# 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

- (B) Radiation

- (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

- (C) Temperature gradient

- (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

- (D) Intensity of
radiation

**8. If the energy radiated per second per sq. cm. of the surface for wave lengths lying between λ, and λ +**

*d*λ is represented by (*e*λ.*d*λ), then*e*λ 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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