# EnggPedia - The Engineering Encyclopedia

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# Basics of Heat Transfer | Modes of Heat Transfer

Heat  is a form of energy which travel from hot body to cold body. There is a small difference between heat and temperature. No medium need exist between the two bodies for heat transfer to take. The basic effect of heat transfer is that the particles of one substance collide with the particles of another substance.  Temperature is the degree of hotness and coldness. Heat always flows wherever temperature difference present between two bodies or region. Since heat transfer is concerned with only two things: temperature, and the flow of heat.

 Heat energy is related to kinetic energy of the molecules. Heat transfer methods are used in numerous disciplines, such as automotive engineering, thermal management of electronic devices and systems, climate control, insulation, materials processing, and power plant engineering. Several material properties serve to modulate the heat transferred between two regions at differing temperatures. Examples include thermal conductivities, specific heats, material densities, fluid velocities, fluid viscosities, surface emissivities, and more. Taken together these properties serve to make the solution of many heat transfer problems an involved process.

There three modes of heat transfer according to movement of molecules. Conduction: conduction is the mode of heat transfer through vibration of atoms or kinetic energy of molecules. The best example of conduction is the heat transfer in metal. Fourier Law of Heat Conduction: heat flow from the region of high gradient to a region of lo gradient when there is a temperature gradient exist, this is known as Fourier law of heat conduction. Convection: Heat energy transfers between a solid and a fluid when there is a temperature difference between the fluid and the solid. Generally, convection heat transfer cannot be ignored when there is a significant fluid motion around the solid. Blowing air over the solid by using external devices such as fans and pumps can also generate a fluid motion. This is known as "forced convection”. Fluid mechanics plays a major role in determining convection heat transfer.

For each kind of convection heat transfer, the fluid flow can be either laminar or turbulent. Laminar flow generally occurs in relatively low velocities in a smooth laminar boundary layer over smooth small objects, while turbulent flow forms when the boundary layer is shedding or breaking due to higher velocities or rough geometries. Radiation: Radiation heat transfer is concerned with the exchange of thermal radiation energy between two or more bodies. Thermal radiation is defined as electromagnetic radiation in the wavelength range of 0.1 to 100 microns (which encompasses the visible light regime), and arises as a result of a temperature difference between 2 bodies.

The human body must maintain a consistent internal temperature in order to maintain healthy bodily functions. Therefore, excess heat must be dissipated from the body to keep it from overheating. When a person engages in elevated levels of physical activity, the body requires additional fuel which increases the metabolic rate and the rate of heat production. The body must then use additional methods to remove the additional heat produced in order to keep the internal temperature at a healthy level. In order to ensure that one portion of the body is not significantly hotter than another portion, heat must be distributed evenly through the bodily tissues. Blood flowing through blood vessels acts as a convective fluid and helps to prevent any buildup of excess heat inside the tissues of the body. This flow of blood through the vessels can be modeled as pipe flow in an engineering system. The heat carried by the blood is determined by the temperature of the surrounding tissue, the diameter of the blood vessel.