Heat Transfer

Heat is energy transferred between materials due to temperature differences. There are three mechanisms which are responsible for thermal energy transfer conduction, convection and radiation. These mechanisms occur individually or in combination.

1. Conduction:-

                Conduction is process of heat transfer by molecular transport and microscopic interactions. Conduction can occur in solids, liquids and gases but usually is only a major contributor in heat transfer through solids. Conduction through material is explained by Fourier’s  Law.

2. Convection:-

                Convection is the process of heat transfer by macroscopic movement of molecules. There are two mechanisms responsible for convection

                1. Molecular Diffusion:-

                                It involves random motion of molecules due to their internal energy content.

                2. Macroscopic Motion:-

                                It occurs because of either forced movement of fluids or natural convection that results                       from density changes due to temperature difference.

3. Radiation:-

                In most systems radiation is seldom a significant contributor to the total heat flux; however when vacuum is present or extremely large temperature difference greater than 100°C exist,  radiation effects should be considered. There are many systems where radiation from sun is the only contributor such as solar drying of grass, fruit or lumber.  

                Radiation is the energy transfer by electromagnetic waves which can include both visible as well as invisible. Radiation does not require a medium such as fluid or solid to transfer energy. Radiation can be reflected, absorbed or transmitted between surfaces. A black body is defined as a material that can absorbs 100% of incident radiation and reflects none. Emissivity of any body is important factor used during radiation. Emissivity is defined as ratio of emitted energy by a material to that of a black body at same temperature. For most non-metallic materials emissivity ranges from 0.90-0.95 and for metallic materials emissivity varies from 0.02-0.9 depending on surface finish and material composition.