BME Thermodynamics 3 — Questions and Answers
Question 1: For a steam power plant operating as a Rankine cycle, the purpose of the feedwater pump is to:
- Heat the feedwater
- Condense steam back to liquid
- Increase pressure of the liquid condensate (Correct answer)
- Superheat the steam
Correct answer: Increase pressure of the liquid condensate
The feedwater pump increases the pressure of the liquid water from condenser pressure to boiler pressure, requiring relatively little work.
Question 2: Maxwell's relations in thermodynamics are derived from which thermodynamic potentials?
- Internal energy only
- Gibbs free energy only
- Internal energy, enthalpy, Helmholtz, and Gibbs functions (Correct answer)
- Enthalpy and entropy only
Correct answer: Internal energy, enthalpy, Helmholtz, and Gibbs functions
Maxwell's relations are derived from the exactness of the differentials of the four thermodynamic potentials: U, H, A, and G.
Question 3: In a refrigeration system using the vapor-compression cycle, the throttling device (expansion valve) reduces pressure by:
- Doing work on the refrigerant
- An isentropic expansion
- An irreversible isenthalpic process (Correct answer)
- Absorbing heat from surroundings
Correct answer: An irreversible isenthalpic process
The expansion valve is an isenthalpic (constant enthalpy) throttling device where pressure drops irreversibly without work or heat transfer.
Question 4: The triple point of water is approximately:
- 0°C and 1 atm
- 100°C and 1 atm
- 0.01°C and 611.7 Pa (Correct answer)
- 4°C and 1 atm
Correct answer: 0.01°C and 611.7 Pa
The triple point of water occurs at approximately 0.01°C (273.16 K) and 611.7 Pa, where all three phases coexist.
Question 5: For an Otto cycle (ideal spark-ignition engine), thermal efficiency depends on:
- Compression ratio and heat added
- Only the working fluid properties
- Compression ratio and specific heat ratio γ (Correct answer)
- Only the compression ratio
Correct answer: Compression ratio and specific heat ratio γ
Otto cycle efficiency η = 1 - (1/r^(γ-1)), where r is the compression ratio and γ is the specific heat ratio.
Question 6: Which property is NOT a state function in thermodynamics?
- Internal energy
- Entropy
- Heat transfer (Correct answer)
- Enthalpy
Correct answer: Heat transfer
Heat transfer (Q) is a path function that depends on the process path, not just initial and final states.
Question 7: In a real gas, the compressibility factor Z = PV/nRT is less than 1 when:
- Intermolecular repulsion dominates
- Temperature is very high
- Intermolecular attraction dominates (Correct answer)
- The gas behaves ideally
Correct answer: Intermolecular attraction dominates
When attractive forces between molecules dominate, Z < 1 because the actual volume is less than the ideal gas prediction.
BME Thermodynamics 3
For a steam power plant operating as a Rankine cycle, the purpose of the feedwater pump is to: