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）︰2013.1.10 考试时限（分钟）：110分 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : 1.Please answer the following questions: (a)Define entropy and exergy. Give also their physical meaning. (b)What are the precesses that make up the Carnot, Stirling and Ericsson cycles? Sketch the T-s and P-v diagrams. (c)What is an ideal vapor-compression refrigeration cycle? When selecting a refrigerant for a certain application, what qualities would you look for in the refrigerant? (d)Propose methods to increase the efficiency of the Rankine cycle? 2.The thermal efficiency η of a cold air-standard diesel cycle can be expressed by k r - 1 1 ┌ c ┐ η=1-───│─────│ k-1 └ k(r - 1) ┘ r c where r is compression ratio and r is cutoff ratio and k is the specific heat ratio. Derive this expression. 3.(a)Consider a simple Brayton cycle using air as the working fluid; has a pressure ratio of 12;has a maximum cycle temperature of 600°C; and operates the compressor inlet at 90kPa and at 15°C. Which will have the greatest impact on the back-work ratio:a compressor is isentropic efficiency of 90% or a turbine isentropic efficiency of 90%? The properties of air at room temperature are C = 1.005kJ/kg·K and k=1.4 p (b)Repeat problem (a) using exergy analysis. Q ↙ in → ┌──┐ → ┌──┤ ├──┐ │ └──┘ │ ▕╲│ ｜╱︳Turbine ▕ ╲ ╱ ︳ W Compressor▕ ▕================︳ ╞=→ ▕ ／ ╲ ︳ ▕／│ ｜╲︳ │ ┌──┐ │↓ └──┤ ├──┘ ← └──┘ ↙ Q out 4.A 200 m^3 rigid tank initially contains atmospheric air at 100kPa and 300K and is to be used as storage vessel for compressed air at 1MPa and 300K. Compressed air is to be supplied by a compressor that makes in atmospheric air at P_0 = 100KPa and T_0 = 300K. Determine the minimum work required for this process. The gas constant of air is R=0.287 kJ/kg·K. ┌──────┐ │ Air │ │ 3 │ │ V=200m │ │ │ │ 100kPa │ Compressor │ ↓ │ ↙ │ 1MPa ╞═╤═╗ │ │ ║ ║ │ 300K │ ╙═╣ └──────┘ ║ ↖100kPa 300K 5.A simple Rankine cycle uses water as the working fluid. The boiler operates at 6000kPa and the condencer at 50 kPa. At the entrance to the turbine, the temperature is 450°C. The isentropic efficiency of the turbine is 94%, pressure and pump losses are negligible, and the water leaving the condencer is subcooled by 6.3°C. The boiler is sized for a mass flow rate os 20 kg/s. Determine the rate at which heat is added in the boiler, the power required to operate the pumps, the net power produced by the cycle, and the thermal efficiency. --

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