chapter_3 forced convection
DESCRIPTION
Heat TransferTRANSCRIPT
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BKF2422 HEAT TRANSFERBKF2422 HEAT TRANSFERCHAPTER 3CHAPTER 3Part 1 Principles of steady-state heat Principles of steady-state heat transfer in convectiontransfer in convection
FORCED CONVECTIONFORCED CONVECTION
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TOPIC OUTCOMESTOPIC OUTCOMES
Define and differentiate between forced convection and natural convection
Solve problems involve with forces convection inside pipe Determine the heat transfer coefficient and solve problems
for heat transfer in system with fluid flow across plate, tube cylinder, sphere and bank of tubes.
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CONTENTCONTENT Forced Convection Heat Transfer Inside Pipe
For laminar flow inside pipeFor turbulent flow inside pipeFor transition flow inside pipeEntrance-region effect on heat transfer coefficientLiquid-metal heat transfer coefficient
Log mean Temperature Difference Heat Transfer Outside Various Geometries In Forced Convection
Flow Parallel to Flat PlateFlow Past Cylinder With Axis PerpendicularFlow Past Single SphereFlow Past Banks of Tubes or CylindersFlow Past in Packed Beds
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Natural convectionFrom planes and cylinders In enclosed spaces
BoilingNucleate boilingFilm boiling
CondensationFilm-condensation coefficient for vertical surfacesFilm-condensation coefficient for horizontal cylinders
CONTENTCONTENT
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Convection: Heat transfer using movement of fluids. Heat transfer is considered as convection with the presence of
bulk fluid motion. Fluid motion enhances heat transfer where the higher the fluid velocity, the higher the rate of heat transfer.
2 main classification of convective heat transfer;
1. Forced Convection : fluid flow by pressure differences, a pump, a fan and so on
2. Natural Convection: motion of fluid results from the density changes in heat transfer
CONVECTION HEAT TRANSFERCONVECTION HEAT TRANSFER
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The rate of heat transfer :
Tw = 80 oC
To = 30 oCq
q Ah(Tw To)The convection coefficient is a measure of how effective a fluid is at carrying heat to and away from the surface.h = heat transfer coefficient
(W/m2.K)A= surface area (m2)
Fluid flow
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CONVECTION HEAT TRANSFER
Metal wall
Warm fluid A
Cold fluid B
q
Turbulence absent
T2
T3 Turbulence region
T1
q = hA (T-Tw)
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FKKSA
FORCED CONVECTION INSIDE PIPESFORCED CONVECTION INSIDE PIPES
Forced convection – fluid forced to flow by pressure differences
Types of fluid, laminar or turbulent
– great effect on heat-transfer coefficient
More turbulent– greater heat-transfer coefficient
Reynolds number, NRe
NRe
D
wherev = velocity of fluid (m/s) = viscosity of fluid (Pa.s)= density of fluid (kg/m3)
D = diameter of pipe (m)
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FKKSA
FORCED FORCED CONVECTIONCONVECTION
where
= viscosity of fluid (Pa.s)= density of fluid (kg/m3)k = thermal conductivity of fluid (W/m.K)cP = heat capacity of fluid (J/kg.K)h = heat transfer coefficient (W/m2.K)
D = diameter of pipe (m)
Prandtl number, NPr
Dimensionless numbers:
Nusselt number, NNu
kμc
ρckρ
μN P
P
Pr
khDN
Nu
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FKKSA
LAMINAR FLOW INSIDE HORIZONTAL PIPELAMINAR FLOW INSIDE HORIZONTAL PIPE
whereD = inside diameter of pipe (m)
L = length of pipe (m)
b = viscosity of fluid at bulk temperature (Pa.s)
w = viscosity of fluid at wall temperature (Pa.s)ha = average heat transfer coefficient (W/m2.K)
NNu
ah
aD
k1.86 N
ReN
PrDL
13
b
w
0.14
NRe 2100 & NReNP r 100 :L
D
All physical properties at except w 2bi
Tbo
T
mean bT
q = haA∆Ta where 2
boT
wT
biT
wT
aΔT
100
2100
PrRe
Re
L
DNN
N
Limitations
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FKKSA
TURBULENT FLOW INSIDE HORIZONTAL PIPE
where
NNu
hLD
k0.027 N
Re0.8N
Pr
13
b
w
0.14
NRe 6000 , 0.7 ≤ NP r ≤ 16000 & 60:DL
Rate of heat transfer is greater
cP = heat capacity of fluid (J/kg.K)
D = inside diameter of pipe (m)
k = thermal conductivity of fluid (W/m.K)
b = viscosity of fluid at bulk average temperature (Pa.s)
hL = heat transfer coefficient based on the log mean driving force ∆Tlm (W/m2.K)
w = viscosity of fluid at wall temperature (Pa.s)
Many industrial heat transfer processes in the turbulent region
60
160007.0
6000
Pr
Re
D
LL
DN
NLimitations
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FKKSA
TURBULENT FLOW INSIDE HORIZONTAL PIPE
where
D = inside diameter of pipe (m)
= velocity of fluid (m/s)
hL3.52
0.8
D0.2
hL1429 10.0146 ToC
0.8
D0.2
hL423
0.8
D0.2
Air at 1 atm total pressure (NRe 2100) :
Water at T = 4 to 105oC :
Organic liquids :
Flow inside helical coils :
Hcoil = hstraight pipes + (1 + 3.5D/Dcoil)
2100Re N
Limitations
CT o105 to4
Limitations
4Re 10N
Limitations
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EXAMPLE 4.5-1EXAMPLE 4.5-1 Page 262: Heating of Air in Page 262: Heating of Air in Turbulent FlowTurbulent Flow
Air at 206.8 kPa and an average of 477.6 K is being heated as it flows through a tube of 25.4mm inside diameter at velocity of 7.62 m/s. The heating medium of 488.7 K steam condensing on the outside of the tube. Since the heat-transfer coefficient of condensing steam is several thousand W/m2.K and the resistance of the metal wall is very small, it will be assumed that the surface wall temperature of the metal in contact with the air is 488.7 K. Calculate the heat-transfer coefficient for an L/D > 60 and also the heat-transfer flux q/A.
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boT
K 7.488 Steam, wT
steamo hh
L
mm 5.42air
biTkPa 8.206
m/s 62.7
K 6.477
P
v
Tave
3
Pr
5
kg/m 74.0
W/m03894.0
686.0
Pa.s 106.2
K 477.6 kPa, 101.32at A.3,Appendix From
k
N
TTP
b
bmave
Pa.s 1064.2
K 7.884at A.3,Appendix From5
w
wT
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3kPa 8.206
21
1
2
1
212
kg/m 509.135.101
8.20674.0
on depend is
,For
TT
T
T
P
P
P & TRT
PM
RTPMRTV
mPM
RTM
mPVnRTPV
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)6000(10122.1
106.2
)509.1)(62.7(104.25
4
5
3
Re
D
N
.K W/m2.63
0264.0
0260.0686.010122.1027.0
03894.0
)104.25(
027.0
2
14.0
3
18.043
14.0
3
1
Pr8.0
Re
L
L
w
bLNu
h
h
NNk
DhN
2 W/m1.701
6.4777.4882.63
bmwL TThA
q
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FKKSA
TRANSITION FLOW INSIDE A PIPETRANSITION FLOW INSIDE A PIPE
2100 NRe 6000 :
whereG = mass velocity of fluid (kg/s.m2) =
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EXAMPLE 4.5-2 Page 264: Water Heated by Steam and Trial and error Solution
Water is flowing in a horizontal 1-in schedule 40 steel pipe at an average temperature of 65.6oC and a velocity of 2.44 m/s. it is being heated by condensing steam at 107.8oC on the outside of pipe wall. The steam side coefficient has been estimated as ho = 10500 W/m2.K.
a) Calculate the convective coefficient hi for water inside the pipe
b) Calculate the overall coefficient Ui based on the inside surface area
c) Calculate the heat-transfer rate q for 0.305m of pipe with the water at an average temperature of 65.6oC
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boT
C 8.107 Steam, owT
.K W/m10500 2oh
m 305.0
mm 6.642water
biT m/s 44.2
C 6.65 o
v
Tbm
mm 3.43
3
Pr
3
o
kg/m 9.981
W/m6629.0
72.2
Pa.s 10432.0
C, 5.56at A.3,Appendix From
k
N
T
b
bm
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Pa.s 1056.3C 80at
C 80 l,first triafor C80or 107.8 and 65.5between way the
thirdoneabout as assumed be willand needed is metal inside of re temperatuThe
4o
oo
ww
w
T
T
)6000(10473.1
1032.4
)980)(44.2(0266.0
5
4
Re
D
N
.K W/m13324
1056.3
1032.472.210473.1027.0
663.0
)0266.0(
027.0
2
14.0
4
4
3
18.05
14.0
3
1
Pr8.0
Re
L
L
w
bLNu
h
h
NNk
DhN
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again. re temperatufor the make weassumption check the tohave We
2m 0255.0
)305.0)(0266.0(
LDA ii
2m 032.0
)305.0)(0334.0(
LDA oo
:follow as are areas variousThe
002633.0)305.0()45( 2
)0266.0/0334.0ln(
2
)ln( 1/2 kL
rrRpipe
002976.0)032.0(10500
11
ooo Ah
R
002943.0)0255.0(13324
11
iii Ah
R
008552.0 R
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necessary.not is trialSecond
C. 80 of estimate original the toclose quite is C.This 1.805.145.65T Hence,
C 14.5K 5.14)2.42(008852.0
002943.0)2.42( drop eTemperatur
is film water theacross drop re temperatuThe
C 42.265.6) (107.8 is difference re temperatuoverall The
oow
o
o
R
Ri
(b)
.K W/m4586
)008552.0(0255.0
1
1
2
RAU
TTAUR
TTq
ii
ioiiio
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(c)
W4935
)2.42)(0255.0(4586
C 2.426.658.107
C, 65.6 of re temperatuaveragean at with watero
o
ioii
io
TTAUq
TT
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FKKSA
ENTRANCE-REGION EFFECT ON HEAT-ENTRANCE-REGION EFFECT ON HEAT-TRANSFER COEFFICIENTTRANSFER COEFFICIENT
At entrance, h = ∞
0.7
LLD1
hh
LD61
hh
L
2 20DL
20 60DL
h = average heat transfer coefficient for a tube of finite length L
hL= heat transfer coefficient for a very long tube
where
At L/D 60, h = hL
202 D
L
Limitations
6020 D
LLimitations
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FKKSA
LIQUID METAL HEAT-TRANSFER COEFFICIENT
NPe = Peclet number = NReNPr
where
Turbulent flow, NRe 6000 :
0.4Pe
N 6250k
DL
h
NuN .
0.8Pe
N 02505.0k
DL
h
NuN .
Constant wall temperature ( 60, NPe 100 ) :DL
DL
Uniform heat flux, 100 NPe 104 & 60 :
4
Re
10100
60
2100
PeN
D
L
N
Limitations
100
60
PeND
L
Limitations
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EXAMPLE 4.5-3EXAMPLE 4.5-3 Page 266: Liquid-metal Heat Page 266: Liquid-metal Heat Transfer Inside a TubeTransfer Inside a Tube
A liquid metal flows at a rate of 4.00kg/s through a tube having an inside diameter of 0.05m. The liquid enters at 500K and is heated to 505K in the tube. The tube wall is maintained at a temperature of 30K above the fluid bulk temperature and constant heat flux is maintained. Calculate the required tube length. The average physical properties are as follows: μ = 7.1 x 10-4 Pa.s, ρ = 7400 kg/m3, cp = 120 J/kg.K, k = 13 w/m.K
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kg/s 4m
K 30 bmw TT
metal liquidK 500biTK 505boT
?L
m 05.0iD
3
4
kg/m 7400
W/m.K13
kJ/kg.K 120
Pa.s 101.7
K, 5.502at A.3,Appendix From
k
c
T
p
b
bm
K 5.5022
505500
2
bobi
bmean
TTT
wT
bmT
q
corhbibopbmeanw TTcmTThAq
LDA ii
K 30 bmw TT
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)2000(10435.1
05.04
101.7
)4(05.0
5
24
Re
xA
mDN
bmT
wT
4
2i
x
DA
)10100(940
)1055.6(10435.1 4
35
PrRe
Pe
Pe
N
NNN
3
4
Pr
1055.6
13
)101.7(120
k
cN p
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.K W/m2512
)940(625.013
)05.0(
625.0
2
4.0
4.0
i
i
Peii
Nu
h
h
Nk
DhN
W2400
)500505)(120(4
bibop TTcmq
2m10185.3
2400)30(2512
2400
2
i
ibmeanwi
i
A
A
ATTh
A
q
m 203.0
)05.0(10185.32
L
L
LDA ii
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bmoobmii TAUTAUq
cicocphihohp TTcmTTcmq
1
2
12
2
1
21
lnlnTT
TT
TT
TTTlm
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flowrent countercur
flow parallel
hoThiT
hiT hoT
coT
coTciT
ciT
1T
1T
2T
2T
hiT
hiT
hoT
hoT
ciT
ciT
coT
coT
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2
bobiwiaiaa
TTTAhTAhq
2
1
21
lnTT
TTAhTAhq iailma
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EXAMPLE 4.5-4 Page 268: Heat Transfer Area and EXAMPLE 4.5-4 Page 268: Heat Transfer Area and Log Mean Temperature DifferenceLog Mean Temperature Difference
A heavy hydrocarbon oil which has a cpm = 2.30kJ/kg is being cooled in a heat exchanger from 371.9 K to 349.7 K and flows inside the tube at a rate of 3630 kg/h. A flow of 1450kg water/h enters at 288.6K for cooling and lows outside the tube.
a) Calculate the water outlet temperature and heat-transfer area if the overall Ui = 340 W/m2.K and the streams are countercurrent
b) Repeat for parallel flow
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flowrent countercur (a)
1T
2T
hiTK 1.397
K7.349hoT
K 6.288ciT
coT
kg/h 3630 oil, m
kg/h 1450 water, m
W51490
3600/)7.3499.371)(3.2(3630
hihohp TTcmq
?
.K W/m340
kJ/kg.K 187.4
kJ/kg.K 3.2
2
i
i
waterp
oilp
A
U
c
c
K 1.319
)6.288)(187.4(145051490
co
co
cicocp
T
T
TTcmq
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K 9.56
8.521.61
ln
8.521.61
ln2
1
21
TT
TTTlm
2m 66.2
)9.56(34051490
i
i
lmii
A
A
TAUq
K 1.616.2887.349
K 8.521.3199.371
2
1
T
T
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flow parallel (b)
K 3.836.2889.371
K 6.301.3197.349
2
1
T
T
K 7.52
6.303.83
ln
6.303.83
ln2
1
21
TT
TTTlm
2m 87.2
)7.52(34051490
i
i
lmii
A
A
TAUq
forces. driving peraturelarger tem
gives wscounterflo because occurs This w.counterflofor than arealarger a is This
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EXAMPLE 4.5-5EXAMPLE 4.5-5 Page 269 : Laminar Heat Page 269 : Laminar Heat Transfer and Trial and ErrorTransfer and Trial and Error
A hydrocarbon oil at 150oF enters inside a pipe with an inside diameter of 0.0303 ft and a length of 15 ft with a flow rate of 80 Ibm/h. The inside pipe surface is assumed constant at 350oF since steam is condensing outside the pipe wall and has a very large heat-transfer coefficient. The properties of the oil are cpm= 0.5 btu/Ibm.oF and km = 0.083 btu/h.ft.oF. The viscosity of the oil varies with temperature as follows: 150oF, 6.50 cp; 200oF, 5.05 cp; 250oF, 3.80 cp; 300oF, 2.82 cp; 350oF, 1.95cp. Predict the heat-transfer coefficient and the oil outlet temperature, Tbo
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Ibm/h 80m
F 350 owT
oil
F 150 obiT?boT
ft 15
ft 0303.0
4
2DAx
Fbtu/h.ft. 083.0
Fbtu/Ibm. 5.0o
o
m
oilp
k
c
Ib/ft.h 23.12cp1
Ib/ft.h 2.41915.05cp
F 2002
150250
F 250 if valueAssume
o
o
bmean
bo
T
T
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)2100(5.275
0303.04
23.12
)80(0303.0
2
Re
xA
mDN
7.73 083.0
)23.12(5.0
Pr
k
cN p
4115
0303.0)7.73(5.275PrRe
L
DNN
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F.btu/h.ft 1.20
4191.295.1
23.124186.1
083.0
)0303.0(
86.1
o2
14.0
3
1
14.03
1
PrRe
h
h
L
DNN
k
hDN
w
bNu
again. re temperatufor the make weassumption check the tohave We
150)5.0(0.80 bobibopm TTTcmq
aa TAhq
bobobobi
wa TTTT
TT 5.02752
150350
2
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F 255
5.0275150)5.0(0.80o
bo
bobo
aabibopm
T
TT
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EXAMPLE 4.6-1 Page: 272 Cooling a Copper Fin
A smooth, flat, thin fin of copper extending out from a tube is 50 mm by 51 mm square. Its temperature is approximately uniform at 82.2oC. Cooling air at 15.6oC and 1 atm abs flows parallel to the fin at a velocity of 12.2 m/s.
a) For laminar flow, calculate the heat-transfer coefficient, h
b) If the leading edge of the fin is rough so that all of the boundary layer o film next to the fin is rough so that alls o the boundary layer or film next to the fin is completely turbulent, calculate h
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EXAMPLE 4.6-2 Cooling of a Sphere
Using same condition as Example 4.6-1, where air at 1 atm abs pressure and 15.6oC is flowing at velocity of 12.2 m/s, predict the average heat-transfer coefficient for air flowing by a sphere having a diameter of 51mm and an average surface temperature of 82.2oC. Compare this with the value of h = 77.2 W/m2.K for the flat plate in turbulent flow.
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EXAMPLE 4.6-3 Page 273: Heating Air by a Bank of Tubes
Air at 15.6oC and 1 atm abs flows across a bank of tubes containing four transverse rows in the direction of flow and 10 rows normal to the flow at a velocity of 7.62 m/s as the air approaches the bank of tubes. The tube surfaces are maintained at 57.2oC. The outside diameter of the tubes is 25.4mm and the tubes are in-line to the flow. The spacing Sn of the tubes normal to the flow is 38.1mm and also Sp is 38.1mm parallel to the flow. For a 0.305m length of the tube bank, calculate the heat-transfer rate
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