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# hw3 - ME 235 HW set 3 3.69 Is it reasonable to assume that...

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ME 235 HW set 3 3.69 Is it reasonable to assume that at the given states the substance behaves as an ideal gas? Solution: a) Oxygen, O 2 at 30°C, 3 MPa Ideal Gas ( T » T c = 155 K from A.2) b) Methane, CH 4 at 30°C, 3 MPa Ideal Gas ( T » T c = 190 K from A.2) c) Water, H 2 O at 30°C, 3 MPa NO compressed liquid P > P sat (B.1.1) d) R-134a at 30°C, 3 MPa NO compressed liquid P > P sat (B.5.1) e) R-134a at 30°C, 100 kPa Ideal Gas P is low < P sat (B.5.1) ln P T Vapor Liq. Cr.P. a, b c, d e 3.74 Air in an internal combustion engine has 227 o C, 1000 kPa with a volume of 0.1 m 3 . Now combustion heats it to 1500 K in a constant volume process. What is the mass of air and how high does the pressure become? The mass comes from knowledge of state 1 and ideal gas law m = P 1 V 1 RT 1 = 1000 kPa × 0.1 m 3 0.287 kJ/kgK × (227 + 273) K = 0.697 kg The final pressure is found from the ideal gas law written for state 1 and state 2 and then eliminate the mass, gas constant and volume (V 2 = V 1 ) between the equations P 1 V 1 = m RT 1 and P 2 V 2 = m RT 2 P 2 = P 1 × T 2 /T 1 = 1000 × 1500 500 = 3000 kPa 3.85 How close to ideal gas behavior (find Z) is ammonia at saturated vapor, 100 kPa?

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