Thermal Diffusivity ,Convective heat transfer coefficient ,Extended surfaces

Thermal Diffusivity

Thermal diffusivity is the ratio of thermal conductivity (k) to the product of density of substance and its specific heat. That is.

It’s unit is m²/s. It tells us that how fast heat is propagated or diffused through a material during change of temperature with time. The larger the value of thermal diffusivity the shorter is the time required for the applied heat to penetrate heat deeper into the solid.

Convective heat transfer coefficient (h):

Convective heat transfer coefficient is defined as the ratio of heat flux (W/m²) and temperature (K). Its unit becomes W/m²K. It significantly varies with the type of fluid and temperature. The value of h in natural convection varies between 5 to 28 and for forced convection it varies between 20 to 500.

Extended surfaces:

The heat conducted through solids, walls or boundaries has to be continuously dissipated to the surroundings or environment to maintain the system in a steady state condition. In many engineering applications large quantities of heat have to be dissipated from smaller areas. Heat transfer by convection between a surface and the fluid surrounding can be increased by attaching to surface thin strips of metal called fins. The fins increase the effective area of the surface thereby increasing the heat transfer by convection. The fins are also referred to extended surfaces. Fins are manufactured in different geometries, depending upon practical applications. The ribs attached along the length of a tube are called longitudinal fins. The concentric annular discs around a tube are termed circumferential fins. Pin fins or spines are the rods protruding from the surfaces.

The fins may be uniform or variable cross section. They have many different practical applications, viz cooling of electronic components, cooling of motorcycle engines, compressors, electric motors, transformers, refrigerators, high-efficiency boilers. Solid gas turbine blades often act as fins, conducting heat down their length to a cool disc, use of fin theory is also made in estimating the error in temperature measurement by thermometers or thermocouples.

The problem of determination of heat flow through a fin requires the knowledge of temperature distribution through it. This can be obtained by regarding the fin as metallic plate connected at its base to a heated wall and transferring heat to a fluid by convection. The heat flow through the fin is by conduction. Thus the temperature distribution in a fin will depend upon the properties of both the fin material and surrounding fluid.