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Heat Transfer

Physics defines heat as the transfer of a certain amount of thermal energy in a thermodynamic system which is transferred across a well defined boundary. Heat is not statistically contained inside the matter and is the characteristic of a process. Heat in engineering context, is synonymous to the thermal energy and does not necessarily involve its transfer. A temperature difference leads to heat transfer form the high temperature gradient to low temperature gradient

There are various mechanisms which can be considered the fundamental modes for transferring heat

1. Conduction- In the process of conduction, the heat is transferred by the adjacent atoms of a system which vibrate against each other. It also transferred by the movement of electrons from one atom to another. Conduction is the most widely occurring method in heat transfer within solid a solid or between two or more solids which are in thermal contact with each other. Gases are less conductive in comparison to solids. The study conductance between two or more solid bodies which are in contact with each other is called thermal contact conductance. When the temperature difference is constant during the process of conduction, the conduction is called steady state conductance and when the temperature changes within an object as function of time, the process is called transient conduction.

2. Convection- When the movement of fluids cause the transfer of heat from on place to another, the process is called convection or convective heat transfer. When the fluid flows in bulk motion, there is enhance heat transfer between the solid surface and the fluid. Convection occurs mostly in liquids and gases and describes the combined effects of fluid flow and when the temperature variations in the fluid lead to density variations, the buoyancy forces are created which causes the fluid motion of the liquid. This is termed as free or natural convection. When external means (like fans stirrers, pumps etc) are used to create the necessary force for the flow of liquid, they create a convection current which is artificially induced. This is called forced convection.

3. Radiation- All matters process thermal energy. A pool of thermal energy emits electromagnetic waves which have a temperature above absolute zero. These waves are called as thermal radiations. The random movement of atoms and molecules in a matter directly lead to emission of thermal radiation. These atoms and molecules are also composed of other charged particles like electrons and protons. The emission resulting from movements of these charged particles are termed as electromagnetic radiation.

4. Mass transfer- In the process of mass transfer, the physical transfer of hot or cold objects form one place to another causes the transfer of energy. The movements of iceberg in changing ocean currents, placing of hot water in bottle are some examples of the process of mass transfer.

The phase change or transition also causes transfer of heat. The phase changes from water to ice, ice to water, water to steam etc involve significant changes in energy levels and the process is used in refrigerators, steam engines etc. Heat transfer during the process of boiling is complex, and is characterized by an S shaped curve which relates the heat flux to the surface temperature difference. During the process of condensation, the latent heat of vaporization must be released.

Modeling approaches to heat transfer

Complex heat transfer is modeled in different ways

By Heat equation- The partial differential equation in the form of heat equation is used which describes the distribution of heat in a specific region over a certain time period. Some cases have exact solutions while others have equations which are solved numerically using computational methods.

By Lumped system analysis- It is a type of approximation technique used in the process of transient conduction. It is used whenever the heat conduction in an object is faster than heat conduction across the boundary of an object.

Applications of heat transfer

Heat transfer provides broad application for numerous device and systems. Heat transfer principles are use to preserve, increase or decrease temperature across various type of circumstances.

The thermal insulators are designed for reducing the flow of heat by limiting convection, conduction or both. Radiant barriers are those materials which reflect radiation and are therefore used to reduce the flow of heat from radiation sources. Good insulators not always make good radiant barriers and vice versa. Heat exchangers cause the heat transfer from one fluid to another and are used in air conditioning, space heating, power generation and chemical processing. Heat dissipation is carried out by heat sinks which transfer the heat generated within a solid material to a fluid medium. These heat sinks are used in air conditioning and refrigerating systems as well as in car’s radiators. Heat sinks are also used in CPU’s, light emitting diodes, higher power lasers etc.