Difference between free convection and forced convection

Difference between free convection and forced convection

Introduction

Convection Can be defined as the transfer of heat through a fluid  (liquid or gas) caused by molecular motion. It can also be defined as a process of transfer of heat or electricity by Utilizing currents in liquids or gases resulting from changes in temperatures.

Conduction is the transmission through or a conductor.

Difference between free convection and forced convection

1. Free convection

It is a type of convection where the motion of the fluid is only due to its density difference because of temperature variation. A good example of this is the boiling of water.

At heating the density change in the boundary layer will cause the fluid to rise and be replaced by cooler fluid in that also, will heat and rise and this is a continuous phenomenon.

2. Forced convection

It is a mode of heat transfer where some external source of heat like a blower, fan, pump or mixer is used to cause the motion of the fluid i.e. air.

Solving convection problems

When solving convection problems, one should have the following tips:-

• If the velocity of the fluid is mentioned in the problem, you can straight away do the calculation for forced convection.
• If the velocity of the fluid is negligible, then it is free convection.

REMARKS

• This is because in forced convection heat transfer depends on Reynolds number(which is a function of velocity) whereas in free convection heat transfer depends on Rayleigh number (does not contain velocity terms).
• Forced convection is a type of transport in which fluid motion is generated by an external source of heat and it is one of the main methods used in heat transfer as significant amounts of heat energy can be transported very efficiently.
• Forced convection also occurs in electronics cooling by adding a fan or a blower e.g, in general, it relies on a device to enhance heat transfer. Through active cooling technologies, the rate of fluid flow increases during convection.
• Forced convection involves fluid motion as well as conduction. It increases the convective transfer rate. Rate of heat transfer in forced convection depends on the velocity of air. The higher the air velocity the higher will be the convection heat transfer.

EQUATIONS RELATED TO HEAT TRANSFER CONCERNING TO THE FORCED AND FREE CONVECTION

As stated earlier heat transfer can be defined as the process of transfer of heat from an object at a higher temperature to another object at a lower temperature and therefore heat is the measure of kinetic energy possessed by the particles in a given system.

HEAT TRANSFER EQUATIONS

Let us consider a system of mass M kg. To compute the relation between the rises in temperature with the amount of heat supplied, we have to multiply the specific heat of the system by the mass of the system and the rise in the temperature.

Heat transferred from one system to another system is given by the following equation,

Q = M × C ×∆T.  Where,

Q-Is the heat transferred/supplied to the system

M-Is the mass of the system

C-Is the specific heat of the system

∆T-Is the difference in temperature of the system

The transfer of heat occurs through three different processes:-

• Conduction
• Convection
• Radiation

But our study will single out only convection. And therefore heat transferred by the process of convection is given by:

Q=H_C A(T_{HOT –} T_COLD ), Where

Q is the heat transferred/supplied

H_c is the convective heat transfer coefficient

T_HOT  is hot temperature

T_COLD  is the cold temperature

A is  the area of surface

EXAMPLE 1

A system of weight 7kg is heated from its initial temperature of 30 ⁰C to its final temperature of   60⁰ C. Find out the total heat obtained by the system if the specific heat of the system is 0.45kg  per kg K

Solution

Q ⁼ M ×C × ∆T

⁼ 7×0.45×(60 -30)

⁼  94.5j

EXAMPLE 2

Fluid flows over a plane surface 1m by 1m. The surface temperature is 50 ⁰C, the fluid temperature is 20 ⁰C and the convection heat transfer coefficient is 2000w/  ⁰C.

Find the convective heat transfer between the hotter surface and the colder air.

Q=H_c  A (T_{HOT ⁻}T_COLD)

=2000×1×1×(50-20)

=60000w

Or

=60kw