Experimental study of forced convective heat transfer in composite packed beds of spheres

In the present paper, the forced convective heat transfer was experimentally studied in a square channel fully filled with steel spherical particles. Three kinds of randomly packed beds were constructed, including two kinds of uniform packings with mono-sized particles of d_p=6mm or 12mm, respectively, and a kind of composite packing with both particles of d_p=6mm and 12mm. The packing structures of these packed beds were reconstructed with discrete element method (DEM), and the radial porosity distributions were analyzed and compared with each other. Furthermore, the pressure drop, heat transfer and overall heat transfer performances of different packings were also carefully investigated. Firstly, it is found that, for mono-sized packing, the packed bed with lower tube to particle diameter ratio (D/d_p) would be less compact and the tube-wall effect would be remarkable. With composite packing method, the tube-wall effect in the packed bed with low D/d_p would be restrained in the near tube wall region and the air flow rate in inner region would be enhanced. Secondly, the pressure drop and heat transfer capacity of mono-sized packing with large D/d_p were found to be much higher than those of packing with low D/d_p, while the overall heat transfer capacities were close with each other.