BME Thermodynamics 4 — Questions and Answers
Question 1: The Diesel cycle differs from the Otto cycle in that heat addition occurs at constant:
- Volume
- Temperature
- Pressure (Correct answer)
- Entropy
Correct answer: Pressure
In the Diesel cycle, fuel injection and combustion occur at constant pressure, unlike the Otto cycle where heat is added at constant volume.
Question 2: The Gibbs phase rule (F = C - P + 2) for a single-component system at a point on the saturation curve gives how many degrees of freedom?
- 0
- 1 (Correct answer)
- 2
- 3
Correct answer: 1
For a single component (C=1) with two phases (P=2): F = 1 - 2 + 2 = 1, meaning only one intensive variable can be specified independently.
Question 3: For an ideal gas mixture, Dalton's Law states that the total pressure equals:
- The pressure of the most abundant component
- The average of all component pressures
- The sum of the partial pressures of all components (Correct answer)
- The product of all component pressures
Correct answer: The sum of the partial pressures of all components
Dalton's Law states that the total pressure of an ideal gas mixture equals the sum of the partial pressures each gas would exert alone.
Question 4: The effectiveness of a heat exchanger is defined as the ratio of:
- Outlet to inlet temperature difference
- Actual heat transfer to maximum possible heat transfer (Correct answer)
- Hot fluid heat loss to cold fluid heat gain
- Overall heat transfer coefficient to surface area
Correct answer: Actual heat transfer to maximum possible heat transfer
Effectiveness ε = Q_actual / Q_max, where Q_max is the maximum heat transfer achievable with an infinitely long counterflow exchanger.
Question 5: The Helmholtz free energy A is defined as:
- A = H - TS
- A = U + PV
- A = U - TS (Correct answer)
- A = H + TS
Correct answer: A = U - TS
Helmholtz free energy A = U - TS, and it represents the maximum work obtainable from a closed system at constant temperature and volume.
Question 6: Superheating steam in the Rankine cycle primarily serves to:
- Reduce pump work
- Increase condenser pressure
- Increase turbine work output and reduce moisture content (Correct answer)
- Decrease boiler pressure
Correct answer: Increase turbine work output and reduce moisture content
Superheating raises steam temperature and enthalpy before the turbine, increasing work output while keeping steam quality high to protect turbine blades.
Question 7: The Carnot COP of a refrigerator operating between TL = 250 K and TH = 300 K is:
- 1.2
- 5.0 (Correct answer)
- 0.83
- 6.0
Correct answer: 5.0
COP_Carnot = TL / (TH - TL) = 250 / (300 - 250) = 250/50 = 5.0.
BME Thermodynamics 4
The Diesel cycle differs from the Otto cycle in that heat addition occurs at constant: