NCEES Heat Transfer 1 — Questions and Answers
Question 1: According to Fourier's Law of heat conduction, the heat flux q" is proportional to which of the following?
- Positive temperature gradient
- Negative temperature gradient (Correct answer)
- Thermal diffusivity only
- Convection coefficient
Correct answer: Negative temperature gradient
Fourier's Law states q" = -k(dT/dx); the negative sign shows heat flows in the direction of decreasing temperature.
Question 2: A flat wall has thermal conductivity k = 2 W/(m·K), area A = 5 m², thickness L = 0.2 m, and surface temperatures of 100°C and 20°C. What is the conductive heat transfer rate?
- 2 kW
- 4 kW (Correct answer)
- 8 kW
- 16 kW
Correct answer: 4 kW
Using q = kA(ΔT/L) = 2 × 5 × 80/0.2 = 4,000 W = 4 kW.
Question 3: The thermal resistance for convection from a surface with area A and convection coefficient h is expressed as:
- L/(kA)
- 1/(hA) (Correct answer)
- hA
- k/(LA)
Correct answer: 1/(hA)
The convective thermal resistance is R_conv = 1/(hA), analogous to Ohm's law with Q = ΔT/R.
Question 4: Which dimensionless number represents the ratio of convective heat transfer to conductive heat transfer at a surface?
- Reynolds number
- Prandtl number
- Nusselt number (Correct answer)
- Biot number
Correct answer: Nusselt number
The Nusselt number Nu = hL/k quantifies convective enhancement over pure conduction across the same fluid layer.
Question 5: For a composite plane wall with three layers in series, the total thermal resistance is:
- Product of individual resistances
- Sum of individual resistances (Correct answer)
- Harmonic mean of individual resistances
- Geometric mean of individual resistances
Correct answer: Sum of individual resistances
Thermal resistances in series add directly: R_total = R₁ + R₂ + R₃, analogous to electrical resistors in series.
Question 6: The Stefan-Boltzmann Law for a blackbody states that emissive power is proportional to which power of absolute temperature?
- Second power (T²)
- Third power (T³)
- Fourth power (T⁴) (Correct answer)
- First power (T)
Correct answer: Fourth power (T⁴)
The Stefan-Boltzmann Law gives E_b = σT⁴, where σ = 5.67 × 10⁻⁸ W/(m²·K⁴).
Question 7: The Biot number (Bi = hLc/k) less than 0.1 indicates that which simplifying assumption is valid?
- Quasi-steady conduction
- Lumped capacitance method (Correct answer)
- Blackbody radiation dominance
- Turbulent natural convection
Correct answer: Lumped capacitance method
When Bi < 0.1, spatial temperature gradients within the solid are negligible and the lumped capacitance (lumped system) method applies.
NCEES Heat Transfer 1
According to Fourier's Law of heat conduction, the heat flux q" is proportional to which of the following?