Heat and Mass Transfer – Set 08

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Q1. Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be

(A) One
(B) Two
(C) Three
(D) Four

Answer: (C) Three


Q2. Fourier’s law of heat conduction gives the heat flow for

(A) Irregular surfaces
(B) Nonuniform temperature surfaces
(C) One dimensional cases only
(D) Two dimensional cases only

Answer: (C) One dimensional cases only


Q3. Two plates spaced 150 mm apart are maintained at 1000°C and 70°C. The heat transfer will take place mainly by

(A) Convection
(B) Radiation
(C) Forced convection
(D) Free convection

Answer: (B) Radiation


Q4. Reynolds number (RN) is given by (where h = Film coefficient, l = Linear dimension, V = Velocity of fluid, k = Thermal conductivity, t = Temperature, ρ = Density of fluid, cp = Specific heat at constant pressure, and μ = Coefficient of absolute viscosity)

(A) RN = hl/k
(B) RN = μ cp/k
(C) RN = ρ V l /μ
(D) RN = V²/t.cp

Answer: (C) RN = ρ V l /μ


Q5. The ratio of the thickness of thermal boundary layer to the thickness of hydrodynamic boundary layer is equal to (Prandtl number) n, where n is equal to

(A) -1/3
(B) -2/3
(C) 1
(D) -1

Answer:(A) -1/3


Q6. In a heat exchanger with one fluid evaporating or condensing, the surface area required is least in

(A) Parallel flow
(B) Counter flow
(C) Cross flow
(D) All of these

Answer: (D) All of these


Q7. In counter current flow heat exchanger, the logarithmic temperature difference between the fluids is __ as compared to parallel flow heat exchanger.

(A) Same
(B) Less
(C) Greater
(D) None of these

Answer: (C) Greater


Q8. The critical temperature is the temperature

(A) Below which a gas does not obey gas laws
(B) Above which a gas may explode
(C) Below which a gas is always liquefied
(D) Above which a gas will never liquefied

Answer: (D) Above which a gas will never liquefied


Q9. 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) Wien’s law
(B) Stefan’s law
(C) Kirchhoff’s law
(D) Planck’s law

Answer: (C) Kirchhoff’s law


Q10. The unit of Stefan-Boltzmann constant is

(A) Watt/mK
(B) Watt/m²K²
(C) Watt/m²K4
(D) Watt/mK²

Answer:(C) Watt/m²K4


Q11. The process of heat transfer from one particle of the fluid to another by the actual movement of the fluid particles due to difference of density caused by temperature of the particle is known as

(A) Conduction
(B) Free convection
(C) Forced convection
(D) Radiation

Answer: (B) Free convection


Q12. Which of the following is expected to have highest thermal conductivity?

(A) Steam
(B) Solid ice
(C) Melting ice
(D) Water

Answer: (B) Solid ice


Q13. The value of Prandtl number for air is about

(A) 0.1
(B) 0.3
(C) 0.7
(D) 1.7

Answer:(C) 0.7


Q14. In regenerator type heat exchanger, heat transfer takes place by

(A) Direct mixing of hot and cold fluids
(B) A complete separation between hot and cold fluids
(C) Flow of hot and cold fluids alternately over a surface
(D) Generation of heat again and again

Answer: (C) Flow of hot and cold fluids alternately over a surface


Q15. Sensible heat is the heat required to

(A) Change vapour into liquid
(B) Change liquid into vapour
(C) Increase the temperature of a liquid of vapour
(D) Convert water into steam and superheat it

Answer: (C) Increase the temperature of a liquid of vapour


Q16. Cork is a good insulator because it has

(A) Free electrons
(B) Atoms colliding frequency
(C) Low density
(D) Porous body

Answer: (D) Porous body


Q17. The amount of heat flow through a body by conduction is

(A) Directly proportional to the surface area of the body
(B) Directly proportional to the temperature difference on the two faces of the body
(C) Dependent upon the material of the body
(D) All of the above

Answer: (D) All of the above


Q18. Thermal diffusivity is

(A) A dimensionless parameter
(B) Function of temperature
(C) Used as mathematical model
(D) A physical property of the material

Answer: (D) A physical property of the material


Q19. A non-dimensional number generally associated with natural convection heat transfer is

(A) Grashoff number
(B) Nusselt number
(C) Weber number
(D) Prandtl number

Answer: (A) Grashoff number


Q20. The unit of overall coefficient of heat transfer is

(A) kcal/m²
(B) kcal/hr °C
(C) kcal/m² hr °C
(D) kcal/m hr °C

Answer:(C) kcal/m² hr °C


Q21. In free convection heat transfer, Nusselt number is function of

(A) Grashoff number and Reynold number
(B) Grashoff number and Prandtl number
(C) Prandtl number and Reynold number
(D) Grashoff number, Prandtl number and Reynold number

Answer: (B) Grashoff number and Prandtl number


Q22. A grey body is one whose absorptivity

(A) Varies with temperature
(B) Varies with the wave length of incident ray
(C) Varies with both
(D) Does not vary with temperature and wave length of the incident ray

Answer: (D) Does not vary with temperature and wave length of the incident ray


Q23. The ratio of Nusselt number and the product of Reynold’s number and Prandtl number is equal to

(A) Stanton number
(B) Biot number
(C) Peclet number
(D) Grashoff number

Answer: (A) Stanton number


Q24. The product of Reynolds number and Prandtl number is known as

(A) Stanton number
(B) Biot number
(C) Peclet number
(D) Grashoff number

Answer: (C) Peclet number


Q25. A perfect black body is one which

(A) Is black in colour
(B) Reflects all heat
(C) Transmits all heat radiations
(D) Absorbs heat radiations of all wave lengths falling on it

Answer: (D) Absorbs heat radiations of all wave lengths falling on it


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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