Heat transfer and flow structure from dimples in an internal cooling passage

Experimental results, measured on and above the dimpled test surface placed on the bottom wall of the channel, are presented for different Reynolds numbers Re_H, channel aspect ratios, ratios of air inlet-stagnation temperature to local wall temperature T_oi/T_w, and top wall configurations (opposite to the dimpled surface). Results include instantaneous qualitative flow structural characteristics using smoke patterns, time- averaged flow structure measurements, surface static pressure measurements, and surface heat transfer measurements.; Flow visualization images show vortex pairs and vortical fluid shed from the dimples, and the large up-wash region and packets of fluid emanating from the central regions of each dimple. When protrusions are added to the top wall, they result in additional vortical fluid and flow mixing.; Time-averaged surveys show the deficits in pressures and velocities as a result of vortex shedding from the dimples. Streamwise vorticity distributions over a spanwise-normal plane give the magnitude, location, and size of time-averaged vortex pairs. Streamwise turbulence intensity distributions show that the maximum turbulent mixing occurs just downstream of each dimple. Friction factors, deduced from time-averaged surface static pressure measurements, are generally unaffected by Re_H and vary by small amounts as the aspect ratio is changed. Friction factors in the channel increase when top wall protrusions are present.; Local Nusselt number ratios Nu/Nu_o increase significantly near the downstream edge of a dimple due to the vortex pairs shed from the dimples and locally enhanced turbulent mixing induced by these vortical fluids. The effects of the secondary flows become more prominent when T_oi/T _w decreases. Smaller H/D values and top wall protrusions also increase Nu/Nu_o near these locations. As a result, Nu/Nu_o increases as either H/D or T_oi/T_w decreases and when top wall protrusions are present. Nu/Nu_o are about constant as Re_H changes.; Thermal performance parameters generally change over a small range as H/D changes at constant Re_H. They increase significantly as T _oi/T_w decreases. These magnitudes are always higher for H/D = 0.25 regardless of Re_H and T_oi/T_w value. With protrusions on the top wall, thermal performance parameters are lower than with other channel geometries.