Basic Thermal engineering VIVA questions and answers

Hired Thermal engineering VIVA questions and answers:

1. Define heat transfer?
Heat transfer can be defined as the transmission of energy from one region to another due to temperature difference.

2. What are the modes of heat transfer?
1. Conduction
2. Convection
3. Radiation.

3. What is conduction?
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.

4. State Fourier’s law of conduction.
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.

5. Define Thermal conductivity.
Thermal conductivity is defined as the ability of a substance to conduct heat.

6. Write down the equation for conduction of heat through a slab or plane wall.
Heat transfer, Q = ∆ Toverall / R
Where, ∆ T = T1 – T2
 R = L / kA – Thermal resistance of slab
 L – Thickness of slab
 K – Thermal conductivity of slab
 A – Area

7. What are the factors affecting the thermal conductivity?
a. Moisture b. Density of material c. Pressure
d. Temperature e. Structure of material.

8. 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.

9. 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

10. Define Stanton number [St].
It is the ratio of Nusselt number to the product of Reynolds number and
Prandtl number. St = Nu / Re x Pr.

11. What is meant by Newtonion and non-newtonion fluids?
The fluids which obey the Newton’s law of viscosity are called Newtonion fluids and those which do not obey are called no-newtonion fluids.

12. 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.

13. Define Convection.
Convection is a process of heat transfer that will occur between a solid surface and a fluid medium when they are at different temperatures.

14. Define Reynolds number [Re].
It is defined as the ratio of inertia force to viscous force.
 Re = Inertia force / Viscous force

15. Define Prandtl number [Pr].
It is the ratio of the momentum diffusivity to the thermal diffusivity.
 Pr = Momentum diffusivity / Thermal diffusivity

16. Define Nusselt Number [Nu].
It is defined as the ratio of the heat flow by convection process under an unit temperature gradient to the heat flow rate by conduction under an unit temperature gradient through a stationary thickness [L] of metre.

 Nusselt Number [Nu] = qconv /qcond

17. 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.

18. What is forced convection?
If the fluid motion is artificially created by means of an external force like a blower or fan, that type of heat transfer is known as forced convection.

19. What are the dimensionless parameters used in forced convection?
1. Reynolds number [Re].
2. Nusselt number [Nu].
3. Prandtl number [Pr].

20. Mention Stefan boltzman contant.
σ = Stefan Boltzman constant = 5.6697 x 10-8 W/ (m² K4)

21. Define Stefan boltzman contant.
Stefan Boltzman law states that the total emissive power of a perfect black body is proportional to
fourth power of the absolute temperature of black body surface
Eb = σT
4
σ = Stefan Boltzman constant = 5.6697 x 10-8 W/ (m² K4)

22. Define Emissive power [Eb].
The emissive power is defined as the total amount of radiation emitted by a body per unit time and unit area. It is expressed in W/m2

23. Define monochromatic emissive power. [Ebλ]
The energy emitted by the surface at a given length per unit time per unit area in all directions is known as monochromatic emissive power.

24. What is meant by absorptivity?
Absorptivity is defined as the ratio between radiation absorbed and incident radiation. Absorptivity, α = Radiation absorbed / Incident radiation.

25. Define Radiation.
The heat transfer from one body to another without any transmitting medium is known as radiation. It is an electromagnetic wave phenomenon.

26. Define Emissivity.
It is defined as the ability of the surface of a body to radiate heat. It is also defined as the ratio of emissive power of any body to the emissive power of a black body of equal temperature.

Emissivity, ε = E / Eb.

27. Define Emissive power [Eb].
The emissive power is defined as the total amount of radiation emitted by a body per unit time and unit area. It is expressed in W/m2.

28. Define monochromatic emissive power. [Ebλ]
The energy emitted by the surface at a given length per unit time per unit area in all directions is known as monochromatic emissive power.

29. What is meant by absorptivity?
Absorptivity is defined as the ratio between radiation absorbed and incident radiation.
 Absorptivity, α = Radiation absorbed / Incident radiation.

30. What is meant by reflectivity?
Reflectivity is defined as the ratio of radiation reflected to the incident radiation.
Reflectivity, ρ = Radiation reflected / Incident radiation.

31. What is meant by transmissivity?
Transmissivity is defined as the ratio of radiation transmitted to the incident radiation.
Transmissivity, τ = Radiation transmitted / Incident radiation.

32. What is black body?
 Black body is an ideal surface having the following properties.
1. A black body absorbs all incident radiation, regardless of wav e length and direction.
2. For a prescribed temperature and wave length, no surface can emit more energy than black body.

33. What is meant by gray body?
If a body absorbs a definite percentage of incident radiation irrespective of their wave length, the body is known as gray body. The emissive power of a gray body is always less than that of the black body.

34. What is heat exchanger?
A heat exchanger is defined as an equipment which transfers the heat from a hot fluid to a cold fluid.

35. 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.

36. 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.

37. What is meant by parallel flow heat exchanger?
In this type of heat exchanger, hot and cold fluids move in the same direction.

38. What is meant by counter flow heat exchanger?
In this type of heat exchanger, hot and cold fluids move in parallel but opposite directions.

39. What is meant by cross flow heat exchanger?
In this type of heat exchanger, hot and cold fluids move at right angles to each other.

40. What is meant by Shell and tube heat exchanger?
In this type of heat exchanger, one of the fluids moves through a bundle of tubes enclosed by a shell. The other fluid is forced through the shell and it movesover the outside surface of the tubes.

41. 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-efficient [W/m2K], A=Area, m2
 [∆T]m = Logarithmic mean temperature difference.

42. What is meant by Effectiveness?
The heat exchanger effectiveness is defined as the ratio of actual heat transferto the maximum possible heat transfer.
Effectiveness ε = Actual heat transfer / Maximum possible heat transfer = Q / Qmax

43. Power requirement of a refrigerator is___________.
Inversely proportional to COP.

44. In SI units, one ton of refrigeration is equal to _________.
210 kJ/min.

45. Define tons of refrigeration and COP.
A tonne of refrigeration is defined as the quantity of heat required to beremoved from one tonne of water [1000 kg] at 00C to convert that into ice at 00C in24 hours. In actual practice,1 tonne of refrigeration = 210kJ/min = 3.5kW.

46. The capacity of a domestic refrigerator is in the range of ___________.
1 to 3 tonne.

47. Name four important properties of a good refrigerant.
1. Low boiling point.
2. High critical temperature & pressure.
3. Low specific heat of liquid.

48. 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.

49. Name any four commonly used refrigerants.
1. Ammonia [NH3].
2. Carbon dioxide [CO2].
3. Sulphur di oxide [SO2].
4. Freon – 12. Dr. N.N.C.E MECH/ VI Sem TE LAB II - LM52

50. What are the advantages and disadvantages of air refrigeration system?
Advantages:
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.
Disadvantages:
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.

51. What is net refrigerating effect of the refrigerant?
Refrigerating effect is the total heat removed from the refrigerant in the evaporator.
 COP = Refrigeration effect / Work done.
 Refrigeration effect = COP x Work done.

52. Define refrigerant.
Any substance capable of absorbing heat from another required substance canbe used as refrigerant.