Heat And Mass Transfer MCQ Questions & Answers | Mechanical Engineering

Kirchhoff's law states that For an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, the emissivity is equal to the absorptivity.

In a heat exchanger with one fluid evaporating or condensing, the surface area required is least in -Parallel flow - Counter flow - Cross flow

A steam pipe is to be lined with two layers of insulating materials of different thermal conductivities. For the minimum heat transfer,The better insulation must be put inside

The ratio of Nusselt number and the product of Reynold's number and Prandtl number is equal to Stanton number The Stanton number is a dimensionless number that measures the ratio of heat transferred into a fluid to the thermal capacity of fluid.

The product of Reynolds number and Prandtl number is known as Peclet number
Peclet number, Pe, is a dimensionless group representing the ratio of heat transfer by motion of a fluid to heat transfer by thermal conduction

Upto the critical radius of insulation,Added insulation will increase heat loss
It is the radius of insulation before heat transfer increases and after heat transfer decreases.

Fourier's law of heat conduction is $${\text{kA}}\frac{{{\text{dT}}}}{{{\text{dx}}}}$$
Where Q = Amount of heat flow through the body in unit time, A = Surface area of heat flow, taken at right angles to the direction of heat flow, dT = Temperature difference on the two faces of the body, dx = Thickness of the body, through which the heat flows, taken along the direction of heat flow and k = Thermal conductivity of the body.
The law of heat conduction, also known as Fourier's law, states that the time rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area, at right angles to that gradient, through which the heat flows.

The exchanger is performing at its best when the outlet temperatures are equal. 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.

Option A: Both the fluids at inlet (of heat exchanger where hot fluid enters) are in their coldest state
This is incorrect because in a counter flow heat exchanger, the hot fluid enters the exchanger in its hottest state, and the cold fluid enters the exchanger in its hottest state from the opposite end. Therefore, it is not possible for both fluids to be in their coldest state at the same inlet.

Option B: Both the fluids at inlet are in their hottest state
This is correct. In a counter flow heat exchanger, the hot fluid enters at its hottest state from one end, and the cold fluid enters at its hottest state from the opposite end. This creates a temperature gradient that facilitates efficient heat transfer throughout the exchanger.

Option C: Both the fluids at exit are in their hottest state
This is incorrect. In a counter flow heat exchanger, the fluids flow in opposite directions. At the exit, the hot fluid will be in its coldest state (after transferring heat) and the cold fluid will be in its coldest state (after receiving heat). Therefore, both fluids cannot be in their hottest state at the exit.

Option D: One fluid is in hottest state and other in coldest state at inlet
This is incorrect because, in a counter flow heat exchanger, both fluids enter at their respective hottest states from opposite ends. If one fluid is in its hottest state at the inlet, the other fluid will also be in its hottest state but from the opposite end, not at the same inlet.

Conclusion:

Option B is the correct answer because, in counter flow heat exchangers, both fluids enter the exchanger in their hottest states from opposite ends, which creates an optimal temperature gradient for heat transfer.

The energy distribution of an ideal reflector at higher temperatures is largely in the range of Shorter wavelength