MAE 311 Practice Problem Set 5 Questions Spring 2024 Problem 1: A large stationary diesel engine produces 5 MW with a thermal efficiency of 40%. The exhaust gas, which we assume is air, flows out at 800 K and the intake air is 290 K. How large a mass flow rate is that, assuming this is the only way we reject heat? Problem 2: Given that the hot thermal reservoir temperature is 25 o
C and the cold thermal reservoir temperature is -10 o
C: For each of the following cases, determine if a heat pump satisfies the first law (energy equation) and if it violates the second law. a. Q
H
̇
= 6 kW, Q
L
̇
= 4 kW, W
̇
= 2 kW b. Q
H
̇
= 6 kW, Q
L
̇
= 0 kW, W
̇
= 6 kW c. Q
H
̇
= 6 kW, Q
L
̇
= 2 kW, W
̇
= 5 kW d. Q
H
̇
= 6 kW, Q
L
̇
= 6 kW, W
̇
= 0 kW Problem 3: Find the power output and the low T heat rejection rate for a Carnot cycle heat engine that receives 6 kW at 250 o
C and rejects heat at 30 o
C. Problem 4: A refrigerator should remove 400 kJ from some food. Assume the refrigerator works in a Carnot cycle between –
15 o
C and 45 o
C with a motor-compressor of 400 W. How much time does it take if this is the only cooling load? Problem 5: A household freezer operates in a room at 20°C. Heat must be transferred from the cold space at a rate of 2 kW to maintain its temperature at −30°C. What is the theoretically smallest (power) motor required to operate this freezer?
