midterm2_solution

midterm2_solution - Problem 1 Steam expands in a turbine...

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Unformatted text preview: Problem 1 Steam expands in a turbine steadily at a rate of 25 , 000 kg/h , entering at 6 MPa and 450 ◦ C and leaving at 20 kPa as saturated vapor. If the power generated by the turbine is 4 MW , determine the rate of entropy generation for this process. Assume the surrounding medium is at 25 ◦ C. Solution: State in: 6 MPa , 450 ◦ C, → h 1 = 3302 . 9 kJ/kg , s 1 = 6 . 7219 kJ/kg · K State out: 20 kPa , x = 1, → h 2 = 2608 . 9 kJ/kg , s 2 = 7 . 0973 kJ/kg · K ˙ W = 6 MW , ˙ m = 25000 3600 kg/s Energy Eq.: ˙ Q- ˙ W = ˙ m ( h 2- h 1 ) ⇒ ˙ Q = ˙ W + ˙ m ( h 2- h 1 ) =- 819 . 4 kW This means heat is being rejected from the turbine. Entropy Eq.:- ˙ Q out T amb + ˙ m ( s 1- s 2 ) + ˙ S gen = 0 ⇒ ˙ S gen = ˙ Q out T amb- ˙ m ( s 1- s 2 ) ≈ 11kW / K Problem 2 Consider an ideal refrigeration cycle with R134a as the working fluid. The compressor power input is 1 . 5 kW , bringing the R134a from 200 . 74 kPa to 1200 kPa by com- pression. The cold space evaporator is a heat exchanger that cools atmosphere air from 30pression....
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This test prep was uploaded on 04/20/2008 for the course ME 235 taught by Professor Borgnakke during the Winter '07 term at University of Michigan.

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midterm2_solution - Problem 1 Steam expands in a turbine...

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