Experimental study on laminar air flow and heat transfer through a spiral channel filled with steel balls

The demand in the industry for high heat transfer performance is increasing. Studies have been greatly carried out on the new emerging technologies and creating new arrangements in engineering design. Porous media or curved channel increases heat transfer. Studies have been carried out for many years in heat transfer in curved channels and porous media separately. However, in this study, the heat transfer in the case of both curved and porous media is investigated experimentally. The experiments are conducted at constant heat flux in laminar airflow regime. A total of 169 sets of experiments are carried out in 8 experimental groups, in two different radii of curvature, in the unfilled spiral channel and in spiral packed bed formed by locating 3 different ball diameters (2.00, 2.38 and 3.17 mm). Nusselt number (Nu) correlations are presented separately for both the inner surface, the outer surface and taking the average of the inner and outer surface temperatures in unfilled spiral channel. Nusselt numbers (Nu_i, Nu_o, Nu_a) for inner surface, outer surface and the average of the inner and outer surface temperature are obtained as Nu_i=0.93De^0.45, Nu_o=0.50De^0.58, Nu_a=1.16De^0.44 respectively. Heat transfer in spiral packed beds has not been investigated or reported in the available literature. By defining a modified Dean number (De_m) to reveal the effect of curvature in spiral packed beds, a correlation between the modified Dean number (De_m) and the Nusselt number (Nu_p) based on ball diameter is given. Nusselt number for spiral packed beds are presented as Nu_p=0.19De_m^0.81. This correlation is also suitable for straight packed beds when the curvature ratio is neglected.