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HW2_sol - Thermal Fluids Engineering I Spring 2006...

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Thermal & Fluids Engineering I. Spring 2006. Homework #2 Solution. WebCT post date: 02/02/2006 2-17 Calculate the work, in joules, that is done in the quasi-equilibrium process from state 1 to state 2 shown in the figure. Approach: The work done is the area under the curve of P versus V . Use geometry to calculate this area. Assumptions: 1. The process is quasi-equilibrium. Solution: The work for a quasi-equilibrium process is W = PdV The area under the curve from state 1 to state 2 is represented as W = (100kPa)(2-1)cm 3 + (50kPa)(3-2)cm 3 = (150 kPa cm 3 ) 1000Pa 1kPa 3 1 100 3 3 m cm 0.15 J W = Answer
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Thermal & Fluids Engineering I. Spring 2006. Homework #2 Solution. Page 2 2-25 In the figure below, a piston is resting on a set of stops. The cylinder contains CO 2 initially at –30 ° C and 45 kPa. The mass of the piston is 1.2 kg and its diameter is 0.06 m. Assuming atmospheric pressure is 101 kPa, how much heat must be added to just lift the piston off the stops. Approach: A force balance on the piston can be used to calculate the gas pressure at piston lift-off. The ideal gas law is used to determine the final temperature. As a last step, apply the first law to find heat added. Assumptions: 1. Specific heat is constant. 2. Carbon dioxide behaves like an ideal gas under these conditions. Solution: The piston will just lift off when the pressure inside equals the pressure outside plus the weight of the piston per unit area.
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