Turbulent flow and heat transfer in channels with combined rough and smooth surfaces.

Abstract

A two-part experimental investigation is reported on the effects of transverse square rib roughening on fluid flow and heat transfer in channels with uniform and non-uniform boundary conditions. The first part of the experimental programme consisted of providing detailed measurements of mean and basic turbulent characteristics of fully developed, flow in two rectangular ducts of aspect ratios 1.63 and 3.0. In each duct only one wall was roughened. In channels having low aspect ratios secondary flows play an important part in momentum transfer, and an interpretation of their effect on the measured Reynolds shear stress distribution has been attempted. In the second part of the experimental programme mean velocity and temperature profiles, friction factors and Stanton numbers were measured in an internally roughened pipe and annuli composed of a rough inner rod and either a smooth or a rough outer pipe. Heating was always applied on the outer surface. In all the geometries the mean velocities near the rough walls were found to be represented by logarithmic straight lines. The gradients of these lines were independent of Reynolds number but differed for various geometries. The mean temperature profiles, measured in the rough pipe and the fully rough annulus, showed that these could also be represented by logarithmic straight lines, but the slopes of these profiles were markedly different from those of the velocity profiles. These results cast considerable doubt on the validity of the 'universal law of the wall' and Nikuradse's drag law for ribbed surfaces, as well as on the correctness of applying Dipprey and Sabersky's heat-momentum analogy to such surfaces. Finally, the various transformation methods proposed for the interpretation of measured pressure drop and heat transfer in channels with mixed rough and smooth surfaces have been analysed. A new simple transformation method has been proposed for the overall pressure drop results.