Modes of Heat Transfer : Conduction, Convection and Radiation

Heat is a foam of energy that is transferred from hot to cold body or from higher to lower temperature. Total heat transfer is the sum of heat transferred by all three modes: Conduction, Convection and Radiation.

In this article we will discuss various modes of heat transfer, conduction, convection and radiation. And how to calculate their values.

Conductive Heat Transfer

Transfer of Heat, from hot to cold thermally contacted solid bodies due to temperate difference is known as conductive heat transfer. 

As shown in image, when a conductive metal plate is heated at one end, Transfer of heat takes place from heated end to cooler end. Thermally conductive surface is required for conductive heat transfer. 

Conductive Heat Transfer Calculation

Qc = Conductive heat transfer per unit time in watt

A = cross section area in meter ^ 2

k = Material Thermal Conductivity

dT = (T2-T1)

L = Thickness or length of the part

The rate of heat transfer through conduction is governed by the Fourier’s law of heat conduction.

It is directly proportional to contact area, material thermal conductivity and temperature difference and inversely proportional to thickness of the material. 

Conductive Heat transfer (Qc) = – K A dT / L

From this we can conclude conductive heat transfer will be high in copper compared to wood, plastic of aluminum. You can also use this calculator to calculate conductive hear transfer.

Convective Heat Transfer

Transfer of heat from one place to another by the movement of fluids (Air or liquid). Convective heat transfer involves conduction and fluid flow of molecules.

As shown when water is heated in a bowl. Transfer of heat and molecules takes place from heated bottom surface to top surface.

Convective heat Transfer  Calculation

The rate of convective heat transfer is governed by the Newton’s law of cooling.

Convection 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/square m K)

dT = Temperature Difference

Types of Convective Heat Transfer

1) Natural Convection 

Natural convection is caused by buoyancy forces due to density and temperature variation of fluid. During natural convection hot fluid/air rise up and is replaced by cooler fluid/air that will also heat and rise.

2) Forced Convection

Forced convective heat transfer occurs when flow of fluid/air is caused by an external force such as fan.

Convective Heat Transfer Coefficient

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 .

Radiation Heat Transfer

Heat transfer from one body to another by thermal radiations and electromagnetic waves is known as radiation heat transfer. It does not require any media for heat transfer. 

Radiation heat transfer can happen in vacuum as well. For example heat radiations from sun is transferred to earth through vacuum space.

Radiation Heat Transfer Calculations

The rate of heat radiation emitted by a surface is calculated by Stefan-Boltzmann law. It says Radiation heat transfer per unit time is directly proportional to the fourth power of the Temperature

Radiation Heat Transfer per unit time (Qr) = ε σ T^4 A

Where

σ (Stefan-Boltzmann Constant) = 5.6703 10^8 (W/m2K4)

T = absolute temperature in Kelvin

A = area of the emitting body in m2

ε = emissivity coefficient of the body, Value of emissivity for black body is “one”

Conclusion:

To sum up, Transfer of heat from one body to another takes place by three modes of heat transfer : conduction, convection and radiation. Total heat transfer is sum of all three heat transfers. You can also use calculators to calculate amount of heat transferred.

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