Heat and Mass Transfer - Set 05 - ObjectiveBooks

# Practice Test: Question Set - 05

1. In convection heat transfer from hot flue gases to water tube, even though flow may be turbulent, a laminar flow region (boundary layer of film) exists close to the tube. The heat transfer through this film takes place by
(A) Convection
(C) Conduction
(D) Both convection and conduction

2. The heat transfer by conduction through a thick cylinder (Q) is given by (where T₁ = Higher temperature, T₂ = Lower temperature, r₁ = Inside radius, r₂ = Outside radius, l = Length of cylinder, and k = Thermal conductivity)
(A) Q = [2πlk (T₁ - T₂)]/2.3 log (r₂/r₁)
(B) Q = 2.3 log (r₂/r₁)/[2πlk (T₁ - T₂)]
(C) Q = [2π (T₁ - T₂)]/2.3 lk log (r₂/r₁)
(D) Q = = 2πlk/2.3 (T₁ - T₂) log (r₂/r₁)

3. 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

4. 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

5. 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
(D) Thermal conductivity

6. The value of the wave length for maximum emissive power is given by
(A) Kirchhoff's law
(B) Stefan's law
(C) Wine’s law
(D) Planck's law

7. The rate of energy emission from unit surface area through unit solid angle, along a normal to the surface, is known as
(A) Emissivity
(B) Transmissivity
(C) Reflectivity

8. If the energy radiated per second per sq. cm. of the surface for wave lengths lying between λ, and λ + dλ is represented by (eλ.dλ), then eλ is called
(A) Absorptive power
(B) Emissive power
(C) Emissivity
(D) None of these

9. 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

10. When absorptivity (α) = 1, reflectivity (ρ) = 0 and transmissivity (τ) = 0, then the body is said to be a
(A) Black body
(B) Grey body
(C) Opaque body
(D) White body

11. 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

12. 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)

13. 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

14. The critical temperature is the temperature
(A) Below which a gas does not obey gas laws
(B) Above which a gas may explode
(C) Below which a gas is always liquefied
(D) Above which a gas will never liquefied

15. A non-dimensional number generally associated with natural convection heat transfer is
(A) Grashoff number
(B) Nusselt number
(C) Weber number
(D) Prandtl number

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