forced convection heat transfer from a circular cylinder embedded in ...

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NOMENCLATURE L Diameter of the porous Diameter of the cylinder (16 mm) Darcy Number, Permeability Thermal conductivity of Spheres Thermal conductivity of air Thermal conductivity, ( ) Length of the bed Prandtl number Q Heat flux ( ) Heat flux per unit area ( ) Reynolds number Inlet temperature Surface temperature of the cylinder Average velocity Diffusivity Porosity Nu Nusselt number (( – ) ) P Pressure Pe Peclet number, Kinematic viscosity ρ Density of air µ Viscosity 3. EXPERIMENTAL METHOD The experiment includes a bed of porous media in an insulated duct. Inlet velocity was measured by hot wire transducer and pressure drop was measured before and after the porous bed using a monometer. A power supply was connected to a cylindrical heater embedded in the porous media to generate heat ( ). T-type thermocouples were used to measure the surface temperature of the cylinder ( ) as well as the flow temperature in the downstream, (figures 1 & 2). The porosity ( ) was found to be 0.388. Subsequently, permeability was obtained by using Kozeny-Carman equation: ( ) where is the diameter of the spheres, (6 mm). Thermal conductivity of the porous material was taken as 1.3 W/m °C. Permeability was found to be and Darcy number was calculated as . 2
Figure 1 : Experimental Setup (Front View) each air flow five different heat fluxes were supplied to the element. By changing the heat flux and waiting for a while to reach to a steady state surface temperature, all temperatures were automatically recorded by the means of a data acquisition system. The whole process was done for both the clear flow and flow in the porous medium. To compare the heat transfer rate for each medium, Nusselt number versus Reynolds number were plotted for all the cases. 4. RESULTS AND DISCUSSION Figure 2 : Experimental Setup (top view) The air flow was supplied by the use of a suction type wind tunnel. The inlet temperature, the surface temperature of the cylindrical heater, and downstream temperature in the porous media, were recorded through the data acquisition system. The experiment was carried out for three different air velocities, which were measured using the hot wire transducer. Pressure drop before and after the porous bed was measures using a manometer. For Table 1 shows different heat fluxes and their corresponding steady state surface temperature for various numbers. The difference between the surface temperature of the cylindrical heater and inlet temperature in the case of porous media is much lower than the case of clear fluid. For example, in the case of Re = 1800 and Q=15.9 W the difference between the inlet temperature and the surface temperature is 17.2 ºc in the porous medium case and 50.1 ºc in the case of clear flow. One of the main aims of such setup is to enhance the heat transfer and to decrease the temperature of the element to the lowest degree possible. Here, we can see the difference in the heat transfer effectiveness. That is, the temperature of the surface of the cylinder recorded 35.7 ºc 3
Page 1: FORCED CONVECTION HEAT TRANSFER FRO
Page 5 and 6: Variation of Nu in porous medium us

forced convection heat transfer from a circular cylinder embedded in ...

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