heat chap07 073

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Chapter 7External Forced Convection

7-63

Special Topic: Thermal Insulation

7-73C Thermal insulation is a material that is used primarily to provide resistance to heat flow. It differsfrom other kinds of insulators in that the purpose of an electrical insulator is to halt the flow of electriccurrent, and the purpose of a sound insulator is to slow down the propagation of sound waves.

7-74C In cold surfaces such as chilled water lines, refrigerated trucks, and air conditioning ducts,insulation saves energy since the source of coldness is refrigeration that requires energy input. In thiscase heat is transferred from the surroundings to the cold surfaces, and the refrigeration unit must now workharder and longer to make up for this heat gain and thus it must consume more electrical energy.

7-75C The R-valueof insulation is the thermal resistance of the insulating material per unit surface area.Forflatinsulation theR-value is obtained by simply dividing the thickness of the insulation by its thermalconductivity. That is,R-value =L/k. Doubling the thicknessL doubles theR-value of flat insulation.

7-76C The R-value of an insulation represents the thermal resistance of insulation per unit surface area (orper unit length in the case of pipe insulation).

7-77C Superinsulations are obtained by using layers of highly reflective sheets separated by glass fibers inan evacuated space. Radiation between two surfaces is inversely proportional to the number of sheets usedand thus heat loss by radiation will be very low by using this highly reflective sheets. Evacuating the spacebetween the layers forms a vacuum which minimize conduction or convection through the air space.

7-78C Yes, hair or any other cover reduces heat loss from the head, and thus serves as insulation for thehead. The insulating ability of hair or feathers is most visible in birds and hairy animals.

7-79C The primary reasons for insulating are energy conservation, personnel protection and comfort,maintaining process temperature, reducing temperature variation and fluctuations, condensation andcorrosion prevention, fire protection, freezing protection, and reducing noise and vibration.

7-80C The optimum thickness of insulation is the thickness that corresponds to a minimum combined costof insulation and heat lost. The cost of insulation increases roughly linearly with thickness while the cost ofheat lost decreases exponentially. The total cost, which is the sum of the two, decreases first, reaches aminimum, and then increases. The thickness that corresponds to the minimum total cost is the optimumthickness of insulation, and this is the recommended thickness of insulation to be installed.

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7-64

7-81 The thickness of flatR-8 insulation in SI units is to be determined when the thermal conductivity of thematerial is known.

AssumptionsThermal properties are constant.

Properties The thermal conductivity of the insulating material is given to be k= 0.04 W/mC.

Analysis The thickness of flat R-8 insulation (in m2.C/W) is determinedfrom the definition ofR-value to be

R L

kL R kvalue value

2m . C / W)(0.04 W / m. C) (8 0.32 m

7-82E The thickness of flat R-20 insulation in English units is to be determined when the thermalconductivity of the material is known.

AssumptionsThermal properties are constant.

Properties The thermal conductivity of the insulating material is given to be k= 0.02 Btu/hftF.

Analysis The thickness of flat R-20 insulation (in hft2F/Btu) is determinedfrom the definition ofR-value to be

ft0.4 F)Btu/h.ft.2F/Btu)(0.0.h.ft20(2

valuevalue kRLk

L

R

R-8

L

R-20

L

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7-65

7-83 A steam pipe is to be covered with enough insulation to reduce the exposed surface temperature to

30C . The thickness of insulation that needs to be installed is to be determined.

Assumptions 1 Heat transfer is steady since there is no indication of any change with time. 2 Heat transferis one-dimensional since there is thermal symmetry about the centerline and no variation in the axial

direction. 3 Thermal properties are constant. 4 The thermal contact resistance at the interface is negligible.

Properties The thermal conductivities are given to be k = 52 W/mC for cast iron pipe and k= 0.038

W/mC for fiberglass insulation.Analysis The thermal resistance network for this problem involves 4 resistances in series. The inner radiusof the pipe is r1 = 2.0 cm and the outer radius of the pipe and thus the inner radius of insulation is r2 = 2.3cm. Letting r3 represent the outer radius of insulation, the areas of the surfaces exposed to convection for aL = 1 m long section of the pipe become

A r L

A r L r r r

1 1

3 3 3 3 3

2 2 0 02

2 2 2

( . m)(1 m) = 0.1257 m

(1 m) = m ( in m)

2

2

Then the individual thermal resistances are determined to be

C/W9944.0)mC)(0.1257.W/m(80

1122

1conv,1

AhRR

ii

C/W00043.0)mC)(1W/m.(522

)02.0/023.0ln(

2

)/ln(

1

12pipe1

Lk

rrRR

R Rr r

k L

rr2

3 2

2

33

2

00234188 0 023

insulation

2 (0.038 W/ m. C)(1 mC / W

ln( / ) ln( / . )

). ln( / . )

R Rh A r r

oo

conv,2 2 21

(22 W / m . C)(2 mC / W

1 1

13823 3 3 ) .

Noting that all resistances are in series, the total resistance is

C/W)2.138/(1)023.0/ln(188.400043.009944.0 3321total rrRRRRR oi

Then the steady rate of heat loss from the steam becomes

C/W])2.138/(1)023.0/ln(188.400043.009944.0[

C)22110(

33total

rrR

TTQ oi

Noting that the outer surface temperature of insulation is specified to be 30C, the rate of heat loss can alsobe expressed as

( )

QT T

R rro

o

3

33

30 221106

C

1/ (138.2 ) C / W

Setting the two relations above equal to each other and solving for r3 gives r3 = 0.0362 m. Then theminimum thickness of fiberglass insulation required is

t= r3 - r2 = 0.0362 - 0.0230 = 0.0132 m = 1.32 cm

Therefore, insulating the pipe with at least 1.32 cm thick fiberglass insulation will ensure that the outer

surface temperature of the pipe will be at 30C or below.

RiTi

Rinsulation RoTo

R i e

T1 T2 T3

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7-66

7-84"!PROBLEM 7-84"

"GIVEN"

T_i=110 "[C]"

T_o=22 "[C]"

k_pipe=52 "[W/m-C]"

r_1=0.02 "[m]"

t_pipe=0.003 "[m]""T_s_max=30 [C], parameter to be varied"

h_i=80 "[W/m^2-C]"

h_o=22 "[W/m^2-C]"

k_ins=0.038 "[W/m-C]"

"ANALYSIS"

L=1 "[m], 1 m long section of the pipe is considered"

A_i=2*pi*r_1*L

A_o=2*pi*r_3*L

r_3=r_2+t_ins*Convert(cm, m) "t_ins is in cm"

r_2=r_1+t_pipe

R_conv_i=1/(h_i*A_i)

R_pipe=ln(r_2/r_1)/(2*pi*k_pipe*L)

R_ins=ln(r_3/r_2)/(2*pi*k_ins*L)R_conv_o=1/(h_o*A_o)

R_total=R_conv_i+R_pipe+R_ins+R_conv_o

Q_dot=(T_i-T_o)/R_total

Q_dot=(T_s_max-T_o)/R_conv_o

Ts, max [C] tins [cm]

24 4.45

26 2.489

28 1.733

30 1.319

32 1.055

34 0.871

36 0.7342

38 0.6285

40 0.5441

42 0.4751

44 0.4176

46 0.3688

48 0.327

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7-67

20 25 30 35 40 45 500

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Ts,max [C]

tins

[cm]

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7-69

To determine the thickness of insulation whose cost is equal to annual energy savings, we repeat thecalculations above for 2, 3, . . . . 15 cm thick insulations, and list the results in the table below.

Insulationthickness

Rate of heat lossW

Cost of heat lost$/yr

Cost savings$/yr

Insulation cost$

0 cm 133,600 12,157 0 01 cm 15,021 1367 10,790 2828

5 cm 3301 300 11,850 3535

10 cm 1671 152 12,005 9189

11 cm 1521 138 12,019 9897

12 cm 1396 127 12,030 10,604

13 cm 1289 117 12,040 11,310

14 cm 1198 109 12,048 12,017

15 cm 1119 102 12,055 12,724

Therefore, the thickest insulation that will pay for itself in one year is the one whose thickness is 14 cm.

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7-70

7-86 A cylindrical oven is to be insulated to reduce heat losses. The optimum thickness of insulation and theamount of money saved per year are to be determined.

Assumptions 1 Steady operating conditions exist. 2 Heat transfer through the insulation is one-dimensional.3 Thermal conductivities are constant. 4 The thermal contact resistance at the interface is negligible. 5 Thesurface temperature of the furnace and the heat transfer coefficient remain constant. 6 The surfaces of thecylindrical oven can be treated as plain surfaces since its diameter is greater than 1 m.

Properties The thermal conductivity of insulation is given to be k= 0.038 W/mC.

Analysis We treat the surfaces of this cylindrical furnace as plain surfaces since its diameter isgreater than 1 m, and disregard the curvature effects. The exposed surface area of the furnace is

222sidebase m69.70m)m)(65.1(2m)5.1(2222 rLrAAAo

The rate of heat loss from the furnace before the insulation is installed is

W101,794=C)2775)(mC)(70.69.W/m30()( 22 TTAhQ soo

Noting that the plant operates 5280 = 4160 h/yr, the annual heat lost fromthe furnace is

Q Q t ( . 101794 kJ / s)(4160 3600 s/ yr) =1.524 10 kJ / yr9

The efficiency of the furnace is given to be 78 percent. Therefore, to generate thismuch heat, the furnace must consume energy (in the form of natural gas) at a rate of

Q Qin oven9 910 kJ / yr) / 0.78 =1.954 10 kJ / yr =18,526 therms/ yr / ( . 1524

since 1 therm = 105,500 kJ. Then the annual fuel cost of this furnace before insulation becomes

Annual Cost Q Unit cost therm / yr)($0.50 / therm) = $9,263 / yrin ( ,18526

We expect the surface temperature of the furnace to increase, and the heat transfer coefficient to decreasesomewhat when insulation is installed. We assume these two effects to counteract each other. Then the rateof heat loss for 1-cm thick insulation becomes

W445,11

C.W/m30

1CW/m.038.0

m01.0

C)2775)(m69.70(

1

)(

2

2

ins

insconvinstotalins

o

soss

hk

t

TTA

RR

TT

R

TT

Q

Also, the total amount of heat loss from the furnace per year and the amount and cost of energyconsumption of the furnace become

Q Q t

Q Q

Q

ins ins8

in,ins ins oven8 8

in,ins

kJ / s)(4160 3600 s / yr) =1.714 10 kJ / yr

10 kJ / yr) / 0.78 = 2.197 10 kJ / yr = 2082 therms

Annual Cost Unit cost therm / yr)($0.50 / therm) = $1041/ yr

( .

/ ( .

(

11445

1714

2082

Cost savings = Energy cost w/o insulation - Energy cost w/insulation = 9263 - 1041 = $8222/yr

The unit cost of insulation is given to be $10/m2 per cm thickness, plus $30/m2 for labor. Then the total cost

of insulation becomes

Insulation Cost Unit cost)(Surface area) =[($10/ cm)(1 cm) +$30/ m m ) = $28282 2 ( ]( .7069

RoT

RinsulationTs

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7-71

To determine the thickness of insulation whose cost is equal to annual energy savings, we repeat thecalculations above for 2, 3, . . . . 15 cm thick insulations, and list the results in the table below.

InsulationThickness

Rate of heat lossW

Cost of heat lost$/yr

Cost savings$/yr

Insulation cost$

0 cm 101,794 9263 0 01 cm 11,445 1041 8222 2828

5 cm 2515 228 9035 3535

9 cm 1413 129 9134 8483

10 cm 1273 116 9147 9189

11 cm 1159 105 9158 9897

12 cm 1064 97 9166 10,604

Therefore, the thickest insulation that will pay for itself in one year is the one whose thickness is 9 cm. The10-cm thick insulation will come very close to paying for itself in one year.

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7-72

7-87E Steam is flowing through an insulated steel pipe, and it is proposed to add another 1-in thick layerof fiberglass insulation on top of the existing one to reduce the heat losses further and to save energy andmoney. It is to be determined if the new insulation will pay for itself within 2 years.

Assumptions 1 Heat transfer is steady since there is no indication of any change with time. 2 Heat transferis one-dimensional since there is thermal symmetry about the centerline and no variation in the axial

direction. 3 Thermal properties are constant. 4 The heat transfer coefficients remain constant. 5 The thermalcontact resistance at the interface is negligible.

Properties The thermal conductivities are given to be k = 8.7 Btu/hftF for steel pipe and k = 0.020

Btu/hftF for fiberglass insulation.

Analysis The inner radius of the pipe is r1 = 1.75 in, the outer radius of the pipe is r2 = 2 in, and the outerradii of the existing and proposed insulation layers are r3 = 3 in and 4 in, respectively. Considering a unitpipe length ofL = 1 ft, the individual thermal resistances are determined to be

R Rh A h r L

ii i

conv,1 21

(30 Btu / h.ft . F)[2 (1.75/ 12 ft)(1 ft)h. F / Btu

1 1

20 0364

1 1( ) ].

R Rr r

k L1

2 1

12

2 175000244

pipe

2 (8.7 Btu / h.ft. F)(1 fth. F / Btu

ln( / ) ln( / . )

).

Current Case:

Rr r

k Linsulation

ins 2 (0.020 Btu / h.ft. F)(1 fth. F / Btu

ln( / ) ln( / )

).3 2

2

3 23227

R Rh A h r

oo o

conv,2 21

(5 Btu / h.ft . F)[2 ft)(1 fth. F / Btu

1 1

2 3 1201273

3 3( ) ( / )].


( )

. . . . ) ..Q

T

R

T T

R R R R

i o

i ocurrent

total pipe ins

F

( h F / BtuBtu / h

400 600 0364 0 00244 3 227 01273

1002

Proposed Case:

Rr r

k Linsinsulation

2 (0.020 Btu / h.ft. F)(1 fth. F / Btu

ln( / ) ln( / )

).3 2

2

4 25516

R Rh A h r

oo o

conv,2 21

(5 Btu / h.ft . F)[2 ft)(1 fth. F / Btu

1 1

2 4 1200955

3 3( ) ( / )].


( )

. . . . ) ..Q

T

R

T T

R R R R

i o

i oprop

total pipe ins

F

( h F / BtuBtu / h

400 600 0364 0 00244 5 516 0 0955

602

Therefore, the amount of energy and money saved by the additional insulation per year are

yr/504.3$)Btu1000/01.0)($Btu/yr400,350()costUnit(=savedMoney

Btu/yr400,350)h/yr8760)(Btu/h0.40(

Btu/h0.402.602.100

saved

savedsaved

currentpropsaved

Q

tQQ

QQQ

or $7.01 per 2 years, which is barely more than the $7.0 minimum required. But the criteria is satisfied, andthe proposed additional insulation isjustified.

Ri

Ti

Rinsulation Ro

To

R i e

T1 T2 T3

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7-73

7-88 The plumbing system of a plant involves some section of a plastic pipe exposed to the ambient air.The pipe is to be insulated with adequate fiber glass insulation to prevent freezing of water in the pipe. Thethickness of insulation that will protect the water from freezing under worst conditions is to be determined.

Assumptions 1 Heat transfer is transient, but can be treated as steady at average conditions. 2 Heat transferis one-dimensional since there is thermal symmetry about the centerline and no variation in the axial

direction. 3 Thermal properties are constant. 4 The water in the pipe is stationary, and its initial temperature

is 15C. 5 The thermal contact resistance at the interface is negligible. 6 The convection resistance inside

the pipe is negligible.

Properties The thermal conductivities are given to be k = 0.16 W/mC for plastic pipe and k = 0.035

W/mC for fiberglass insulation. The density and specific heat of water are to be = 1000 kg/m3 and Cp =

4.18 kJ/kg.C (Table A-15).

Analysis The inner radius of the pipe is r1 = 3.0 cm and the outer radius of the pipe and thus the innerradius of insulation is r2 = 3.3 cm. We let r3 represent the outer radius of insulation. Considering a 1-m

section of the pipe, the amount of heat that must be transferred from the water as it cools from 15 to 0 C isdetermined to be

kJ177.3=C0)-C)(15kJ/kg.kg)(4.18827.2(

kg827.2m)](1m)(0.03)[kg/m1000()(

total

2321

TmCQ

LrVm

p

Then the average rate of heat transfer during 60 h becomes

,

(.Q

Q

tave

total J

s)W

177 300

60 36000821

The individual thermal resistances are

R Rr r

k L1

2 1

2

0 033 0 0300948

pipe

pipe 2 (0.16 W/ m. C)(1 mC / W

ln( / ) ln( . / . )

).

Rr r

k L

rrinsulation

2 (0.035 W/ m. C)(1 mC / W

ln( / ) ln( / . )

). ln( / . )3 2

2

33

2

00334 55 0 033

R R h A r ro

o conv 2 2

1

(30 W / m . C)(2 m C / W

1 1

18853 3 3 ) .

Then the rate of average heat transfer from the water can be expressed as

[ . ( )]

. . ln( / . ) / ( . ).

,Q

T T

R r rr

i ave o

total

0.821 WC

[ ] C / Wm

7 5 10

0 0948 4 55 0 033 1 1885350

3 33

Therefore, the minimum thickness of fiberglass needed to protect the pipe from freezing is

t= r3 - r2 = 3.50 - 0.033 = 3.467 m

which is too large. Installing such a thick insulation is not practical, however, and thus other freezeprotection methods should be considered.

Ri 0

Ti

Rinsulation RoTo

R i e

T1 T2 T3

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7-74

7-89 The plumbing system of a plant involves some section of a plastic pipe exposed to the ambient air. Thepipe is to be insulated with adequate fiber glass insulation to prevent freezing of water in the pipe. Thethickness of insulation that will protect the water from freezing more than 20% under worst conditions is tobe determined.

Assumptions 1 Heat transfer is transient, but can be treated as steady at average conditions. 2 Heat transferis one-dimensional since there is thermal symmetry about the centerline and no variation in the axial

direction. 3 Thermal properties are constant. 4 The water in the pipe is stationary, and its initial temperature

is 15C. 5 The thermal contact resistance at the interface is negligible. 6 The convection resistance insidethe pipe is negligible.

Properties The thermal conductivities are given to be k = 0.16 W/mC for plastic pipe and k = 0.035

W/mC for fiberglass insulation. The density and specific heat of water are to be = 1000 kg/m3 and Cp =

4.18 kJ/kg.C (Table A-15).

Analysis The inner radius of the pipe is r1 = 3.0 cm and the outer radius of the pipe and thus the inner radiusof insulation is r2 = 3.3 cm. We let r3 represent the outer radius of insulation. The latent heat of freezing ofwater is 33.7 kJ/kg. Considering a 1-m section of the pipe, the amount of heat that must be transferred from

the water as it cools from 15 to 0C is determined to be

kJ0.3667.1883.177

kJ7.188)kJ/kg7.333)(kg827.2(2.02.0

kJ177.3=C0)-C)(15kJ/kg.kg)(4.18827.2(

kg827.2m)](1m)(0.03)[kg/m1000()(

freezingcoolingtotal

freezing

total

2321

QQQ

mhQ

TmCQ

LrVm

if

p

Then the average rate of heat transfer during 60 h becomes

,

(.Q

Q

tave

total J

s)W

366 000

60 36001694

The individual thermal resistances are

R Rr r

k L1

2 1

2

0 033 0 0300948

pipe

pipe 2 (0.16 W/ m. C)(1 mC / W

ln( / ) ln( . / . )

).

R r r

k L

r rinsulation2 (0.035 W/ m. C)(1 m

C / W

ln( / ) ln( / . ))

. ln( / . )3 2

2

33

20033 4 55 0 033

R Rh A r r

oo

conv 2 21

(30 W / m . C)(2 mC / W

1 1

18853 3 3 ) .

Then the rate of average heat transfer from the water can be expressed as

[ . ( )]

. . ln( / . ) / ( . ).

,Q

T T

R r rr

i ave o

total

1.694 WC

[ ] C / Wm

7 5 10

0 0948 4 55 0 033 1 18850312

3 33

Therefore, the minimum thickness of fiberglass needed to protect the pipe from freezing is

t= r3 - r2 = 0.312 - 0.033 = 0.279 m

which is too large. Installing such a thick insulation is not practical, however, and thus other freezeprotection methods should be considered.

Ri 0

Ti

Rinsulation RoTo

R i e

T1 T2 T3

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7-75

Review Problems

7-90 Wind is blowing parallel to the walls of a house. The rate of heat loss from the wall is to bedetermined.

Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Recr = 5105. 3

Radiation effects are negligible. 4 Air is an ideal gas with constant properties. 5 The pressure of air is 1 atm.

Properties Assuming a film temperature ofTf= 10C for theoutdoors, the properties of air are evaluated to be (Table A-15)

7336.0Pr

/sm10426.1

CW/m.02439.0

25-

k

AnalysisAir flows along 8-m side. The Reynolds number in this case is

625

10792.7/sm10426.1

m)(8m/s)3600/100050(Re

LVL

which is greater than the critical Reynolds number. Thus we have combined laminar and turbulent flow.Using the proper relation for Nusselt number, heat transfer coefficient is determined to be

C.W/m78.30)096,10(m8

CW/m.02439.0

096,10)7336.0(871)10792.7(037.0Pr)871Re037.0(

2

3/18.063/18.00

NuL

kh

k

LhNu

o

L

The thermal resistances are

2m24=m)m)(83( wLAs

C/W0014.0)mC)(24.W/m78.30(

11

C/W1408.0

m24

C/W.m38.3)38.3(

C/W0052.0)mC)(24.W/m8(

11

22

2

2

22

so

o

s

valueinsulation

sii

AhR

A

RR

AhR

Then the total thermal resistance and the heat transfer rate through the wall are determined from

W122.1

C/W1474.0

C)422(

C/W1474.00014.01408.00052.0

21

total

oinsulationitotal

R

TTQ

RRRR

Ri Rinsulation Ro

T1 T2

Air

V = 50 km/h

T2 = 4C

L = 8 m

WALL

T1 = 22 C

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7-76

7-91 A car travels at a velocity of 60 km/h. The rate of heat transfer from the bottom surface of the hotautomotive engine block is to be determined for two cases.

Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Recr = 5105. 3 Air is

an ideal gas with constant properties. 4 The pressure of air is 1 atm. 5 The flow is turbulent over the entire

surface because of the constant agitation of the engine block. 6 The bottom surface of the engine is a flatsurface.

Properties The properties of air at 1 atm and the film temperature of

(Ts + T)/2 = (75+5)/2 = 40C are (Table A-15)

7255.0Pr

/sm10702.1

CW/m.02662.0

25-

k

AnalysisThe Reynolds number is

525

10855.6/sm10702.1

m)(0.7m/s)3600/100060(Re

LV

L

which is less than the critical Reynolds number. But we will assumeturbulent flow because of the constant agitation of the engine block.

C.W/m97.58)1551(m7.0

CW/m.02662.0

1551)7255.0()10855.6(037.0PrRe037.0

2

3/18.053/18.0

NuL

kh

k

hLNu L

W1734=C5)-(75m)m)(0.7(0.6C).W/m97.58()( 2 ssconv TThAQ

The heat loss by radiation is then determined from Stefan-Boltzman law to be

W181K)273+(10K)273+(75).KW/m10(5.67)m7.0)(m6.0)(92.0()(

4442-8

44

surrssrad TTAQ

Then the total rate of heat loss from the bottom surface of the engine block becomes

W1915 1811734radconvtotal QQQ

The gunk will introduce an additional resistance to heat dissipation from the engine. The total heat transferrate in this case can be calculated from

W1668=

m)0.7m6.0)(CW/m.3(

)m002.0(

m)]m)(0.7C)[(0.6.W/m97.58(

1

C5)-(75

12

ss

s

kA

L

hA

TTQ

The decrease in the heat transfer rate is

1734-1668 = 66 W

Ts = 75C

= 0.92

Air

V = 60 km/h

T = 5C

L = 0.7 m

Engine block

Ts = 10C

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7-77

7-92E A minivan is traveling at 60 mph. The rate of heat transfer to the van is to be determined.


Radiation effects are negligible. 4 Air flow is turbulent because of the intense vibrations involved. 5 Air is

an ideal gas with constant properties. 5 The pressure of air is 1 atm.

Properties Assuming a film temperature of Tf = 80F, theproperties of air are evaluated to be (Table A-15E)

7290.0Pr

/sft101697.0

FBtu/h.ft.01481.023-

k

AnalysisAir flows along 11 ft long side. The Reynolds number in this case is

6

2310704.5

/sft101697.0

ft)(11]ft/s)3600/528060[(Re

LV

L

which is greater than the critical Reynolds number. The air flow is assumed to be entirely turbulent becauseof the intense vibrations involved. Then the Nusselt number and the heat transfer coefficient are determinedto be

F.Btu/h.ft39.11)8461(ft11

FBtu/h.ft.01481.0

8461)7290.0()10704.5(037.0PrRe037.0

2

3/18.063/18.0

NuL

kh

k

LhNu

o

L

o


2ft8.240ft)ft)(116(+ft)ft)(112.3(+ft)ft)(62.3(2 sA

F/Btuh.0004.0)ftF)(240.8.Btu/h.ft39.11(

11

F/Btuh.0125.0)ft(240.8

F/Btu.h.ft3)3(

F/Btuh.0035.0)ftF)(240.8.Btu/h.ft2.1(

11

22

2

2

22

soo

s

valueinsulation

si

i

AhR

A

RR

AhR

Then the total thermal resistance and the heat transfer rate into the minivan are determined to be

Btu/h1220

F/Btuh.0164.0

F)7090(

F/Btuh.0164.00004.00125.00035.0

21

total

oinsulationitotal

R

TTQ

RRRR

Air

V = 60 mph

T = 50F

L = 11 ft

Minivan

Ri Rinsulation Ro

T1 T2

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7-78

7-93 Wind is blowing parallel to the walls of a house with windows. The rate of heat loss through thewindow is to be determined.


Radiation effects are negligible. 4 Air flow is turbulent because of the intense vibrations involved. 5 The

minivan is modeled as a rectangular box. 6 Air is an ideal gas with constant properties. 6 The pressure of airis 1 atm.

Properties Assuming a film temperature of 5C, the propertiesof air at 1 atm and this temperature are evaluated to be (TableA-15)

7350.0Pr

/sm10382.1

CW/m.02401.0

25-

k

AnalysisAir flows along 1.2 m side. The Reynolds number in this case is

625

10447.1/sm10382.1

m)(1.2m/s)3600/100060(Re

LVL

which is greater than the critical Reynolds number. Thus we have combined laminar and turbulent flow.Using the proper relation for Nusselt number, heat transfer coefficient is determined to be

C.W/m93.40)2046(m2.1

CW/m.02401.0

2046)7350.0(871)10447.1(037.0Pr)871Re037.0(

2

3/18.063/18.0

NuL

kh

k

hLNu L


2m5.4=m)m)(1.52.1(3sA

C/W0045.0)mC)(5.4.W/m93.40(

11

C/W0012.0)mC)(5.4W/m.(0.78

m005.0

C/W0231.0)mC)(5.4.W/m8(

11

22,

2

22,

so

oconv

s

cond

siiconv

AhR

kA

LR

AhR

Then the total thermal resistance and the heat transfer rate through the 3 windows become

W833.3

C/W0288.0

C)]2(22[

C/W0288.00045.00012.00231.0

21

,,

total

oconvcondiconvtotal

R

TTQ

RRRR

AirV = 60 km/h

T2 = -2C

L = 1.2 m

WINDOW

T1 = 22 C

7/30/2019 Heat Chap07 073

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7-79

7-94 A fan is blowing air over the entire body of a person. The average temperature of the outer surface ofthe person is to be determined.

Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 The

pressure of air is 1 atm. 4 The average human body can be treated as a 30-cm-diameter cylinder with anexposed surface area of 1.7 m2.

Properties We assume the film temperature to be 35C. The

properties of air at 1 atm and this temperature are (Table A-15)

7268.0Pr

/sm10655.1

CW/m.02625.0

25-

k


4

2510063.9

/sm10655.1

m)m/s)(0.3(5Re

DV

The proper relation for Nusselt number corresponding to this Reynolds number is

6.203

000,282

10063.91

7268.0/4.01

)7268.0()10063.9(62.03.0

000,282

Re

1Pr/4.01

PrRe62.0

3.0

5/48/5

4

4/13/2

3/15.04

5/48/5

4/13/2

3/15.0

khD

Nu

Then

C.W/m02.18)6.203(m3.0

CW/m.02655.0 2

NuD

kh

Considering that there is heat generation in that person's body at a rate of 90 W and body gains heat byradiation from the surrounding surfaces, an energy balance can be written as

Q Q Qgenerated radiation convection

Substituting values with proper units and then application of trial & error method yields the averagetemperature of the outer surface of the person.

C36.2K309.2

s

ss

ssssurrs

T

TT

TThATTA

)]27332()[7.1)(02.18(])27340)[(1067.5)(7.1)(9.0(90

)()(W90

448

44

V = 5 m/s

T = 32C

Person, Ts90 W

= 0.9

D = 0.3 m

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7-80

7-95 The heat generated by four transistors mounted on a thin vertical plate is dissipated by air blown overthe plate on both surfaces. The temperature of the aluminum plate is to be determined.


Radiation effects are negligible. 4 The entire plate is nearly isothermal. 5 The exposed surface area of the

transistor is taken to be equal to its base area. 6 Air is an ideal gas with constant properties. 7 The pressureof air is 1 atm.

Properties Assuming a film temperature of 40C, theproperties of air are evaluated to be (Table A-15)

7255.0Pr

/sm10702.1

CW/m.02662.0

25-

k

AnalysisThe Reynolds number in this case is

425

10386.5/sm10702.1

m)(0.22m/s)60/250(Re

LV

L

which is smaller than the critical Reynolds number. Thus we have laminar flow. Using the proper relationfor Nusselt number, heat transfer coefficient is determined to be

C.W/m75.16)5.138(m22.0

CW/m.02662.0

5.138)7255.0()10386.5(664.0PrRe664.0

2

3/15.043/15.0

NuL

kh

khLNu L

The temperature of aluminum plate then becomes

C50.0

])m22.0(2)[C.W/m75.16(

W)124(C20)(

22s

ssshA

QTTTThAQ

Discussion In reality, the heat transfer coefficient will be higher since the transistors will cause turbulencein the air.

Ts

12 W

V = 250 m/min

T = 20C

L= 22 cm

7/30/2019 Heat Chap07 073

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7-81

7-96 A spherical tank used to store iced water is subjected to winds. The rate of heat transfer to the icedwater and the amount of ice that melts during a 24-h period are to be determined.

Assumptions 1 Steady operating conditions exist. 2 Thermal resistance of the tank is negligible. 3 Radiation

effects are negligible. 4 Air is an ideal gas with constant properties. 5 The pressure of air is 1 atm.

Properties The properties of air at 1 atm pressure and the free stream temperature of 30C are (Table A-15)

7282.0Pr

kg/m.s10729.1

kg/m.s10872.1

/sm10608.1

CW/m.02588.0

5

C0@,

5

25-

s

k

Analysis(a) The Reynolds number is

625

10304.1/sm10608.1

m)(3.02m/s1000/3600)(25Re

DV

The Nusselt number corresponding to this Reynolds number is determined from

105610729.1

10872.1)7282.0()10304.1(06.0)10304.1(4.02

PrRe06.0Re4.02

4/1

5

54.03/265.06

4/1

4.03/25.0

sk

hDNu

and C.W/m05.9)1056(m02.3

CW/m.02588.0 2

NuD

kh

The rate of heat transfer to the iced water is

W7779 C)030(]m)(3.02C)[.W/m05.9())(()(222

TTDhTThAQ sss

(b) The amount of heat transfer during a 24-hour period is

kJ079,672s)3600kJ/s)(24779.7( tQQ

Then the amount of ice that melts during this period becomes

kg2014kJ/kg7.333

kJ079,672

ifif

h

QmmhQ

1 cmIced water

Di = 3 m

0C

Q

Ts = 0CV = 25 km/hT = 30C

7/30/2019 Heat Chap07 073

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7-82

7-97 A spherical tank used to store iced water is subjected to winds. The rate of heat transfer to the icedwater and the amount of ice that melts during a 24-h period are to be determined.

Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 7 Thepressure of air is 1 atm.

Properties The properties of air at 1 atm pressure and the free stream temperature of 30C are (Table A-15)

kg/m.s10872.1

/sm10608.1

CW/m.02588.0

5

25-

k

7282.0Pr

kg/m.s10729.1 5

C0@,

s

Analysis(a) The Reynolds number is

625

10304.1/sm10608.1

m)(3.02m/s1000/3600)(25Re

DV

The Nusselt number corresponding to this Reynolds number is determined from

105610729.1

10872.1)7282.0()10304.1(06.0)10304.1(4.02

PrRe06.0Re4.02

4/1

5

54.03/265.06

4/1

4.03/25.0

sk

hDNu

and C.W/m05.9)1056(m02.3

CW/m.02588.0 2

NuD

kh

In steady operation, heat transfer through the tank by conduction is equal to the heat transfer from the outersurface of the tank by convection and radiation. Therefore,

( ) ( ), ,

, ,

Q Q Q

QT T

Rh A T T A T T

s out s in

sphereo o surr s out o surr s out

through tank from tank, conv+rad

4 4

where C/W10342.2m)m)(1.50C)(1.51W/m.15(4

m)50.151.1(

4

5

21

12

rkr

rrRsphere

222 m28.65m)02.3( DAo

Substituting,

]K)273(K)27315)[(.KW/m1067.5)(m65.28)(9.0(

C))(30mC)(28.65.W/m05.9(C/W1034.2

C0

4,

44282

,22

5

,

outs

outsouts

T

TT

Q

whose solution is

kW9.63 W9630andC23.0 QTs

(b) The amount of heat transfer during a 24-hour period is

kJ032,832s)3600kJ/s)(2463.9( tQQ

Then the amount of ice that melts during this period becomes

kg2493kJ/kg7.333

kJ032,832

ifif

h

QmmhQ

1 cmIced water

Di = 3 m

0C

Ts, outV = 25 km/h

T = 30C 0C

7/30/2019 Heat Chap07 073

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7-98E A cylindrical transistor mounted on a circuit board is cooled by air flowing over it. The maximumpower rating of the transistor is to be determined.

Assumptions 1 Steady operating conditions exist. 2 Radiation effects are negligible. 3 Air is an ideal gas

with constant properties. 4 The pressure of air is 1 atm.

Properties The properties of air at 1 atm and the film temperatureof F1502/)120180( fT are (Table A-15)

7188.0Pr

/sft10210.0

FBtu/h.ft.01646.023-

k


5.727/sft10210.0

ft)/12ft/s)(0.22(500/60Re

23

DV

The Nusselt number corresponding to this Reynolds number is

72.13

000,282

5.7271

7188.0/4.01

)7188.0()5.727(62.03.0

000,282

Re1

Pr/4.01

PrRe62.03.0

5/48/5

4/13/2

3/15.0

5/48/5

4/13/2

3/15.0

k

hDNu

and F.Btu/h.ft32.12)72.13(ft)12/22.0(

FBtu/h.ft.01646.0 2

NuD

kh

Then the amount of power this transistor can dissipate safely becomes

Btu/h)3.412=W(1

C)120180(ft)2ft)(0.25/1(0.22/12F).Btu/h.ft32.12(

)()(

2

W0.26=Btu/h0.887

TTDLhTThAQ sss

Air500 ft/min

120F

Powertransistor

D = 0.22 in

heat chap07 073

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