# Convection in Heat Transfer

Convection, as we know, is the method of heat transfer in the fluid or the gas medium.  Or in other words, the heat energy transferred between a surface and moving fluid with different temperatures is known as convection.

Convection is divided into two categories –

• Free convection
• Forced convection

Free convection is caused by the densities difference between the fluids and the Forced convection, on the other hand, is caused when the fluid is forced by an external means such as a fan.

The process of heat convection  was first discovered by Newton as the newtons law of cooling and mathematically it was given as

Where Q is the heat transfer rate

A=area of the surface

dt= temperature difference between the bulk fluid and the surface

h=convective heat transfer coefficient

In some cases, the fluid is air and the outside temperature is called the ambient temperature denoted by the symbol of infinity.

Typically the convective heat transfer coefficient is generally low for the laminar flow as compared to the convective heat transfer of the turbulent flow.

Another useful quantity is known as the Nusselt number. It is defined as the ratio of convective heat transfer to the conductive heat transfer. mathematically

Nu=convective heat transfer ÷ conductive heat transfer

That means hL ÷kf

This means enhancement of heat transfer through a fluid layer as a result of convective relative to conduction across the same fluid layer.

Please comment below for any question!