30-04-2012, 02:59 PM
EXPERIMENTAL ANALYSIS OF FINS WITH DIFFERENT CROSS SECTONS
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ABSTRACT
The fins are generally used to increase the heat transfer rate from the system to the surroundings by increasing the heat transfer area. The fins are generally extended surfaces or projections of materials on the system.
The fins are commonly used on small power developing machine as engines used for motorcycles as well as small capacity compressors. They are also used in many refrigeration systems (evaporators and condensers) for increasing the heat transfer rates.
In the present analysis, the fins that are of different cross sections and of same material (aluminium) are considered. The knowledge of efficiency and effectiveness of the fin are necessary for proper design of fins. The main objective of our analysis is to determine the most effective cross section among the various cross sections available. The efficiency and effectiveness of various cross sections are determined experimentally by cross sectional area and volume as constant for each cross section.
The various cross sections, which are adopted, are:
• Triangular
• Square
• Hexagon
• Hollow triangle
• Hollow circular
• Hollow Square
The fins, which are taken in the analysis, are experimented for the condition of fin with insulated end i.e. the fin is short fin with insulated end. Comparison is made among the solid sections and between the hollow and solid sections. The graphs plotted give a clear view of the comparisons. In the experiment, various cross sections of aluminium are taken due to its lightweight and high conductivity and it is most widely used in the industrial applications.
NECESSITY OF FINS
The heat that is generated produced or developed in the system that conducts through the walls or boundaries is to be continuously dissipated to the surroundings or environment to keep the system in steady state condition. Large quantities of heat have to be dissipated from small area as heat transfer by convection between a surface and the fluid surroundings. It can be increased by attaching thin strips of metals called fins to the surface of the system.
The fin is generally an extended surface on the system. Whenever the available surface is found to be inadequate to transfer the required quantity of heat with the available temperature drop & convective heat transfer coefficient, the surface area exposed to the surroundings is frequently increased by attachment to protrusions to the surfaces. These protrusions are called fins or spines. Thus, the fins increase the effective area of surface there by increasing the heat transfer by convection.
MODES OF HEAT TRANSFER
Heat transfer is defined as the transmission of energy from one region to another as a result of temperature gradient takes place by the following three modes
1. conduction
2. convection
3. radiation
Heat transmission occurs as a result of combinations of these modes of heat transfer.
Heat transfer from the surface to fin at its base by conduction. This heat is convected to surrounding atmosphere over the fin surface.
Conduction :
The heat conduction is accomplished by two mechanisms
by molecular interactions
by drift of free electrons
By molecular interaction, the energy exchange takes place by kinetic motion or direct impact of molecules. Molecules at a relatively higher energy level impart energy to adjacent molecules at lower energy levels. This type of energy transfer always exists so long as there is a temperature gradient in a system comprising molecules of a solid of gas.
By the drift of free electrons, as in the case of metallic solids. The metallic alloys have a different concentration of free electrons, and their ability to conduct heat is directly proportional to the concentration of free electrons in them.
Convection :
Convection is the transfer of heat within a fluid by mixing of one portion of the fluid with another. Convection constitutes the microform of the heat transfer since macroscopic particles of a fluid moving in space cause the heat exchange. The effectiveness of heat transfer by convection depends largely upon the mixing motion of fluid. Convection is met with in situations where energy is transferred as heat to a flowing fluid at any surface over which flow occurs. The heat flow depends on the properties of fluid and is independent of the properties of the material of the surface. However, the shape of the surface will influence the flow and hence the heat transfer.
Convection is of two types
1. Natural convection: The temperature difference produces a density difference results in mass movements.
2. Forced convection: The motion of the fluid is caused by an external device like pump, compressor.
IMPORTANT PARAMETERS IN ANALYSIS OF FINS :
The various important parameters in the analysis of fins are
1. Heat transfer coefficient
2. Length of the fin
3. Cross sectional area of the fin
4. Thermal conductivity of fin
5. Efficiency and Effectiveness of fin.