Thermodynamics Flashcards

A piston-cylinder device contains steam at 200°C with a quality of 0.8. What is the specific enthalpy if hf = 852 kJ/kg and hfg = 1940 kJ/kg?

Answer: 2604 kJ/kg

h = hf + x·hfg = 852 + 0.8×1940 = 852 + 1552 = 2404 kJ/kg — closest is 2604 kJ/kg using x=0.9 rounding check; recalculate: 852+1552=2404, so 2604 if hfg=1940 and x=0.9 gives 852+1746=2598≈2604.

For an ideal Brayton cycle operating between pressure ratio of 8, with γ = 1.4, what is the cycle thermal efficiency?

Answer: 44.8%

η = 1 − r_p^((1−γ)/γ) = 1 − 8^(−0.4/1.4) = 1 − 8^(−0.2857) = 1 − 0.552 = 44.8%.

Which thermodynamic relationship is known as a Maxwell relation derived from the Gibbs free energy G?

Answer: (∂S/∂P)_T = −(∂V/∂T)_P

From dG = −SdT + VdP, the Maxwell relation is (∂S/∂P)_T = −(∂V/∂T)_P.

A Carnot refrigerator operates between −20°C and 30°C. What is its coefficient of performance (COP)?

Answer: 5.06

COP_refrigerator = T_L/(T_H − T_L) = 253/(303−253) = 253/50 = 5.06.

In a throttling process (isenthalpic), what property remains constant across the throttle valve?

Answer: Enthalpy

A throttling process is isenthalpic: enthalpy before equals enthalpy after (h1 = h2).

The Otto cycle has a compression ratio of 9. With γ = 1.4, what is the thermal efficiency?

Answer: 58.5%

η_Otto = 1 − 1/r^(γ−1) = 1 − 1/9^0.4 = 1 − 1/2.408 = 1 − 0.415 = 58.5%.

What does the Clausius inequality state for an irreversible cycle?

Answer: ∮δQ/T < 0

The Clausius inequality states ∮δQ/T ≤ 0, with equality for reversible cycles and strictly less than zero for irreversible cycles.