Air enters a turbine operating at steady state with a pressure of 75 lbf/in.2 , a temperature of 800 R, and a velocity of 400 ft/s. At the turbine exit, the conditions are 15 lbf/in.2 , 600 R, and 100 ft/s. Heat transfer from the turbine to its surroundings takes place at a location where the average surface temperature is 620 R. The rate of heat transfer is 10 Btu per lb of air passing through the turbine.
(a) For a control volume including only the turbine and its contents, determine the work developed, in Btu, and the rate at which entropy is produced, in Btu/ R, each per lb of air flowing.
(b) For a control volume including the turbine and a portion of its immediate surroundings so that the heat transfer occurs at the ambient temperature, 40 F, determine the rate of entropy production in Btu/ R per lb of air passing through the turbine.
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Explain why the entropy production value of part (b) differs from that calculated in part (a).