Heat and Mass Transfer Objective Questions with Answers Set – 11

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All questions answers in the last paragraph

Q1. Which of the following statement is correct?

(A) A grey body is one which absorbs all radiations incident on it.
(B) At thermal equilibrium, the emissivity and absorptivity are same.
(C) The energy absorbed by a body to the total energy falling on it, is called emissivity.
(D) A perfect body is one which is black in colour.

Q2. An ordinary passenger aircraft requires a cooling system of capacity.

(A) 2 TR
(B) 4 TR
(C) 8 TR
(D) 10 TR

Q3. The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body

(A) Move actually
(B) Do not move actually
(C) Affect the intervening medium
(D) Does not affect the intervening medium

Q4. Which of the following has least value of conductivity?

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

Q5. Joule sec is the unit of

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

Q6. 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

Q7. 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

Q8. 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

Q9. 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

Q10. Which of the following is expected to have highest thermal conductivity?

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

Q11. The value of Prandtl number for air is about

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

Q12. Stefan Boltzmann law is applicable for heat transfer by

(A) Conduction
(B) Convection
(C) Radiation
(D) Conduction and radiation combined

Q13. 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

Q14. The product of Reynolds number and Prandtl number is known as

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

Q15. An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m²). The critical thickness of insulation will be

(A) 25 mm
(B) 40 mm
(C) 160 mm
(D) 800 mm

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