## Thermal Engineer Interview Questions & Answers

1. Question 1. Define Heat Transfer?

Answer :Heat transfer can be defined as the transmission of energy from one region to another due to temperature difference.

2. Question 2. What Are The Modes Of Heat Transfer?

1. Conduction
2. Convection
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4. Question 3. What Is Conduction?

Answer :Heat conduction is a mechanism of heat transfer from a region of high temperature to a region of low temperature within a medium [solid, liquid or gases] or different medium in direct physical contact.

5. Question 4. State Fourier’s Law Of Conduction?

Answer :The rate of heat conduction is proportional to the area measured normal to the direction of heat flow and to the temperature gradient in that direction.
Q α – A dT / dx
Q = -kA dT /dx
Where, A – Area in m2
dT / dx – Temperature gradient, K/m
k – Thermal conductivity, W/mK.

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7. Question 5. Define Thermal Conductivity?

Answer :Thermal conductivity is defined as the ability of a substance to conduct heat.

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9. Question 6. What Are The Factors Affecting The Thermal Conductivity?

1. Moisture
2. Density of material
3. Pressure
4. Temperature e. Structure of material.
10. Question 7. What Is Meant By Free Or Natural Convection?

It is fluid motion is produced due to change in density resulting from temperature gradients, the mode of heat transfer is said to be free or natural convection.

12. Question 8. Define Grashof Number [gr].?

It is defined as the ratio of product of inertia force and buoyancy force to the square of viscous force.

Gr = Inertia force x Buoyancy force / [Viscous force]2

13. Question 9. Define Stanton Number [st].?

It is the ratio of Nusselt number to the product of Reynolds number andPrandtl number.

St = Nu / Re x Pr.

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15. Question 10. What Is Meant By Newtonian And Non-newtonian Fluids?

The fluids which obey the Newton’s law of viscosity are called Newtonian fluids and those which do not obey are called no-newtonian fluids.

16. Question 11. What Is Meant By Laminar Flow ?

Laminar flow: Laminar flow is sometimes called stream line flow. In this type of flow, the fluid moves in layers and each fluid particle follows a smooth continuous path. The fluid particles in each layer remain in an orderly sequence without mixing with each other.

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18. Question 12. What Are The Dimensionless Parameters Used In Forced Convection?

1. Reynolds number [Re].
2. Nusselt number [Nu].
3. Prandtl number [Pr].
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20. Question 13. Define Reynolds Number [re]?

It is defined as the ratio of inertia force to viscous force.

Re = Inertia force / Viscous force

21. Question 14. State Newton’s Law Of Convection?

Heat transfer from the moving fluid to solid surface is given by the equation.

Q = h A = [Tw – T∞]

This equation is referred to as Newton’s law of cooling.
Where h = Local heat transfer coefficient in W/m2K.
A = Surface area in m2
Tw = Surface [or] Wall temperature in K.
T∞ = Temperature of fluid in K.

22. Question 15. Mention Stefan Boltzmann Constant?

σ = Stefan Boltzmann constant = 5.6697 x 10-8 W/ (m² K4).

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24. Question 16. What Is Meant By Absorptivity?

25. Question 17. What Is Meant By Direct Heat Exchanger [or] Open Heat Exchanger?

In direct contact heat exchanger, the heat exchange takes place by direct mixing of hot and cold fluids.

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27. Question 18. What Is Meant By Indirect Contact Heat Exchanger?

In this type of heat exchangers, the transfer of heat between two fluids could be carried out by transmission through a wall which separates the two fluids.

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29. Question 19. What Is Meant By Parallel Flow Heat Exchanger?

In this type of heat exchanger, hot and cold fluids move in the same direction.

30. Question 20. What Is Meant By Lmtd?

We know that the temperature difference between the hot and cold fluids in the heat exchanger varies from point to point. In addition various modes of heat transfer are involved. Therefore based on concept of appropriate mean temperature difference, also called logarithmic mean temperature difference, the total heat transfer rate in the heat exchanger is expressed as

Q = U A [∆T]m

Where, U=Overall heat transfer co-coefficient [W/m2K], A=Area, m2
[∆T]m = Logarithmic mean temperature difference.

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32. Question 21. What Is Meant By Effectiveness?

The heat exchanger effectiveness is defined as the ratio of actual heat transfer to the maximum possible heat transfer.

Effectiveness ε = Actual heat transfer / Maximum possible heat transfer = Q / Qmax

33. Question 22. What Is The Difference Between Air Conditioning And Refrigeration?

Refrigeration is the process of providing and maintaining the temperature in space below atmospheric temperature. Air conditioning is the process of supplying sufficient volume of clean aircontaining a specific amount of water vapour and maintaining the predeterminedatmospheric condition with in a selected enclosure.

34. Question 23. What Are The Advantages And Disadvantages Of Air Refrigeration System?

• 1. The refrigerant used namely air is cheap and easily available.
• 2. There is no danger of fire or toxic effects due to leakages.
• 3. The weight to tonne of refrigeration ratio is less as compared to other systems.

• 1. The quantity of refrigerant used per tonne of refrigeration is high as compared to other system.
• 2. The COP of the system is very low. Therefore running cost is high.
• 3. The danger of frosting at the expander valves is more as the air contains moisture content.
35. Question 24. What Is Net Refrigeration Effect Of The Refrigeration?

Refrigeration effect is the total heat removed from the refrigerant in the evaporator.

COP = Refrigeration effect / Work done.
Refrigeration effect = COP x Work done.