Heat And Mass Transfer MCQ Questions & Answers | Mechanical Engineering

Kirchhoff's law states that the emissivity and the absorptivity of a surface at a given temperature and wavelength are equal. The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as Kirchhoff's law

Thermal conductivity is a material property.
It will not differ with the dimensions of a material, but it is dependent on the temperature, the density and the moisture content of the material.
The thermal conductivity of a material depends on its temperature, density and moisture content.

The intensity of radiation is defined as the rate of emission of radiation in a given direction from a surface per unit solid angle and per unit projected area of a radiating surface on a plane perpendicular to the direction of radiation.

Radiation refers to the emission of energy in rays or waves. Heat moves through space as energy waves. It is the type of heat one feels when sitting in front of a fireplace or around a campfire.

Absorptive power is a measure of heat absorbed by an object. When heat is incident on the surface an object, a part of it is absorbed and the remaining part is reflected. Usually the absorbed radiation is converted to thermal energy, increasing the object's temperature.

Sensible Heat Ratio (SHR) - is used to describe the ratio of sensible heat load to total heat load and can be expressed as:
$$\eqalign{ & {\text{SHR}} = \frac{{{{\text{q}}_{\text{s}}}}}{{{{\text{q}}_{\text{t}}}}} \cr & = \frac{{{{\text{q}}_{\text{s}}}}}{{{{\text{q}}_{\text{s}}} + {{\text{q}}_l}}}......\left( {\text{1}} \right) \cr} $$
where,
SHR = sensible heat ratio (ratio of sensible heat load to total heat load)
$${{{\text{q}}_{\text{s}}}}$$ = sensible heat load (kW, Btu/hr)
$${{{\text{q}}_{\text{t}}}}$$ = total heat load - sensible heat and latent heat (kW, Btu/hr)
$${{{\text{q}}_l}}$$ = latent heat load (kW, Btu/hr)
For an air flow (1) can be modified to:
$${\text{SHR}} = \frac{{{{\text{c}}_{\text{p}}}\left( {{{\text{t}}_{\text{o}}} - {{\text{t}}_{\text{i}}}} \right)}}{{{{\text{h}}_{\text{o}}} - {{\text{h}}_{\text{i}}}}}......\left( {\text{2}} \right)$$
where,
$${{{\text{c}}_{\text{p}}}}$$ = specific heat air (1.005 kJ/kg°C, 0.240 Btu/lb°F)
$${{{\text{t}}_{\text{o}}}}$$ = outlet air temperature (°C, °F)
$${{{\text{t}}_{\text{i}}}}$$ = inlet air temperature (°C, °F)
$${{{\text{h}}_{\text{o}}}}$$ = outlet moist air enthalpy (kJ/kg, Btu/lb)
$${{{\text{h}}_{\text{i}}}}$$ = inlet moist air enthalpy (kJ/kg, Btu/lb)

Counter flow heat exchangers are inherently more efficient than parallel flow heat exchangers because they create a more uniform temperature difference between the fluids, over the entire length of the fluid path.

The automobile radiator is a heat exchanger of Cross flow type
A radiator is a type of heat exchanger. It is designed to transfer heat from the hot coolant that flows through it to the air blown through it by the fan.

A grey body is an idealized physical body used in thermal radiation studies.

It is defined as a body whose absorptivity and emissivity remain constant across all wavelengths and temperatures, although the value is less than 1 (unlike a black body which has a value of 1).

Option A: Varies with temperature
This is incorrect because a grey body’s absorptivity does not vary with temperature. It is assumed to remain constant for analysis purposes.

Option B: Varies with wavelength of the incident ray
This is incorrect. If the absorptivity varies with wavelength, the body is not grey. Grey body assumes constant absorptivity across all wavelengths.

Option C: Is equal to its emissivity
While Kirchhoff’s law states that emissivity equals absorptivity at thermal equilibrium, this is not the defining property of a grey body. This condition can apply to other bodies as well.

Option D: Does not vary with temperature and wavelength of the incident ray
This is correct. A grey body is defined as one whose absorptivity does not change with wavelength or temperature.

Correct Answer: Option D: Does not vary with temperature and wavelength of the incident ray

In free convection heat transfer transition from laminar to turbulent flow is governed by the critical value of the Prandtl number, Grashoff's number