课程名称︰热力学 课程性质︰必修 课程教师︰陈瑶明 开课学院：工学院 开课系所︰机械系 考试日期（年月日）︰2014/1/9 考试时限（分钟）：120min 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : (附Table:saturated refrigerant-134a-Pressure + Superheated refrigerant-134a ) (配分每大题20%) 1.Please answer the following questions: (1)What are the Kelvin-Planck and Clausius expressions of the second law of thermodynamics? (2)What's entropy? What's its physical significance? (3)What's exergy? How is it defined? (4)What's Rankine cycle? How can you improve its thermal efficiency? (5)How can you select the right refrigerant for a vapor-compression refrigeration cycle? 2.Air is compresses steadily by a reversible compressor from an inlet state of 100 kPa and 300K to an exit pressure of 900kPa. Determine the compressor work per unit mass for (1)isentropic compression with k=1.4 (2)polytropic compression with n=1.3 (3)isothermal compression, and ideal two-stage compression with intercooling with a polytropic exponent of 1.3. Ideal gas with constant specific heats can be assumed. The properties of air at room temperature are:Cp=1.005kL/kgK, Cv=0.718kJ/kgK, R=0.287kJ/kgK, k=1.4 3.What is a Carnot cycle? Illustrate on a P-Vdiagram and an S-Tdiagram. Derive the efficiency of an engine using the Carnot cycle. (You can assume the cycle is reversible and the gas used is an ideal gas.) 4.An ideal Diesel cycle has a compression ratio of 20 and a cutoff ratio of 1.3. Determine the maximum temperature of the air and the rate of heat addition to this cycle when it produces 250kW of power and the state of the air at the beginning of the compression is 90kPa and 15*C. Use constant specific heats at room temperature. The properties of air at room temperature are:(as the end of question 2 provided.) 5.A refrigerator operating on the vapor-compression refrigeration cycle using refrigerant-134a as the refrigerant is considered. The temperature of the cooled space and the ambient air are at-13*C and 27*C, respectively. R-134a enters the compressor at 140kPa as a saturated vapor and leaves at 1MPa and 70*C. The refrigerant leaves the condenser as a saturates liquid. The rate of cooling provided by the system is 13kW. Determine (1)the mass flow rate of R-134a and the COP, (2)the exergy destruction in each component of the cycle and the exergy efficiency of the compressor, and (3)the second-law efficiency of the cycle. --

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