Important Questions and Answers - Heat and Mass Transfer - Convection

1. 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.

2. What is meant by free or natural convection?

If the 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.

3. 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.

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

1. Reynolds number (Re)

2. Nusselt number (Nu)

3. Prandtl number (Pr)

5. Define boundary layer thickness.

The thickness of the boundary layer has been defined as the distance from the surface at which the local velocity or temperature reaches 99% of the external velocity or temperature.

6. What is meant by laminar flow and turbulent 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.

Turbulent flow: In addition to the laminar type of flow, a distinct irregular flow is frequency observed in nature. This type of flow is called turbulent flow. The path of any individual particle is zig–zag and irregular. Fig. shows the instantaneous velocity in laminar and turbulent flow.

7. What is hydrodynamic boundary layer?

In hydrodynamic boundary layer, velocity of the fluid is less than 99% of free stream velocity.

8. What is thermal boundary layer?

In thermal boundary layer, temperature of the fluid is less than 99% of free stream velocity.

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 / Buyoyancy force (Viscous force)2

10. Define Reynolds number (Re).

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

Re = Inertia force / Viscous force

11. Define prandtl number (Pr).

It is the ratio of the momentum diffusivity of the thermal diffusivity.

Pr = Momentum diffusivity / Thermal diffusivity

12.Whichmode heattransfer is the convection heat transfer coefficient usually higher,natural or forced convection ? why ?

Convection heat transfer coefficient is usually higher in forced convection than in natural convection, because it mainly depends upon the factors such as fluid density, velocity and viscosity.