Influence of Roughness Shape and Spacing on the Performance of Three-Dimensional Helically Dimpled Tubes

Abstract

A comparison of measured and predicted heat-transfer and friction-factor values is first presented for two newly developed doubly enhanced tubes. The internal enhancement consists of three-dimensional roughness elements that are essentially spherical segments and are formed by externally dimpling the outside surface. Very good agreement was obtained with these experimental data and the heat-transfer and friction-factor values obtained with prediction methods developed by Taylor and Hodge (1992a, 1992b) for three-dimensional roughened tubes. These prediction methods were then used to determine the variations of the inside heat-transfer enhancement level and the efficiency index (heat-transfer enhancement divided by the friction-factor increase) for different spherical shapes and spacings. This analysis revealed the following: significant performance improvements are possible with a greater roughness curvature and closer roughness spacings. The maximum enhancement level and efficiency-index value of 3.5 and 1.7, respectively, were obtained for water with touching half-spheres. These predicted values exceed all previously published values for passive enhancement geometries. However, these values are obtained for roughness geometries that are very different from those used for validation of the prediction algorithms developed by Taylor and Hodge (1992a, 1992b).