Modes of Heat Transfer : Conduction, Convection and Radiation

Heat is a foam of energy that is transferred from hot body to cold body. In another words, heat is transferred from higher temperature to lower temperature.

Total heat transfer is the sum of total heat transferred by all three modes. Three modes of heat transfer and given below :

  1. Conduction
  2. Convection
  3. Radiation

Conductive Heat Transfer

Conductive heat transfer is heat transfer between two or more thermally contacted solid bodies due to temperate difference between them. Transfer of Heat takes place from hot body to cold body.

For Example When a heat conductive metal bar is heated with candle. Conductive heat transfer takes place from heated end to other end.

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

Rate of conductive heat transfer 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

Qc = Conductive heat transfer per unit time in watt

A = cross section area in meter ^ 2

k = Material Thermal Conductivity

dT = Temperature Difference ( T2-T1)

L = Thickness or length of the part

Convective Heat Transfer

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

For example when water is heated in a bowl. Transfer of heat and molecules takes place from bottom to top.

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

Convection Heat transfer (Qv) = hc A dT

Qv = Convective heat transfer per unit time in Watt

A = heat transfer area in meter ^ 2

hc= Convective Heat Transfer Coefficient (W/m^2K)

dT = Temperature Difference

Types of Convective Heat Transfer

  1. Forced Convection
  2. Natural Convection

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.

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.

Its value for free convection (medium-air) varies from 5 – 25 (W/m ^2K) and for forced convection it’s value varies from 10 – 200 (W/m2K)

Convective heat transfer coefficient for Air Calculation

hc = 10.45 – v + 10 v1/2

v = Relative speed of the object through the air (in the range of 2 to 20 m/s)

Radiation Heat Transfer

Transfer of heat 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. In another words, radiation heat transfer can happen in vacuum as well.

For example heat radiations from sun is transferred to earth through space that is completely vacuum.

The rate of heat radiation emitted by a surface at any temperature is calculated by Stefan-Boltzmann law. 

Radiation Heat Transfer Calculations

Value of 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

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”

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