ME or MEng Master of Engineering Thermodynamics and Heat Transfer 2 — Questions and Answers
Question 1: The effectiveness-NTU method in heat exchanger analysis defines effectiveness as:
- Ratio of actual heat transfer to maximum possible heat transfer (Correct answer)
- Ratio of outlet to inlet temperature
- Number of transfer units divided by flow rate
- Ratio of hot-side to cold-side heat capacity rates
Correct answer: Ratio of actual heat transfer to maximum possible heat transfer
Effectiveness ε = Q_actual / Q_max, where Q_max is the heat transferred if one fluid underwent the maximum possible temperature change.
Question 2: In the Rankine cycle used in steam power plants, which component increases the pressure of the working fluid?
- Turbine
- Condenser
- Boiler
- Pump (Correct answer)
Correct answer: Pump
The pump raises the pressure of the condensate from condenser pressure to boiler pressure, requiring work input.
Question 3: Stefan-Boltzmann Law states that thermal radiation emitted by a blackbody is proportional to:
- T²
- T³
- T⁴ (Correct answer)
- T^0.5
Correct answer: T⁴
A blackbody emits radiation q = σT⁴, where σ is the Stefan-Boltzmann constant (5.67 × 10⁻⁸ W/m²K⁴).
Question 4: Superheat in a Rankine cycle is added primarily to:
- Reduce pump work
- Increase condenser pressure
- Increase turbine efficiency and reduce moisture content (Correct answer)
- Lower boiler operating temperature
Correct answer: Increase turbine efficiency and reduce moisture content
Superheating raises the turbine inlet temperature, increasing cycle efficiency and reducing moisture that erodes turbine blades.
Question 5: The Biot number in heat transfer is defined as:
- hL/k, ratio of convective to conductive resistance (Correct answer)
- k/(ρcL), thermal diffusivity parameter
- ρVL/μ, inertial to viscous ratio
- hL/α, convection-diffusivity ratio
Correct answer: hL/k, ratio of convective to conductive resistance
Bi = hL_c/k compares internal conductive resistance (L_c/k) to external convective resistance (1/h) for a solid body.
Question 6: In a refrigeration cycle, the Coefficient of Performance (COP) is defined as:
- Work input divided by heat removed from the cold space
- Heat removed from the cold space divided by work input (Correct answer)
- Heat rejected to the hot reservoir divided by work input
- Work output divided by heat absorbed
Correct answer: Heat removed from the cold space divided by work input
COP_refrigerator = Q_L / W_net, measuring how much heat is removed per unit of work input.
ME or MEng Master of Engineering Thermodynamics and Heat Transfer 2
The effectiveness-NTU method in heat exchanger analysis defines effectiveness as: