Effect of Surface Roughness on Heat Transfer from Horizontal Immersed Tubes in a Fluidized Bed

Abstract

Experimental results of the total heat transfer coefficient between 12.7 mm dia copper tubes with four different rough surfaces and glass beads of three different sizes as taken in a 0.305 m × 0.305 m square fluidized bed as a function of fluidizing velocity are reported. The comparison of results for the rough and technically smooth tubes suggests that the heat transfer coefficient strongly depends on the ratio of pitch (Pf) to the average particle diameter (dp), where Pf is the distance between the two corresponding points on consecutive threads or knurls. By the proper choice of (Pf/dp) ratio, the maximum total heat transfer coefficient for V-thread tubes (hwfb) can be increased by as much as 40 percent over the value for a smooth tube with the same outside diameter. However, for values of (Pf/dp) less than 0.95, the maximum heat transfer coefficient for the V-thread rough tubes is smaller than the smooth tube having the same outside diameter. The qualitative variation of the heat transfer coefficient for rough tubes with (Pf/dp) is explained on the basis of the combined effect of contact geometry between the solid particles and the heat transfer surface, and the solids renewal rate at the surface. The present findings are critically compared with somewhat similar investigations from the literature on the heat transfer from horizontal or vertical rough tubes and tubes with small fins.