Heat Transfer Flashcards

According to Fourier's Law of heat conduction, the heat flux q" is proportional to which of the following?

Answer: Negative temperature gradient

Fourier's Law states q" = -k(dT/dx); the negative sign shows heat flows in the direction of decreasing temperature.

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?

Answer: 4 kW

Using q = kA(ΔT/L) = 2 × 5 × 80/0.2 = 4,000 W = 4 kW.

The thermal resistance for convection from a surface with area A and convection coefficient h is expressed as:

Answer: 1/(hA)

The convective thermal resistance is R_conv = 1/(hA), analogous to Ohm's law with Q = ΔT/R.

Which dimensionless number represents the ratio of convective heat transfer to conductive heat transfer at a surface?

Answer: Nusselt number

The Nusselt number Nu = hL/k quantifies convective enhancement over pure conduction across the same fluid layer.

For a composite plane wall with three layers in series, the total thermal resistance is:

Answer: Sum of individual resistances

Thermal resistances in series add directly: R_total = R₁ + R₂ + R₃, analogous to electrical resistors in series.

The Stefan-Boltzmann Law for a blackbody states that emissive power is proportional to which power of absolute temperature?

Answer: Fourth power (T⁴)

The Stefan-Boltzmann Law gives E_b = σT⁴, where σ = 5.67 × 10⁻⁸ W/(m²·K⁴).

The Biot number (Bi = hLc/k) less than 0.1 indicates that which simplifying assumption is valid?

Answer: Lumped capacitance method

When Bi < 0.1, spatial temperature gradients within the solid are negligible and the lumped capacitance (lumped system) method applies.