高等工程热力学试卷和答案

1True or false(20pts)1) Mass can cross the system boundary in the Open system .(T)2) Heat and work can not cross in the Closed system. (F)3)Intensive property is a property that is independent of the extent or mass of the system.(T)4)Heat exchangers are devices that transfer energy between fluids at different temperatures.(T)5) Heat engine can have a thermal efficiency of 100%. (F)6)Heat Engines are Devices that are used to convert heat to work.(T)7)The decrease in quality is always accompanied by an increase in entropy.(T)8)Reversible processes actually can occur in nature.F)9)Energy can be neither created or destroyed;it can only change forms.（T）10)Both Heat and Work are associated with a process and a state.(F)11)Once the temperature equality is established,the heat transfer stops.(T)12)The direction of heat transfer is always from the lower temperature body to the higher temperature.body.(F)13)Work is not a property of a system or its surroundings.(T)14)Closed Feedwater Heaters require a separate pump for each heater.(F)15)The reheat process in general can significantly change the cycle efficiency.(T)2Chosing a correct answer(30pts)1) A thermodynamic system generally has a boundary, this boundary （b）a)can be personally defined from your own interestsb)can only be defined following certain rolesc)is depend on the physical boundariesd)is independent on the physical as well as personal wills2) A system is called a closed system when （c）a)there is only heat exchange between the system and its surroundingsb)there is only work interactions between the system and its surroundingsc)there are both heat exchange and work interactoins between the system and its surroundingsd)there are neither heat exchange nor work interactions between the system and its surroundings3)The specific internal energy,u,other than U,of a system is determined by the（c）of the systema) volume b) pressure c) temperature d) mass4) An example of internal property of a system is （c？）a)mass b)total volume c)pressure d)enthalpy5) A steady state process is related to （a ？）a) a closed system with invarible propertiesb) a control volume with its property changesc) an open system keeping its properties constantd) a closed system having its properties changed6) A good approximation for a stream flowing through a porous plug is （a ）a) h in = h out b) P in = P out c)T in = T out d) a and b e) b and c7) Δ T= 0 is true （d ）a) for an adiabatic process. b) if no work is done.c) for an isentropic process. d) for a reversible process.8) Characteristics of Heat Engines include （e ）a) They receive heat from a high-temperature source.1b)They convert part of this heat to work.c)They reject the remaining waste heat to low-temperature sink.d) They operate on a cycle.e)all of above9) Quality （a）a) is defined by M /M + M ) liq liq vapb) is defined by v = x v + (1-x) v f gc)equals zero for a saturated vapor.d)is only defined in the two phase region.e)is the fraction of mass that is liquid.10) A good approximation for a stream flowing through a heat exchanger is （e？）a) h = h in out b) P = P in out c) T = T in outd) a and b e) b and c11)In an adiabatic process:（b）a)temperature cannot change.b)there is no heat flow.c)internal energy cannot change.d) a and be)a,b and c12) Which of the following is an extensive property ? （b）a)temperature b)weight c)compositiond)pressure e)none of the above13) What is the efficiency of an ideal Carnot heat engine cycle operating between 38 C and 482 C ? （c）a) 90 % b) 89 % c) 59 % d) 16 % e) 11 %14) Which of the following does not have units of energy? （d）a) N-m b) kw-hr c) Pa-m3 d) All have units of energy15)Extensive properties（d）a)apply at a point.b) do not depend on the size of the system.c)depend on position within a system.d)are proportional to the mass of a system.e)are printed in the tables in the back of your book.3Provide definition for the following terms(10pts)1)Internal energy:Internal energy is really the kinetic and potential energy of the atoms in the molecules.2)Heat:Energy crossing the system boundary,not associated with mass,due to a temperature difference.3)Work:Work is the energy transfer associated with a force acting through a distance.4) Closed system: No mass crosses the boundary. Heat and work can cross.5) Open system: Mass can cross the system boundary.4Answering and calculations(40pts)1) Feedwater Heater:Inlet 1 T1 = 200 ºC, p1 = 700 kPa, m1 40 kg/s2Inlet 2 T2 = 40 ºC, p2 = 700 kPa,Exit sat. liquid, p3 = 700 kPa,Find m 2 ?, m 3 ? andV 2 ? A2 = 25 cm2 Steady State m i m e m 1 m 2 m 3AVvInlet 2: compressed liquidTable A-4, v2 = 0.001008 m3/kgExit: saturated liquid Table A-5, v3 = 0.001108 m3/kgm 3 (AV)3v 3 0.060.00110854.15 kg/s m 2 m 3 m 1= 54.15 – 40 = 14.15 kg/sV m 2v 2 (14.15)(0.001008) 5.7 m/s2) A rigid tank contains 10 kg of water at 90 ºC. If 8 kg of the water is in the liquid form and the rest is in the vapor form. Determine the pressure in the tank and the volume of the tank . Table A-4, Psat = 70.14 kPa, P = Psat = 70.14 kPaTable A-4, vf = 0.001036 m3/kg, vg = 2.361 m3/kg v = vf + x(vg -vf) = 0.001036 + 0.2(2.361 – 0.001036) = 0.473 m3/kg V = mv = 10(0.473) = 4.73 m33)Heat is transferred to a heat engine from a furnace at a rate of 80 MW. If the rate of waste heat rejection to a nearby river is 50 MW, determine the net power output and the thermal efficiency. W Q H Q L 80 50 30 MWQ H 80 MW, Q L 50 MW0.375Q H 80 m A V 2 A 0.00252(AV)3 0.06 m 3 /sW304) A heat engine receives 600 kJ of heat from a high-temperature source at 1000 K during a cycle. It converts150kJ of this heat to work and rejects the remaining450kJ to a low-temperature sink at300 K. Determine if this heat engine violates the 2nd law of thermodynamics on the basis of(a)the Clausius inequality.(b)the Carnot principle.(a)Clausius inequalityQ QH QL600450T T T10003003H L= - 0.9 kJ/K < 0(b) Carnot principleth 1 Q Q H L 1 0.25T 300ηth < ηrev5)A heat pump maintains a house at a specified temperature. The rate of heat loss of the house and the power consumption of the heat pump are given. The exterior temperature outside of the house is T L , 20C. The interior temperature e inside of the house is T H ,220C. The rate of heat loss of the house is Q H , 110.000kj/h. It is to be determined if this heat pump can do thejob.Assumptions: The heat pump operates steadily.Analysis: The power input to a heat pump will be a minimum when the heat pump operates in a reversible manner. The coefficient of performance of a reversible heat pump depends on the temperature limits in the cycle only, and is determined fromCOP HPjev =11（T /T ）LH = 11（2273K ）/(22273K ) 14.75The required power input to this reversible heat pump is determined from the definition of the coefficient of performance to beW net.jev.min= Q H = 110.000kj /h ( 1h ) =2.07kwThis heat pump is powerful enough since 8 kw >2.07 kw.6) 0.5 kg of air undergoes a Carnot cycle with η = 0.5.Given the initial pressure p1 = 700 kPa, initial volume V1 = 0.12 m3 and heat transfer during the isothermal expansion process Q12 = 40 kJ, Find the highest and the lowest temperatures in the cycle.p 1V 1 (700)(0.12)mR (0.5)(0.287)T H T 1 585.4 K H COP P 14.75 3600srev 1 T L 1 10000.7 Hth T T Q T，T L = 292.7 K4 H1 L 1 L Q H T H 1 1 0.5 0.5 L。