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ME -251 MIDTERM EXAM (Fall 2008 - Instructor: R. Betti) Closed books and closed notes. A single page cheat sheet is allowed Due at 12:20 pm PROBLEM 1 (33%) In a vapor-compression refrigeration cycle, ammonia exits the evaporator as saturated vapor at –20oC. The refrigerant enters the condenser at 16 bar and 160oC, and saturated liquid exits at 16 bar. There is no heat transfer between the compressor and its surroundings and the refrigerant passes through the evaporator without loss of pressure. If the refrigerant capacity is 200kW, Determine: (a) the mass flow rate of refrigerant in kg/s (11%) (c) the isentropic efficiency of the compressor (11%) (d) the coefficient of performance (11%) PROBLEM 2 (34%) In a turboprop engine, air enters a diffuser where decelerates essentially to zero velocity. At the diffuser inlet, the pressure is 40kPa, the temperature is 240K, the velocity is 200m/s and the volumetric flow rate is 90m3/s. After the diffuser, a compressor raises the air pressure to 400kPa. The air goes through a combustion chamber leaving with a temperature of 1200K before entering a turbine that drives the compressor as well as the propeller (IMPORTANT: the turbine provides the work required to drive the compressor and the propeller). Air leaves the turbine with a pressure of 50kPa and goes through a nozzle where it expands to the atmospheric pressure of 40kPa. Assuming the flow is isentropic through the diffuser, compressor, turbine and nozzle, and using the cold air-standard analysis (constant specific heats), determine: (a) the power delivered to the propeller in MW (17%) (b) the air velocity at the nozzle exit (17%) Neglect the kinetic energy except at the diffuser inlet and the nozzle exit. PROBLEM 3 (33%) Air enters the compressor of a gas turbine power plant at 100kPa, 300K. The air is compressed to 1MPa with intercooling to 300K between stages. The turbine inlet temperature is 1520K and the expansion occurs in two stages, with reheat to 1500K between stages at a pressure of 330kPa. The compressor and turbine stages are isentropic. The net power developed is 2MW. (a) Choose the optimum intecooling pressure in the compressor and explain why you choose that value [you can use the results of the standard cold-air analysis (constant specific heats) without deriving it]. If you can’t answer this question then choose the intecooling pressure of 300kPa. (10%) (b) Determine the thermal efficiency (15%) (c) Determine the volumetric flow rate (8%)