INTRODUCTIONConvection is the movement of molecules within fluids (i.e. liquids, gases). It cannot take place insolids, since either bulk current flows or significant diffusion can take place in solids. Convection isone of the major modes of heat transfer and mass transfer. Forced convection is a mechanism, or type of heat transport in which fluid motion isgenerated by an external source (like a pump, fan, suction device, etc.). It should be considered asone of the main methods of useful heat transfer as significant amounts of heat energy can betransported very efficiently and this mechanism is found very commonly in everyday life,including central heating, air conditioning, steam turbines and in many other machines. Forcedconvection is often encountered by engineers designing or analyzing heat exchangers, pipe flow,and flow over a plate at a different temperature than the stream (the case of a shuttle wing during re-entry, for example). However, in any forced convection situation, some amount of natural convectionis always present whenever there are g-forces present (i.e., unless the system is in free fall). Whenthe natural convection is not negligible, such flows are typically referred to as mixed convection. The removal of excessive heat from system components is essential to avoid damagingeffects of burning or overheating. Therefore, the enhancement of heat transfer is an importantsubject of thermal engineering. Extended surfaces (fins) are frequently use in heat exchangingdevices for the purpose of improve the heat transfer between a primary surface and the surroundingfluid.
THEORYHeat transfer from an object can be improve by increasing the surface area in contact with the air byadding fins or pins normal to the surface. This can be seen in Newton’s Law of Cooling that statesthat the rate of heat loss of a body is proportional to the difference in temperatures between the bodyand its surroundings, which defines the convection heat transfer rate. The constant of proportionality h is termed the convection heat-transfer coefficient. The heattransfer coefficient h is a function of the fluid flow, so, it is influenced by the surface geometry, thefluid motion in the boundary layer and the fluid properties as well. The effect of the surfaces can bedemonstrated by comparing finned and unfinned surfaces with a flat plate under the same conditionsof power and flow. A heated surface dissipates heat to the surrounding fluid primarily through a process calledconvection. Heat is also dissipated by conduction and radiation, however these effects are notconsidered in this experiment. Air in contact with the hot surface is heated by the surface and risesdue to reduction in density. The heated air is replaced by cooler air, which is in turn heated by thesurface, and rises. This process is called free convection. Convection heat transfer from an object can be improved by increasing the surface area incontact with the air. In practical it may be difficult to increase the size of the body to suit. In thesecircumstances the surface area in contact with the air may be increased by adding fins or pinsnormal to the surface. These features are called extended surfaces. A typical example is the use offins on the cylinder and head on an air-cooled petrol engine. The effect of extended surfaces can bedemonstrated by comparing finned and pinned surfaces with a flat under the same conditions ofpower input and airflow.