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%)