Experimental investigation of heat transfer and drag on surfaces with spherical dimples

Abstract

The results of an experimental investigation of the heat transfer and the hydraulic drag in air flow past models with different configurations of vortex reliefs in the form of spherical dimples in a plane surface are considered. To increase the reliability of the results and to reduce their uncertainty the experiments were performed on two aligned models, one of which was smooth (reference model), while the other was coated with the relief under study. Both the thermal and the hydraulic parameters of the two surfaces were simultaneously recorded. The drag coefficient was determined by directly weighing the models under study in the form of floating elements using a one-component strain-gauge balance. The heat transfer coefficient was determined by recording the unsteady heat transfer process and solving the time-dependent three-dimensional heat conduction equation using the measured temperature fields on the surfaces under study. The two-dimensional fields of the heat transfer coefficients on the model surfaces were obtained and the flow over the dimpled surfaces was visualized. The Reynolds number dependences of the drag, heat transfer, and heat-hydraulic efficiency were determined, Re being based on the boundary layer length. The dependences of the mean drag (cx/cx0) and heat transfer (St/St0) coefficients on the dimple arrangement density (streamwise and spanwise pitches) were obtained. For the given set of parameters a heat-hydraulic optimum geometry is determined; for this geometry the mean Reynolds analogy factor RAF=1.1 at St/St0=1.21 and cx/cx0=1.1.