exam Midterm 1 MECH 3800 Engineering Thermodynamics II
Date: October 10th, 2008 Time: 13h30 to 15h30 Open books, open notes exam No questions allowed Question #1 (4 points) Give a short written answer to this question: Indicate the advantages regarding refrigeration capacity, compressor work and COP, that a multistage cycle with flash and mixing chamber has over a standard one stage refrigeration cycle using the same working fluid and operating between the same minimum and maximum pressures. Clearly show these advantages by comparing the T-s diagrams for these two cycles. Question #2 (7 points) As shown on the following figure, a steam turbine having an isentropic efficiency of 90% drives an air compressor having an isentropic efficiency of 85%. Operational data is provided on the figure. Assume the ideal gas model for air and ignore stray heat transfer and kinetic and potential energy effects. Determine the mass flow rate of the steam entering the turbine, in kg of steam per kg of air exiting the compressor. Room: MA222 and B311 Professor: Dr. Dominic Groulx Total: 4 questions (35 points)
Question #3 (10 points) The rigid tank illustrated in the figure has a volume of 0.06 m3 and initially contains a two-phase liquid-vapor mixture of H2O at a pressure of 15 bar and a quality of 20%. As the tank contents are heated by an heat source at 377C, a pressure-regulating valve keeps the pressure in constant the tank by allowing saturated vapor to escape. Neglecting kinetic and potential energy effects, determine: a) the final total mass in the tank, in kg, and the amount of heat transfer, in kJ, if heating continues until the final quality is x = 0.5; b) the amount of entropy being generated during this process.
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Question #4 (14 points) A vapor-compression refrigeration system operates with the cascade arrangement presented in the following figure. R-22 is the working fluid in the high-temperature cycle and R-134a is used in the low temperature cycle. For the R-134a cycle, the working fluid enters the compressor as saturated vapor at -35C and is compressed to 300 kPa, this compressor having an isentropic efficiency of 88%. The R134a leaves the heat exchanger at 0C. For the R-22 cycle, the working fluid enters the co...