Solution manual heat and mass transfer a practical approach 2nd edition cengel ch 9

The rate of heat loss from the pipe by natural convection and radiation is to be determined.. The rate of heat loss from the cylindrical side surface of the pan by natural convection and

Chapter 9 NATURAL CONVECTION

Physical Mechanisms of Natural Convection

9-1C Natural convection is the mode of heat transfer that occurs between a solid and a fluid which moves

under the influence of natural means Natural convection differs from forced convection in that fluid motion in natural convection is caused by natural effects such as buoyancy

9-2C The convection heat transfer coefficient is usually higher in forced convection because of the higher

fluid velocities involved

9-3C The hot boiled egg in a spacecraft will cool faster when the spacecraft is on the ground since there is

no gravity in space, and thus there will be no natural convection currents which is due to the buoyancy force

9-4C The upward force exerted by a fluid on a body completely or partially immersed in it is called the

buoyancy or “lifting” force The buoyancy force is proportional to the density of the medium Therefore, the buoyancy force is the largest in mercury, followed by in water, air, and the evacuated chamber Note that in an evacuated chamber there will be no buoyancy force because of absence of any fluid in the medium

9-5C The buoyancy force is proportional to the density of the medium, and thus is larger in sea water than

it is in fresh water Therefore, the hull of a ship will sink deeper in fresh water because of the smaller buoyancy force acting upwards

9-6C A spring scale measures the “weight” force acting on it, and the person will weigh less in water

because of the upward buoyancy force acting on the person’s body

9-7C The greater the volume expansion coefficient, the greater the change in density with temperature, the

greater the buoyancy force, and thus the greater the natural convection currents

9-8C There cannot be any natural convection heat transfer in a medium that experiences no change in

volume with temperature

9-9C The lines on an interferometer photograph represent isotherms (constant temperature lines) for a gas,

which correspond to the lines of constant density Closely packed lines on a photograph represent a large temperature gradient

9-10C The Grashof number represents the ratio of the buoyancy force to the viscous force acting on a fluid

The inertial forces in Reynolds number is replaced by the buoyancy forces in Grashof number

9-11 The volume expansion coefficient is defined as

β 1 For an ideal gas, P=ρRT or

RT , and thus

T T RT

P T RT

P T

RT P

P

11

11

/1

ρ

∂

∂ρβ

Natural Convection Over Surfaces

9-12C Rayleigh number is the product of the Grashof and Prandtl numbers

9-13C A vertical cylinder can be treated as a vertical plate when D L

Gr

≥ 351 4/

9-14C No, a hot surface will cool slower when facing down since the warmer air in this position cannot rise

and escape easily

9-15C The heat flux will be higher at the bottom of the plate since the thickness of the boundary layer

which is a measure of thermal resistance is the lowest there

9-16 A horizontal hot water pipe passes through a large room The rate of heat loss from the pipe by natural

convection and radiation is to be determined

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

atmospheric pressure is 1 atm 4 The temperature of the outer surface of the pipe is constant

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

of (Ts+T∞)/2 = (65+22)/2 = 43.5°C are (Table A-15)

2 5

K00316.0K)2735.43(

11

1

CW/m

02688

0

=+

/sm10735.1(

)m06.0)(

K2265)(

K00316.0)(

m/s81.9(Pr)(

2 2 5

3 -1

)805,692(387.06.0Pr

/559.01

387.06

.0

2

27 / 8 16 / 9

6 / 1 2

27 / 8 16 / 9

6 / 1

m06.0(

C.W/m893.5)15.13(m06.0

CW/m

02688.0

k

h

s

W 477.6

9-17 A power transistor mounted on the wall dissipates 0.18 W The surface temperature of the transistor is

to be determined

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas with constant properties 3 Any heat transfer from the base surface is disregarded 4 The local atmospheric pressure is 1 atm 5 Air properties are

evaluated at 100°C

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

temperature of 100°C are (Table A-15)

1 -

2 5

K00268.0K)273100(

11

2

C W/m

03095

0

=+

Air 35°C

Power transistor, 0.18 W

D = 0.4 cm

ε = 0.1

The transistor loses heat through its cylindrical surface as well as its top surface For convenience,

we take the heat transfer coefficient at the top surface of the transistor to be the same as that of its side surface (The alternative is to treat the top surface as a vertical plate, but this will double the amount of calculations without providing much improvement in accuracy since the area of the top surface is much smaller and it is circular in shape instead of being rectangular) The characteristic length in this case is the outer diameter of the transistor, L c = D=0.004m Then,

6.292)7111.0()

/sm10306.2(

)m004.0)(

K35165)(

K00268.0)(

m/s81.9(Pr)(

2 2 5

3 -1

)6.292(387.06

.0Pr

/559.01

387.06

.0

2

27 / 8 16 / 9

6 / 1 2

27 / 8 16 / 9

6 / 1

Nu

2 2

2

2

m0000691

04/m)004.0()m0045.0)(

m004.0(4/

C.W/m78.15)039.2(m004.0

CW/m

03095.0

=+

=+

π

πDL D

A

Nu D

)K27325()273(

1067.5)(

m0000691

0)(

1.0(

C)35)(

m0000691

0)(

C.W/m8.15(W

18

0

)(

)(

4 4

8 2

2 2

4 4

×+

surr s s s

s

T

T T

T T A T T hA

which is relatively close to the assumed value of 165°C To improve the accuracy of the result, we repeat the Rayleigh number calculation at new surface temperature of 187°C and determine the surface temperature to be

"T_infinity=35 [C], parameter to be varied"

10 15 20 25 30 35 40155

9-19E A hot plate with an insulated back is considered The rate of heat loss by natural convection is to be determined for different orientations

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

atmospheric pressure is 1 atm

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

(Ts+T∞)/2 = (130+75)/2 = 102.5°F are (Table A-15)

1 -

2 3

R001778.0R)4605.102(

11

0

FBtu/h.ft

01535

0

=+

Analysis (a) When the plate is vertical, the characteristic length is

the height of the plate L c = L=2ft Then,

3 -1

2

2

3

10503.5)7256.0()

/sft101823.0(

)ft2)(

R75130)(

R001778.0)(

ft/s2.32(Pr)(

7256.0

492.01

)10503.5(387.0825.0

Pr

492.01

Ra387.0825

.0

2

27 / 8 16 / 9

6 / 1 8 2

27 / 8 16 / 9

6 / 1

×+

2

ft4)ft2(

F.Btu/h.ft7869.0)6.102(ft

2

FBtu/h.ft

01535.0

ft244

A

Then,

6 2

2 3

3 -1

2

2

3

10598.8)7256.0()

/sft101823.0(

)ft5.0)(

R75130)(

R001778.0)(

ft/s2.32(Pr)(

Ra

24.29)10598.8(54.054

5.0

FBtu/h.ft

01535

10598

=

Ra

62.14)10598.8(27.027

5.0

FBtu/h.ft

01535

Btu/h 98.7

9-20E"!PROBLEM 9-20E"

T s [F] Q a [Btu/h] Q b [Btu/h] Q c [Btu/h]

9-21 A cylindrical resistance heater is placed horizontally in a fluid The outer surface temperature of the resistance wire is to be determined for two different fluids

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas with constant properties 3 The local atmospheric pressure is 1 atm 4 Any heat transfer by radiation is ignored 5 Properties are evaluated at

500°C for air and 40°C for water

Properties The properties of air at 1 atm and 500°C are (Table A-15)

1 -

2 5

K001294.0K)273500(

11

7

C W/m

05572

0

=+

2 6

K000377

C W/m

Analysis (a) The solution of this problem requires a trial-and-error approach since the determination of the

Rayleigh number and thus the Nusselt number depends on the surface temperature which is unknown We start the solution process by “guessing” the surface temperature to be 1200°C for the calculation of h We will check the accuracy of this guess later and repeat the calculations if necessary The characteristic length

in this case is the outer diameter of the wire, L c = D=0.005m Then,

)/sm10804.7(

)m005.0(C)201200)(

K001294.0)(

m/s81.9(Pr)(

3 -1

)7.214(387.06

.0Pr

/559.01

387.06

.0

2

27 / 8 16 / 9

6 / 1 2

27 / 8 16 / 9

6 / 1

m005.0(

C.W/m38.21)919.1(m005.0

CW/m

05572.0

T

T

T T hA Q

C)20)(

m01571.0)(

C.W/m38.21(W

400

)(

2 2

&

which is sufficiently close to the assumed value of 1200°C used in the evaluation of h, and thus it is not necessary to repeat calculations

(b) For the case of water, we “guess” the surface temperature to be 40°C The characteristic length in this

case is the outer diameter of the wire, L c = D=0.005m Then,

197,92)32.4()

/sm106582.0(

)m005.0)(

K2040)(

K000377.0)(

m/s81.9(Pr)(

2 2 6

3 -1

)197,92(387.06.0Pr

/559.01

387.06

.0

2

27 / 8 16 / 9

6 / 1 2

27 / 8 16 / 9

6 / 1

Nu

CW/m

631

1134(W

400

)(

T

T

T T hA Q

s

s s

&

which is sufficiently close to the assumed value of 40°C in the evaluation of the properties and h The film

temperature in this case is (Ts+T∞)/2 = (42.5+20)/2 =31.3°C, which is close to the value of 40°C used in the

evaluation of the properties

9-22 Water is boiling in a pan that is placed on top of a stove The rate of heat loss from the cylindrical side surface of the pan by natural convection and radiation and the ratio of heat lost from the side surfaces of the pan to that by the evaporation of water are to be determined

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas

with constant properties 3 The local atmospheric pressure is 1 atm

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

(Ts+T∞)/2 = (98+25)/2 = 61.5°C are (Table A-15)

Vapor

2 kg/h

Water 100°C

2 5

K00299.0K)2735.61(

11

1

CW/m

02819

0

=+

2 5

3 -1

2

2

3

10299.7)7198.0()

/sm10910.1(

)m12.0)(

K2598)(

K00299.0)(

m/s81.9(Pr)(

/ 1 6

4

/

1

35and thus

0.25

<

07443.0)7198.0/10299.7(

)12.0(3535

Gr

L D Gr

7198.0

492.01

)10299.7(387.0825.0

Pr

492.01

Ra387.0825

.0

2

27 / 8 16 / 9

6 / 1 6 2

27 / 8 16 / 9

6 / 1

×+

+

=

Nu

2 2

m09425.0)m12.0)(

m25.0(

C.W/m720.6)60.28(m12.0

CW/m

02819.0

)K27325()K27398(.KW/m1067.5)(

m09425.0)(

95.0(

kg/s3600/2

1254

1.562.46

f

9-23 Water is boiling in a pan that is placed on top of a stove The rate of heat loss from the cylindrical side surface of the pan by natural convection and radiation and the ratio of heat lost from the side surfaces of the pan to that by the evaporation of water are to be determined

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas

with constant properties 3 The local atmospheric pressure is 1 atm

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

(Ts+T∞)/2 = (98+25)/2 = 61.5°C are (Table A-15)

1 -

2 5

K00299.0K)2735.61(

11

1

CW/m

02819

0

=+

2 5

3 -1

2

2

3

10299.7)7198.0()

/sm10910.1(

)m12.0)(

K2598)(

K00299.0)(

m/s81.9(Pr)(

/ 1 6

4

/

1

35and thus

0.25

<

07443.0)7198.0/10299.7(

)12.0(3535

Gr

L D Gr

7198.0

492.01

)10299.7(387.0825.0

Pr

492.01

Ra387.0825

.0

2

27 / 8 16 / 9

6 / 1 6 2

27 / 8 16 / 9

6 / 1

×+

+

=

Nu

2 2

m09425.0)m12.0)(

m25.0(

C.W/m720.6)60.28(m12.0

CW/m

02819.0

)K27325()K27398(.KW/m1067.5)(

m09425.0)(

10.0(

kg/s3600/2

1254

9.52.46

f

9-24 Some cans move slowly in a hot water container made of sheet metal The rate of heat loss from the four side surfaces of the container and the annual cost of those heat losses are to be determined

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas with constant properties 3 The local atmospheric pressure is 1 atm 3 Heat loss from the top surface is disregarded

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

(Ts+T∞)/2 = (55+20)/2 = 37.5°C are (Table A-15)

Water bath 55°C Aerosol can

1 -

2 5

K003221.0K)2735.37(

11

1

CW/m

02644

0

=+

2 5

3 -1

2

2

3

10565.3)7261.0()

/sm10678.1(

)m5.0)(

K2055)(

K003221.0)(

m/s81.9(Pr)(

7261.0

492.01

)10565.3(387.0825.0

Pr

492.01

Ra387.0825

.0

2

27 / 8 16 / 9

6 / 1 8 2

27 / 8 16 / 9

6 / 1

×+

m5.0()m1)(

m5.0(2

C.W/m75.4)84.89(m5.0

CW/m

02644.0

=+

k

h

and

W1.748C)2055)(

m5.4)(

C.W/m75.4()

W/m1067.5)(

m5.4)(

7.0(

)(

4 4

4 2 8 2

4 4

=+

−+

Q& ε σ

Then the total rate of heat loss becomes

W 1499

=+

=+

= rad 748.1 750.9

convection natural

Q& & &

The amount and cost of the heat loss during one year is

kWh131,13 h)8760)(

kW499.1

=Δ

Q total &total

9-25 Some cans move slowly in a hot water container made of sheet metal It is proposed to insulate the side and bottom surfaces of the container for $350 The simple payback period of the insulation to pay for itself from the energy it saves is to be determined

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas with constant properties 3 The local atmospheric pressure is 1 atm 3 Heat loss from the top surface is disregarded

Properties Insulation will drop the outer surface temperature to a value close to the ambient temperature The solution of this problem requires a trial-and-error approach since the determination of the Rayleigh number and thus the Nusselt number depends on the surface temperature, which is unknown We assume the surface temperature to be 26°C The properties of air at the anticipated film temperature of (26+20)/2=23°C are (Table A-15)

1 -

2 5

K00338.0K)27323(

11

1

CW/m

02536

0

=+

Cm

5.0

=

= L

L c

Aerosol can insulation

Water bath 55°C

7 2

2 5

3 -1

2

2

3

10622.7)7301.0()

/sm10543.1(

)m5.0)(

K2026)(

K00338.0)(

m/s81.9(Pr)(

7301.0

492.01

)10622.7(387.0825.0

Pr

492.01

Ra387.0825

.0

Nu

2

27 / 8 16 / 9

6 / 1 7 2

27 / 8 16 / 9

6 / 1

×+

m5.0()m10.1)(

m5.0(2

C.W/m868.2)53.56(m5.0

CW/m

02536.0

=+

97

])K27320()K27326)[(

.KW/m1067.5)(

m7.4)(

1

0

(

+

C)2026)(

m7.4)(

C.W/m868

2

(

)(

)(

4 4

4 2 8 2

2 2

4 4

=

+

−+

−

=+

=

−

s s rad

Q

In steady operation, the heat lost by the side surfaces of the tank must be equal to the heat lost from the exposed surface of the insulation by convection and radiation, which must be equal to the heat conducted through the insulation The second conditions requires the surface temperature to be

m05.0

C)(55)mC)(4.7W/m

035.0(W97.5

T L

T T kA Q

W5.97

=Δ

Q &

Cost=(853.7 kWh)($0.085/kWh)=$72.6

Then money saved during a one-year period due to insulation becomes

1043

$6.72

$1116

$Cost

Costsaved

insulation with insulation

without

where $1116 is obtained from the solution of Problem 9-24

The insulation will pay for itself in

days 122

=

yr 0.3354

$

350

$savedMoney

Costriod

Payback pe

Discussion We would definitely recommend the installation of insulation in this case

9-26 A printed circuit board (PCB) is placed in a room The average temperature of the hot surface of the

board is to be determined for different orientations

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas with

constant properties 3 The local atmospheric pressure is 1 atm 3 The heat

loss from the back surface of the board is negligible

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

temperature of (Ts+T∞)/2 = (45+20)/2 = 32.5°C are (Table A-15)

1 -

2 5

K003273.0K)2735.32(

11

1

CW/m

02607

0

=+

Analysis The solution of this problem requires a trial-and-error approach since the determination of the

Rayleigh number and thus the Nusselt number depends on the surface temperature which is unknown

(a) Vertical PCB We start the solution process by “guessing” the surface temperature to be 45°C for the

evaluation of the properties and h We will check the accuracy of this guess later and repeat the calculations

if necessary The characteristic length in this case is the height of the PCB, L c = L=0.2m Then,

7 2

2 5

3 -1

2

2

3

10756.1)7275.0()

/sm10631.1(

)m2.0)(

K2045)(

K003273.0)(

m/s81.9(Pr)

7275.0

492.01

)10756.1(387.0825.0

Pr

492.01

Ra387.0825

.0

Nu

2

27 / 8 16 / 9

6 / 1 7 2

27 / 8 16 / 9

6 / 1

×+

m15.0

(

C.W/m794.4)78.36(m2.0

CW/m

02607.0

2 2

4 4

)K27320()273(

1067.5)(

m03.0)(

8.0(C)20)(

m03.0)(

C.W/m

)(

+

−+

×+

surr s s s

s

T T

T T A T T

hA

Its solution is

which is sufficiently close to the assumed value of 45°C for the evaluation of the properties and h

(b) Horizontal, hot surface facing up Again we assume the surface temperature to be 45 and use the properties evaluated above The characteristic length in this case is

°C

m

0429.0)m15.0m2.0(2

)m15.0)(

m20.0

2 5

3 -1

2

2

3

10728.1)7275.0()

/sm10631.1(

)m0429.0)(

K2045)(

K003273.0)(

m/s81.9(Pr)(

02607

=

= k Nu

h

Heat loss by both natural convection and radiation heat can be expressed as

])K27320()273)[(

1067.5)(

m03.0)(

8.0(C)20)(

m03.0)(

C.W/m

)(

4 4

8 2

2 2

4 4

+

−+

×+

surr s s s

s

T T

T T A T T

hA

Its solution is

which is sufficiently close to the assumed value of 45°C in the evaluation of the properties and h

(c) Horizontal, hot surface facing down This time we expect the surface temperature to be higher, and

assume the surface temperature to be 50 We will check this assumption after obtaining result and repeat calculations with a better assumption, if necessary The properties of air at the film temperature of

°C

C352

2050

2 5

K003247.0K)27335(

11

1

CW/m

02625

0

=+

/sm10655.1(

)m0429.0)(

K2050)(

K003247.0)(

m/s81.9(Pr)(

2 2 5

3 -1

Ra

453.5)379,166(27.027

CW/m

02625

1067.5)(

m03.0)(

8.0(C)20)(

m03.0)(

C.W/m

)(

4 4

8 2

2 2

4 4

+

−+

×+

surr s s s

s

T T

T T A T T

9-28 Absorber plates whose back side is heavily insulated is placed horizontally outdoors Solar radiation is incident on the plate The equilibrium temperature of the plate is to be determined for two cases

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

atmospheric pressure is 1 atm

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

temperature of (Ts+T∞)/2 = (115+25)/2 = 70°C are (Table A-15)

2 5

K002915.0K)27370(

11

1

CW/m

02881

0

=+

properties and h We will check the accuracy of this guess later and repeat the calculations if necessary

The characteristic length in this case is 0.24m

)m8.0m2.1(2

)m8.0)(

m2.1

2 5

3 -1

2

2

3

10414.6)7177.0()

/sm10995.1(

)m24.0)(

K25115)(

K002915.0)(

m/s81.9(Pr)(

CW/m

02881

m8.0

=

s

A

In steady operation, the heat gain by the plate by absorption of solar radiation

must be equal to the heat loss by natural convection and radiation Therefore,

W6.584)m96.0)(

W/m700)(

87.0

1067.5)(

m96.0)(

09.0(C)25)(

m96.0)(

C.W/m801

)(

4 4

8 2

2 2

4 4

+

−+

×+

sky s s s

s

T T

T T A T T

hA

Its solution is T s =115.6 C°

which is identical to the assumed value Therefore there is no need to repeat calculations

If the absorber plate is made of ordinary aluminum which has a solar absorptivity of 0.28 and an emissivity of 0.07, the rate of solar gain becomes

W2.188)m96.0)(

W/m700)(

28.0

])K27310()273)[(

1067.5)(

m96.0)(

07.0(C)25)(

m96.0)(

C.W/m801

)(

4 4

8 2

2 2

4 4

+

−+

×+

sky s s s

s

T T

T T A T T hA

Its solution is Ts = 55.2°C

Repeating the calculations at the new film temperature of 40°C, we obtain

h = 4.524 W/m2.°C and Ts = 62.8°C

9-29 An absorber plate whose back side is heavily insulated is placed horizontally outdoors Solar radiation

is incident on the plate The equilibrium temperature of the plate is to be determined for two cases

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

atmospheric pressure is 1 atm

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

temperature of (Ts+T∞)/2 = (70+25)/2 = 47.5°C are (Table A-15)

1 -

2 5

K00312.0K)2735.47(

11

1

CW/m

02717

0

=+

properties and h We will check the accuracy of this guess later and repeat the calculations if necessary

The characteristic length in this case is 0.24m

)m8.0m2.1(2

)m8.0)(

m2.1

2 5

3 -1

2

2

3

10379.4)7235.0()

/sm10774.1(

)m24.0)(

K2570)(

K00312.0)(

m/s81.9(Pr)

Ra

93.43)10379.4(54.054

CW/m

02717

m8.0

=

s

A

In steady operation, the heat gain by the plate by absorption of solar radiation must be equal to the heat loss

by natural convection and radiation Therefore,

W6.658)m96.0)(

W/m700)(

98.0

1067.5)(

m96.0)(

98.0(C)25)(

m96.0)(

C.W/m973

)(

4 4

8 2

2 2

4 4

+

−+

×+

surr s s s

s

T T

T T A T T

hA

Its solution is T s =73.5 C°

which is close to the assumed value Therefore there is no need to repeat calculations

For a white painted absorber plate, the solar absorptivity is 0.26 and the emissivity is 0.90 Then the rate of solar gain becomes

W7.174)m96.0)(

W/m700)(

26.0

above for convenience (actually, we should calculate the new h using data at a lower temperature, and

iterating if necessary for better accuracy),

])K27310()273)[(

1067.5)(

m96.0)(

90.0(C)25)(

m96.0)(

C.W/m973

)(

4 4

8 2

2 2

4 4

+

−+

×+

°

−

°

−+

surr s s s

s

T T

T T A T T hA

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas

with constant properties 3 The local atmospheric pressure is 1 atm

Analysis The heat transfer surface area of the cylinder is

2m05027.0)m8.0)(

m02.0

=

=πDL π

A

Noting that in steady operation the heat dissipated from the outer

surface must equal to the electric power consumed, and radiation

is negligible, the convection heat transfer is determined to be

C W/m

m05027.0(

W40)

( )

T T A

Q h

T T hA

Q

s s s

.KW/m1067.5)(

m05027.0)(

1.0(

)(

4 4

4 2 8 2

4 4

=+

−+

Q& ε σ

Therefore, the fraction of heat loss by radiation is

%8.111175.0W40

W7.4fraction

which is greater than 5% Therefore, the radiation effect is still more than acceptable, and corrections must

be made for the radiation effect

9-31 A thick fluid flows through a pipe in calm ambient air The pipe is heated electrically The power rating of the electric resistance heater and the cost of electricity during a 10-h period are to be determined.√

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

atmospheric pressure is 1 atm

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

(Ts+T∞)/2 = (25+0)/2 = 12.5°C are (Table A-15)

2 5

K003503.0K)2735.12(

11

1

CW/m

02458

0

=+

2 5

3 -1

2

2

3

10106.8)7330.0()

/sm10448.1(

)m3.0)(

K025)(

K003503.0)(

m/s81.9(Pr)(

Ra

7330.0/559.01

)10106.8(387.06.0Pr

/559.01

387.06

.0

2

27 / 8 16 / 9

6 / 1 7 2

27 / 8 16 / 9

6 / 1

m3.0(

C.W/m366.4)29.53(m3.0

CW/m

02458.0

m25.94)(

C.W/m366.4()

.KW/m1067.5)(

m25.94)(

8.0(

)(

4 4

4 2 8 2

4 4

=+

−

−+

Q& ε σ

Then,

kW 29.1

=

=+

=+

= radiation 10,287 18,808 29,094 W

convection natural

Q& & &

The total amount and cost of heat loss during a 10 hour period is

kWh9.290h)kW)(101.29

=Δ

=Q t

Q &

Cost=(290.9kWh)($0.09/kWh)=$26.18

9-32 A fluid flows through a pipe in calm ambient air The pipe is heated electrically The thickness of the

insulation needed to reduce the losses by 85% and the money saved during 10-h are to be determined

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

atmospheric pressure is 1 atm

Properties Insulation will drop the outer surface temperature to a value close to the ambient temperature,

and possible below it because of the very low sky temperature for radiation heat loss For convenience, we use the properties of air at 1 atm and 5°C (the anticipated film temperature) (Table A-15),

1 -

2 5

K003597.0K)2735(

11

1

CW/m

02401

0

=+

ε = 0.1

Tsky = -30°C

T∞ = 0°C

Analysis The rate of heat loss in the previous problem was

obtained to be 29,094 W Noting that insulation will cut down

the heat losses by 85%, the rate of heat loss will be

W4364W094,2915.0)

85.01

Q saved total &saved

Moneysaved=(Energysaved)(Uni costofenergy)=(247.3 kWh)($0.09/kWh)=$22.26

The characteristic length in this case is the outer diameter of the insulated pipe,

where t insul

insul

L = +2 =0.3+2 insul is the thickness of insulation in m Then the problem can be

formulated for Ts and tinsul as follows:

)7350.0()

/sm10382.1(

)23.0(K]

)273)[(

K003597.0)(

m/s81.9(Pr)(

2 2 5

3 -1

2

27 / 8 16 / 9

6 / 1 2

27 / 8 16 / 9

6 / 1

7350.0/559.01

387.06

.0Pr

/559.01

387.06

.0

Nu

m))(1002

3.0(

CW/m

02401.0

s

c c

t L

D

A

Nu L

Nu L

The total rate of heat loss from the outer surface of the insulated pipe by convection and radiation becomes

])K27330()[

.KW/m1067.5()1.0(+)273(

4364

)(

)(

4 4

4 2 8

4 4

−

=+

=

−

∞

s s

s s

surr s s s

s rad conv

T A

T hA

T T A T T hA Q

Q

In steady operation, the heat lost by the side surfaces of the pipe must be equal to the heat lost from the exposed surface of the insulation by convection and radiation, which must be equal to the heat conducted through the insulation Therefore,

]3.0/)23.0ln[(

K)m)(298C)(100W/m

035.0(2W4364 )/ln(

)(

2 tank

insul

s o

s insulation

t

T D

D

T T kL Q

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

atmospheric pressure is 1 atm

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

temperature of (Ts+T∞)/2=(140+75)/2=107.5°F are (Table A-15)

1 -

2 3

R001762.0R)4605.107(

11

0

FBtu/h.ft

01546

0

=+

properties and h We will check the accuracy of this guess later and repeat the calculations if necessary

The characteristic length in this case is the outer diameter of the furnace, L c = D=8ft Then,

10 2

2 3

3 -1

2

2

3

10996.3)7249.0()

/sft101851.0(

)ft8)(

R75140)(

R001762.0)(

ft/s2.32(Pr)(

)10996.3(387.06.0Pr

/559.01

387.06

.0

2

27 / 8 16 / 9

6 / 1 10 2

27 / 8 16 / 9

6 / 1

ft8(

F.Btu/h.ft7287.0)9.376(ft

8

FBtu/h.ft

01546.0

( therms/h)48

)(

82.0

.RBtu/h.ft10

1714.0)(

m7.326)(

85.0(

)]

R46075()[

ft7.326(F).Btu/h.ft7287.0(Btu/h360

,

39

)(

)(

4 4

4 2 8

2

2 2

4 4

+

−

×+

surr s s s

s

T T

T T A T T hA

which is very close to the assumed value Therefore, there is no need to repeat calculations

The total amount of heat loss and its cost during a-2800 hour period is

Btu10102.1 h)2800)(

Btu/h360,39

=Δ

Q total &total

Cost=(1.102×108/100,000 therm)($0.65/therm)=$716.4

9-34 A glass window is considered The convection heat transfer coefficient on the inner side of the window, the rate of total heat transfer through the window, and the combined natural convection and radiation heat transfer coefficient on the outer surface of the window are to be determined

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas

with constant properties 3 The local atmospheric pressure is 1 atm

Q&

Outdoors -5°C

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

(Ts+T∞)/2 = (5+25)/2 = 15°C are (Table A-15)

1 -

2 5

K003472.0K)27315(

11

1

CW/m

02476

0

=+

2 5

3 -1

2

2

3

10986.3)7323.0()

/sm10471.1(

)m2.1)(

K525)(

K34720.0)(

m/s81.9(Pr)(

Ra

7.189

7323.0

492.01

)10986.3(387.0825.0

Pr

492.01

Ra387.0825

.0

2

27 / 8 16 / 9

6 / 1 9 2

27 / 8 16 / 9

6 / 1

×+

+

=

Nu

2m4.2m)m)(22.1

(

)7.189(m2.1

CW/m

02476.0

k

(b) The sum of the natural convection and radiation heat transfer from the room to the window is

W9.187C)525)(

m4.2)(

C.W/m915.3()

.KW/m1067.5)(

m4.2)(

9.0(

)(

4 4

4 2 8 2

4 4 radiation

=+

−+

A

Q&total=Q&convection+Q&radiation =187.9+234.3=422.2 W

(c) The outer surface temperature of the window can be determined from

C65.3)m4.2)(

CW/m

78.0(

)m006.0)(

W346(C5)

(

2

total , , ,

s i s s

kA

t Q T T T

T t

5(65.3)[

m4.2(

W346)

(

or

)(

2 ,

,

total combined

, , combined total

o o s s

o o s s

T T A

Q h

T T A h

Q

&

&

Note that and thus the thermal resistance R of a layer is proportional to the temperature drop

across that layer Therefore, the fraction of thermal resistance of the glass is equal to the ratio of the temperature drop across the glass to the overall temperature difference,

ΔT QR= &

4.5%)(or 045.0)5(25

65.35total glass

Δ

=

TR

T R

R

which is low Thus it is reasonable to neglect the thermal resistance of the glass

9-35 An insulated electric wire is exposed to calm air The temperature at the interface of the wire and the plastic insulation is to be determined

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

atmospheric pressure is 1 atm

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

temperature of (Ts+T∞)/2 = (50+30)/2 = 40°C are (Table A-15) Air

1 -

2 5

K003195.0K)27340(

11

1

CW/m

02662

0

=+

properties and h We will check the accuracy of this guess later and repeat the calculations if necessary The characteristic length in this case is the outer diameter of the insulated wire L c = D = 0.006 m Then,

3.339)7255.0()

/sm10702.1(

)m006.0)(

K3050)(

K003195.0)(

m/s81.9(Pr)(

2 2 5

3 -1

)3.339(387.06

.0Pr

/559.01

387.06

.0

2

27 / 8 16 / 9

6 / 1 2

27 / 8 16 / 9

6 / 1

C.W/m327.9)101.2(m006.0

CW/m

02662.0

.KW/m1067.5)(

m2262.0)(

9.0(

C)30)(

m226.0)(

C.W/m327.9(W

80

)(

)(

4 4

4 2 8 2

2 2

4 4

+

−+

×+

surr s s s

s

T T

T T A T T hA

Its solution is

C6

=

°

°

=+

CW/m

15.0(2

)3/6ln(

)W80(+C6.522

)/ln(

)()/

T D

D

kL

&

9-36 A steam pipe extended from one end of a plant to the other with no insulation on it The rate of heat loss from the steam pipe and the annual cost of those heat losses are to be determined

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

atmospheric pressure is 1 atm

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

(Ts+T∞)/2 = (170+20)/2 = 95°C are (Table A-15)

2 5

K002717.0K)27395(

11

2

CW/m

0306

0

=+

2 5

3 -1

2

2

3

10231.1)7121.0()

/sm10252.2(

)m0603.0)(

K20170)(

K002717.0)(

m/s81.9(Pr)(

)10231.1(387.06.0Pr

/559.01

387.06

.0

2

27 / 8 16 / 9

6 / 1 6 2

27 / 8 16 / 9

6 / 1

C.W/m823.7)42.15(m0603.0

CW/m

0306.0

=

=

+

−+

×+

27,388

])K27320()K273170)[(

.KW/m1067.5)(

m37.11)(

7.0(

C)20170)(

m37.11)(

C.W/m823

)(

4 4

4 2 8 2

2 2

4 4σε

&

The total amount of gas consumption and its cost during a one-year period is

therms/yr496

,10s/h)3600h/yr8760(kJ105,500

therm178.0

kJ/s388.27

9-38 A steam pipe extended from one end of a plant to the other It is proposed to insulate the steam pipe for $750 The simple payback period of the insulation to pay for itself from the energy it saves are to be determined

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

atmospheric pressure is 1 atm

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

temperature of (Ts+T∞)/2 = (35+20)/2 = 27.5°C are (Table A-15)

2 5

K003328.0K)2735.27(

11

1

CW/m

0257

0

=+

We will check the accuracy of this guess later and repeat the calculations if necessary The characteristic length in this case is the outer diameter of the insulated pipe, L c = D=0.1603m Then,

6 2

2 5

3 -1

2

2

3

10856.5)7289.0()

/sm10584.1(

)m1603.0)(

K2035)(

K003328.0)(

m/s81.9(Pr)

)10856.5(387.06.0Pr

/559.01

387.06

.0

2

27 / 8 16 / 9

6 / 1 6 2

27 / 8 16 / 9

6 / 1

C.W/m884.3)23.24(m1603.0

CW/m

0257.0

])K27320()K27335)[(

.KW/m1067.5)(

m22.30)(

1

0

(

+

C)2035)(

m22.30)(

C.W/m884

3

(

)(

)(

4 4

4 2 8 2

2 2

4 4

=

+

−+

−

=+

=

−

s s rad

Q

In steady operation, the heat lost from the exposed surface of the insulation by convection and radiation must be equal

to the heat conducted through the insulation This requirement gives the surface temperature to be

)m60)(

CW/m

038.0(2

)03.6/03.16ln(

C)170(W

2039 2

)/ln( 2 1 .

s s

i s ins

s i

kL

D D T T R

T T Q

Q& &

It gives 30.8°C for the surface temperature, which is somewhat different than the assumed value of 35°C Repeating the calculations with other surface temperatures gives

W1988and

C3

Q&

Therefore, the money saved by insulation will be 0.921×($5647/yr) = $5237/yr which will pay for the cost

of $750 in $750/($5237/yr)=0.1432 year = 52.3 days

9-39 A circuit board containing square chips is mounted on a vertical wall in a room The surface temperature of the chips is to be determined

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas with

constant properties 3 The local atmospheric pressure is 1 atm 4 The heat

transfer from the back side of the circuit board is negligible

L = 30 cm

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

temperature of (Ts+T∞)/2 = (35+25)/2 = 30°C are (Table A-15)

1 -

2 5

K0033.0K)27330(

11

1

CW/m

02588

0

=+

properties and h We will check the accuracy of this guess later and repeat the calculations if necessary

The characteristic length in this case is the height of the board, L c = L=0.3m Then,

7 2

2 5

3 -1

2

2

3

10463.2)7282.0()

/sm10608.1(

)m3.0)(

K2535)(

K0033.0)(

m/s81.9(Pr)(

.0

492.01

)10463.2(387.0825.0Pr

492.01

Ra387.0825

.0

2

27 / 8 16 / 9

6 / 1 7 2

27 / 8 16 / 9

6 / 1

×+

+

=

Nu

2 2

2

m09.0m)3.0

(

C.W/m50.3)57.40(m3.0

CW/m

02588.0

k

h

Considering both natural convection and radiation, the total rate of heat loss can be expressed as

])K27325()K273)[(

.KW/m1067.5)(

m09.0)(

7.0(

C)25)(

m09.0)(

C.W/m50.3(W)05.0

121

(

)(

)(

4 4

4 2 8 2

2 2

4 4

+

−+

×+

surr s s s

s

T T

T T A T T hA

9-40 A circuit board containing square chips is positioned horizontally in a room The surface temperature

of the chips is to be determined for two orientations

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas with constant properties 3 The local atmospheric pressure is 1 atm 4 The heat transfer from the back side of the circuit board is negligible

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

temperature of (Ts+T∞)/2 = (35+25)/2 = 30°C are (Table A-15) Air

T∞ = 25°C

Tsurr = 25°C

PCB, Ts

ε = 0.7 121×0.05 W

L = 30 cm

1 -

2 5

K0033.0K)27330(

11

1

CW/m

02588

0

=+

properties and h The characteristic length for both cases is determined from

m

075.0m)]

3.0(+m)3.02[(

m)3.0

2 5

3 -1

2

2

3

10848.3)7282.0()

/sm10608.1(

)m075.0)(

K2535)(

K00333.0)(

m/s81.9(Pr)

2

m09.0m)3.0

(

C.W/m641.4)45.13(m075.0

CW/m

02588.0

k

h

Considering both natural convection and radiation, the total rate of heat loss can be expressed as

])K27325()K273)[(

.KW/m1067.5)(

m09.0)(

7.0(

C)25)(

m09.0)(

C.W/m641.4(W)05.0

121

(

)(

)(

4 4

4 2 8 2

2 2

4 4

+

−+

×+

surr s s s

s

T T

T T A T T hA

Its solution is Ts = 32.5°C

which is sufficiently close to the assumed value Therefore, there is no need to repeat calculations

(b) Chips (hot surface) facing up:

725.6)10848.3(27.027

CW/m

02588

.KW/m1067.5)(

m09.0)(

7.0(

C)25)(

m09.0)(

C.W/m321.2(W)05.0

121

(

)(

)(

4 4

4 2 8 2

2 2

4 4

+

−+

×+

surr s s s

s

T T

T T A T T hA

Its solution is Ts = 35.0°C

which is identical to the assumed value in the evaluation of properties and h Therefore, there is no need to

repeat calculations

9-41 It is proposed that the side surfaces of a cubic industrial furnace be insulated for $550 in order to reduce the heat loss by 90 percent The thickness of the insulation and the payback period of the insulation

to pay for itself from the energy it saves are to be determined

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas with

constant properties 3 The local atmospheric pressure is 1 atm

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

(Ts+T∞)/2 = (110+30)/2 = 70°C are (Table A-15)

2 5

K002915.0K)27370(

11

1

CW/m

02881

0

=+

2 5

3 -1

2

2

3

10301.3)7177.0()

/sm10995.1(

)m2)(

K30110)(

K002915.0)(

m/s81.9(Pr)(

7177.0

492.01

)10301.3(387.0825.0

Pr

492.01

Ra387.0825

.0

2

27 / 8 16 / 9

6 / 1 10 2

27 / 8 16 / 9

6 / 1

×+

+

=

Nu

2 2

2

m16)m2(

4

C.W/m318.5)2.369(m2

CW/m

02881.0

k

h

Then the heat loss by combined natural convection and radiation becomes

W119

,

15

])K27330()K273110)[(

.KW/m1067.5)(

m16)(

7.0(

C)30110)(

m16)(

C.W/m318

5

(

)(

)(

4 4

4 2 8 2

2 2

4 4

=

+

−+

×+

9.01(

W607,13 W119,159.09

.0

insulation no loss

insulation no saved

Q Q

s/yr)3600(8760kJ105,500

therm178.0

kJ/s607.13saved

=Q&saved t

Money saved=(Energy saved)(Unit cost ofenergy)=(5215 therms)($0.55/therm)=$2868Therefore, the money saved by insulation will pay for the cost of $550 in

550/($2868/yr)=0.1918 yr = 70 days

Insulation will lower the outer surface temperature, the Rayleigh and Nusselt numbers, and thus the convection heat transfer coefficient For the evaluation of the heat transfer coefficient, we assume the surface temperature in this case to be 50°C The properties of air at the film temperature of (Ts+T∞)/2 = (50+30)/2 = 40°C are (Table A-15)

1 -

2 5

K003195.0K)27340(

11

1

CW/m

02662

0

=+

2 5

3 -1

2

2

3

10256.1)7255.0()

/sm10702.1(

)m2)(

K3050)(

K003195.0)(

m/s81.9(Pr)(

.0

492.01

)10256.1(387.0825.0Pr

492.01

Ra387.0825

.0

2

27 / 8 16 / 9

6 / 1 10 2

27 / 8 16 / 9

6 / 1

×+

+

=

Nu

C.W/m620.3)0.272(m2

CW/m

02662

.KW/m1067.5()7.0(+C)30(C).W/m620

)(

4 4

4 2 8 2

4 4

+

−+

−

=+

=

−

∞

s s

surr s s s

s rad conv

T A

T A

T T A T T hA Q

insul

s s

ins

s s

insulation

t

T A

t

T T

kA Q

Q&= & = ( furnace− ) → 1512W=(0.038W/m.°C) (110− )°C

Solving the two equations above by trial-and error (or better yet, an equation solver) gives

Ts = 48.4°C and tinsul = 0.0254 m = 2.54 cm

9-42 A cylindrical propane tank is exposed to calm ambient air The propane is slowly vaporized due to a

crack developed at the top of the tank The time it will take for the tank to empty is to be determined

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

atmospheric pressure is 1 atm 4 Radiation heat transfer is negligible

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

(Ts+T∞)/2 = (-42+25)/2 = -8.5°C are (Table A-15)

1 -

2 5

K003781.0K)2735.8(

11

1

CW/m

02299

0

=+

Analysis The tank gains heat through its cylindrical surface as well as its circular end surfaces For

convenience, we take the heat transfer coefficient at the end surfaces of the tank to be the same as that of its side surface (The alternative is to treat the end surfaces as a vertical plate, but this will double the amount

of calculations without providing much improvement in accuracy since the area of the end surfaces is much smaller and it is circular in shape rather than being rectangular) The characteristic length in this case is the outer diameter of the tank, L c = D=1.5m Then,

10 2

2 5

3 -1

2

2

3

10869.3)7383.0()

/sm10265.1(

)m5.1](

K)42(25)[(

K003781.0)(

m/s81.9(Pr)(

)10869.3(387.06.0Pr

/559.01

387.06

.0

2

27 / 8 16 / 9

6 / 1 10 2

27 / 8 16 / 9

6 / 1

Nu

2 2

2

2

m38.224/m)5.1(2)m4)(

m5.1(4/2

C.W/m733.5)1.374(m5.1

CW/m

02299.0

=+

=+

π

A

Nu D

42(25)[(

m38.22)(

C.W/m733.5()

kJ/s598.8

kg4107)m4(4

)m5.1()kg/m581(4

2 3

=πρ

=ρ

kg4107

9-43E The average surface temperature of a human head is to be determined when it is not covered

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

atmospheric pressure is 1 atm 4 The head can be approximated as a 12-in.-diameter sphere

Properties The solution of this problem requires a trial-and-error approach since the determination of the Rayleigh number and thus the Nusselt number depends on the surface temperature which is unknown We start the solution process by “guessing” the surface temperature to be 120°F for the evaluation of the

properties and h We will check the accuracy of this guess later and repeat the calculations if necessary The properties of air at 1 atm and the anticipated film temperature of (Ts+T∞)/2 = (120+77)/2 = 98.5°F are (Table A-15E)

1 -

2 3

R001791.0R)4605.98(

11

0

FBtu/h.ft

01525

0

=+

2 3

3 -1

2

2

3

10943.6)7262.0()

/sft10180.0(

)ft5.0)(

R7795)(

R001791.0)(

ft/s2.32(Pr)(

7262.0

469.01

)10943.6(589.02

Pr

469.01

589.0

16 / 9

4 / 1 6

9 / 4 16 / 9

4 / 1

×+

+

Nu

2 2

2

2

ft7854.0)ft5.0(

F.Btu/h.ft7744.0)39.25(ft

1

FBtu/h.ft

01525.0

.RBtu/h.ft10

1714.0)(

m7854.0)(

9.0(

F)77)(

ft7854.0(F).Btu/h.ft7744.0(Btu/h)

)(

4 4

4 2 8

2

2 2

4 4

+

−+

×+

surr s s s

s

T T

T T A T T hA

Its solution is

Ts = 125.9°F

which is sufficiently close to the assumed value in the evaluation of the properties and h Therefore, there is

no need to repeat calculations

9-44 The equilibrium temperature of a light glass bulb in a room is to be determined

Assumptions 1 Steady operating conditions exist 2 Air is an ideal gas with constant properties 3 The local atmospheric pressure is 1 atm 4 The light bulb is approximated as an 8-cm-diameter sphere

Properties The solution of this problem requires a trial-and-error approach since the determination of the Rayleigh number and thus the Nusselt number depends on the surface temperature which is unknown We start the solution process by “guessing” the surface temperature to be 170°C for the evaluation of the

properties and h We will check the accuracy of this guess later and repeat the calculations if necessary The properties of air at 1 atm and the anticipated film temperature of (Ts+T∞)/2 = (170+25)/2 = 97.5°C are (Table A-15)

1 -

2 5

K002699.0K)2735.97(

11

2

C W/m

03077

0

=+

3 -1

/sm10279.2(

)m08.0)(

K25170)(

K002699.0)(

m/s81.9(Pr)(

)10694.2(589.02Pr

/469.01

589.02

9 / 4 16 / 9

4 / 1 6

9 / 4 16 / 9

4 / 1

=+

×+

=+

2

2

m02011.0m)08.0(

C.W/m854.7)42.20(m08.0

CW/m

03077.0

.KW/m1067.5)(

m02011.0)(

9.0(

C)25)(

m02011.0)(

C.W/m854.7(W)60

)(

4 4

4 2 8 2

2 2

4 4

+

−+

×+

surr s s s

s

T T

T T A T T hA

Its solution is

Ts = 169.4°C

which is sufficiently close to the value assumed in the evaluation of properties and h Therefore, there is no

need to repeat calculations