Heat and Mass Transfer – Set 03

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Q1. Wien’s law states that the wave length corresponding to __ is proportional to the absolute temperature.

(A) Minimum energy
(B) Maximum energy
(C) Both (A) and (B)
(D) None of these

Answer: (B) Maximum energy

Q2. When heat is transferred by molecular collision, it is referred to as heat transfer by

(A) Conduction
(B) Convection
(C) Radiation
(D) Scattering

Answer: (B) Convection

Q3. Thermal conductivity of wood depends on

(A) Moisture
(B) Density
(C) Temperature
(D) All of the above

Answer: (D) All of the above

Q4. The thermal diffusivities for gases are generally

(A) More than those for liquids
(B) Less than those for liquids
(C) More than those for solids
(D) Dependent on the viscosity

Answer: (A) More than those for liquids

Q5. Thermal conductivity of solid metals with rise in temperature normally

(A) Increases
(B) Decreases
(C) Remain constant
(D) May increase or decrease depending on temperature

Answer: (B) Decreases

Q6. Heat is closely related with

(A) Liquids
(B) Energy
(C) Temperature
(D) Entropy

Answer: (C) Temperature

Q7. The time constant of a thermocouple is

(A) The time taken to attain the final temperature to be measured
(B) The time taken to attain 50% of the value of initial temperature difference
(C) The time taken to attain 63.2% of the value of initial temperature difference
(D) Determined by the time taken to reach 100°C from 0°C

Answer:(C) The time taken to attain 63.2% of the value of initial temperature difference

Q8. Which of the following has maximum value of thermal conductivity?

(A) Aluminium
(B) Steel
(C) Brass
(D) Copper

Answer: (A) Aluminium

Q9. Heat conducted through per unit area and unit thick face per unit time when temperature difference between opposite faces is unity, is called

(A) Thermal resistance
(B) Thermal coefficient
(C) Temperature gradient
(D) Thermal conductivity

Answer: (D) Thermal conductivity

Q10. Fourier’s law of heat conduction is valid for

(A) One dimensional cases only
(B) Two dimensional cases only
(C) Three dimensional cases only
(D) Regular surfaces having non-uniform temperature gradients

Answer: (A) One dimensional cases only

Q11. The energy distribution of an ideal reflector at higher temperatures is largely in the range of

(A) Shorter wavelength
(B) Longer wavelength
(C) Remain same at all wavelengths
(D) Wavelength has nothing to do with it

Answer: (A) Shorter wavelength

Q12. The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as

(A) Kirchoff’s law
(B) Stefan’s law
(C) Wien’ law
(D) Planck’s law

Answer: (A) Kirchoff’s law

Q13. The highest thermal diffusivity is of

(A) Iron
(B) Lead
(C) Concrete
(D) Wood

Answer: (B) Lead

Q14. A designer chooses the values of fluid flow rates and specific heats in such a manner that the heat capacities of the two fluids are equal. A hot fluid enters the counter flow heat exchanger at 100°C and leaves at 60°C. A cold fluid enters the heat exchanger at 40°C. The mean temperature difference between the two fluids is

(A) 20°C
(B) 40°C
(C) 60°C
(D) 66.7°C

Answer: (A) 20°C

Q15. The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body

(A) Move actually
(B) Do not move actually
(C) Affect the intervening medium
(D) Does not affect the intervening medium

Answer: (B) Do not move actually

Q16. In free convection heat transfer transition from laminar to turbulent flow is governed by the critical value of the

(A) Reynold’s number
(B) Grashoff’s number
(C) Reynold’s number, Grashoff’s number
(D) Prandtl number, Grashoff’s number

Answer: (D) Prandtl number, Grashoff’s number

Q17. The thickness of thermal and hydrodynamic boundary layer is equal if Prandtl number is

(A) Equal to one
(B) Greater than one
(C) Less than one
(D) Equal to Nusselt number

Answer: (A) Equal to one

Q18. The natural convection air cooled condensers are used in

(A) Domestic refrigerators
(B) Water coolers
(C) Room air conditioners
(D) All of these

Answer: (A) Domestic refrigerators

Q19. The logarithmic mean temperature difference ™ is given by (where Δt1 and Δt2 are temperature differences between the hot and cold fluids at entrance and exit)

(A) tm = (Δt1 – Δt2)/ loge (Δt1/Δt2)
(B) tm = loge (Δt1/Δt2)/ (Δt1 – Δt2)
(C) tm = tm = (Δt1 – Δt2) loge (Δt1/Δt2)
(D) tm = loge (Δt1 – Δt2)/ Δt1/Δt2

Answer: (A) tm = (Δt1 – Δt2)/ loge (Δt1/Δt2)

Q20. The value of the wave length for maximum emissive power is given by

(A) Kirchhoff’s law
(B) Stefan’s law
(C) Wine’s law
(D) Planck’s law

Answer: (C) Wine’s law

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Heat and Mass Transfer – Set 01
Heat and Mass Transfer – Set 02
Heat and Mass Transfer – Set 03
Heat and Mass Transfer – Set 04
Heat and Mass Transfer – Set 05
Heat and Mass Transfer – Set 06
Heat and Mass Transfer – Set 07
Heat and Mass Transfer – Set 08