1. From the concept of kinetic theory, mean travel velocity of the gas molecules is prescribed by the
relation
a) V = (3M/GT) 1/2
b) V = (3GT/M) 1/2
c) V = (6T/GM) 1/2
d) V = (3GT/2M) 1/2
Explanation: Heat transfer by conduction in gases occurs through transport of the kinetic energy of molecular motion resulting from the random movement and collisions of the molecules.
Where, G = Universal gas constant
M = Molecular weight of the gas
T = Absolute temperature.
2. Consider the following parameters
(i) Composition
(ii) Density
(iii) Porosity
(iv) Structure
Then, thermal conductivity of glass wool varies from sample to sample because of variation is
a) i and ii
b) i, ii, iii and iv
c) i and iii
d) i, ii and iii
Explanation: There is a variation due to all the above options.
3. The thermal conductivity and the electrical conductivity of a metal at absolute temperature are
related as
a) k/σ T
b) k/σ
c) k σ/T
d) k/T
Explanation: it is defined as the ratio of thermal conductivity to the product of electrical conductivity and temperature.
4. The value of Lorenz number in 10-8 W ohms/K2 is
a) 2.02
b) 2.35
c) 2.56
d) 2.45
Explanation: Lorenz no. = k/α T
5. The relation Ϫ2 t =0 is referred to as
a) Poisson’s equation
b) Fourier heat conduction equation
c) Solution for transient conduction
d) Laplace equation
Explanation: In the absence of internal heat generation or release of energy within the body, equation reduces to Laplace equation.
6. The unit of thermal diffusivity is
a) m2/hr-K
b) kcal/m2-hr
c) m2/hr
d) m/hr-K
Explanation: The quantity α = k/pc is called thermal diffusivity.
7. The diffusion equations
Ɏ2t + q g = (1/α) (d t/d r)
Governs the temperature distribution under unsteady heat flow through a homogeneous and isotropic
material. The Fourier equation follows from this expression when
a) Temperature doesn’t depends on time
b) There is no internal heat generation
c) Steady state conditions prevail
d) There is no internal heat generation but unsteady state condition prevails
Explanation: In unsteady state condition, there is no internal heat generation.
8. The temperature distribution in a large thin plate with uniform surface temperature will be
(Assume steady state condition)
a) Logarithmic
b) Hyperbolic
c) Parabolic
d) Linear
Explanation: The temperature increases with increasing value of x. Temperature gradient will be positive i.e. linear.
9. Let us assume two walls of same thickness and cross-sectional area having thermal conductivities in
the ratio 1/2. Let us say there is same temperature difference across the wall faces, the ratio of heat
flow will be
a) 1
b) 1/2
c) 2
d) 4
Explanation: Q1 = k1 A1 d t1/δ1 and Q2 = k2A2 d t2/δ 2 Now, δ1 = δ2 and A1 = A2 and d t1 = d t2 So, Q1/Q2 = ½.
10. The interior of an oven is maintained at a temperature of 850 degree Celsius by means of a suitable
control apparatus. The oven walls are 500 mm thick and are fabricated from a material of thermal
conductivity 0.3 W/m degree. For an outside wall temperature of 250 degree Celsius, workout the
resistance to heat flow
a) 0.667 degree/W
b) 1.667 degree/W
c) 2.667 degree/W
d) 3.667 degree/W
Explanation: R t = 0.5/0.3 = 1.667 degree/W.