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

ρcp/k

Answer

k·ρ·cp

Answer

cp/(ρk)

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

(r₂ − r₁) / (2πkL)

Answer

ln(r₂/r₁) / (πkL)

Answer

2πkL / ln(r₂/r₁)

Question 3

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

Accepted Answer

Heat exchanger performance

Answer

Transient heat conduction in solids

Answer

Radiation heat transfer between surfaces

Answer

Natural convection from vertical plates

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 always has lower effectiveness

Answer

Both configurations yield identical heat transfer rates

Answer

Parallel flow allows the cold fluid to exceed the hot fluid outlet temperature

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

Ratio of fin surface area to fin base area

Answer

Fin heat transfer divided by total bare-wall heat transfer

Answer

Ratio of fin length to fin thickness

Question 6

For radiation exchange between two large parallel plates with emissivities ε₁ and ε₂, the net heat flux is given by:

Accepted Answer

q" = σ(T₁⁴ − T₂⁴) / (1/ε₁ + 1/ε₂ − 1)

Answer

q" = σ(T₁⁴ − T₂⁴)·ε₁·ε₂

Answer

q" = σ(T₁² − T₂²)·(ε₁ + ε₂)

Answer

q" = ε₁·ε₂·σ(T₁ − T₂)⁴

Question 7

The Prandtl number (Pr) for a fluid is defined as the ratio of:

Accepted Answer

Momentum diffusivity to thermal diffusivity (ν/α)

Answer

Thermal diffusivity to momentum diffusivity (α/ν)

Answer

Buoyancy force to viscous force

Answer

Convective to conductive heat transfer