Title Heat Transfer Viva Questions Heat Transfer Heat Viscosity Fluid Dynamics Thermal Conduction 138.2 KB 4
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It is the ratio of the momentum diffusivity to the thermal diffusivity. Pr = Momentum diffusivity / Thermal

diffusivity

15. 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

16. 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.

17. 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.

18. What are the dimensionless parameters used in forced convection?

1. Reynolds number [Re]. 2. Nusselt number [Nu]. 3. Prandtl number [Pr].

19. What is fin?

Fins are extended surfaces used primarily to enhance the heat transfer rate between the solid fins and an

20. List out the Fin types.

1. Straight fin of uniform cross section

2. Straight fin of non-uniform cross section

3. Annular fin

4. Pin fin

21. Mention Stefan boltzman contant.

_ = Stefan Boltzman constant = 5.6697 x 10-8 W/ (m² K4)

22. 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 = _T4;

_ = Stefan Boltzman constant = 5.6697 x 10-8 W/ (m² K4)

23. 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.

24. Define monochromatic emissive power. [Eb_]

The energy emitted by the surface at a given length per unit time per unit area in all directions

25. What is meant by absorptivity?

Absorptivity is defined as the ratio between radiation absorbed and incident radiation.

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The heat transfer from one body to another without any transmitting medium is known as radiation. It is an

electromagnetic wave phenomenon.

27. 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, _ = E / Eb.

28. 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.

29. What is meant by reflectivity?

Reflectivity is defined as the ratio of radiation reflected to the incident radiation.

30. What is meant by transmissivity?

Transmissivity is defined as the ratio of radiation transmitted to the incident radiation.

31. 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.

32. 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.

33. What is heat exchanger?

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

34. 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.

35. 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.

36. What is meant by parallel flow heat exchanger?

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

37. What is meant by counter flow heat exchanger?

In this type of heat exchanger, hot and cold fluids move in parallel but opposite directions.

38. 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.

39. What is meant by Shell and tube heat exchanger?

In this type of heat exchanger, one of the fluids moves through a bundle oftubes enclosed by a shell. The

other fluid is forced through the shell and it moves over the outside surface of the tubes.

40. 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