Convection

Convection may occur within a body or between two bodies if they are brought into contact.  The fluid motion will almost certainly occur if one of the bodies is a liquid or gas.  Convection is the conduction process between a solid surface and a moving fluid.  The motion of the fluid may be natural or forced.  Convection cannot occur in a vacuum.  Convection above a hot surface occurs because hot air expands, becomes less dense, and rises.  Hot water is less dense than cold water and rises, causing convection currents, which transport energy.  Convection can lead to circulation in a liquid, as in heating a pot of water over a flame.  Heated water expands and becomes more buoyant.  Cooler, denser water near the surface descends, and patterns of circulation are formed.

Conduction

Conduction is the only method of heat transfer in opaque solids.  If the temperature at one end of a metal rod is raised by heating, then heat is conducted to the colder end raising its temperature.  Conduction is heat transfer utilizing molecular agitation within a material without any motion of the material.  Suppose one end of a metal rod is at a higher temperature. In that case, energy will be transferred down the rod toward the colder end because higher-speed particles will collide with the slower ones, with a net transfer of energy to the slower ones.

Radiation

Radiation fundamentally differs from conduction and convection in that the bodies exchanging heat are not in contact.  They are many times separated by a vacuum space.  All substances emit radiant energy merely by having a positive absolute temperature.  The higher the temperature, the higher the amount of energy radiated.  All substances also are capable of absorbing radiated energy.  Generally, dull, rough surfaces absorb more heat than bright polished surfaces.  Bright surfaces reflect more energy than dull surfaces.  It is generally accepted that good emitters are sound absorbers, while poor emitters are poor absorbers.

General Heat Conduction Equation

Consider that a rectangular body of length “l” and opposite faces of area “A”, has one face at temperature t’ and the other face at temperature t.  Then (t’-t)/l is the temperature drop per unit distance called the temperature gradient.  The quantity of heat transmitted per second (Q) from one face to the opposite face is proportional to the area of the face and the temperature gradient.

The proportionality constant “k” depends upon the nature of the substance conducting the heat.  The value “k” is the substance’s thermal conductivity (coefficient of thermal conductivity).

It logically follows that “k” for a given substance is the amount of heat conducted per unit time per unit area per unit temperature gradient.

When Q is expressed in terms of BTU/Hour, area in square feet, (t’-t”) in degrees F, and “l” in feet, then k must be expressed in terms of BTU / Hour /degree F / foot.

In engineering work, the area is expressed in square feet, the temperature gradient in degrees F per inch, and the rate of heat conduction in BTU / Hour.  Using these units, k for a given substance is the number of BTU’s conducted per hour per square foot for a temperature gradient of 1 degree F per inch of thickness.

Q = k * area * temperature gradient = kA ((t’-t”)/l)

General Radiation Heat Transfer Equation

The Stefan-Boltzmann equation relates the total amount of radiation emitted by an object to its temperature:

P = e * s * A *(Th⁴-Tc⁴)
P = power radiated in watts / square meter / second
e = emissivity
s = 5.6703 * 10^-8 watt /( meter²K⁴)(Stefan-Boltzmann Constant)
A = radiating area in square meters
Th= Temperature of warm surface in K
Tc= Temperature of cold surface in K

Emissivity is the ratio of radiation emitted by a blackbody or a surface and the theoretical radiation predicted.  A material’s surface emissivity is a measure of the energy emitted when a surface is directly viewed.  Surface emissivity is generally measured indirectly by assuming that e = 1 reflectivity.  A single energy bounce is measured and the reflected energy measured.

Material Emmissivity Chart