第二章 热力学第一定律

第二章 热力学第一定律

英文习题

1. Cooling of a hot fluid in a tank

A rigid tank contains a hot fluid that is cooled while being stirred by a

paddle wheel. Initially, the internal energy of the fluid is 800 kJ. During the cooling process, the fluid loses 500 kJ of heat, and the paddle wheel does 100 kJ of work on the fluid. Determine the final internal energy of the fluid. Neglect the energy stored in the paddle

wheel.

2. Heating of a gas by a resistance heater

A piston-cylinder device initially contains 0.5 m 3

of nitrogen gas at 400 kPa and 27℃. An electric heater within the device is turned on and is allowed to pass a current of 2 A for 5 min from a 120-V source. Nitrogen expands at constant pressure, and a heat loss of 2800 J occurs

during the process. Determine the final temperature of nitrogen.

3. Cooling of an iron block by water

contains 0.5 A 50-kg iron block at 80℃ is dropped into an insulated tanks that equilibrium

m 3

of liquid water at 25℃. Determine the temperature when thermal is reached.

4. Deceleration of air in a diffuser

Air at 10℃ and 80 kPa enter the diffuser of a jet engine steadily with a velocity of 200 m/s. The inlet area of the diffuser is 0.4 m 2

. The air leaves the diffuser with a velocity that is

very small compared with the inlet velocity. Determine (a) the

mass flow rate of the air and (b) the temperature of the air leaving the diffuser.

5. Energy balance in turbine

Consider a gas turbine power plant with air as the working fluid, Air enters at 100 kPa, 20ºC (ρ=1.19 kg/m 3

), with a velocity of 130m/s through an opening 0.112 m 2

cross-sectional area. After being compressed, heated and expanded through a turbine, the air leaves at 180 kPa, 150ºC (ρ=1.48 kg/m 3

), through an opening of 0.100 m 2

cross-sectional area. The power output of the plant is 375 kW. The internal energy and enthalpy of the air are given in kJ/kg by

U=0.717T and h=1.004T, where T is

temperature on the Kelvin scale. Determine the net amount

of

heat

added to

the air in kJ/kg.

FIGURE 2-1

FIGURE 2-2

FIGURE 2-3

FIGURE 2-4

2

113

111112

.0/130/19.120100m A s m c m kg C

T kPa p f ===︒==ρ21

1

2

Ws

Q

2

23

222100.0/48.1150180m A m kg C

T kPa

p ==︒==ρFIGURE 2-5

6. Air is compressed in a frictionless steady-flow process

Air is compressed in a frictionless steady-flow process from 90 kPa, 150ºC (v=0.918 m 3

/kg) to 130 kPa in such a manner that p(v+0.250)=constant, where v is in m 3

/kg, inlet velocity is negligible small, and

discharge velocity is 110 m/s. Calculate the work required per kilogram of air.

7. Mixture process

A mixture of air and water vapor with an enthalpy of 120 kJ/kg enters the dehumidifying section of an air-conditioning system at a rate of 320 kg/hr, liquid water drains out of the dehumidifier with an enthalpy of 42 kJ/kg at a rate of 7.0 kg/hr. An air vapor mixture leaves with an enthalpy of 47 kJ/kg. Determine the rate of heat removal from the fluids passing through the dehumidifier.

8. Reviews problem

A piston-cylinder

device contains helium gas initially at 150 kPa,

20℃, and

0.5 m 3

. The helium is now compressed in a polytropic process (PV n

=constant) to 400 kPa and 140℃. Determine the heat loss or gain

during this process.

9. Two rigid tanks are connected by a valve. Tank A

contains 0.2 m 3

of water at 400 kPa and 80 percent quality. Tank B contains 0.5 m 3

of water at 200 kPa and 250℃. The valve is now opened, and the two tanks eventually come to the same state. Determine the pressure and the amount of heat transfer when

the system reaches thermal equilibrium with the surrounding at 25 ℃.

10. Consider a well-insulated horizontal rigid cylinder that is

divided

1

1

2

2

3

3

dehumidifier

kg

kJ h hr kg m /126/32011==∙

kg

kJ h hr kg m /42/0.722==∙

kg

kJ h /473=FIGURE 2-7

FIGURE 2-8

W in

1

1

2

2

/918.09013

11===f c kg m v kPa p s

m c kPa p f /11013022==FIGURE 2-6

FIGURE 2-9

FIGURE 2-10