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Mechanical - 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.

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Mechanical : Heat and Mass Transfer : Convection : Important Questions and Answers - Heat and Mass Transfer - Convection |

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