Convection

1. Introduction

Convection is a mode of heat transfer in which heat is transferred from one place to another by the actual motion of a fluid (liquid or gas). It always involves both heat transfer and mass movement of the fluid.

Convection occurs when a fluid is in contact with a surface at a different temperature or when temperature differences exist within the fluid itself.

2. Mechanism of Convection

Convection heat transfer occurs in two steps:

Conduction of heat between the solid surface and the fluid particles in immediate contact with it.
Bulk movement of fluid particles, carrying heat from one region to another.

Thus, convection is a combined effect of conduction and fluid motion.

3. Types of Convection

Convection is classified into two main types:

3.1 Natural (Free) Convection

In natural convection, fluid motion is caused by buoyancy forces that arise due to density differences created by temperature variation in the fluid.

Hot fluid becomes lighter and rises
Cold fluid becomes denser and sinks
No external device is used to move the fluid

Examples:

Heating of air near a hot radiator
Circulation of water in a heated tank
Cooling of electronic components in still air

3.2 Forced Convection

In forced convection, fluid motion is produced by external means such as fans, pumps, or blowers.

Fluid velocity is controlled externally
Higher heat transfer rates compared to natural convection

Examples:

Air blown over car radiators
Water circulation in boilers and heat exchangers
Cooling of engines using fans

4. Newton’s Law of Cooling

The rate of heat transfer by convection is given by Newton’s law of cooling: $Q = hA(T_s – T_f)$ Q=hA(Ts−Tf)

Where:

$Q$ Q = Rate of heat transfer (W)
$h$ h = Convective heat transfer coefficient (W/m²·K)
$A$ A = Surface area (m²)
$T_s$ Ts = Surface temperature
$T_f$ Tf = Fluid temperature

The value of $h$ h depends on:

Nature of fluid
Flow velocity
Surface geometry
Type of convection (natural or forced)

5. Convective Heat Transfer Coefficient (h)

The convective heat transfer coefficient represents the effectiveness of convection heat transfer.

Typical Values of hhh:

Condition	$h$ h (W/m²·K)
Natural convection (air)	5 – 25
Forced convection (air)	25 – 250
Forced convection (water)	500 – 10,000

6. Boundary Layer Concept

When a fluid flows over a solid surface, a boundary layer is formed near the surface.

Velocity of fluid is zero at the surface
Velocity increases gradually away from the surface
Temperature also changes across the boundary layer

Types of boundary layers:

Velocity boundary layer
Thermal boundary layer

The thickness of these layers greatly affects the heat transfer rate.

7. Factors Affecting Convection Heat Transfer

Temperature difference between surface and fluid
Velocity of fluid flow
Properties of fluid (density, viscosity, thermal conductivity)
Surface area and geometry
Nature of flow (laminar or turbulent)

8. Dimensionless Numbers in Convection

Convection analysis often uses dimensionless numbers:

Reynolds Number (Re): Indicates flow regime (laminar or turbulent)
Prandtl Number (Pr): Ratio of momentum diffusivity to thermal diffusivity
Nusselt Number (Nu): Ratio of convective to conductive heat transfer

These numbers help in determining the value of the heat transfer coefficient $h$ h.

9. Comparison of Natural and Forced Convection

Aspect	Natural Convection	Forced Convection
Cause of flow	Buoyancy forces	External devices
Heat transfer rate	Low	High
Control	Difficult	Easy
Equipment required	No	Yes

10. Applications of Convection

Cooling of automobile engines
Heat transfer in boilers and condensers
Air conditioning and refrigeration systems
Cooling of electronic components
Heating and ventilation of buildings