NCEES Heat Transfer 3

Question 1

For the lumped capacitance method, the temperature response of a body cooling in a fluid follows which mathematical form?

Accepted Answer

Exponential decay with time constant τ = ρVcp/(hAs)

Answer

The lumped capacitance solution is (T−T∞)/(Ti−T∞) = exp(−t/τ), an exponential decay with time constant τ = ρVcp/(hAs).

Question 2

A blackbody at 1000 K emits thermal radiation at what power per unit area? (σ = 5.67 × 10⁻⁸ W/m²·K⁴)

Accepted Answer

56,700 W/m²

Answer

E_b = σT⁴ = 5.67×10⁻⁸ × (1000)⁴ = 5.67×10⁻⁸ × 10¹² = 56,700 W/m².

Question 3

In the NTU-effectiveness (ε-NTU) method for heat exchanger analysis, effectiveness ε is defined as:

Accepted Answer

Actual heat transfer rate divided by maximum possible heat transfer rate

Answer

Effectiveness ε = q_actual/q_max, where q_max = Cmin(Th,i − Tc,i) is the thermodynamic maximum.

Question 4

For fully developed laminar flow inside a circular tube with a constant heat flux boundary condition, the Nusselt number is approximately:

Accepted Answer

4.36

Answer

Nu = 4.36 for fully developed laminar flow in a circular tube with uniform (constant) heat flux; Nu = 3.66 applies for constant wall temperature.

Question 5

Which dimensionless number characterizes the ratio of buoyancy forces to viscous forces and is the key parameter in natural convection analysis?

Accepted Answer

Grashof number (Gr = gβΔTL³/ν²)

Answer

The Grashof number Gr = gβΔTL³/ν² represents the ratio of buoyancy to viscous forces driving natural convection.

NCEES Practice Test