Download 幻灯片 1

Chapter 1
Basic Concepts
1. A gas in a piston-cylinder device is
compressed, and as a result its temperature
rises. Is this a heat or work interaction?
2. On a P-v diagram, what does the area under
the process curve represent?
3. Determine the energy required to accelerate a
2000-kg car from 20 to 70 km/h on an uphill
road with a vertical rise of 40m.
4.
Consider a device with one inlet and one exit.
If the volume flow rates at the inlet and at the
exit are the same, is the flow through the
device necessarily steady？ Why?
5. A 2 m3 rigid tank contains nitrogen gas at
500kPa and 300K. Now heat is transferred to
the nitrogen in the tank and the pressure of
nitrogen rises to 800kPa. The work done during
this process is
(A) 600 kJ
(B) 1000 kJ
(C) 0 kJ
(D) 500 kJ
(E) 1600 kJ
6. A 2 m3 rigid tank contains nitrogen gas at
500kPa and 300K. Now heat is transferred to
the nitrogen in the tank and the pressure of
nitrogen rises to 800kPa. The work done during
this process is
(A) 600 kJ
(D) 500 kJ
(B) 1000 kJ
(E) 1600 kJ
(C) 0 kJ
CHAPTER 2
PROPERTIES OF PURE SUBSTANCES
2.1 A rigid tank contains 5kg of an ideal gas at 4atm and
40℃.Now a valve is opened, and half of mass of the gas is
allowed to escape .If the final pressure in the tank is
1.5atm.the final temperature in the tank is
[a] -38 ℃
[e]144 ℃
[b]30 ℃
[c]40 ℃
[d] 53 ℃
2.2 A 5  m rigid vessel contains steam at 20 MPa and 400
℃.The mass of the steam is
3
[a] 5.0kg
[e]680kg
[b] 0.322kg
[c] 322kg
[d] 503kg
2.3 A 500  m3 rigid tank is filled with saturated
liquid-vapor mixture of water at 200kPa.If 20
percent of the mass is liquid and the 80 percent
of the mass is vapor ,the total mass in the tank
is
[a] 705kg
[b] 500kg
[c]258kg
[d] 635kg
[e]2809
Chapter 3
The first Law of Thermodynamics

A reversible cycle plots as a perfect circle
on a T-S diagram with maximum and
minimum temperatures 600 K and 300K and a
maximum and minimum entropy of 600 kJ/K
and 300kJ/K.
a) What is the net work of this cycle?
b) Indicate the path in the cycle along which
heat is rejected. How do you know?(A sentence
or two is required, perhaps bolstered by an
equation).
c) What is the thermodynamic efficiency of this
cycle?
 2.
The sketch below shows a perfectly
insulated container with two compartments
separated by a non-adiabatic, frictionless
piston. Both compartments are at the same
pressure (PA= PB) and contain an equal
amount of the same gas ( mA =mB ). Initially
compartment A is at a higher temperature than
compartment B(TA>TB).
 a) Which way, if at all, does the piston move? Indicate
by an arrow on a drawing in your exam book (A
sentence or two is expected, perhaps bolstered by
some equations).
 b) Is the final internal energy Ufinal of the system
(both compartments together) lower, equal or higher
than initial internal energy Uinitial of the system? Why?
(A sentence or two is expected, perhaps bolstered by
some equations.)
 c) What is the final temperature of the system Tfinal
expressed in terms of TA and TB ?
 b) What is the change in entropy of compartment A?
What is the change in entropy of compartment B?
 e) What is the change in entropy of the surroundings?
 f) Is this process reversible or irreversible? How do you
know? (Note: you do not have to prove your answer
mathematically to get full credit. Explain in a sentence
how you know whether the process is reversible or
irreversible)
 10 kg of air contained in a piston cylinder
apparatus at 5 bar,300K, expands isothermally until
the pressure is 1 bar. Determine:

(i) the work done.

(ii) the internal energy change of the air.

(iii) the heat transfer.
 Assume that air behaves as an ideal gas and
pressure variation is gradual and continuous.
 A system is capable of executing a cyclic process as
indicated in the P V sketch; it may be executed either
clockwise abca or counterclockwise adca.
(a)
When
going clockwise to state c, 80KJ of heat flow from the
system. Find the work along the path ca.
(b)
When
going counterclockwise to state c, 70KJ of heat flow
from the system. Find the work during the process
adc.
CHAPTER 5
THE SECOND LAW OF THERMODYNAMICS
5.1 The drinking water needs of an office are met by
cooling tab water in a refrigerated water fountain from 22℃
to 8 ℃ at an average rate of 8kg/h .If the COP of this
refrigerator is 3.1,the required power input to this
refrigerator is
[a] 28W
[b] 42W
[c] 88W
[d] 130W
[e] 403W
5.2 A heat engine cycle is executed with steam
in the saturation dome. The pressure of steam
1MPa during heat addition, and 0.5MPa during
heat rejection. The highest possible efficiency of
this heat engine is
[a] 6.2％
[b] 15.6 ％
[c] 50.0 ％
[d] 93.8 ％
[e] 100 ％
5.3 A heat engine receives heat from a source at
100 ℃ and rejects the waste heat to a sink at 50
℃ .If heat is supplied to this engine at a rate of
100kj/s,the maximum power this heat engine can
produce is
[a] 25.4kW
[c] 174.6 kW
[e] 100.0kW
[b] 55.4kW
[d] 95.0kW
5.4 Steam is compressed from 8 MPa and 300 ℃ to 10
MPa isentropically. The final temperature of the steam is
[a] 290 ℃
[b] 300 ℃
[c] 320 ℃
[d] 330 ℃
[e] 340 ℃
5.5 Helium gas is compressed from 1 atm and 25
℃ to a pressure of 10 atm adiabatically. The
lowest temperature of helium after compression is
[a] 25 ℃
[c] 250 ℃
[e] 476 ℃
[b] 63 ℃
[d] 384 ℃
5.6 Steam expands in an adiabatic turbine from
10MPa and 500 ℃ to 0.1MPa at a rate of 8kg/s.If
steam leaves the turbine as saturated vapor,the
power output of the turbine is
[a] 5586kw
[c] 2136 kw
[e]26,990kw
[b] 698kw
[d] 12,452kw
5.7 A unit mass of a substance an irreversible process
from state 1 to state 2 while gaining heat from the
surroundings at temperature T in the amount of q.If the
entropy of the substance is S1 at state 1,and S 2 at state
2,the entropy change of the substance s during this
process is
[a] S  S2  S1
[b] S  S2  S1
[c] S  S2  S1  q / T
[d] S  S2  S1  q / T
[e] S  S2  S1
5.8 Air is compressed from room conditions to a
specified pressure in a reversible manner by two
compressor :one isothermal and the other
adidbatic. If the entropy change of air s during the
reversible adiabatic compression, the correct
statement regarding entropy change of air per unit
mass is
[a] Sisot  Sadia  0
[b] Sisot  Sadia  0
[c] Sisot  Sadia  0
[d] Sisot  Sadia  0
[e] Sisot  Sadia  0
5.9 Heat is lost through a plane wall steadily at a rate
of 600W.If the inner and outer surface temperature
of the wall are 20 ℃ and5℃, respectively, the rate of
entropy generation within the wall is
[a] 0.11W/K
[c] 2.10 W/K
[e] 90.0W/K
[b] 4.21W/K
[d] 142.1W/K
5.10 Air at 15 ℃ is compressed steadily and isothermally
from 100KPa to700 KPa at a rate of 0.12kg/s.The minimum
power input to the compressor is
[a] 1.0kW
[e] 16.1kW
[b] 11.2kW
[c] 25.8kW
[d] 19.3 kW
5.11 Air is to be compressed steadily and
isentropically from 1atm to 16atm by a two-stage
compressor. To minimize the total compression
work, the intermediate pressure between the two
stages must be
[a] 2atm
[c] 8 atm
[e] 12atm
[b] 4atm
[d] 10 atm
5.12 Liquid water enters an adiabatic piping system
at 15 ℃ at a rate of 5kg/s.If the water temperature
rises by 0.5 ℃ during flow due to friction.the rate of
entropy generation in the pipe is
[a] 36W/K
[c] 685W/K
[e] 8370W/K
[b] 29 W/K
[d] 920W/K
5.13 Steam enters an adiabatic turbine at 4 MPa
and 500 ℃ at a rate of 15kg/s,and exits at 0.2MPa
and 300 ℃.The rate of entropy generation in the
turbine is
[a] 0.8kW/K
[c] 12.0kW/K
[e] 17.4kW/K
[b] 1.2kW/K
[d] 15.1kW/K
5.13
 A quantity of air undergoes a thermodynamic cycle
consisting of three processes in series.
 Process 1-2: constant-volume heating from P1=0.1 MPa,
T1=288 K, V1=0.02 m 3 to P2=0.42 MPa;
 Process 2-3: constant-pressure cooling ;
 Process 3-1:
isothermal heating to the initial state ;
 Employing the ideal gas model with Cp=1 kJ/kgK,
evaluate the change in entropy for each process. Sketch
the cycle on p-v coordinates.
CHAPTER 8
THERMODYNAMICS OF HIGH-SPEED GAS FLOW
8.1 In which medium will sound travel fastest for a given
temperature: air,helium,or argon?
8.2 Is it possible to accelerate a gas to a supersonic
velocity in a converging nozzle?
8.3 Air enters a converging-diverging nozzle at a pressure
of 1.2MPa with negligible velocity. What is the lowest
pressure that can be obtained at the throat of the nozzle?
8.4 Air enters a nozzle at 0.2MPa,350K,and a
velocity of 150m/s.Assuming isentropic flow,
determine the pressure and temperature of air at
a location where the air velocity equals the
velocity of sound. What is the ratio of the area at
this location to the entrance area?
8.5 Air enters a converging-diverging nozzle at
0.8MPa with negligible velocity. Assuming the flow
to be isentropic, determine the back pressure that
will result in an exit Mach number of 1.8.
Air enters an adiabatic nozzle at 800 K and 140 KPa
and emerges at 7 KPa. The isentropic efficiency of the
nozzle is known to be 0.96. Determine the discharge
velocity. Sketch the process on a T-S diagram and
label the states. You may assume the working fluid is
a perfect gas with Cp=1 kJ/kgK.
8.6 Steam enters an converging nozzle at 30.MPa and 600
℃ with a negligible velocity, and it exits at 1.8MPa.For a
nozzle exit area of 24c㎡,determine the exit. Velocity, mass
flow rate, and exit Mach number if the nozzle (a) is isentropic
and (b) has an efficiency of 90percent
CHAPTER 9
GAS POWER CYCLES
9.1 A Carnot cycle operates between the temperature
limits of 300K and 1500K,and produces 600kw of net
power. The rate of entropy change of the working fluid
during the heat addition process is
[a] 0
[c] 0.5 kW/K
[e] 5.0kW/K
[b] 0.4kW/K
[d] 2.0kW/K
 a) A simple engine uses a perfect gas as the working
fluid in a piston-cylinder system. The gas is first heated
at constant pressure from state 1 to state 2, then cooled
at constant volume to state 3 where T3=T1, and then
cooled at constant temperature, thereby returning to
state 1. Derive expressions for the amounts of energy
transfer as work and heat (per kg of gas) for each
process in terms of the temperatures and pressures at
each state and the constants of the gas. Suppose
T1=300K, P1=0.2 MPa, T2=800K, K=Cp/Cv=1.4.
Calculate the cycle efficiency (net work output/energy
input as heat)
 b) When the engine of part (a) is reversed, it
becomes a refrigeration device. Calculate the amount
of energy transfer as heat from the cold space for this
cycle (per kg of gas), and the cycle COP (energy
transfer as heat from cold space/net work input). You
can assume air for the working fluid.
CHAPTER 10
REFRIGERATION CYCLES
10.1 A refrigerator removes heat from a refrigerated
space at-5 ℃ at a rate of 0.35kj/s and rejects it to an
environment at 20 ℃ .The minimum requires power
input is
[a] 30W
[b] 33W
[c] 56W
[d] 124W
[e] 350W
10.2 An ideal vapor compression refrigeration
cycle with R-134a as the working fluid operates
between the pressure limits of 140kPa and
900kpa .The mass fraction of the refrigerant
that is in the liquid phase at the inlet of the
evaporator is
[a] 0.75
[c] 0.50
[e] 0.25
[b] 0.65
[d] 0.35
10.3 Consider a refrigeration that operates on the
vapor compression refrigerant cycle with R-134a
as the working fluid. The refrigerant enters the
compressor as saturated vapor at 140kPa ,and
exits at 800kPa and 60 ℃,and leaves the
condenser as saturated liquid at 800kPs.The
coefficient of performance of this refrigerator is
[a] 0.41
[c] 1.8
[e] 3.4
[b] 0.1
[d] 2.5
 There are 528 kJ/min of heat removed from a body by
a refrigerator operating between the limits of 244.5 K
and 305.5 K. If its coefficient of performance is three
fourths of that of a Carnot refrigerator working
between the same temperature limits, find (a) the
heat rejected and (b) the work input, kw. (c) What
are the COP and the heat if this device is used to
deliver heat ?