1. Define critical Reynolds number. What is the typical value for flow over a flat plate and flow through a pipe?

The Reynolds number at which flow changes from laminar to turbulent is called critical Reynolds number.

            For plate, it is 5 * 10⁵

            For Pipe it is 2300.           

2. What is hydrodynamic and thermal boundary layer?

The velocity of the fluid is less than 99% of free stream velocity, it is said to be “hydro dynamic boundary layer”.

The temperature of the fluid is less than 99% of free stream temperature, it is said to be “thermal boundary layer”.

3. Define Reynolds number & Prandtl number.

      Reynolds number (Re) is defined as the ratio of inertia force to the viscous force.

      Prandtl number (Pr) is defined as the ratio of the momentum diffusivity to the thermal diffusivity.

4. Define Nussselt number & Groshof number.

Nusselt number (Nu) is defined as the ration 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 mater.

Groshof number (Gr) is defined as the ratio of product of inertia force and buyoncy force to the square of the viscous force.

5. How does laminar flow differ from turbulent flow?

Laminar flow

Laminar flow sometimes called stream line flow.

In this type of flow, the fluid moves in a layers and each fluid particle follows a smooth continuous path.

The fluid particles in each layer remain in orderly sequence without mixing with each other.

Turbulent flow

 In addition to the laminar type of flow a distinct irregular flow is frequently observed in nature.

 This type of flow is called turbulent flow.

 The path of any individual particle is zig-zag and irregular.

6. Define boiling and condensation.

Boiling

The change of phase from liquid to vapour state is known as boiling.

Condensation

The change of phase from vapour to liquid state is known as Condensation.

7. Give the application of boiling and condensation.

  (i) Thermal and nuclear power plant

  (ii) Refrigeration systems

  (iii) Air conditioning systems

  (iv) Process of heating and cooling

8. What is meant by pool boiling?

If heat is added to a liquid from a submerged solid surface, the boiling process is referred to as pool boiling.

In this case the liquid above the hot surface is essentially stagnant and its motion near the surface is due to free convection and mixing induced by bubble growth and detachment.

9. What is meant by film wise condensation?

The liquid condensate wets the solid surface, spreads out and forms a continuous film over the entire surface is known as film wise condensation.

10. What is meant by drop wise condensation? Give its merits.

The vapour condensation, the vapour condenses into small liquid droplets of various sizes which fall down the surface in a random fashion.

Merits:

A large portion of the area of the plate is directly exposed to vapour. The heat transfer rate in drop wise condensation is 10 times higher than in film condensation.