MODES OF HEAT TRANSFER : CONDUCTION,CONVECTION AND RADIATION EXPLAINED !!

Heat, energy that is transferred from one body to another as the result of a difference in temperature.Heat will always be transferred from higher temperature to lower temperature independent of the mode. The energy transferred is measured in Joules (kcal or Btu). The rate of energy transfer, more commonly called heat transfer, is measured in Joules/second  (kcal/hr or Btu/hr). 

Heat is transferred by three primary modes:

• Conduction (Energy transfer in a solid)
• Convection (Energy transfer in a fluid)
• Radiation (Does not need a material to travel through)

CONDUCTION :

Conduction is the transfer of heat between substances that are in direct contact with each other. The better the conductor, the more rapidly heat will be transferred.If one body is at a higher temperature than the other, the motion of the molecules in the hotter body will vibrate the molecules at the point of contact in the cooler body and consequently result in increase in temperature.   The amount of heat transferred by conduction depends upon the temperature difference, the properties of the material involved, the thickness of the material, the surface contact area, and the duration of the transfer. 

Metals are good conductors of heat, while gaseous substance, having low densities or widely spaced molecules, are poor conductors of heat. Poor conductors of heat are usually called insulators. The measure of the ability of a substance to insulate is its thermal resistance. This is commonly referred to as the R-value (RSI in metric).  The R-value is generally the inverse of the thermal conductivity, the ability to conduct heat.

Typical units of measure for conductive heat transfer are:

Per unit area (for a given thickness)

Metric (SI) :  Watt per square meter (W/m)

Overall 

Metric (SI) :  Watt (W)  or kilowatts (kW)

CONVECTION :

When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal energy along. This type of heat transfer is called convection. The fluid above a hot surface expands, becomes less dense, and rises.There are two types of convection: natural and forced. In case of natural convection, the fluid in contact with or adjacent to a high temperature body is heated by conduction. As it is heated, it expands, becomes less dense and consequently rises. This begins a fluid motion process in which a circulating current of fluid moves past the heated body, continuously transferring heat away from it. In the case of forced convection, the movement of the fluid is forced by a fan, pump or other external means.  A centralized hot air heating system is a good example of forced convection.  

Units of measure for rate of convective heat transfer are:

Metric (SI) : Watt (W) or kilowatts (kW)

RADIATION:

Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object as is the case with conduction and convection. Heat can be transmitted through empty space by thermal radiation often called infrared radiation. This is a type electromagnetic radiation . No mass is exchanged and no medium is required in the process of radiation. Examples of radiation is the heat from the sun, or heat released from the filament of a light bulb.

Typical units of measure for rate of radiant heat transfer

Metric (SI) ——Watt per square meter 

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