The Friction and Heat Transmission Coefficients of Rough Pipes

Abstract

In a previous paper (Cope 1937)† an account was given of the simultaneous measurement of the heat transmission and friction coefficients of a series of smooth pipes of various shapes of cross-section. The results formed the first section of a comprehensive research into the problem of heat transmission. The present paper, which describes the results of tests on rough pipes, forms the second section. Three pipes were tested, their internal surfaces being artificially roughened by a special knurling process which produced a series of pyramids geometrically similar in form but varying in absolute size from pipe to pipe. The roughness ratios (radius of pipe/height of pyramid) were approximately 8/1, 15/1, and 45/1. The apparatus used was basically the same as that used in the previous tests; the working fluid was water, and the Reynolds number ranged from 2,000 to 60,000. The results indicate that when fully turbulent conditions are established the roughness has very little effect on the heat transmission coefficient, but that in the transition region between laminar and fully turbulent flow, the roughness may increase that coefficient to considerably more than its value for a smooth pipe; and that the heat transmission graphs for the three pipes are in better agreement if shearing force velocity is used instead of mean velocity, in forming the non-dimensional parameters used for plotting purposes. The broad conclusions from the practising engineer's standpoint are (1) for a given pressure drop across a heat transmission apparatus more heat will be transmitted if the pipes be smooth than if they be rough; (2) the velocity of the working fluid will, of course, be greater with smooth pipes; and (3) the smooth pipe is more efficient if the comparison is made on a basis of heat transmission for equal power.