The Effect of Roughness on the Heat Transfer from a Pipe to a Moving Fluid.

Experiments were carried out on water flowing turbulently inside each of the five following steam heated pipes:- 1. Three copper pipes, their internal surfaces being artificially roughened by a special knurling process which produced a pattern of frasta of pyramid geometrically similar in form But varying in absolute size from pipe to pipe. The conventional roughness ratios (height of frustum of pyramid / radius of pipe) were 0.0309, 0.0228 and 0.0147. 2. A smooth copper pipe of the same inner diameter as the three artificially roughened pipes. 3. A smooth Brass pipe of 1.25 inch inside diameter. In case 1, the effect of roughness on the heat transfer and friction was investigated, and the empirical equations correlating the average heat transfer and overall roughness, By introducing the friction velocity, were deduced. Also, the average heat transfer data and friction data were correlated By assuming the eddy diffusivity for heat to Be equal to that for momentum. To show the direct effect of the height of roughness on the correlation Between Nusselt number, Prandtl number and Reynolds number, the conventional roughness ratio e/r was introduced as a parameter. The friction data for all tubes with and without heat addition are investigated. Also, a correlation Between roughness function and roughness characteristic was deduced and the equivalent sand-grain roughness was found. The results of the writer's experiments on rough tubes were correlated with those in previous research on air flowing inside tubes having square-thread type roughness and those on water flowing inside tubes having pyramid-type roughness. In Case (2) the average coefficients of heat transfer and the coefficients of friction were compared with those of the rough tubes. In Case (3) the average coefficients of heat transfer and the coefficients of friction were calculated in order to compare them with those obtained in Cases 1 and 2. In Cases 1 and 2, the efficiency of each tube was investigated By comparing the horse-power transmitted as heat with the horse-power expended in forcing the fluid through the same length of tube. In all cases the change in slope of the curve representing the relation Between Nu/Pr0.4 and Re was investigated. Finally, the effect of evaluating the physical properties of water at Bulk temperature, film temperature and surface temperature, on the equation correlating Nusselt number, Prandtl number and Reynolds number was investigated in all three cases