Heat and Mass Transfer – Set 07

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Q1. The emissive power of a body depends upon its

(A) Temperature
(B) Wave length
(C) Physical nature
(D) All of the above

Answer: (D) All of the above


Q2. 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) k. A. (dT/dx)


Q3. 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) All of the above


Q4. 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) All of the above


Q5. 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) Inertia force to viscous force


Q6. A composite slab has two layers of different materials with thermal conductivities k₁ and k₂. If each layer has the same thickness, then the equivalent thermal conductivity of the slab will be

(A) k₁ k₂
(B) (k₁ + k₂)
(C) (k₁ + k₂)/ k₁ k₂
(D) 2 k₁ k₂/ (k₁ + k₂)

Answer: (D) 2 k₁ k₂/ (k₁ + k₂)


Q7. When α is absorptivity, ρ is reflectivity and τ is transmissivity, then for a diathermanous body,

(A) α = 1, ρ = 0 and τ = 0
(B) α = 0, ρ = 1 and τ = 0
(C) α = 0, ρ = 0 and τ = 1
(D) α + ρ = 1 and τ = 0

Answer: (D) α + ρ = 1 and τ = 0


Q8. The expression Q = ρ AT4 is called

(A) Fourier equation
(B) Stefan-Boltzmann equation
(C) Newton Reichmann equation
(D) Joseph-Stefan equation

Answer: (B) Stefan-Boltzmann equation


Q9. 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) The better insulation must be put inside


Q10. Which of the following has least value of conductivity?

(A) Glass
(B) Water
(C) Plastic
(D) Air

Answer: (D) Air


Q11. Joule sec is the unit of

(A) Universal gas constant
(B) Kinematic viscosity
(C) Thermal conductivity
(D) Planck’s constant

(B) Kinematic viscosity


Q12. Absorptivity of a body will be equal to its emissivity

(A) At all temperatures
(B) At one particular temperature
(C) When system is under thermal equilibrium
(D) At critical temperature

Answer: (C) When system is under thermal equilibrium


Q13. 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) Radiation


Q14. 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) Convert water into steam and superheat it


Q15. In heat transfer, conductance equals conductivity (kcal/hr/sq.m/°C/cm) divided by

(A) Hr (time)
(B) Sq. m (area)
(C) °C (temperature)
(D) K.cal (heat)

Answer: (D) K.cal (heat)


Q16. 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) All of the above


Q17. 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) 1: 2


Q18. The concept of overall coefficient of heat transfer is used in case of heat transfer by

(A) Conduction
(B) Convection
(C) Radiation
(D) Conduction and convection

Answer: (D) Conduction and convection


Q19. The unit of Stefan Boltzmann constant is

(A) watt/cm² °K
(B) watt/cm⁴ °K
(C) watt²/cm °K⁴
(D) watt/cm² °K⁴

Answer: (D) watt/cm² °K⁴


Q20. Depending on the radiating properties, a body will be black when
(Where a = absorptivity, p = reflectivity, X = transmissivity.)

(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

Answer: (A) P = 0, x = 0 and a = 1


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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