspendingmetallic solid fine particles into the base fluid is expectedto improve the thermal conductivity of that fluid. Theenhancement of thelma I conductivity of conventional

fluids by the suspension of solid particles, such asmillimeter- or micrometer-sized particles, has been wellknown for more than 100 years. However, they have notbeen of interest for practical applications due to problemssuch as sedimentation, erosion, fouling and increasedpressure drop of the flow channel. The recentadvancement in materials technology has made it possibleto produce nanometer-sized particles that can overcomethese problems. Innovative heat transfer fluids-suspendedby nanometer-sized solid particles are called 'Nanofluids'.

II. MODEL / CORRELATIO DEVELOPME T

This analysis is concerned about pool boiling heattransfer using nanofluids, a subject of severalinvestigations over the past few years. The work ismotivated by the controversial results reported in theliterature and the potential impact of nanofluids on heattransfer intensification. Systematic calculation are carriedout to formulate stable aqueous based nanofluidscontaining Ti02 nanoparticles and to investigate their heattransfer behavior under nucleate pool boiling conditions.The details are elaborated in the following paragraphs.

In this analysis, Ti02 nanoparticles are used in wateras a base fluid for the formulation of nanofluid. As it iswell known, nanoparticles have a strong tendency toagglomerate due to relatively strong Vander Waalsattraction between particles in both dry and wetenvironments. Dry nanoparticles frequency occur in theform of agglomerates, particularly formed due tosintering, and are difficult to break even by usingprolonged ultrasonication and magnetic stirring [1].

In order to prevent formation of agglomerates,surfactants and/or dispersants are often used butsurfactants may experience changes in their propertiesand even fail at elevated temperatures. The electrostaticstabilization method is adopted in the above case. Such amethod makes use of repulsion due to electric doublelayers surrounding around individual nanoparticles.

64 NIET Journal of Engineering & Technology, Vol. 1, Issue 2, 2013

In analytical experiment, nanofluid, of a presetconcentration was prepared and filled into the boilingvessel. The fluid was then preheated to the saturatedtemperature, followed by the measurements in thenucleate boiling regime under the steady state.Temperature data can be recorded at each heat flux, qcalculated by

q=~ (I)

The steady state heat diffusion equation was adoptedto obtain the boiling surface temperature.

The heat transfer coefficient, is calculated by

qh

In the nucleate boiling regime, the rate of heat transferstrongly depends on the nature of nucleation (the numberof active nucleation sites on the surface, rate of bubbleformation at each site, etc.), which is difficult to predict.The type and condition of the heated surface also affectthe heat transfer. These complications pose difficult todevelop theoretical relations for heat transfer in thenucleate boiling regime, and we had to rely on relationbased on experimental data. The most widely usedcorrelation for the rate of heat transfer in the nucleateboiling is Rohsenow correlation [4], and is expressed as

In the above correlation 'Pl ' (density of the liquid),'u,' (viscosity of the liquid), Pr ': (Pranndtl no. of theliquid) are formulated for a simple based liquid orconventional heat transfer fluids. CsJ (experimentalconstant) that depends on surface fluid combination, hasgreat affects on the formulation of the heat flux. Theseproperties are creating a huge change in the abovecorrelation when it is subjected to pool boiling ofnanofluids depending on the heating surface, the volumefraction of the nanoparticles and the base fluid used. Ingeneral, the correlation changes its form whennanoparticle is dispersed in the base fluid. The changesoccur in the properties which are then formulated in theabove correlation and are discussed in the next section.

2.1. Prediction of the pool boiling heat transfer properties

The present study is aimed at developing a correlationto predict the heat flux of nucleate pool boiling of Ti02 -

water nanofluid at different particle volume fractions with

respect to the temperature on a mechanically polished flatstainless steel plate. The following section describes theexpressions of different nanofluid properties.

2.1.1. Density of the nanofluid [2)

The density of nanofluid is written as

Pnf = (1 - rp)Pr rpPp

2.1.2. Specific heat of the nanofluid [2)

(5)

(2)

The specific heat ofnanofluid is written asC - (l-<P)PfCt+<PPpCp

nf - (l-<p)Pf <PPp

2.1.3. Viscosity of the nanofluid [2)

The viscosity of nanofluid is written as

(6)

(7)

(3)2.1.4. Thermal conductivity of the nanofluid [2)

The thermal conductivity of nanofluid is written as

knf =1+ 3(a-l)q> (8)kr (a+2)-(a-I)q>

2.1.5. Volumefraction of the nanoparticles [2)

The volume fraction ofnanoparticles is written as

<P= (~) pp +1 (9)<Pm Pr

2.1.6. Prandtl number of the nanofluid [2)

The Prandtl number of nanofluid is written as

P_ /-lnfCnf

rnf- knf

(10)

Putting equation (5) (6) (7) (8) (9) (10) in equation (4) thefollowing new equation is obtained.

Thus newly developed generalized correlation may beused knowing different thermal properties of nanofluidand heating surface properties for boiling heat transfercoefficient.

III. RESULTS AND DISCUSSIONS

The variation of heat transfer coefficient, at differentnanoparticle concentration for Ti02-water nanofluid havebeen studied in detail. The nanoparticle volumeconcentration, fluid temperature and surface temperaturehave been varied to observe their effects. The results ofheat flux/heat transfer coefficient obtained for theoretical

NIET Journal of Engineering & Technology, Vol. 1, Issue 2, 2013 65

correlation is compared with experimental data forvalidation of the predicted model. The following sectiondescribes in detail.

The predicted heat flux results are then compared withexperimental results [3], the prediction gives goodagreement with the experimental results [3].The resultregarding the comparison of heat flux at 0.32 wt. %nanoparticle concentrations is shown in Fig.l.

1~'--A~t~O~.3~~w~t~%~----------------~----------~

120

•

;tQ> 1000-

X::l 80~'"<I>.c: 60

'"cE ~.;::<I>Cl.. 20x

W

+30%

-30%

O~------~------~------~----~~----~o 50 100 150

P re:dicted heat flux. q pre200

Fig.! Comparison of predicted heat flux with experimental results [3]at 0.32 wt.% ofTi02 -nanofluid

Effect of varying nanoparticle concentration on heattransfer coefficient on varying nanoparticleconcentrations. The heat transfer coefficient of Ti02-

water nanofluid compared with that obtained fromRohsenow correlation for water shown in Fig.2. Theresults show significant improvement on the heat transfercoefficient of nucleate pool boiling due to the presence ofnanoparticles. The improvement increases withnanoparticle concentration at high heat fluxes. At thenanoparticle concentration of 0.72 wt. %, approximately22% increase in heat transfer coefficient is achieved.

IV. CONCLUSIONS AND FUTURE SCOPE OF STUDY

The present study represents the development oftheoretical correlation for pool boiling of nanofluidhaving Ti02 as nanoparticle and water as base fluid on amechanically polished stainless steel flat plate which is aboiling surface, where the concentration of the Ti02

particles in water increases from 0.32 wt. % to 0.72 wt. %to observe the effects on boiling heat transfer coefficient.

Based on the study, it is summarized that1. The theoretical correlation for prediction of heat

transfer coefficient is developed.

2. The surface -liquid temperature difference decreaseswith increasing nanoparticle concentration.

3. The theoretical heat flux compared with theexperimental heat flux is within ±30% error bandagreement.

4. Heat transfer coefficient of nucleate pool boiling usingnanofluid increases with nanoparticle concentration athigh heat fluxes. Approximately 22% increase in heattransfer coefficient is observed for 0.72 wt. %nanoparticle concentration.

NOMENCLATURE

C Specific heat, J/Kg KCsr Experimental constant that depends on surface-fluid

combinationD Diameterg Gravitational acceleration, m/s'h Heat transfer coefficienthrg Enthalpy of vaporization, J/Kgk Thermal conductivity, W/mk

140 ------------------------------------,

-+- 032 wt % predicted.c'. 0.42 wt % predicted-t- 0.52 wt % predicted- ...- ..._ .. 0.62 wt % predicted- -- - 0.72 wt % predicted

• Rohsenow, water

120

100

90

.7

t 60

s:

40

2O

250

200 600 900 1000

Fig. 2 Comparison of heat transfer coefficient with heat flux withdifferent nanoparticle concentration

m Mass, Kg p Density, Kg/rn'n Shape factor <p Volume fractionPr PrandtI number rpm Mass fractionQ Heat, J 0' Surface tension of liquid-q Heat flux, W/m 2 vapor interface, N/mR Heater resistance '¥ SphericityT Temperature, K f base fluidU Voltage I liquidV Volume, m J nf nanofluida Ratio ofthermal conductivities p particle11 Viscosity, Kg/ms v vapour

REFERENCES

[I] S.K. Das, N. Putra, P. Thiesen, and W Roetzel, "Temperaturedependence of thermal conductivity enhancement for nanofluids,'Transactions of the ASME,J heat transfer.2003a, vol- 125, pp. 567-574.

[2] W Yu, D.M.France, S.U.S.Choi, and J.L.Routbort, "Review andassessment of nanofluid technology for transportation and otherapplications. Energy systems Division," Argonne NationalLaboratory, April 2007 , pp. 14.

[3] YDing, H.Chen, L.Wang, C.Y.Yang, YHe, WYang!, WLee,L.Zhang and R.Huo, "Heat Transfer Intensification UsingNanofluids," 2007.

[4] YA.Cengel, Heat Transfer-A practical Approach" Tata McGraw-Hill, New Delhi, 2002.

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