Solution manual heat and mass transfer a practical approach 2nd edition cengel ch 10

Chapter 10 BOILING AND CONDENSATION Boiling Heat Transfer 10-1C Boiling is the liquid-to-vapor phase change process that occurs at a solid-liquid interface when the surface is heated a

Chapter 10 BOILING AND CONDENSATION

Boiling Heat Transfer

10-1C Boiling is the liquid-to-vapor phase change process that occurs at a solid-liquid interface when the

surface is heated above the saturation temperature of the liquid The formation and rise of the bubbles and the liquid entrainment coupled with the large amount of heat absorbed during liquid-vapor phase change at essentially constant temperature are responsible for the very high heat transfer coefficients associated with nucleate boiling

10-2C Yes Otherwise we can create energy by alternately vaporizing and condensing a substance

10-3C Both boiling and evaporation are liquid-to-vapor phase change processes, but evaporation occurs at

the liquid-vapor interface when the vapor pressure is less than the saturation pressure of the liquid at a

given temperature, and it involves no bubble formation or bubble motion Boiling, on the other hand,

occurs at the solid-liquid interface when a liquid is brought into contact with a surface maintained at a temperature Ts sufficiently above the saturation temperature Tsat of the liquid

10-4C Boiling is called pool boiling in the absence of bulk fluid flow, and flow boiling (or forced

convection boiling) in the presence of it In pool boiling, the fluid is stationary, and any motion of the fluid

is due to natural convection currents and the motion of the bubbles due to the influence of buoyancy

10-5C Boiling is said to be subcooled (or local) when the bulk of the liquid is subcooled (i.e., the

temperature of the main body of the liquid is below the saturation temperature Tsat), and saturated (or bulk)

when the bulk of the liquid is saturated (i.e., the temperature of all the liquid is equal to Tsat)

10-6C The boiling curve is given in Figure 10-6 in the text In the natural convection boiling regime, the

fluid motion is governed by natural convection currents, and heat transfer from the heating surface to the

fluid is by natural convection In the nucleate boiling regime, bubbles form at various preferential sites on

the heating surface, and rise to the top In the transition boiling regime, part of the surface is covered by a

vapor film In the film boiling regime, the heater surface is completely covered by a continuous stable

vapor film, and heat transfer is by combined convection and radiation

10-7C In the film boiling regime, the heater surface is completely covered by a continuous stable vapor

film, and heat transfer is by combined convection and radiation In the nucleate boiling regime, the heater surface is covered by the liquid The boiling heat flux in the stable film boiling regime can be higher or lower than that in the nucleate boiling regime, as can be seen from the boiling curve

10-8C The boiling curve is given in Figure 10-6 in the text The burnout point in the curve is point C The

burnout during boiling is caused by the heater surface being blanketed by a continuous layer of vapor film

at increased heat fluxes, and the resulting rise in heater surface temperature in order to maintain the same heat transfer rate across a low-conducting vapor film Any attempt to increase the heat flux beyond will cause the operation point on the boiling curve to jump suddenly from point C to point E However, the surface temperature that corresponds to point E is beyond the melting point of most heater materials, and burnout occurs The burnout point is avoided in the design of boilers in order to avoid the disastrous explosions of the boilers

&max

q

10-9C Pool boiling heat transfer can be increased permanently by increasing the number of nucleation sites

on the heater surface by coating the surface with a thin layer (much less than 1 mm) of very porous material, or by forming cavities on the surface mechanically to facilitate continuous vapor formation Such

surfaces are reported to enhance heat transfer in the nucleate boiling regime by a factor of up to 10, and the critical heat flux by a factor of 3 The use of finned surfaces is also known to enhance nucleate boiling heat transfer and the critical heat flux

10-10C The different boiling regimes that occur in a vertical tube during flow boiling are forced

convection of liquid, bubbly flow, slug flow, annular flow, transition flow, mist flow, and forced convection of vapor

10-11 Water is boiled at 1 atm pressure and thus at a saturation (or boiling) temperature of Tsat = 100°C in

a mechanically polished stainless steel pan whose inner surface temperature is maintained at Ts = 110°C

The rate of heat transfer to the water and the rate of evaporation of water are to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the heater and the boiler are

h fg l pl

3 3

mechanically polished stainless steel surface (Table 10-3 ) Note

that we expressed the properties in units specified under Eq 10-2

in connection with their definitions in order to avoid unit

2

3 3

1/2 3

3

3 sat ,

2 / 1 nucleate

W/m700,140

75.1)102257(0130.0

)100110(42170589

.0

0.60)-9.8(957.9)

10)(225710

282.0(

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρρμ

&

The surface area of the bottom of the pan is

2 2

2

m04909.04/m)25.0(4

=

=

= nucleate (0.04909m2)(140,700W/m2)boiling A q

Q& s&

(b) The rate of evaporation of water is determined from

kg/s 10

J/s69073

boiling n

evaporatio

fg h

Q m

&

&

That is, water in the pan will boil at a rate of 3 grams per second

10-12 Water is boiled at 1 atm pressure and thus at a saturation (or boiling) temperature of Tsat = 100°C by

a mechanically polished stainless steel heating element The maximum heat flux in the nucleate boiling

regime and the surface temperature of the heater for that case are to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the boiler are negligible

Properties The properties of water at the saturation

h fg l pl

3 3

J / kg

kg m / s

Also, 0.0130 and n = 1.0 for the boiling of water on a mechanically polished stainless steel surface

(Table 10-3) Note that we expressed the properties in units specified under Eqs 10-2 and 10-3 in connection with their definitions in order to avoid unit manipulations For a large horizontal heating

3 sat ,

2 / 1 nucleate

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρρμ

&

3 3

1/2 3

3

75.1)102257(0130.0

)100(42170589

.0

0.60)-9.8(957.9)

10)(225710

282.0(000

C_sf=0.0130 "from Table 8-3 of the text"

n=1 "from Table 8-3 of the text"

C_cr=0.12 "from Table 8-4 of the text"

10-14E Water is boiled at 1 atm pressure and thus at a saturation (or boiling) temperature of Tsat = 212°F

by a horizontal polished copper heating element whose surface temperature is maintained at Ts = 788°F

The rate of heat transfer to the water per unit length of the heater is to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the boiler are negligible

Properties The properties of water at the saturation temperature of 212°F are and

(Table A-9E) The properties of the vapor at the film temperature of

are (Table A-16E)

ρl = 59 82 lbm / ft3

h fg = 970 Btu / lbm

T f =(Tsat+T s) /2=(212 788+ ) /2=500°F

FftBtu/h02267.0

FBtu/lbm4707

.0C

hBtu/lbm0.04564

lbm/ft 02571

Analysis The excess temperature in this case is ΔT T T= s− sat=788 212− =576°F, which is much larger than 30°C or 54°F Therefore, film boiling will occur The film boiling heat flux in this case can be determined to be

2

4 / 1 3

2

sat

4 / 1

sat

3

film

ftBtu/h

600

,

18

)212788()

212788)(

12/5.0)(

04564.0(

)]

212788(4707.04.0970)[

02571.082.59)(

02571.0()02267.0()3600(2

)(

)]

(4.0)[

(62

−

T T D

T T C h

gk

s v

sat s pv fg

v l v v

μ

ρρρ

film

4

3600,184

+

q& & &

Finally, the rate of heat transfer from the heating element to the water is determined by multiplying the heat flux by the heat transfer surface area,

Btu/h 2465

ft12/5.0(

)(

2 total

total total

π

πDL q q

A

10-15E Water is boiled at 1 atm pressure and thus at a saturation (or boiling) temperature of Tsat = 212°F

by a horizontal polished copper heating element whose surface temperature is maintained at Ts = 988°F

The rate of heat transfer to the water per unit length of the heater is to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the boiler are negligible

Properties The properties of water at the saturation temperature of 212°F are and

(Table A-9E) The properties of the vapor at the film temperature of

are, by interpolation, (Table A-16E)

ρl= 59 82 lbm / ft3

h fg = 970 Btu / lbm

T f =(Tsat+T s) /2=(212+988) /2=600°F

FftBtu/h02640.0

FBtu/lbm4799

.0C

hBtu/lbm0.05101

lbm/ft 02395

ft/h2 Note that we expressed the properties in units that will cancel

each other in boiling heat transfer relations Also note that we used vapor properties at 1 atm pressure from Table A-16E instead of the properties of saturated vapor from Table A-9E since the latter are at the saturation pressure of 1541 psia (105 atm)

Analysis The excess temperature in this case is ΔT T= s−Tsat =988 212− =776°F, which is much larger than 30°C or 54°F Therefore, film boiling will occur The film boiling heat flux in this case can be determined from

2

4 / 1 3

2

sat

4 / 1

sat

3

film

ftBtu/h

144

,

25

)212988()

212988)(

12/5.0)(

05101.0(

)]

212988(4799.04.0970)[

02395.082.59)(

02395.0()02640.0()3600(2

)(

)]

(4.0)[

(62

−

T T D

T T C h

gk

s v

sat s pv fg

v l v v

μ

ρρρ

film

4

3144,254

+

q& & &

Finally, the rate of heat transfer from the heating element to the water is determined by multiplying the heat flux by the heat transfer surface area,

10-16 Water is boiled at sea level (1 atm pressure) and thus at a saturation (or boiling) temperature of Tsat

= 100°C in a mechanically polished AISI 304 stainless steel pan placed on top of a 3-kW electric burner Only 60% of the heat (1.8 kW) generated is transferred to the water The inner surface temperature of the

pan and the temperature difference across the bottom of the pan are to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the boiler are negligible 3 The

boiling regime is nucleate boiling (this assumption will be checked later) 4 Heat transfer through the

bottom of the pan is one-dimensional

Properties The properties of water at the saturation temperature of

100°C are (Tables 10-1 and A-9)

h fg l pl

3 3

J / kg

kg m / s

Also, ksteel = 14.9 W/m⋅°C (Table A-3), Csf = 0.0130 and n = 1.0 for the boiling of water on a

mechanically polished stainless steel surface (Table 10-3 ) Note that we expressed the properties in units specified under Eq 10-2 connection with their definitions in order to avoid unit manipulations

Analysis The rate of heat transfer to the water and the heat flux are

2 2

2 2

2

W/m25.46

=)m9W)/(0.07061800

(/

m07069.04/m)30.0(4/

W1800

= kW8.1 kW360

Then temperature difference across the bottom of the pan is determined directly from the steady dimensional heat conduction relation to be

→Δ

=

CW/m9.14

m))(0.006W/m

460,25(

2

steel steel

k

L q T L

T k

&

temperature can also be used to determine the surface temperature when the heat flux is given

Assuming nucleate boiling, the temperature of the inner surface of the pan is determined from Rohsenow relation to be

3 sat ,

2 / 1 nucleate

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρρμ

&

3 3

1/2 3

3

75.1)102257(0130.0

)100(42170589

.0

0.60)9.8(957.9)

10)(225710

282.0(460

10-17 Water is boiled at 84.5 kPa pressure and thus at a saturation (or boiling) temperature of Tsat = 95°C

in a mechanically polished AISI 304 stainless steel pan placed on top of a 3-kW electric burner Only 60%

of the heat (1.8 kW) generated is transferred to the water The inner surface temperature of the pan and the

temperature difference across the bottom of the pan are to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the boiler are negligible 3 The

boiling regime is nucleate boiling (this assumption will be checked later) 4 Heat transfer through the

bottom of the pan is one-dimensional

Properties The properties of water at the saturation temperature

h fg

l pl

3 3

Also, ksteel = 14.9 W/m⋅°C (Table A-3), Csf = 0.0130 and n = 1.0 for the boiling of water on a

mechanically polished stainless steel surface (Table 10-3) Note that we expressed the properties in units

specified under Eq 10-2 in connection with their definitions in order to avoid unit manipulations

Analysis The rate of heat transfer to the water and the heat flux are

2 2

2

2 2

2

kW/m25.46

=W/m25,460

=)m9W)/(0.07061800

(/

m07069.04/m)30.0(4/

W1800

= kW8.1 kW360

Then temperature difference across the bottom of the pan is determined directly from the steady dimensional heat conduction relation to be

one-C 3

→Δ

=

CW/m9.14

m))(0.006W/m

460,25(

2

steel steel

k

L q T L

T k

2 / 1 nucleate

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρρμ

&

3 3

1/2 3

3

85.1)102270(0130.0

)95(42120599

.0

0.50)9.8(961.5)

10)(227010

297.0(460

10-18 Water is boiled at sea level (1 atm pressure) and thus at a saturation (or boiling) temperature of Tsat

= 100°C by a stainless steel heating element The surface temperature of the heating element and its power rating are to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the coffee maker are negligible 3

The boiling regime is nucleate boiling (this assumption will be checked later)

Properties The properties of water at the saturation temperature of

100°C are (Tables 10-1 and A-9)

h fg l pl

3 3

J / kg

kg m / s

Also, C 0.0130 and n = 1.0 for the boiling of water on a stainless steel surface (Table 10-3 ) Note

that we expressed the properties in units specified under Eq 10-2 connection with their definitions in order

to avoid unit manipulations

sf =

P = 1 atm

1 LWater, 100°C

Coffeemaker

Analysis The density of water at room temperature is very nearly 1 kg/L, and thus the mass of 1 L water at

18°C is nearly 1 kg The rate of energy transfer needed to evaporate half of this water in 25 min and the heat flux are

2 2

2 2

W/m29,940

= kW/m29.94

=)m513 kW)/(0.027523

.0(/

m02513.0m)m)(0.204.0(

kW7523.0s)

60(25

kJ/kg) kg)(22575

.0(

=

→

=Δ

=

s s

fg fg

A Q

t Q

temperature can also be used to determine the surface temperature when the heat flux is given

Assuming nucleate boiling, the temperature of the inner surface of the pan is determined from Rohsenow relation to be

3 sat ,

2 / 1 nucleate

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρρμ

&

3 3

1/2 3

3

75.1)102257(0130.0

)100(42170589

.0

0.60)9.8(957.9)

10)(225710

282.0(940

10-19 Water is boiled at sea level (1 atm pressure) and thus at a saturation (or boiling) temperature of Tsat

= 100°C by a copper heating element The surface temperature of the heating element and its power rating

are to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the coffee maker are negligible 3

The boiling regime is nucleate boiling (this assumption will be checked later)

Properties The properties of water at the saturation temperature

of 100°C are (Tables 10-1 and A-9)

h fg l pl

3 3

J / kg

kg m / s

Also, 0.0130 and n = 1.0 for the boiling of water on a copper surface (Table 10-3 ) Note that we

expressed the properties in units specified under Eq 10-2 connection with their definitions in order to avoid unit manipulations

C sf =

P = 1 atm

1 LWater, 100°C

Coffeemaker

Analysis The density of water at room temperature is very nearly 1 kg/L, and thus the mass of 1 L water at

18°C is nearly 1 kg The rate of energy transfer needed to evaporate half of this water in 25 min and the heat flux are

2 2

2 2

W/m29,940

= kW/m29.94

=)m513 kW)/(0.027523

.0(/

m02513.0m)m)(0.204.0(

kW7523.0s)

60(25

kJ/kg) kg)(22575

.0(

=

→

=Δ

=

s s

fg fg

A Q

t Q

The Rohsenow relation which gives the nucleate boiling heat flux for a specified surface temperature can also be used to determine the surface temperature when the heat flux is given

Assuming nucleate boiling, the temperature of the inner surface of the pan is determined from Rohsenow relation to be

3 sat ,

2 / 1 nucleate

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρρμ

&

3 3

1/2 3

3

75.1)102257(0130.0

)100(42170589

.0

0.60)9.8(957.9)

10)(225710

282.0(940

10-20 Water is boiled at a saturation (or boiling) temperature of Tsat = 120°C by a brass heating element

whose temperature is not to exceed Ts = 125°C The highest rate of steam production is to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the boiler are negligible 3 The

boiling regime is nucleate boiling since ΔT T T= s− sat=125 120− = °5 C which is in the nucleate boiling range of 5 to 30°C for water

Properties The properties of water at the saturation temperature of 120°C are (Tables 10-1 and A-9)

h fg l pl

3 3

Also, 0.0060 and n = 1.0 for the boiling of water on a brass surface (Table 10-3) Note that we

expressed the properties in units specified under Eq 10-2 in connection with their definitions in order to avoid unit manipulations

1/2 3

3

3 sat ,

2 / 1 nucleate

W/m190,290

44.1)102203(0060.0

)120125(42440550

.0

)12.19.8(943.4)

10)(220310

232.0(

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρρμ

&

The surface area of the heater is

2

m04084.0m)m)(0.6502.0

m04084.0

s3600J/kg102203

J/s852,113

boiling n

evaporatio

fg h

Q m

&

&

Therefore, steam can be produced at a rate of about 20 kg/h by this heater

10-21 Water is boiled at 1 atm pressure and thus at a saturation (or boiling) temperature of Tsat = 100°C by

a horizontal nickel plated copper heating element The maximum (critical) heat flux and the temperature jump of the wire when the operating point jumps from nucleate boiling to film boiling regime are to be

determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the boiler are negligible

Properties The properties of water at the saturation temperature of 100°C are (Tables 10-1 and A-9)

P = 1 atm

qmax Ts

h fg l pl

3 3

J / kg

kg m / s

Also, 0.0060 and n = 1.0 for the boiling of water on a nickel plated surface (Table 10-3 ) Note that

we expressed the properties in units specified under Eqs 10-2 and 10-3 in connection with their definitions

in order to avoid unit manipulations The vapor properties at the anticipated film temperature of T

C sf =

f =

(T s +Tsat )/2 of 1000°C (will be checked) (Table A-16)

s kg/m10762.4

CW/m1362.0

CJ/kg 2471

kg/m1725.0

5 3

60.0(12.0

*12.0

1.2

<

60.00589

.0

60.09.957(8.9)0015.0()(

*

25 0 25

0

2 / 1 2

/ 1

g L

L

cr

v l

σ

ρ ρ

Then the maximum or critical heat flux is determined from

temperature can also be used to determine the surface temperature when the heat flux is given Substituting the maximum heat flux into the Rohsenow relation together with other properties gives

3 sat ,

2 / 1 nucleate

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρ ρ μ

&

4

3)(

)(

)]

(4.0)[

(62

.04

sat 4 sat

4 / 1

sat

3 rad

film

T T D

T T C h

gk q

q

s v

sat s pv fg

v l v v

−+

−

=+

μ

ρ ρ ρ

4 / 1 5

3 3

)273100()273(

KW/m1067.5)(

100)(

003.0)(

10762.4(

)]

100(24714.0102257)[

1725.09.957)(

1723.0()1362.0(81.962

⋅

×+

s

T

T T

T

Solving for the surface temperature gives Ts = 1871°C Therefore, the temperature jump of the wire when the operating point jumps from nucleate boiling to film boiling is

Temperature jump: ΔT =Ts,film−T s,crit =1871−109=1762°C

Note that the film temperature is (1871+100)/2=985°C, which is close enough to the assumed value of 1000°C for the evaluation of vapor paroperties

10-22"!PROBLEM 10-22"

"GIVEN"

L=0.3 "[m]"

D=0.003 "[m]"

"epsilon=0.5 parameter to be varied"

P=101.3 "[kPa], parameter to be varied"

C_sf=0.0060 "from Table 8-3 of the text"

n=1 "from Table 8-3 of the text"

T_vapor=1000-273 "[C], assumed vapor temperature in the film boiling region"

rho_v_f=density(Fluid$, T=T_vapor, P=P) "f stands for film"

C_v_f=CP(Fluid$, T=T_vapor, P=P)*Convert(kJ/kg-C, J/kg-C)

10-23 Water is boiled at sea level (1 atm pressure) and thus at a saturation (or boiling) temperature of Tsat

= 100°C in a teflon-pitted stainless steel pan placed on an electric burner The water level drops by 10 cm

in 30 min during boiling The inner surface temperature of the pan is to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the pan are negligible 3 The boiling

regime is nucleate boiling (this assumption will be checked later)

Properties The properties of water at the saturation

temperature of 100°C are (Tables 10-1 and A-9)

h fg l pl

3 3

J / kg

kg m / s

Also, 0.0058 and n = 1.0 for the boiling of water on a teflon-pitted stainless steel surface (Table

10-3) Note that we expressed the properties in units specified under Eq 10-2 connection with their

definitions in order to avoid unit manipulations

C sf =

Analysis The rate of heat transfer to the water and the heat flux are

2 2

2 2

2 evap

3 evap

evap

W/m2,402,000

=)m2W)/(0.0314470

,75(/

m03142.04/m)20.0(4/

kW47.75 kJ/kg)7 kg/s)(22503344

.0(

kg/s03344.0s

6030

m)0.10m0.2)(

kg/m9.957(

Δ

=Δ

=

s s

fg

A Q

q

D

A

h m

Q

t

V t

m m

πρ

temperature can also be used to determine the surface temperature when the heat flux is given Assuming nucleate boiling, the temperature of the inner surface of the pan is determined from Rohsenow relation to

be

3 sat ,

2 / 1 nucleate

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρρμ

&

3 3

1/2 3

3

75.1)102257(0058.0

)100(42170589

.0

0.60)9.8(957.9)

10)(225710

282.0(000

10-24 Water is boiled at sea level (1 atm pressure) and thus at a saturation (or boiling) temperature of Tsat

= 100°C in a polished copper pan placed on an electric burner The water level drops by 10 cm in 30 min

during boiling The inner surface temperature of the pan is to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the pan are negligible 3 The boiling

regime is nucleate boiling (this assumption will be checked later)

Properties The properties of water at the saturation

temperature of 100°C are (Tables 10-1 and A-9)

h fg l pl

3 3

J / kg

kg m / s

Also, 0.0130 and n = 1.0 for the boiling of water on a copper surface (Table 10-3) Note that we

expressed the properties in units specified under Eq 10-2 connection with their definitions in order to avoid unit manipulations

C sf =

Analysis The rate of heat transfer to the water and the heat flux are

2 2

2 2

2 evap

3 evap

evap

W/m2,402,000

=)m2W)/(0.0314470

,75(/

m03142.04/m)20.0(4/

kW47.75 kJ/kg)7 kg/s)(22503344

.0(

kg/s03344.0s

6030

m)0.10m0.2)(

kg/m9.957(

Δ

=Δ

=

s s

fg

A Q

q

D

A

h m

Q

t

V t

m m

πρ

temperature can also be used to determine the surface temperature when the heat flux is given Assuming nucleate boiling, the temperature of the inner surface of the pan is determined from Rohsenow relation to

be

3 sat ,

2 / 1 nucleate

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρρμ

&

3 3

1/2 3

3

75.1)102257(0130.0

)100(42170589

.0

0.60)9.8(957.9)

10)(225710

282.0(000

10-25 Water is boiled at a temperature of Tsat = 150°C by hot gases flowing through a mechanically

polished stainless steel pipe submerged in water whose outer surface temperature is maintained at Ts = 165

°C The rate of heat transfer to the water, the rate of evaporation, the ratio of critical heat flux to current

heat flux, and the pipe surface temperature at critical heat flux conditions are to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the boiler are negligible 3 The

boiling regime is nucleate boiling since ΔT T T= s− sat=165 150− = °15 C which is in the nucleate boiling range of 5 to 30°C for water

Properties The properties of water at the saturation temperature of

150°C are (Tables 10-1 and A-9)

h fg l pl

3 3

J / kg

kg m / s

mechanically polished stainless steel surface (Table 10-3) Note that

we expressed the properties in units specified under Eq 10-2 in

connection with their definitions in order to avoid unit manipulations

C sf =

Analysis (a) Assuming nucleate boiling, the heat flux can be determined from Rohsenow relation to be

Water, 150°CBoiler

Hot gases

Vent

Ts,pipe = 165°C

2

3 3

1/2 3

3

3 sat ,

2 / 1 nucleate

W/m000,383,1

16.1)102114(0130.0

)150165(43110488

.0

)55.29.8(916.6)

10)(211410

183.0(

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρ ρ μ

&

The heat transfer surface area is

2

m854.7m)m)(5005.0

=

=

= nucleate (7.854m2)(1,383,000W/m2)boiling A q

thusand1.2

>

*(since 12.0

0.12

>

7.100488

.0

)55.26.916(8.9)025.0()(

*

2 / 1 2

/ 1

L C

g L

L

cr

v l

Then the maximum or critical heat flux is determined from

(d) The surface temperature of the pipe at the burnout point is determined from Rohsenow relation at the

critical heat flux value to be

C 166.5°

cr s

n l fg sf

cr s l p v

l fg l

T

T

h C

T T C g

h q

,

3 3 , 1/2

3 3

3 sat , , 2 / 1 cr

nucleate,

16.1)102114(0130.0

)150(

43110488

.0

)55.29.8(916.6)

10)(211410

183.0(000

)(

σ

ρ ρ μ

&

10-26 Water is boiled at a temperature of Tsat = 160°C by hot gases flowing through a mechanically

polished stainless steel pipe submerged in water whose outer surface temperature is maintained at Ts = 165

°C The rate of heat transfer to the water, the rate of evaporation, the ratio of critical heat flux to current

heat flux, and the pipe surface temperature at critical heat flux conditions are to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the boiler are negligible 3 The

boiling regime is nucleate boiling since ΔT T T= s− sat=165 160− = °5 C which is in the nucleate boiling range of 5 to 30°C for water

Properties The properties of water at the saturation temperature of

160°C are (Tables 10-1 and A-9)

kg / m

kg / m

N / mPr

3 3

h fg l pl

3 3

J / kg

kg m / s

mechanically polished stainless steel surface (Table 10-3 ) Note that

we expressed the properties in units specified under Eq 10-2 in

connection with their definitions in order to avoid unit manipulations

C sf =

Analysis (a) Assuming nucleate boiling, the heat flux can be determined from Rohsenow relation to be

Water, 150°CBoiler

Hot gases

Vent

Ts,pipe = 160°C

2

3 3

1/2 3

3

3 sat ,

2 / 1 nucleate

W/m359,61

09.1)102083(0130.0

)160165(43400466

.0

)26.39.8(907.4)

10)(208310

170.0(

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρρμ

&

The heat transfer surface area is

2

m854.7m)m)(5005.0

=

=

= nucleate (7.854m2)(61,359W/m2)boiling A q

and thus1.2

>

*(since 12.0

0.12

>

9.100466

.0

)26.34.907(8.9)025.0()(

*

2 / 1 2

/ 1

L C

g L

L

cr

v l

Then the maximum or critical heat flux is determined from

(d) The surface temperature of the pipe at the burnout point is determined from Rohsenow relation at the

critical heat flux value to be

C 176.1°

cr s

n l fg sf

cr s l p v

l fg l

T

T

h C

T T C g

h q

,

3 3 , 1/2

3 3

3 sat , , 2 / 1 cr

nucleate,

09.1)102083(0130.0

)160(

43400466

.0

)26.39.8(907.4)

10)(208310

170.0(000

)(

σ

ρρμ

&

10-27E Water is boiled at a temperature of Tsat = 250°F by a nickel-plated heating element whose surface

temperature is maintained at Ts = 280°F The boiling heat transfer coefficient, the electric power consumed, and the rate of evaporation of water are to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the boiler are negligible 3 The

boiling regime is nucleate boiling since ΔT T T= s− sat=280 250− =30°F which is in the nucleate boiling range of 9 to 55°F for water

Properties The properties of water at the saturation temperature of 250°F are (Tables 10-1 and A-9E)

3 3

2

h fg

l pl

=

lbm/ h ft

Heating element

Also, g = 32.2 ft/s2 and 0.0060 and n = 1.0 for the boiling of water on a nickel plated surface (Table

10-3) Note that we expressed the properties in units that will cancel each other in boiling heat transfer relations

C sf =

Analysis (a) Assuming nucleate boiling, the heat flux can be determined from Rohsenow relation to be

2

3 1/2

3 sat ,

2 / 1 nucleate

ftBtu/h221,475,3

43.1)946(0060.0

)250280(015.11208

.0

)0723.032.2(58.82)

)(946556.0(

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρρμ

=

kW 1(since

=

Btu/h811,909)ftBtu/h221ft)(3,475,2

ft12/5.0()

kW 266.6

W&e & & s &

(c) Finally, the rate of evaporation of water is determined from

lbm/h 961.7

=

=

=

Btu/lbm946

Btu/h811,909boiling n

evaporatio

fg h

Q m

&

&

10-28E Water is boiled at a temperature of Tsat = 250°F by a platinum-plated heating element whose

surface temperature is maintained at Ts = 280°F The boiling heat transfer coefficient, the electric power consumed, and the rate of evaporation of water are to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the boiler are negligible 3 The

boiling regime is nucleate boiling since ΔT T T= s− sat=280 250− =30°F which is in the nucleate boiling range of 9 to 55°F for water

Properties The properties of water at the saturation temperature of 250°F are (Tables 10-1 and A-9E)

3 3

2

h fg

l pl

=

lbm/ h ft

Heating element

Also, g = 32.2 ft/s2 and C 0.0130 and n = 1.0 for the boiling of water on a platinum plated surface

(Table 10-3) Note that we expressed the properties in units that will cancel each other in boiling heat transfer relations

sf =

Analysis (a) Assuming nucleate boiling, the heat flux can be determined from Rohsenow relation to be

2

3 3 1/2

3 sat ,

2 / 1 nucleate

ftBtu/h670,341

43.1)101208.0(0130.0

)250280(015.11208

.0

)0723.032.2(58.82)

)(946556.0(

Pr

)(

)(

s l p v

l fg l

h C

T T C g

h q

σ

ρρμ

=

kW 1(since

=

Btu/h450,89)ftBtu/h0ft)(341,672

ft12/5.0()

kW 26.2

W&e & & s &

(c) Finally, the rate of evaporation of water is determined from

lbm/h 94.6

=

=

=

Btu/lbm946

Btu/h450,89boiling n

evaporatio

fg h

Q m

&

&

10-29E "!PROBLEM 10-29E"

C_sf=0.0060 "from Table 8-3 of the text"

n=1 "from Table 8-3 of the text"

260 265 270 275 280 285 290 295 3000

10-30 Cold water enters a steam generator at 15°C and is boiled, and leaves as saturated vapor at Tsat = 100°C The fraction of heat used to preheat the liquid water from 15°C to saturation temperature of 100°C

is to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses from the steam generator are negligible Properties The heat of vaporization of water at 100°C is hfg = 2257 kJ/kg and the specific heat of liquid water at the average temperature of (15+100)/2 = 57.5°C is Cpl =4.184 kJ/kg⋅°C (Table A-9)

Analysis The heat of vaporization of water represents the amount of heat

needed to vaporize a unit mass of liquid at a specified temperature Using the

average specific heat, the amount of heat needed to preheat a unit mass of

water from 15°C to 100°C is determined to be

Water, 100°C

Steam generator

Water, 15°C

Steam 100°C

kJ/kg355.6

=C)15C)(100 kJ/kg

184.4(preheating =C ΔT = ⋅° − °

and

kJ/kg6.26126.3552257preheating boiling

q

Therefore, the fraction of heat used to preheat the water is

)(or2612.6

6.355 preheat

toFraction

10-31 Cold water enters a steam generator at 20°C and is boiled, and leaves as saturated vapor at boiler pressure The boiler pressure at which the amount of heat needed to preheat the water to saturation temperature is equal to the heat of vaporization is to be determined.

Assumptions 1 Steady operating conditions exist 2 Heat losses from the steam generator are negligible Properties The properties needed to solve this problem are the heat of

vaporization hfg and the specific heat of water Cp at specified temperatures,

and they can be obtained from Table A-9

Water, 100°C

Steam generator

Water, 20°C

Steam 100°C

Analysis The heat of vaporization of water represents the amount of heat

needed to vaporize a unit mass of liquid at a specified temperature, and

represents the amount of heat needed to preheat a unit mass of water

from 20°C to the saturation temperature Therefore,

The solution of this problem requires choosing a boiling temperature, reading

the heat of vaporization at that temperature, evaluating the specific heat at the

average temperature, and substituting the values into the relation above to see

if it is satisfied By trial and error, the temperature that satisfies this condition

is determined to be 315°C at which (Table A-9)

and T

h fg@315°C =1281 kJ / kg ave = (20+315)/2 = 167.5°C → Cp ave, =4 37 kJ / kg C ⋅°Substituting,

0 5 10 15 20 25 309600

10-33 Boiling experiments are conducted by heating water at 1 atm pressure with an electric resistance

wire, and measuring the power consumed by the wire as well as temperatures The boiling heat transfer coefficient is to be determined

Assumptions 1 Steady operating conditions exist 2 Heat losses

from the water are negligible

=

A s

Noting that 3800 W of electric power is consumed when the

heater surface temperature is 130°C, the boiling heat transfer

coefficient is determined from Newton’s law of cooling to be

C W/m

W3800)

( )

sat sat

T T A

Q h

T T

hA

Q

s s s

s

&

&

Condensation Heat Transfer

10-34C Condensation is a vapor-to-liquid phase change process It occurs when the temperature of a vapor

is reduced below its saturation temperature Tsat This is usually done by bringing the vapor into contact with

a solid surface whose temperature Ts is below the saturation temperature Tsat of the vapor

10-35C In film condensation, the condensate wets the surface and forms a liquid film on the surface

which slides down under the influence of gravity The thickness of the liquid film increases in the flow direction as more vapor condenses on the film This is how condensation normally occurs in practice In

dropwise condensation, the condensed vapor forms droplets on the surface instead of a continuous film,

and the surface is covered by countless droplets of varying diameters Dropwise condensation is a much more effective mechanism of heat transfer

10-36C In condensate flow, the wetted perimeter is defined as the length of the surface-condensate

interface at a cross-section of condensate flow It differs from the ordinary perimeter in that the latter refers

to the entire circumference of the condensate at some cross-section

10-37C The modified latent heat of vaporization is the amount of heat released as a unit mass of vapor condenses at a specified temperature, plus the amount of heat released as the condensate is cooled further

to some average temperature between T

h*fg

sat and T s It is defined as h*fg =h fg+0 68 C pl(Tsat−T s) where C

D

pl

is the specific heat of the liquid at the average film temperature

10-38C During film condensation on a vertical plate, heat flux at the top will be higher since the thickness

of the film at the top, and thus its thermal resistance, is lower

10-39C Setting the heat transfer coefficient relations for a vertical tube of height L and a horizontal tube

of diameter D equal to each other yields L= 2 77 ,which implies that for a tube whose length is 2.77

times its diameter, the average heat transfer coefficient for laminar film condensation will be the same whether the tube is positioned horizontally or vertically For L = 10D, the heat transfer coefficient and thus the heat transfer rate will be higher in the horizontal position since L > 2.77D in that case

10-40C The condensation heat transfer coefficient for the tubes will be the highest for the case of

horizontal side by side (case b) since (1) for long tubes, the horizontal position gives the highest heat transfer coefficients, and (2) for tubes in a vertical tier, the average thickness of the liquid film at the lower tubes is much larger as a result of condensate falling on top of them from the tubes directly above, and thus

the average heat transfer coefficient at the lower tubes in such arrangements is smaller

10-41C The presence of noncondensable gases in the vapor has a detrimental effect on condensation heat

transfer Even small amounts of a noncondensable gas in the vapor cause significant drops in heat transfer coefficient during condensation

10-42 The hydraulic diameter Dh for all 4 cases are expressed in terms of the boundary layer thickness δ as follows:

p

w w

h =4 c =4 δ =4δ

p

w w

h =4 c =4 δ =4δ

p

D D

D l

c l l l

h l l l

l l l

μ

ρμ

ρμ

δρμ

10-43 There is film condensation on the outer surfaces of N horizontal tubes arranged in a vertical tier The value of N for which the average heat transfer coefficient for the entire tier be equal to half of the value for

a single horizontal tube is to be determined

Assumptions Steady operating conditions exist

Analysis The relation between the heat transfer coefficients for the two cases

h

12

1

4 / 1 tube

1 , horizontal

tubes N , horizontal

10-44 Saturated steam at atmospheric pressure thus at a saturation temperature of Tsat = 100°C condenses

on a vertical plate which is maintained at 90°C by circulating cooling water through the other side The rate

of heat transfer to the plate and the rate of condensation of steam are to be determined

Assumptions 1 Steady operating conditions exist 2 The plate is isothermal 3 The condensate flow is wavy-laminar over the entire plate (this assumption will be verified) 4 The density of vapor is much

smaller than the density of liquid, ρv <<ρl

Properties The properties of water at the saturation temperature of 100°C are hfg = 2257×103 J/kg and ρv

= 0.60 kg/m3 The properties of liquid water at the film temperature of T f =(Tsat+T s) /2=(100 + 90)/2 = 95°C are (Table A-9),

0 677

3 3

6

5 m

Assuming wavy-laminar flow, the Reynolds number is determined from

1112)

s/m10309.0(

m/s8.9)

J/kg102286)(

s kg/m10297.0(

C)90100(C)W/m677.0(m)3(70.381

4

)(

70.381.4Re

Re

82 0 3 / 1 2 2 6 2 3

3

820 0 3 / 1 2

*

sat wavy

νμ

which is between 30 and 1800, and thus our assumption of wavy laminar flow is verified Then the condensation heat transfer coefficient is determined to be

CW/m6279)

/sm10309.0(

m/s8.92

.5)1112(08.1

C)W/m677.0(1112

2.5Re08.1Re

2 3

/ 1 2 2 6 2 22

1

3 / 1 2 22

1 wavy

J/s850,9413

* on condensati

fg h

Q

&