1000+ Heat & Mass Transfer MCQ for NMAT [Solved]

1. 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

2. Metals are good conductors of heat because
A. Their atoms collide frequently
B. Their atoms are relatively far apart
C. They contain free electrons
D. They have high density
Answer : A

3. The heat transfer takes place according to
A. Zeroth law of thermodynamics
B. First law of thermodynamics
C. Second law of thermodynamics
D. Kirchhoff's law
Answer : C

4. 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

5. Planck's law holds good for
A. Black bodies
B. Polished bodies
C. All coloured bodies
D. All of the above
Answer : A

6. Heat flows from one body to other when they have
A. Different heat contents
B. Different specific heat
C. Different atomic structure
D. Different temperatures
Answer : D

7. Thermal diffusivity of a substance is given by (where h = Thermal diffusivity, ? = Density of substance, S = Specific heat, and k = Thermal conductivity)
A. h = k/ ?S
B. h = ?S/k
C. h = S/?k
D. h = k?/S
Answer : A

8. Upto the critical radius of insulation,
A. Added insulation will increase heat loss
B. Added insulation will decrease heat loss
C. Convective heat loss will be less than conductive heat loss
D. Heat flux will decrease
Answer : A

9. 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

10. Two balls of same material and finish have their diameters in the ratio of 2: 1 and both are heated to same temperature and allowed to cool by radiation. Rate of cooling by big ball as compared to smaller one will be in the ratio of
A. 1 : 1
B. 2 : 1
C. 1 : 2
D. 4 : 1
Answer : C

11. Radiation is the process of heat transfer in which heat flows from a ________, in a straight line, without affecting the intervening medium.
A. Cold body to hot body
B. Hot body to cold body
C. Smaller body to larger body
D. Larger body to smaller body
Answer : B

12. 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

13. When heat is transferred from hot body to cold body, in a straight line, without affecting the intervening medium, it is referred as heat transfer by
A. Conduction
B. Convection
C. Radiation
D. Conduction and convection
Answer : C

14. Thermal conductivity of air at room temperature in kcal/m hr °C is of the order of
A. 0.002
B. 0.02
C. 0.01
D. 0.1
Answer : B

15. The process of heat transfer from one particle of the body to another by the actual motion of the heated particles, is called
A. Conduction
B. Convection
C. Radiation
D. None of these
Answer : B

16. According to Stefan Boltzmann law, ideal radiators emit radiant energy at a rate proportional to
A. Absolute temperature
B. Square of temperature
C. Fourth power of absolute temperature
D. Fourth power of temperature
Answer : C

17. A steam pipe is to be insulated by two insulating materials put over each other. For best results
A. Better insulation should be put over pipe and better one over it
B. Inferior insulation should be put over pipe and better one over it
C. Both may be put in any order
D. Whether to put inferior OIL over pipe or the better one would depend on steam temperature
Answer : A

18. Emissivity of a white polished body in comparison to a black body is
A. Higher
B. Lower
C. Same
D. Depends upon the shape of body
Answer : B

19. Fourier's law of heat conduction is (where Q = Amount of heat flow through the body in unit time, A = Surface area of heat flow, taken at right angles to the direction of heat flow, dT = Temperature difference on the two faces of the body, dx = Thickness of the body, through which the heat flows, taken along the direction of heat flow, and k = Thermal conductivity of the body)
A. k. A. (dT/dx)
B. k. A. (dx/dT)
C. k. (dT/dx)
D. k. (dx/dT)
Answer : A

20. A steam pipe is to be lined with two layers of insulating materials of different thermal conductivities. For the minimum heat transfer,
A. The better insulation must be put inside
B. The better insulation must be put outside
C. One could place either insulation on either side
D. One should take into account the steam temperature before deciding as to which insulation is put where
Answer : A

21. The insulation ability of an insulator with the presence of moisture would
A. Increase
B. Decrease
C. Remain unaffected
D. May increase/decrease depending on temperature and thickness of insulation
Answer : B

22. 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

23. When ? is absorptivity, ? is reflectivity and ? is transmissivity, then for a diathermanous body,
A. ? = 1, ? = 0 and ? = 0
B. ? = 0, ? = 1 and ? = 0
C. ? = 0, ? = 0 and ? = 1
D. ? + ? = 1 and ? = 0
Answer : D

24. The emissivity for a black body is
A. 0
B. 0.5
C. 0.75
D. 1
Answer : D

25. 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

26. 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

27. The logarithmic mean temperature difference (t_m) is given by (where ?t₁ and ?t₂ are temperature differences between the hot and cold fluids at entrance and exit)
A. t_m = (?t₁ - ?t₂)/ log_e (?t₁/?t₂)
B. t_m = log_e (?t₁/?t₂)/ (?t₁ - ?t₂)
C. t_m = t_m = (?t₁ - ?t₂) log_e (?t₁/?t₂)
D. t_m = log_e (?t₁ - ?t₂)/ ?t₁/?t₂
Answer : A

28. Total heat is the heat required to
A. Change vapour into liquid
B. Change liquid into vapour
C. Increase the temperature of a liquid or vapour
D. Convert water into steam and superheat it
Answer : D

29. Which of the following is a case of steady state heat transfer?
A. I.C. engine
B. Air preheaters
C. Heating of building in winter
D. None of the above
Answer : D

30. Heat transfer takes place as per
A. Zeroth law of thermodynamics
B. First law of thermodynamic
C. Second law of the thermodynamics
D. Kirchoff's law
Answer : C

31. The heat transfer by conduction through a thick cylinder (Q) is given by (where T? = Higher temperature, T? = Lower temperature, r? = Inside radius, r? = Outside radius, l = Length of cylinder, and k = Thermal conductivity)
A. Q = [2?lk (T? - T?)]/2.3 log (r?/r?)
B. Q = 2.3 log (r?/r?)/[2?lk (T? - T?)]
C. Q = [2? (T? - T?)]/2.3 lk log (r?/r?)
D. Q = = 2?lk/2.3 (T? - T?) log (r?/r?)
Answer : A

32. According to Kirchoff's law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a
A. Grey body
B. Brilliant white polished body
C. Red hot body
D. Black body
Answer : D

33. 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

34. Thermal conductivity of glass wool varies from sample to sample because of variation in
A. Composition
B. Density
C. Porosity
D. All of the above
Answer : D

35. The expression Q = ? AT⁴ is called
A. Fourier equation
B. Stefan-Boltzmann equation
C. Newton Reichmann equation
D. Joseph-Stefan equation
Answer : B

36. Reynolds number (RN) is given by (where h = Film coefficient, l = Linear dimension, V = Velocity of fluid, k = Thermal conductivity, t = Temperature, ? = Density of fluid, cp = Specific heat at constant pressure, and ? = Coefficient of absolute viscosity)
A. RN = hl/k
B. RN = ? cp/k
C. RN = ? V l /?
D. RN = V²/t.cp
Answer : C

37. 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

38. Thermal conductivity of wood depends on
A. Moisture
B. Density
C. Temperature
D. All of the above
Answer : D

39. Which of the following is the case of heat transfer by radiation?
A. Blast furnace
B. Heating of building
C. Cooling of parts in furnace
D. Heat received by a person from fireplace
Answer : D

40. Sensible heat factor is given by (where S.H. = Sensible heat, and L.H. = Latent heat)
A. S.H/(S.H + L.H)
B. (S.H + L.H) /S.H
C. (L.H - S.H)/S.H
D. S.H/(L.H - S.H)
Answer : A

41. The unit of Stefan-Boltzmann constant is
A. Watt/mK
B. Watt/m²K²
C. Watt/m²K⁴
D. Watt/mK²
Answer : C

42. Heat is transferred by all three modes of transfer, viz. conduction, convection and radiation in
A. Electric heater
B. Steam condenser
C. Boiler
D. Refrigerator condenser coils
Answer : C

43. Which of the following would lead to a reduction in thermal resistance?
A. In conduction, reduction in the thickness of the material and an increase in thermal conductivity.
B. In convection, stirring of the fluid and cleaning the heating surface.
C. In radiation, increasing the temperature and reducing the emissivity.
D. All of the above
Answer : D

44. Total emissivity of polished silver compared to black body is
A. Same
B. Higher
C. More or less same
D. Very much lower
Answer : D

45. The process of heat transfer from one particle of the fluid to another by the actual movement of the fluid particles caused by some mechanical means, is known as
A. Conduction
B. Free convection
C. Forced convection
D. Radiation
Answer : C

46. Fouling factor is used
A. In heat exchanger design as a safety factor
B. In case of Newtonian fluids
C. When a liquid exchanges heat with a gas
D. None of the above
Answer : A

47. The highest thermal diffusivity is of
A. Iron
B. Lead
C. Concrete
D. Wood
Answer : B

48. The value of Prandtl number for air is about
A. 0.1
B. 0.3
C. 0.7
D. 1.7
Answer : C

49. Heat is closely related with
A. Liquids
B. Energy
C. Temperature
D. Entropy
Answer : C

50. A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficients of h?, h? and h? respectively. The average heat transfer coefficient for the cube is
A. h? + h? + h?
B. (h?.h?.h?)1/3
C. 1/h? + 1/h? + 1/h?
D. None of these
Answer : D