Heat And Mass Transfer MCQ Questions & Answers | Mechanical Engineering

Thermal diffusivity. In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure.
It measures the rate of transfer of heat of a material from the hot end to the cold end.
It has the SI derived unit of m²/hr.

Thermocouple can easily sense temperature of 540°C or even upto 1200°C.
For measuring any temperature above 1200°C a radiation pyrometer type thermometer is generally used because this type of temperature sensors is not required to be brought in touch with the object whose temperature to be measured.

The total radiation from a black body per second per unit area is Directly proportional to fourth power of the absolute temperature. This statement is known as Stefan Boltzmann law.

The use of heat exchangers is made in Radiators in automobile, Condensers and boilers in steam plants, Condensers and evaporators in refrigeration and air conditioning units

The heat transfer from a hot body to a cold body is directly proportional to the surface area and difference of temperatures between the two bodies. This statement is called Newton's law of cooling

For metals, the thermal conductivity is mainly a function of the motion of free electrons. As the temperature increases, the molecular vibrations increase (in turn decreasing the mean free path of molecules). So, they obstruct the flow of free electrons, thus reducing the conductivity

Diathermanous body is a body which transmits all the incident radiation without absorbing or reflecting.
(A body with transmitivity 1, reflectivity 0 and absorptivity 0) Air, up to certain extent can be considered as the diathermanous body because, it transmits all the radiation without hearing itself.

Planck's law is a formula for the spectral radiance of an object at a given temperature as a function of frequency (Lf) or wavelength (Lλ).
Planck’s law holds good for black bodies.
It has dimensions of power per solid angle per area per frequency or power per solid angle per area per wavelength.

Thermal conductivity of a material may be defined as the:
- Quantity of heat flowing in one second through one cm cube of material when opposite faces are maintained at a temperature difference of 1°C.
- Quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C.
- Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces.

A perfect blackbody is one that absorbs all incoming light and does not reflect any. At room temperature, such an object would appear to be perfectly black . However, if heated to a high temperature, a blackbody will begin to glow with thermal radiation