Thermodynamics Flashcards

Steam enters a turbine at 4 MPa, 400°C (h1 = 3214 kJ/kg) and exits at 50 kPa (h2 = 2340 kJ/kg). The turbine is adiabatic. What is the specific work output?

Answer: 874 kJ/kg

For an adiabatic turbine, w_turbine = h1 − h2 = 3214 − 2340 = 874 kJ/kg.

Which statement correctly describes the second law of thermodynamics?

Answer: The entropy of an isolated system can never decrease

The second law states that the entropy of an isolated system always increases or remains constant—it never spontaneously decreases.

A gas undergoes a polytropic process where PV^n = constant with n = 1.3. If n = γ = 1.4, the process would be:

Answer: Isentropic

When n = γ, the polytropic process is isentropic (reversible adiabatic), with no entropy change.

For a pure substance on a P-v diagram, the region under the saturation dome represents:

Answer: A two-phase liquid-vapor mixture

The two-phase (liquid-vapor) region lies under the saturation dome on a P-v diagram, bounded by the saturated liquid and saturated vapor curves.

The exergy (availability) of a closed system is defined relative to:

Answer: The dead state (environment conditions)

Exergy is the maximum useful work obtainable as a system reaches equilibrium with the dead state (T0, P0).

An air-standard Diesel cycle has a compression ratio of 18 and a cutoff ratio of 2. With γ = 1.4, what is the thermal efficiency?

Answer: 63.1%

η_Diesel = 1 − [r_c^γ − 1] / [γ·r·(r_c − 1)] = 1 − [2^1.4−1]/(1.4×18^0.4×(2−1)) = 1 − 1.639/(1.4×3.178) = 1 − 0.369 = 63.1%.

In a vapor-compression refrigeration cycle, the condenser rejects heat at 40°C and the evaporator absorbs heat at −10°C. What is the maximum possible COP?

Answer: 5.26

COP_max = T_L/(T_H−T_L) = 263/(313−263) = 263/50 = 5.26.