An investigation of the thermal and hydrodynamic characteristics of an array of protruding elements in single phase forced convection

Experiments were performed to determine the convective heat transfer coefficients for water cooling of inline and staggered arrays of 30 heated protruding elements arranged in six rows. The channel-height-based Reynolds number ranged from 150 to 5150. The channel height was varied over values of 1.2, 1.9, 2.7, and 3.6 element heights. The streamwise and spanwise spacings between elements were varied over a maximum of five values in the range of 0.5 to 6.5 element heights at each channel height. Pressure drops were measured in all cases. The extent of heat transfer enhancement due to vortex generators installed upstream of the array was investigated. The data for all spacings were correlated using an array Reynolds number which accounted for a partitioning of the flow into bypass and array flow. Transition Reynolds numbers were deduced from the heat transfer data, flow visualization and from LDV measurements.

Flow visualization was performed by entraining hydrogen-bubble and dye sheets into the flow. The contributions to mixing of the hydrodynamic features of the flow such as horseshoe vortices, wakes, separated shear layers, reattachment, recirculation, and upwash were investigated using flow visualization. The reattachment length downstream of elements varied from 4 to 1.5 element heights, decreasing both with an increase in Reynolds number, and a decrease in channel height. Mean and fluctuating components of the streamwise and normal velocities were obtained using Laser Doppler Velocimetry. The flow was shown to divide into a high-velocity bypass stream passing over the elements and an array stream passing through the elements. Velocity measurements were used to determine the edge of an "array shear layer". The thickness of the array shear layer was found to increase with distance downstream into the array, showing the spreading influence of the array into the bypass stream. The effect of channel height and streamwise spacing between elements on the mean and fluctuating velocity profiles was investigated. Maximum turbulence intensities were found to occur at the level of the tops of the elements indicating the shear-flow mechanism of turbulence generation.