Experimental Investigation of Enhancement Heat Transfer by Natural Convection from Four Finned Cylinders in a Vertical Array Placed in a Conduit

In this research, experiments have been implemented to enhance heat transfer by free convection from four finned cylinders that have helical fins, fixed horizontally inside conduit and using air as working fluid, the cylinders heated by electrical heaters at constant heat flux (188,341,573,832)W/m2 for Rayleigh number from (5ꓫ104 to 11.5ꓫ104 ) and Prandtl number (0.71)at the ambient temperature, approximately (24–26)oC and pressure at (1 atmosphere). The investigation include designed and manufactured test model, consist from two parts: moving and stationary, the stationary part is a frame made from iron that allows to move the part to inside it, according to ratio(H/D). The ratio (Y/D) changed four times (Y/D=2,4,6,8), the ratio (X/D) changed four times (X/D=1.5,3,5,7) and the ratio (H/D) changed four times (H/D=15,20,25,30). The results showed for all values of heat fluxes, the heat transfer coefficient and Nusselt number increases at separation distances (Y/D=4) & (X/D=3), which gives greater heat transfer rate, and any increasing or reducing lead to reduction in heat transfer rate. But regarding the ratio (H/D), the results showed there was no obvious influence or simple influence on heat transfer. Also, the results showed increased Nusselt number, with increasing Rayleigh number for all cylinders. The experiments have been implemented for single cylinder, placed in free air under constant heat flux, calculated (Nuo) and compared it with (Nui) for cylinders arranged in vertical array at the best case (Y/D=4) & (X/D=3) under the same heat flux (q ͌=341,573,832)W/m2 . The results show, there was obvious enhancement for the cylinders arranged in vertical array compared with singular cylinder in heat transfer, furthermore, this enhancement increases with increasing (Ra). From the results, empirical equation was found, to calculate the value of (Numean) that includes the two variables with (Ramean) as follows: Numean= 0.765Ra0.2 (Y/D) 0.08 (X/D) 0.01 (5ꓫ104≤ Ra ≤ 11.5ꓫ104).