# Heat Transfer Conduction Convection Radiation.doc - Heat...

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Heat Transfer Conduction, Convection, and Radiation Heat Transfer is the study of the rates of thermal energy motion. There are three modes of Heat Transfer: Conduction, Convection, and Radiation. Conduction is concerned with the transfer of thermal energy through a material without bulk motion of the material. This phenomenon is fundamentally a diffusion process that occurs at the microscopic level. Convection is concerned with the transfer of thermal energy in a moving fluid (liquid or gas). Convection is characterized by two physical principles, conduction (diffusion) and bulk fluid motion (advection). The bulk fluid motion can be caused by an external force, for example, a fan, or may be due to buoyancy effects. Finally, Radiation is the transfer of thermal energy through electro-magnetic waves (or photons). It is interesting to note that Radiation requires no medium. Conduction Conduction is the diffusion of thermal energy, i.e., the movement of thermal energy from regions of higher temperature to regions of lower temperature. On a microscopic level, this occurs due to the passing energy through molecular vibrations. Heat flux is denoted as . The units of heat flux are watts. It should be noted that heat flux is a vector quantity. It is often convenient to describe heat flux in terms of the geometry being studied. Thus we define , , and as the heat flux per unit length, area, and volume, respectively. The governing rate equation for conduction is given by Fourier's Law. For one dimension, Fourier's law is expressed as: Where x is the direction of interest, k is a proportionality constant known as thermal conductivity and is the temperature gradient at the location of interest. The negative sign indicates that heat is

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