NCEES Heat Transfer 2

Question 1

Thermal diffusivity α is defined as which of the following combinations of material properties?

Accepted Answer

k/(ρcp)

Answer

Thermal diffusivity α = k/(ρcp) represents how quickly a material conducts heat relative to its ability to store it.

Question 2

For a hollow cylinder with inner radius r₁, outer radius r₂, length L, and thermal conductivity k, the conductive thermal resistance is:

Accepted Answer

ln(r₂/r₁) / (2πkL)

Answer

Integrating Fourier's Law in cylindrical coordinates gives R_cyl = ln(r₂/r₁)/(2πkL).

Question 3

The Log Mean Temperature Difference (LMTD) method is primarily used to analyze:

Accepted Answer

Heat exchanger performance

Answer

LMTD provides the effective mean driving temperature difference for heat exchangers where temperatures vary along the length.

Question 4

In a counterflow heat exchanger compared to a parallel-flow arrangement with the same inlet conditions and UA product, which statement is correct?

Accepted Answer

Counterflow achieves higher LMTD and greater heat transfer

Answer

Counterflow maintains a more uniform temperature difference along its length, yielding a higher LMTD and therefore greater heat transfer capacity.

Question 5

Fin efficiency (η_f) is defined as:

Accepted Answer

Actual fin heat transfer divided by maximum heat transfer if entire fin were at base temperature

Answer

Fin efficiency η_f = q_actual / q_max, where q_max = hA_f(T_b − T∞) assumes the entire fin surface is at the base temperature.

NCEES Practice Test