Heat And Mass Transfer MCQ Questions & Answers | Mechanical Engineering

As the temperature of air increases the molecular diffusion also gets increases and in case of air,
Thermal conductivity is directly proportional to the lattice vibration and molecular diffusion and hence as the temperature increases the thermal conductivity of air increases.

Thermal conductivity is a physical property that describes a material's ability to transfer heat through conduction.

For most liquids, including water, the behavior of thermal conductivity with respect to temperature is different from that of solids.

As the temperature of liquid water increases, its molecules gain kinetic energy and move more rapidly.

This increased molecular motion enhances energy transfer between molecules, causing the thermal conductivity to increase over most of the liquid range.

Therefore, thermal conductivity of water generally increases with rise in temperature, at least within the typical liquid temperature range (0°C to 100°C).

Option B (Decreases) and Option C (Remain constant) are incorrect because thermal conductivity is not constant or decreasing over the normal temperature range.

Option D (May increase or decrease depending on temperature) is also not suitable for this question, as the trend is generally increasing in the liquid phase.

Hence, the correct answer is: Increases (Option A).

Heat is closely related with Temperature

The thermal diffusivities for gases are generally More than those for liquids
the thermal conductivity of the liquid is more the gases thermal diffusivity also depends on density and heat capacity. Which is very small in terms of gases. That's why air has low thermal conductivity but relatively high thermal diffusivity that is 0.19 and water has 0.014.

$$\frac{{{\text{Q with shield}}}}{{{\text{Q without shield}}}} = \frac{1}{{{\text{n}} + 1}}.$$
N = number of parallel shields
0.25n + 0.25 = 1
0.25n = 0.75
n = 3.

Reynolds number (R_N) is given by $${{\text{R}}_{\text{N}}} = \frac{{\rho {\text{V}}l}}{\mu }$$
Where h = Film coefficient, $$l$$ = Linear dimension, V = Velocity of fluid, k = Thermal conductivity, t = Temperature, $$\rho $$ = Density of fluid, c_p = Specific heat at constant pressure and $$\mu $$ = Coefficient of absolute viscosity

An ordinary passenger aircraft requires a cooling system capable of 8 TR capacity and a super constellation requires a cooling system of more than 8 TR capacity

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 $$\frac{1}{{{{\text{h}}_1}}} + \frac{1}{{{{\text{h}}_2}}} + \frac{1}{{{{\text{h}}_3}}}.$$

Moisture would find its way into insulation by vapour pressure unless it is prevented by a vapour seal

The Biot number is the ratio of the internal resistance of a body to heat conduction to its external resistance to heat convection.