# Convection Heat Transfer – Natural and Forced Convection

## What is Convection Heat Transfer?

Transfer of heat from one place to another due to the molecular movement of fluids such as air or liquid is known as convective heat transfer. When molecules move from one place to another they carry energy or heat with them. It is governed by Newton’s Law of Cooling.

For example, air conditioner cools a space by cooling air present in the room and throwing room heat to the outside environment. The rate of cooling of a room depends on cooling air velocity and temperature, cooling space area, and existing room temperature. This mode of heat transfer is known as Convection Heat Transfer

Higher the rate of convective heat transfer, higher will be the rate of cooling inside the room. In this article we will discuss what is convection heat transfer? its types and how to calculate convective heat transfer?

## Newton's Law of Cooling : Convective Heat Transfer Calculation

According to Newton’s law of cooling, Rate of convective heat transfer is directly proportional to heat transfer surface area, convective heat transfer coefficient and temperature difference.

The valve of Convective heat transfer depends on type of flow.

###### Convective Heat transfer (Qv) = hc A dT

where

Qv = Convective heat transfer per unit time in Watt.

A = Heat transfer area in square meter.

hc= Convective heat transfer coefficient (W/m² K).

dT = Temperature difference.

## Types of Convective Heat Transfer

According to the type of flow, convective heat transfer can be classified into two types.

1. Free / Natural Convection
2. Forced Convection

## 1. Natural Convection

Natural convection is the transfer of heat due to movement of liquid or air molecules due to density difference, which is a function of temperature. Force generated due to the liquid or air molecules density difference is known as buoyancy force. Natural Convection occurs without external sources such as a pump or fan.

##### How Natural Convection Heat Transfer Works?

In natural convection hot fluid or air rises up and is replaced by cooler fluid. Further this fluid will also heat up and rise and this cycle continues. Gravity plays a very important role in natural convection.

For example, when cold air comes in contact with a hot object. Air temperature near the hot surface rises up whereas hot body temperature decreases. This Increased air temperature results in the decrease in air density. Due to gravitational forces, high density cold air comes downward and replaces the hot air. This cycle continues till the complete system comes in the equilibrium.

From the above example, we can conclude that the hot air rises upward whereas cold air moves down. One of the best example of this is a refrigerator where the refrigeration unit is placed in the upper section.

##### Examples of Natural Convection heat Transfer

Natural convection plays a key role in natural phenomenon’s on earth and engineering applications. Following are the natural convection heat transfer examples.

• Cyclones.
• Underwater currents.
• Movement of clouds.
• Water heating.
• Cooling in refrigerator
• Convective heating in the microwave.
• Cooling of electronic units and appliances etc.

## 2. Forced Convection

In Forced convective heat transfer, flow of fluid or air is caused by an external force such as fan or pump. Forced convection is used to increase the rate of heat transfer compared to natural convection.

Rate of heat transfer in forced convection depends on the velocity of air. Higher the air velocity, higher will be the rate of heat transfer.

Forced convection involves fluid motion as well as conductive heat transfer. For example, an air conditioner cools a room by forced convection. If we increase AC fan speed, the rate of cooling will also increase because of increased heat transfer coefficient.

##### Forced Convection Application Examples

Following are the examples of forced convection heat transfer.

• Flow of coolant in car engine.
• Cooling of laptop or PC processor using fan.
• Heat exchangers.
• Air conditioning units
• Hair dryer
• Cooling of electronic devices such as servers.

## Convective Heat Transfer Coefficient

The value of Convective heat transfer coefficient (hc) depends on the type of media (gas or liquid), Flow velocity and temperature difference.

###### Convective Heat Transfer coefficient for Air

Natural Convection =  5 – 25 W/m² K

Forced Convection =  10 – 200 W/m² K

###### Formula for Convective heat transfer coefficient Calculation. (for air)

v = Relative speed of the object through the air. This formula works for velocity range  2 to 20 m/s .

## Convective Heat Transfer Calculator

To sum up, Convective heat transfer occurs in liquid or air because of movement of molecules from a hotter place to a colder place. And to improve convective heat transfer forced convection is used.