BME Thermodynamics 5 — Questions and Answers
Question 1: Entropy generation during an irreversible process is always:
- Negative
- Zero
- Positive (Correct answer)
- Equal to heat transfer divided by temperature
Correct answer: Positive
The Second Law requires that entropy generation S_gen > 0 for any irreversible process; it is zero only for reversible processes.
Question 2: In a regenerative Rankine cycle, the regenerator (feedwater heater) is used to:
- Cool the steam before the turbine
- Preheat feedwater using extracted steam, improving efficiency (Correct answer)
- Superheat steam after the boiler
- Reduce condenser size
Correct answer: Preheat feedwater using extracted steam, improving efficiency
Extracted steam from the turbine preheats feedwater before the boiler, reducing the heat input needed and increasing overall cycle efficiency.
Question 3: The Clapeyron equation relates the slope of the saturation curve (dP/dT) to:
- Only the latent heat of vaporization
- Specific volume change and temperature only
- Latent heat, temperature, and specific volume change between phases (Correct answer)
- Only pressure and temperature
Correct answer: Latent heat, temperature, and specific volume change between phases
The Clapeyron equation dP/dT = h_fg / (T·v_fg) relates the saturation pressure slope to enthalpy of phase change, temperature, and volume change.
Question 4: Which dimensionless number represents the ratio of inertial forces to viscous forces in fluid flow, and is often relevant to heat transfer analysis?
- Prandtl number
- Nusselt number
- Reynolds number (Correct answer)
- Grashof number
Correct answer: Reynolds number
The Reynolds number Re = ρVL/μ represents the ratio of inertial to viscous forces and determines whether flow is laminar or turbulent.
Question 5: For a reversible adiabatic compression of an ideal gas from V1 to V2, the work done ON the gas is:
- W = P(V2 - V1)
- W = nCv(T2 - T1) (Correct answer)
- W = nCp(T2 - T1)
- W = nRT ln(V2/V1)
Correct answer: W = nCv(T2 - T1)
For a reversible adiabatic process, Q = 0, so by the first law, W_in = ΔU = nCv(T2 - T1).
Question 6: The Second Law of Thermodynamics, in the context of engines, states that it is impossible to construct a device that:
- Converts heat to work with any efficiency
- Operates in a cycle and produces work while exchanging heat with only one reservoir (Correct answer)
- Converts work to heat
- Operates between two temperature reservoirs
Correct answer: Operates in a cycle and produces work while exchanging heat with only one reservoir
The Kelvin-Planck statement says no heat engine can operate in a cycle while exchanging heat with a single thermal reservoir and producing net work.
Question 7: Van der Waals equation of state corrects the ideal gas law by accounting for:
- Quantum effects and radiation
- Molecular volume and intermolecular attractive forces (Correct answer)
- Chemical reactions between molecules
- Gravitational effects on gas molecules
Correct answer: Molecular volume and intermolecular attractive forces
Van der Waals equation (P + a/v²)(v - b) = RT corrects for intermolecular attractions (term a/v²) and finite molecular volume (term b).
BME Thermodynamics 5
Entropy generation during an irreversible process is always: