Homework # 9 Solutions

#1 5-60 Air is expanded in an adiabatic turbine. The mass flow rate of the air and thepower produced are to be determined.Assumptions 1 This is a steady-flow process since there is no change with time. 2 Theturbine is well-insulated, and thus there is no heat transfer. 3 Air is an ideal gas withconstant specific heats.Properties The constant pressure specific heat of air at the average temperature of(500+150)/2=325°C=598 K is cp = 1.051 kJ/kg·K (Table A-2b). The gas constant of air isR = 0.287 kPa⋅m3/kg⋅K (Table A-1).Analysis (a) There is only one inlet and one exit, and thus mmm &&& == 21 . We take theturbine as the system, which is a control volume since mass crosses the boundary. Theenergy balance for this steady-flow system can be expressed in the rate form as

outin

energies etc. potential, kinetic, internal,in change of Rate

(steady) 0system

mass and work,heat,by nsferenergy tranet of Rate

outin 0

EE

EEE

&&

444 344 21&

43421&&

=

=D=- ä

˜˜¯

ˆÁÁË

Ê -+-=˜

˜¯

ˆÁÁË

Ê -+-=

+˜˜¯

ˆÁÁË

Ê+=˜

˜¯

ˆÁÁË

Ê+

2)(

2

22

22

21

21

22

21

21out

out

22

2

21

1

VVTTcm

VVhhmW

WV

hmV

hm

p&&&

&&&

The specific volume of air at the inlet and the mass flow rate are

/kgm 2219.0kPa 1000

K) 273K)(500/kgmkPa 287.0( 33

1

11 =

+⋅⋅==

P

RTv

kg/s 36.06===/kgm 0.2219

m/s) )(40m 2.0(3

2

1

11

vVA

m&

Similarly at the outlet,

/kgm 214.1kPa 100

K) 273K)(150/kgmkPa 287.0( 33

2

22 =

+⋅⋅==

P

RTv

m/s 78.43m 1

/kg)m 4kg/s)(1.21 06.36(2

3

2

22 ===

A

mV

v&

(b) Substituting into the energy balance equation gives

kW 13,260=

˙˙˚

˘

ÍÍÎ

Ș¯

ˆÁË

Ê-+-⋅=

˜˜¯

ˆÁÁË

Ê -+-=

22

22

22

21

21out

/sm 1000

kJ/kg 1

2

m/s) 78.43(m/s) 40()K150K)(500kJ/kg 051.1(kg/s) 06.36(

2)(

VVTTcmW p&&

Turbine

1 MPa500°C40 m/s

100 kPa150°C

#3 5-122 An evacuated bottle is surrounded by atmospheric air. A valve is opened, andair is allowed to fill the bottle. The amount of heat transfer through the wall of the bottlewhen thermal and mechanical equilibrium is established is to be determined.Assumptions 1 This is an unsteady process since the conditions within the device arechanging during the process, but it can be analyzed as a uniform-flow process since thestate of fluid at the inlet remains constant. 2 Air is an ideal gas with variable specificheats. 3 Kinetic and potential energies are negligible. 4 There are no work interactionsinvolved. 5 The direction of heat transfer is to the air in the bottle (will be verified).Properties The gas constant of air is 0.287 kPa.m3/kg.K (Table A-1).Analysis We take the bottle as the system, which is a control volume since mass crosses the boundary.Noting that the microscopic energies of flowing and nonflowing fluids are represented by enthalpy h andinternal energy u, respectively, the mass and energy balances for this uniform-flow system can beexpressed as

Mass balance:

)0 (since initialout2systemoutin ===ÆD=- mmmmmmm i

Energy balance:

)0 (since initialout22in

energies etc. potential, kinetic, internal,in Change

system

mass and work,heat,by nsferenergy traNet

outin

@@==@=+

D=-

pekeEEWumhmQ

EEE

ii

4342143421

Combining the two balances: ( )ihumQ -= 22in

where

kJ/kg 206.91

kJ/kg 290.16K 290

kg 0.0096)K 290)(K/kgmkPa 0.287(

)m 0.008)(kPa 100(

2

17-A Table2

3

3

2

22

=

=ææææ Ææ==

=⋅⋅

==

u

hTT

RT

Pm

ii

V

Substituting,Qin = (0.0096 kg)(206.91 - 290.16) kJ/kg = - 0.8 kJ

orQout = 0.8 kJ

Discussion The negative sign for heat transfer indicates that the assumed direction iswrong. Therefore, we reverse the direction.

#4 5-78 A hot water stream is mixed with a cold water stream. For a specified mixturetemperature, the mass flow rate of cold water is to be determined.Assumptions 1 Steady operating conditions exist. 2 The mixing chamber is well-insulatedso that heat loss to the surroundings is negligible. 3 Changes in the kinetic and potentialenergies of fluid streams are negligible. 4 Fluid properties are constant. 5 There are nowork interactions.Properties Noting that T < Tsat @ 250 kPa = 127.41°C,the water in all three streams exists as a

H2O(P = 250

kPa)T3 = 42°C

T1 = 80°Cm1 = 0.5kg/s

T2 =20°C·

·

8 LEvacuate

d

100 kPa17°C

compressed liquid, which can be approximated asa saturated liquid at the given temperature. Thus,

h1 @ hf @ 80°C = 335.02 kJ/kgh2 @ hf @ 20°C = 83.915 kJ/kgh3 @ hf @ 42°C = 175.90 kJ/kg

Analysis We take the mixing chamber as the system,which is a control volume. The mass and energybalances for this steady-flow system can be expressedin the rate form as

Mass balance:

321(steady) 0

systemoutin 0 mmmmmm &&&&&& =+æÆæ=D=- ä

Energy balance:

0)peke (since

0

332211

outin

energies etc. potential, kinetic, internal,in change of Rate

(steady) 0system

mass and work,heat,by nsferenergy tranet of Rate

outin

@D@D===+

=

=D=-

WQhmhmhm

EE

EEE

&&&&&

&&

44 344 21&

43421&& Ã

Combining the two relations and solving for &m2 gives ( ) 3212211 hmmhmhm &&&& +=+

& &mh h

h hm2

1 3

3 21=

-

-

Substituting, the mass flow rate of cold water stream is determined to be( )( )

( ) kg/s 0.865=-

-= kg/s 0.5

kJ/kg 83.915175.90

kJ/kg 175.90335.022m&

5-157 The mass flow rate of a compressed air line is divided into two equal streams by aT-fitting in the line. The velocity of the air at the outlets and the rate of change of flowenergy (flow power) across the T-fitting are to be determined.Assumptions 1 Air is an ideal gas with constant specific heats. 2 The flow is steady. 3Since the outlets are identical, it is presumed that the flow divides evenly between thetwo.Properties The gas constant of air is R = 0.287 kPa⋅m3/kg⋅K (Table A-1).Analysis The specific volumes of air at the inlet and outlets are

/kgm 05614.0kPa 1600

K) 273K)(40/kgmkPa 287.0( 33

1

11 =

+⋅⋅==

P

RTv

/kgm 06335.0kPa 1400

K) 273K)(36/kgmkPa 287.0( 33

2

232 =

+⋅⋅===

P

RTvv

1.4 MPa36°C

1.6MPa40°C

50 m/s

1.4MPa36°C

Assuming an even division of the inlet flow rate,the energy balance can be written as

m/s 28.21====æÆæ=2

50

0.05614

06335.0

22 1

1

2

2

132

2

22

1

11 V

A

AVV

VAVA

vv

vv

The mass flow rate at the inlet is

kg/s 0.4372/kgm 0.05614

m/s 50

4

m) 025.0(

4 3

2

1

121

1

111 ====

ppvvVDVA

m&

while that at the outlets is

kg/s 0.21862

kg/s 4372.0

21

32 ====m

mm&

&&

Substituting the above results into the flow power expression produces

kW 0.496-=

-=

-=

/kg)m 14kPa)(0.056 1600)(kg/s 0.4372(/kg)m 35kPa)(0.063 1400)(kg/s 0.2186(2

233

111222flow vv PmPmW &&&

5. 6-9C No. Such an engine violates the Kelvin-Planck statement of the second law ofthermodynamics.

6. 6-15C No. Such an engine violates the Kelvin-Planck statement of the second law ofthermodynamics.