Determination of friction factors and heat transfer coefficients for flow past artificially roughened surfaces

A critical review of the underlying assumptions, theoretical foundations, and supporting experimental evidence for the analytical procedures in current use for the evaluation of roughness effects of heat transfer surfaces of gas cooled reactors are discussed. A method is demonstrated for determination of the slope and intercept of the universal logarithmic dimensionless velocity distribution law for past roughened surfaces without the need for experimental measurement of the velocity profile. The slope is shown to vary with the nature of the roughened surface and to deviate considerably from the slope for turbulent flow past smooth walls in some cases. It is further shown that the intercept, which is a boundary condition equivalent to the roughness parameter for friction, is independent of the width of the velocity profile. The dependence noted by previous investigators was due to their assumption that the slope of the dimensionless velocity profile for flow past rough surfaces has a value equal to that of the slope of this profile in flow past smooth surfaces. A similar method is developed for determination of the slope and intercept of the dimensionless temperature distribution law, but additional experimental investigation is required before the efficacy of this application can be conclusively established.