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1 MAE 320 – Thermodynamics HW 7b Assignment The homework is due Wednesday December 3 rd , 2014. Each problem is worth the points indicated. (1). Multiple choice (18 points ) (1a). (3p) Please circle the valid statement(s) associated with entropy: (A) Entropy is a property. A substance has a fixed entropy at a fixed state; (B) The entropy change between two specified states is the same no matter if the process is reversible or irreversible; (C) On the T-S diagram, the area under the process curve always represents the heat transfer, which has nothing to do with the fact if the process is irreversible or not; (D) Entropy is a path function. The entropy change depends on the path of the process; (E) Entropy generation is a path function, which depends on the path of the process. (1b). (3p) Pease circle the ds equation(s) which is valid for any substance under any process . (A) °± = ²³ ´ + µ²¶ ´ ; (B) °± = · ²´ ´ ; (C) °± = ¸ ²´ ´ + ¹ ²¶ ; (D) °± = ²ℎ ´ ¶²µ ´ . (1c). (3p) Please circle the valid statement(s) associated with an actual steady-operating heat engine cycle: (A) º ̇ »¼½ < 0 ; (B) º ̇ »¼½ = 0 ; (C) º ̇ »¼½ > 0 ; (D) Actual heat engine cycle always involves irreversibility. 1 2 3 4 5 6 18 15 15 15 18 19 100
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2 (1d). (3p) The Isentropic efficiencies of steady-flow devices can be used only when: (A) There is no work transfer; (B) There is no heat transfer; (C) There are no irreversibility; (D) None of the above. (1e). (3p) Entropy can be transferred from the system to the surroundings by: (A) Mass flow; (B) Boundary work; (C) Heat transfer; (D) Shaft work. (1f). (3p) A piston-cylinder device contains superheated steam. During an actual (irreversible) adiabatic process, the entropy of the steam will: (A) Never increase; (B) Sometimes increase; (C) Always increase; (D) It cannot be determined with the given information. (2). Short answer and brief calculation question: (15 Points) (2a). Using the entropy balance equation (Eqn. 7-68), find the entropy balance relation for a steady-flow, single-stream, adiabatic device. (5 Points). (2b). The inner and outer surfaces of a 5 m by 7 m brick wall with a thickness of 20 cm are maintained at temperatures of 20 °C and 5 °C. If the rate of heat transfer through the wall is 1515 W, what is the rate of entropy generation? (5 points ) (2c). Please find the changes in the specific entropy of helium and nitrogen as its state changes from 2000 kPa and 427 °C to 200 kPa and 27 °C? (5 points )
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