BME Heat Transfer 2 — Questions and Answers
Question 1: The Prandtl number is defined as the ratio of:
- Momentum diffusivity to thermal diffusivity (Correct answer)
- Thermal conductivity to viscosity
- Inertia to buoyancy forces
- Heat flux to temperature difference
Correct answer: Momentum diffusivity to thermal diffusivity
Prandtl number Pr = ν/α = momentum diffusivity / thermal diffusivity, indicating the relative thickness of velocity and thermal boundary layers.
Question 2: In a counter-flow heat exchanger, the maximum possible temperature change of the cold fluid is limited by:
- The hot fluid inlet temperature (Correct answer)
- The cold fluid inlet temperature
- The wall temperature
- The log mean temperature difference
Correct answer: The hot fluid inlet temperature
In counter-flow, the cold fluid can theoretically be heated up to the inlet temperature of the hot fluid, giving the highest thermodynamic efficiency.
Question 3: Stefan-Boltzmann law states that radiation heat flux is proportional to:
- T
- T²
- T³
- T⁴ (Correct answer)
Correct answer: T⁴
The Stefan-Boltzmann law states E = εσT⁴, meaning emissive power is proportional to the fourth power of absolute temperature.
Question 4: The LMTD method for heat exchanger analysis stands for:
- Linear Mean Temperature Differential
- Log Mean Temperature Difference (Correct answer)
- Logarithmic Maximum Temperature Drop
- Laminar Mean Thermal Diameter
Correct answer: Log Mean Temperature Difference
LMTD (Log Mean Temperature Difference) is the effective driving temperature difference for heat transfer in a heat exchanger.
Question 5: Which dimensionless number governs natural (free) convection?
- Reynolds number
- Grashof number (Correct answer)
- Peclet number
- Stanton number
Correct answer: Grashof number
The Grashof number Gr = gβΔTL³/ν² represents the ratio of buoyancy forces to viscous forces and governs natural convection.
Question 6: For a blackbody, the emissivity (ε) equals:
- 0
- 0.5
- 1 (Correct answer)
- Depends on temperature
Correct answer: 1
A blackbody is an ideal perfect emitter and absorber, defined by emissivity ε = 1; real surfaces have ε < 1.
BME Heat Transfer 2
The Prandtl number is defined as the ratio of: