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 :
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.
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
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
- Forced 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 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”