Air enters a turbine steadily at a rate of 1 lb/s, a pressure of 75 psia, a temp

ID: 1718049 • Letter: A

Question

Air enters a turbine steadily at a rate of 1 lb/s, a pressure of 75 psia, a temperature of 800 R and a velocity

of 400 ft/s. The air exits the turbine at 15 psia, 600 R, and 100 ft/s. Heat loss to the surroundings at a rate

of 10 Btu, takes place where the surface temperature is 620 R. Assume that the air behaves as an ideal gas

with variable specific heats.

a) For a control volume including only the turbine and its contents, determine (i) the actual power output

of the turbine, and (ii) the rate of entropy production.

b) For an enlarged control volume including the turbine and a portion of its immediate surroundings such

that the heat transfer occurs at the surroundings temperature of 40oF, (i) determine the rate of entropy

production, and (ii) briefly compare this result with the result from Part (a)(ii).

(Answers: (a) 41.3 Btu/s, 0.057 Btu/s-R; (b) 0.061 Btu/s-R)

a) For a control volume including only the turbine and its contents,

(i) the actual power output of the turbine= 41.3 Btu/s

(ii) the rate of entropy production. = 0.057 Btu/s-R

b) For an enlarged control volume including the turbine and a portion of its immediate surroundings such

that the heat transfer occurs at the surroundings temperature of 40oF

(i) determine the rate of entropy = 0.061 Btu/s-R

production

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