NCEES Thermodynamics 5 — Questions and Answers
Question 1: The Joule-Thomson coefficient μ_JT is defined as (∂T/∂P)_h. For an ideal gas, μ_JT equals:
- 1
- cv/cp
- 0 (Correct answer)
- R/cp
Correct answer: 0
For an ideal gas, enthalpy depends only on temperature, so throttling (constant h) causes no temperature change; thus μ_JT = 0.
Question 2: Which statement about entropy is CORRECT?
- Entropy is a path function that depends on process details
- Entropy change for a reversible adiabatic process is zero (Correct answer)
- Entropy always increases in any process
- Entropy can be destroyed in irreversible processes
Correct answer: Entropy change for a reversible adiabatic process is zero
A reversible adiabatic (isentropic) process has ΔS = 0 because δQ = 0 and there is no irreversibility.
Question 3: For a steady-flow device, neglecting kinetic and potential energy changes, the energy balance simplifies to:
- Q − W = m(u2 − u1)
- Q̇ − Ẇ = ṁ(h2 − h1) (Correct answer)
- Q − W = ΔU + ΔKE + ΔPE
- Q̇ = ṁ·cv·(T2 − T1)
Correct answer: Q̇ − Ẇ = ṁ(h2 − h1)
The steady-flow energy equation reduces to Q̇ − Ẇ = ṁ(h2 − h1) when kinetic and potential energy terms are negligible.
Question 4: The reduced pressure and reduced temperature used in the generalized compressibility chart are defined as:
- PR = P − Pc and TR = T − Tc
- PR = P/Pc and TR = T/Tc (Correct answer)
- PR = Pc/P and TR = Tc/T
- PR = P·Vc/(R·Tc) and TR = T/Tc
Correct answer: PR = P/Pc and TR = T/Tc
Reduced properties are dimensionless ratios: PR = P/Pc and TR = T/Tc, where Pc and Tc are critical point values.
Question 5: A heat pump has a COP of 4.5 and consumes 2 kW of power. How much heat does it deliver to the heated space?
- 2.25 kW
- 9.0 kW (Correct answer)
- 6.5 kW
- 4.5 kW
Correct answer: 9.0 kW
COP_HP = Q̇_H / Ẇ → Q̇_H = COP × Ẇ = 4.5 × 2 kW = 9.0 kW.
Question 6: In a Rankine cycle, increasing the boiler pressure (while keeping condenser pressure constant) generally:
- Decreases both efficiency and moisture at turbine exit
- Increases efficiency but increases moisture content at turbine exit (Correct answer)
- Has no effect on cycle efficiency
- Decreases efficiency while decreasing moisture content
Correct answer: Increases efficiency but increases moisture content at turbine exit
Higher boiler pressure increases cycle efficiency (higher average heat addition temperature) but also increases the moisture fraction in the low-pressure turbine stages.
Question 7: The specific heat ratio γ = cp/cv for a monatomic ideal gas is:
- 1.2
- 1.67 (Correct answer)
- 1.4
- 1.3
Correct answer: 1.67
A monatomic ideal gas has cv = (3/2)R and cp = (5/2)R, giving γ = cp/cv = 5/3 ≈ 1.67.
NCEES Thermodynamics 5
The Joule-Thomson coefficient μ_JT is defined as (∂T/∂P)_h.
For an ideal gas, μ_JT equals: