Heat And Mass Transfer MCQ Questions & Answers | Mechanical Engineering

The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body Do not move actually

Thermal diffusivity is defined as the ratio of thermal conductivity to the product of density and specific heat capacity at constant pressure.

It indicates how quickly a material can conduct thermal energy relative to its ability to store it.

Solids generally have much higher thermal conductivity compared to liquids and gases, and although their density and specific heat may also be higher, the conductivity dominates, resulting in higher thermal diffusivity.

Liquids and gases typically have lower thermal conductivity and, in many cases, similar or lower specific heat, which makes their thermal diffusivity values lower than that of solids.

Option A: Less than those for gases
Incorrect. Gases usually have very low thermal diffusivity.

Option B: Less than those for liquids
Incorrect. Liquids have lower thermal diffusivity than most solids.

Option C: More than those for liquids and gases
Correct. Solids, particularly metals, typically have higher thermal diffusivity than both liquids and gases.

Option D: More or less same as for liquids and gases
Incorrect. Thermal diffusivity varies significantly among solids, liquids, and gases.

Correct Answer: Option C: More than those for liquids and gases

Heat transfer through radiation takes place in form of electromagnetic waves mainly in the infrared region.
It depends on nature of the body, its temperature and kind & extent of its surface.

When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal energy along. This type of heat transfer is called convection.

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.

a + p + T = 1
a + 0.40 + 0.15 = 1 (p = 0.40, T = 0.15)
a = 1 - 0.55
a = 0.45

Heat transfer takes place as per Second law of the thermodynamics it states that Heat transfer occurs spontaneously from higher- to lower-temperature bodies but never spontaneously in the reverse direction.

When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal energy along. This type of heat transfer is called convection.

Opaque body are those body where the atoms are closely packed and the light cannot pass through. Whereas a light may or may not pass through a black body depending whether it is opaque or transparent.
Depending on the radiating properties, a body will be opaque when x = 0, a + p = 1.

The larger the LMTD more heat is transferred.
In parallel flow heat exchangers both hot fluid and cold fluid flow in same direction. Counter flow heat exchanger: In counter flow heat exchangers both hot fluid and cold fluid flow in opposite direction.