课程名称︰热力学 课程性质︰必修 课程教师︰黄美娇 开课学院：工学院 开课系所︰机械系 考试日期（年月日）︰2011/01/13 考试时限（分钟）：180 mins 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : Q1.(21%) Answer the following questions clearly but briefly.Note you get no point if no or wrong answer is given to the why-question. (i) A system undergoes a process between two fixed states first in a reversible manner and then in an irreversible manner. For which case is the amount of the integral ∫(δQ/T) greater? Why? (ii) A system undergoes a process involving a same amount of ∫(δQ/T) first in a reversible manner and then in an irreversible manner.For which case is the entropy change of the system greater? Why? (iii) A system undergoes a process between two fixed states in a reversible manner and then in an irreversible manner.For which case is the entropy change of the system greater? Why? (iv) Somebody claims to have developed a new heat-engine cycle that is operating between the temperature limits of 300K ans 800K and has a theoretical efficiency of 0.65. How do you evaluate this claim? (v) Does the area enclosed by the cycle on T-s diagram represent the net work input for the ideal vapor-compression refrigeration cycle? If yes,why so? If not,why not? (vi) What is the criterion that equates the amount of the integral ∫Pdv to the work done by a closed system during a process? (vii) What are the limitation on the thermodynamic properties of a refrigerant for given operating temperatures(ambient temperature T∞ and the refrigerated-space temperature Tc)? Q2.(25%) A dual cucle with air as the working fluid has a compression ratio of 14 and a cutoff ratio of 1.4. At the beginning of the compression process,the working fluid is at 100kPa and 27℃. A total of 600kJ/kg of heat is transferred to air during the whole heat-addition process.Utilizing the cold-air-standard assumption, determine (a)the temperature and pressure of air at the end of each process (b)the work output during the process 3-4-5 and (c)the thermal efficiency. (state 1~5 从dual cycle的P-v图最右下角开始,顺时钟依序标上) Q3.(30%) Air enters a gas turbine with a regenerator at 100kPa and 27℃.The pressure ratio is 12; 600 kJ/kg of heat are added to the air in the combustion chamber;and the adiabatic regenrator increases the temperature of the cold air by 31℃. Assume an isentropic efficiency of 0.92 for the compressor and 0.89 for the turbine.Use the air-standard assumption and answer the following questions. (a)Sketch the T-s diagram of the cycle. Identify the states 1~6. (b)Find the effectiveness εof the regenerator. (c)Determine the overall thermal efficiency. Q4.(30%) Consider a two-stage-cascade multi-purpose refrigeration system using R-134a as the working fluid in both cycles.The refrigerant leaves the condenser at 1MPa and is throttled to 0.4M before it enters into the heat exchanger. The refrigerant enters the refrigerator at -6℃ and the freezer at -20℃.The mass flow rate of the refrigerant in the condenser is 0.1kg/s. The required cooling power in the freezer is 10kW. Assume all components and process are ideal except that the heat exchanger has a heat loss 1.5k. Also assume state 4 (高温cycle进入高压compresor前的状态) and state 10(低温 cycle进入低压compressor前的状态)are both saturated vapor and state 2(高温 cycle 通过condenser後的状态) and state 6(低温cycle通过regenerator後的状态) are saturated liquid. (a)Sketch the T-s diagram of the cycle.Identify the states 1~10. (b)Find the mass flow rate of the refrigerant in the freezer. (c)Find the overall coefficient of performance,COP. --

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