# Heat and Mass Transfer – Set 02

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## Q1. Unit of thermal diffusivity is

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

(A) Conduction
(B) Convection

## Q3. Unit of thermal conductivity in S.I. units is

(A) J/m² sec
(B) J/m °K sec
(C) W/m °K
(D) Option (B) and (C) above.

Answer: (D) Option (B) and (C) above.

## Q4. 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

(A) 0.002
(B) 0.02
(C) 0.01
(D) 0.1

## Q6. Thermal conductivity of a material may be defined as the

(A) Quantity of heat flowing in one second through one cm cube of material when opposite faces are maintained at a temperature difference of 1°C
(B) Quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C
(C) Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces
(D) All of the above

Answer: (D) All of the above

## Q7. 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: (D) All of the above

## Q8. 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) Better insulation should be put over pipe and better one over it

## Q9. Log mean temperature difference in case of counter flow compared to parallel flow will be

(A) Same
(B) More
(C) Less
(D) Depends on other factors

## Q10. 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) All of the above

## Q11. The amount of radiation mainly depends upon the

(A) Nature of the body
(B) Temperature of the body
(C) Type of surface of the body
(D) All of these

## Q12. The rate of energy transferred by convection to that by conduction is called

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

## Q13. 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 h₁, h₂ and h₃ respectively. The average heat transfer coefficient for the cube is

(A) h₁ + h₂ + h₃
(B) (h₁.h₂.h₃)1/3
(C) 1/h₁ + 1/h₂ + 1/h₃
(D) None of these

## Q14. 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) The thermal conductivity of solid metals increases with rise in temperature

## Q15. 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) May increase or decrease depending upon temperature

## Q16. 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) In heat exchanger design as a safety factor

## Q17. In case of liquids and gases, the heat transfer takes place according to

(A) Conduction
(B) Convection
(D) None of these

## Q18. 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

## Q19. Kirchhoff’s law states that

(A) The total radiation from a black body per second per unit area is directly proportional to the fourth power of the absolute temperature
(B) The wave length corresponding to the maximum energy is proportional to the absolute temperature
(C) 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
(D) None of the above

Answer: (C) 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

## Q20. 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)