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
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
Convective Heat Transfer
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
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
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”
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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