NCEES Thermodynamics 3 — Questions and Answers
Question 1: 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?
- 874 kJ/kg (Correct answer)
- 674 kJ/kg
- 974 kJ/kg
- 574 kJ/kg
Correct answer: 874 kJ/kg
For an adiabatic turbine, w_turbine = h1 − h2 = 3214 − 2340 = 874 kJ/kg.
Question 2: Which statement correctly describes the second law of thermodynamics?
- Energy is conserved in all thermodynamic processes
- Heat can spontaneously flow from cold to hot bodies
- The entropy of an isolated system can never decrease (Correct answer)
- All reversible engines have the same efficiency regardless of working fluid
Correct 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.
Question 3: A gas undergoes a polytropic process where PV^n = constant with n = 1.3. If n = γ = 1.4, the process would be:
- Isothermal
- Isobaric
- Isentropic (Correct answer)
- Isochoric
Correct answer: Isentropic
When n = γ, the polytropic process is isentropic (reversible adiabatic), with no entropy change.
Question 4: For a pure substance on a P-v diagram, the region under the saturation dome represents:
- Superheated vapor only
- Compressed liquid only
- A two-phase liquid-vapor mixture (Correct answer)
- Supercritical fluid
Correct 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.
Question 5: The exergy (availability) of a closed system is defined relative to:
- The critical state of the working fluid
- The dead state (environment conditions) (Correct answer)
- The isentropic reference state
- The saturated liquid state at system pressure
Correct answer: The dead state (environment conditions)
Exergy is the maximum useful work obtainable as a system reaches equilibrium with the dead state (T0, P0).
Question 6: 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?
- 63.1% (Correct answer)
- 71.2%
- 58.9%
- 66.4%
Correct 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%.
Question 7: 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?
- 4.26
- 5.26 (Correct answer)
- 3.26
- 6.26
Correct answer: 5.26
COP_max = T_L/(T_H−T_L) = 263/(313−263) = 263/50 = 5.26.
NCEES Thermodynamics 3
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?