工程热力学英语习题.课件.ppt

1、Chapter 1Basic Concepts1.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

2、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 tran

3、sferred 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 kJ6. A 2 m3 rigid tank contains nitrogen gas at 500kPa and 300K. Now heat is transferred to the nitrogen in the tank and the pres

4、sure 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 kJCHAPTER 2PROPERTIES OF PURE SUBSTANCES2.1 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

5、the final pressure in the tank is 1.5atm.the final temperature in the tank isa -38 b30 c40 d 53 e144 2.2 A rigid vessel contains steam at 20 MPa and 400 .The mass of the steam is a 5.0kg b 0.322kg c 322kg d 503kg e680kg35 m2.3 A rigid tank is filled with saturated liquid-vapor mixture of water at 20

6、0kPa.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 c258kg d 635kg e28093500mChapter 3 The first Law of Thermodynamicso A reversible cycle plots as a perfect circle on a T-S diagram with maximum and minimum temperatures 600

7、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

8、 cycle?o 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 t

9、han compartment B(TATB).oa) 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).ob) Is the final internal energy Ufinal of the system (both compartments together) lower, equal or higher t

10、han initial internal energy Uinitial of the system? Why? (A sentence or two is expected, perhaps bolstered by some equations.)oc) What is the final temperature of the system Tfinal expressed in terms of TA and TB ?ob) What is the change in entropy of compartment A? What is the change in entropy of c

11、ompartment B? oe) What is the change in entropy of the surroundings?of) 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)o 1

12、0 kg of air contained in a piston cylinder apparatus at 5 bar,300K, expands isothermally until the pressure is 1 bar. Determine:o (i) the work done.o (ii) the internal energy change of the air.o (iii) the heat transfer.o Assume that air behaves as an ideal gas and pressure variation is gradual and c

13、ontinuous.oA 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

14、state c, 70KJ of heat flow from the system. Find the work during the process adc.CHAPTER 5THE SECOND LAW OF THERMODYNAMICS5.1 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

15、is 3.1,the required power input to this refrigerator is a 28W b 42W c 88W d 130W e 403W5.2 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 isa 6.2

16、 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 b 55.4kW c 174.6 kW d 95.0kW e 100.0kW5.4 Steam is compresse

17、d 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 b 63 c 250 d 384 e 476 5.6 Steam expands

18、 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 b 698kw c 2136 kw d 12,452kw e26,990kw21SSS5.7 A unit mass of a substance an irreversible process from state 1 to state 2 while gaining

19、heat from the surroundings at temperature T in the amount of q.If the entropy of the substance is at state 1,and at state 2,the entropy change of the substance s during this process isa b c d e 1S2S21SSS21/SSSq T21/SSSq T 21SSS5.8 Air is compressed from room conditions to a specified pressure in a r

20、eversible 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 isa b c d e 0isotadiaSS 0isotadiaSS 0isotadiaSS 0isotadiaSS 0isotadiaSS 5.9 Heat

21、 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 b 4.21W/K c 2.10 W/K d 142.1W/K e 90.0W/K5.10 Air at 15 is compressed steadily and isothermally from

22、100KPa to700 KPa at a rate of 0.12kg/s.The minimum power input to the compressor isa 1.0kW b 11.2kW c 25.8kW d 19.3 kW e 16.1kW5.11 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 betw

23、een the two stages must be a 2atm b 4atm c 8 atm d 10 atm e 12atm5.12 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 b 29 W/K c 685W/K d 920W/K e 8370W/K5.

24、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 b 1.2kW/K c 12.0kW/K d 15.1kW/K e 17.4kW/K5.13oA quantity of air undergoes a thermodynamic cycle consisting of three processes in series.o

25、Process 1-2: constant-volume heating from P1=0.1 MPa, T1=288 K, V1=0.02 to P2=0.42 MPa;oProcess 2-3: constant-pressure cooling ;oProcess 3-1: isothermal heating to the initial state ;oEmploying the ideal gas model with Cp=1 kJ/kgK, evaluate the change in entropy for each process. Sketch the cycle on

26、 p-v coordinates.3mCHAPTER 8THERMODYNAMICS OF HIGH-SPEED GAS FLOW8.1 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 pressu

27、re 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

28、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 adiaba

29、tic 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 conver

30、ging 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 90percentCHAPTER 9GAS POWER CYCLES9.1 A Carnot cycle operates

31、 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 b 0.4kW/K c 0.5 kW/K d 2.0kW/K e 5.0kW/Ko a) A simple engine uses a perfect gas as the working fluid in a piston-cylinder system.

32、 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

33、 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)o b) When the engine of part (a) is reversed, it becomes a refrigeration device. Calculate t

34、he 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 10REFRIGERATION CYCLES10.1 A refrigerator removes heat from a refrigerated space at-5

35、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 350W10.2 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 re

36、frigerant that is in the liquid phase at the inlet of the evaporator is a 0.75 b 0.65 c 0.50 d 0.35 e 0.2510.3 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 e

37、xits at 800kPa and 60 ,and leaves the condenser as saturated liquid at 800kPs.The coefficient of performance of this refrigerator is a 0.41 b 0.1 c 1.8 d 2.5 e 3.4oThere 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 ?