BME Heat Transfer 1 — Questions and Answers
Question 1: Fourier's law of heat conduction states that heat flux is proportional to:
- Temperature
- Temperature gradient (Correct answer)
- Thermal diffusivity
- Specific heat
Correct answer: Temperature gradient
Fourier's law states q = -k(dT/dx), meaning heat flux is directly proportional to the negative temperature gradient.
Question 2: Which mode of heat transfer does not require a medium?
- Conduction
- Convection
- Radiation (Correct answer)
- Diffusion
Correct answer: Radiation
Radiation transfers heat via electromagnetic waves and can occur through a vacuum, unlike conduction and convection which require matter.
Question 3: The Biot number (Bi) in transient heat conduction represents the ratio of:
- Convective to conductive resistance (Correct answer)
- Radiation to convection
- Thermal to viscous forces
- Inertia to viscous forces
Correct answer: Convective to conductive resistance
The Biot number Bi = hL/k is the ratio of convective heat transfer resistance at the surface to conductive resistance within the body.
Question 4: In a fin, the efficiency is defined as the ratio of actual heat transfer to:
- Heat transferred by the base without fin
- Maximum possible heat transfer if the entire fin were at base temperature (Correct answer)
- Heat transferred by radiation
- Heat stored in the fin
Correct answer: Maximum possible heat transfer if the entire fin were at base temperature
Fin efficiency compares actual heat transfer to the ideal case where the whole fin surface is at the base (maximum) temperature.
Question 5: What is the thermal resistance for conduction through a flat wall?
- kA/L
- L/(kA) (Correct answer)
- hA
- 1/(hA)
Correct answer: L/(kA)
For a flat wall, thermal resistance R = L/(kA), analogous to electrical resistance, where L is thickness, k is conductivity, and A is area.
Question 6: The Nusselt number (Nu) in convective heat transfer is defined as:
- hL/k (Correct answer)
- VL/ν
- ν/α
- gβΔTL³/ν²
Correct answer: hL/k
The Nusselt number Nu = hL/k is the dimensionless convective heat transfer coefficient, representing the ratio of convective to conductive heat transfer.
BME Heat Transfer 1
Fourier's law of heat conduction states that heat flux is proportional to: