MENG 260 University of San Diego Rate of Entropy Production Midterm Exam 2

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ne082317

Engineering

MENG 260

University of San Diego

MENG

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Academic Integrity Pledge: I have neither given nor received unauthorized Assistance on this examination Dr. MO University of San Diego MENG 260 Midterm Exam 2 Name________________ NOTE: Show all your work clearly. No work = No credit!!! DO NOT COPY YOUR CLASSMATE WORK=Cheating DO NOT ASK OTHER PEOPLE TO TAKE THE EXAM FOR YOU: You will receive zero for plagiarism 1. Steam enters a turbine operating at steady state at inlet pressure of 8 MPa, and inlet temperature of 600℃ with a mass flow rate of 130 kg/min and exits as saturated vapor at 20 KPa, producing power at a rate of 3 MW. Kinetics and potential energy effects can be ignored. (15 points) Determine (a) the rate of heat transfer, in KW, for a control volume including the turbine and its contents, and (b) the rate of entropy production, in KW/K, for an enlarged control volume that includes the turbine and its surroundings that transfer occurs at the ambient temperature, 27℃. 2. Air enters a pipe passing overhead in a warehouse space. The inlet temperature and pressure are 600 K and 600 KPa respectively. Also the mass flow rate enters the pipe is 700 Kg/h. At the pipe exit, the pressure and temperature of the air are 450 KPa and 500 K respectively. Air can be modeled as an ideal gas with K= 1.4. Kinetics and potential energy effects can be ignored. (20 points) Determine at the steady state (a) the rate of heat transfer, KW, for control volume comprising the pipe and its contents, and (b) the rate of entropy production, in KW/K, for an enlarged control volume that includes the pipe and enough of its surroundings that heat transfer occurs at the ambient temperature of 25℃. Based on your result elaborate if the second law of thermodynamics is violated or not. Why or Why not?? 3. Steam enters a turbine operating at steady state at 30 bar and 500℃ with a velocity of 80 m/s. Saturate vapor exits at 0.15 MPa and a velocity of 40 m/s. The elevation of the inlet is 4 meter higher than at the exit. The mass flow rate of the steam is 11 kg/s, and the power developed is 8 MW. (20 points) a) Determine the area at the inlet, in 𝑚2 . b) The rate of heat transfer between the turbine and its surroundings, in KW. 4. Air enters a well - insulated horizontal pipe having a diameter of 2 cm with inlet temperature of 47℃ , and inlet pressure of 500 KPa. The exit pressure and temperature are 100 KPa and 300 K respectively. Applying the ideal gas model, determine at steady state: (20 points) a) The inlet and exit velocities in m/s. b) The mass flow rate in Kg/s. 5. The initial volume for Nitrogen gas in a piston cylinder is 0.75 𝑚3 with an initial pressure and temperature of 0.2 MPa, and 240 K respectively, which undergoes an internally reversible compression during which 𝑃𝑉 1.2 =constant to a final state where temperature is 400 K. Assuming the ideal gas model, determine: (18 points) (a) The pressure at the final state, in bar. (b) The work and heat transfer, each in KJ (c) The entropy change, in KJ/K 6. With an ideal heat pump, determine how much energy can be pumped into a home, which is to be kept at 27℃, from a lake bottom whose water is at 7℃. Calculate the coefficient of performance of this heat pump in this situation. (7 points)
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