Heat transfer and skin friction comparison of dimpled versus protrusion roughness

This paper presents results of skin friction and heat transfer measurements made on flat plates with closely-spaced, three-dimensional surface roughness elements (hemispherical dimples, hemispherical protrusions, and rectangular protrusions) in turbulent boundary-layer flow at velocities ranging from 18 to 40 m/s (roughness Reynolds nos. from 1500 to 4000). The roughness element densities for each plate were chosen using Simpson's sand-grain roughness correlation to study the effects of spacing on drag and heat transfer in the vicinity of the peak of the Simpson-Dvorak correlation curve. The data is presented in terms of the efficiency factor, eta = (St(r)/St(s)/(C-fr/C-fs, to provide a means of comparing the effects on skin friction and heat transfer collectively. Dimpled surfaces have the advantage of significantly increasing heat transfer at a lower penalty in increased drag compared to the protrusion roughness studied.