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

1. In counter flow heat exchangers
A. Both the fluids at inlet (of heat exchanger where hot fluid enters) are in their coldest state
B. Both the fluids at inlet are in their hottest state
C. Both the fluids at exit are in their hottest state
D. One fluid is in hottest state and other in coldest state at inlet
Answer : B

2. Unit of thermal conductivity in M.K.S. units is
A. K cal/kg m² °C
B. K cal m/hr m² °C
C. K cal/hr m² °C
D. K calm/hr °C
Answer : B

3. Film coefficient is defined as the ratio of
A. Thermal conductivity to the equivalent thickness of the film of fluid
B. Temperature drop through the films of fluids to the thickness of film of fluids
C. Thickness of film of fluid to the thermal conductivity
D. Thickness of film of fluid to the temperature drop through the films of fluids
Answer : A

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

5. If the temperature of a solid surface changes from 27°C to 627°C, then its emissive power changes in the ratio of
A. 6
B. 9
C. 27
D. 81
Answer : D

6. Film coefficient is defined as Inside diameter of tube
A. Equivalent thickness of film
B. Thermal conductivity Equivalent thickness of film Specific heat × Viscosity
C. Thermal conductivity Molecular diffusivity of momentum Thermal diffusivity
D. Film coefficient × Inside diameter Thermal conductivity
Answer : B

7. 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

8. Depending on the radiating properties, a body will be white when
A. P = 0, x = 0 and a = 1
B. P=1, T = 0 and a = 0
C. P = 0, x = 1 and a = 0
D. X = 0, a + p = 1 Where a = absorptivity, p = reflectivity, x = transmissivity
Answer : B

9. 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

10. 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

11. The amount of radiation mainly depends on
A. Nature of body
B. Temperature of body
C. Type of surface of body
D. All of the above
Answer : D

12. Cork is a good insulator because it has
A. Free electrons
B. Atoms colliding frequency
C. Low density
D. Porous body
Answer : D

13. The radiation emitted by a black body is known as
A. Black radiation
B. Full radiation
C. Total radiation
D. All of these
Answer : D

14. Which of the following is expected to have highest thermal conductivity?
A. Steam
B. Solid ice
C. Melting ice
D. Water
Answer : B

15. The rate of heat flow through a body is Q = [kA (T? - T?)]/x. The term x/kA is known as
A. Thermal coefficient
B. Thermal resistance
C. Thermal conductivity
D. None of these
Answer : B

16. The thermal diffusivities for solids are generally
A. Less than those for gases
B. Less than those for liquids
C. More than those for liquids and gases
D. More or less same as for liquids and gases
Answer : C

17. Thermal conductivity of water ________ with rise in temperature.
A. Remain same
B. Decreases
C. Increases
D. May increase or decrease depending upon temperature
Answer : D

18. The transfer of heat by molecular collision is known as
A. Conduction
B. Convection
C. Radiation
D. None of these
Answer : B

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

20. Which of the following statement is wrong?
A. The heat transfer in liquid and gases takes place according to convection.
B. The amount of heat flow through a body is dependent upon the material of the body.
C. The thermal conductivity of solid metals increases with rise in temperature
D. Logarithmic mean temperature difference is not equal to the arithmetic mean temperature difference.
Answer : C

21. The heat is transferred by conduction, convection and radiation in
A. Melting of ice
B. Boiler furnaces
C. Condensation of steam in condenser
D. None of these
Answer : B

22. 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

23. 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

24. The use of heat exchangers is made in
A. Radiators in automobile
B. Condensers and boilers in steam plants
C. Condensers and evaporators in refrigeration and air conditioning units
D. All of the above
Answer : D

25. 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

26. In case of liquids and gases, the heat transfer takes place according to
A. Conduction
B. Convection
C. Radiation
D. None of these
Answer : B

27. The critical thickness of insulation for a sphere is
A. k/h?
B. 2k/h?
C. h?/k
D. h?/2k
Answer : B

28. Reynolds number is the ratio of
A. Energy transferred by convection to that by conduction
B. Kinematic viscosity to thermal diffusivity
C. Inertia force to viscous force
D. None of the above
Answer : C

29. 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

30. Unit of thermal diffusivity is
A. m²/hr
B. m²/hr °C
C. kcal/m² hr
D. kcal/m. hr °C
Answer : A

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

32. 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

33. 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

34. In heat exchangers, degree of approach is defined as the difference between temperatures of
A. Cold water inlet and outlet
B. Hot medium inlet and outlet
C. Hot medium outlet and cold water inlet
D. Hot medium outlet and cold water outlet
Answer : D

35. The concept of overall coefficient of heat transfer is used in heat transfer problems of
A. Conduction
B. Convection
C. Radiation
D. Conduction and convection
Answer : D

36. 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

37. 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

38. A grey body is one whose absorptivity
A. Varies with temperature
B. Varies with wavelength of the incident ray
C. Is equal to its emissivity
D. Does not vary with temperature and. wavelength of the incident ray
Answer : C

39. 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

40. Depending on the radiating properties, a body will be black when
A. P = 0, x = 0 and a = 1
B. P= 1, T = 0 and a = 0
C. P = 0, x = 1 and a = 0
D. X = 0, a + p = 0 Where a = absorptivity, p = reflectivity, X = transmissivity.
Answer : A

41. 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

42. 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

43. Pick up the wrong case. Heat flowing from one side to other depends directly on
A. Face area
B. Time
C. Thickness
D. Temperature difference
Answer : C

44. 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

45. The unit of Stefan Boltzmann constant is
A. Watt/cm² °K
B. Watt/cm4 °K
C. Watt²/cm °K?
D. Watt/cm² °K?
Answer : D

46. Thermal diffusivity of a substance is
A. Directly proportional to thermal conductivity
B. Inversely proportional to density of substance
C. Inversely proportional to specific heat
D. All of the above
Answer : D

47. Heat transfer by radiation mainly depends upon
A. Its temperature
B. Nature of the body
C. Kind and extent of its surface
D. All of the above
Answer : D

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. 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

50. 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