Exam Questions Flashcards
What’s the difference between fin efficiency and fin effectiveness? (3 marks)
The fin efficiency is the ratio between the actual heat transfer from the fin and the ideal heat transfer from the fin if the entire fin were at its base temperature.
The fin effectiveness is the ratio between the heat transfer rate from the fin base of area A_b and the heat transfer rate from the surface of area A_b
State Wien’s displacement law and explain its physical significance. (2 marks)
The wavelength at which the peak occurs for a specified temperature is given by:
(λT)_maxpower = 2897.8μmK
Their product between this wavelength and the corresponding temperature is a constant
Give the mathematical formula for the surface resistance due to radiation and explain its physical meaning (2 marks)
R = (1-ε)/Aε
This is the resistance due to the fact that the surface is not a blackbody, but instead gray and so the maximum possible emissive power is not emitted. The emissivity is smaller than 1 in this case
When the linear system resulting from the finite-difference discretization of the heat transfer conduction equation is of small order, which is more likely to be used, a direct or iterative method? Clearly explain why. (2 marks)
Direct method. The resulting matrix is small, so it can easily be inverted. There is no need to iterate (like Gauss-Siedell) to find the solution
Define Kirchhoff’s Law and explain the diagram that shows this (2 marks)
The emissivity and the absorptivity of a surface at a given temperature and wavelength are equal.
Small body inside an enclosure. Small body has A_s, ε, and α inside. Squiggly arrow pointing to small body labelled G, and squiggly arrow coming out of the small body labelled E_emit. T pointing to small body, T pointing to inside surface of enclosure
What’s the mathematical relationship that links absorptivity, reflectivity, and transmissivity? (2 marks)
α + ρ + τ = 1
Discuss the two main methods to study heat exchangers: the log-mean temperature
method and the effectiveness-NTU method. (2 marks)
LMTD: The four temperatures (in/out for the cold and hot flows) and the area are given. The heat transfer can be computed.
Effectiveness-NTU: The heat transfer can be computed without knowing the outlet temperatures
Discuss what the fouling factor, R_f , represents and why it is often used in the study of heat exchangers. (2 marks)
The fouling factor R_f represents the additional resistance to heat transfer as a result of the layer of deposits and accumulation of deposits on the internal surfaces of the heat exchanger.
Was Planck more impressed by Clausius’ or by Kirchhoff’s style of teaching? (3 marks)
More Clausius
Who discovered a simple mathematical relationship that described the effect of a change in temperature on the distribution of blackbody radiation? (4 marks)
Wien
Explain the physical meaning of the summation rule (2 marks)
conservation of energy. Radiation leaving any surface i of an enclosure must be received completely by the other surfaces
What is the physical significance of the Biot number? Justify your answer. (2 marks)
The Biot number represents the ratio of conduction resistance within the body to convection resistance at the surface of the body
Is the Biot number likely to be larger for highly conducting solids or poorly conducting ones? Justify your answer. (2 marks)
The Biot number is more likely to be larger for poorly conducting solids since such bodies have larger resistances against heat conduction
What is the definition and the physical significance of the Prandtl number? Carefully explain the physical meaning of all the quantities you use in the definition of Prandtl number. (2 marks)
The Prandtl number is defined as the ratio between the kinematic viscosity and the thermal diffusivity. It expresses the ratio of molecular diffusion with respect to the thermal diffusion
Define emissivity and explain its physical significance. (2 marks)
Emissivity is the ratio of the radiation emitted by a surface to the radiation emitted by a blackbody at the same temperature