Heat transfer of impinging jet arrays onto half-smooth, half-rough target surfaces

Detailed heat transfer distributions from arrays of impinging jets on a half-smooth, half-rough target surface were experimentally investigated using a transient liquid crystal technique. The target surface was roughened through the creation of rectangular grooves aligned with the jet holes. The grooved regions were designed either parallel (longitudinal grooves) or orthogonal (transverse grooves) to the exit flow direction. Jet-to-jet spacing and jet-to-surface spacing (Z/d) were 4 and 3, respectively. In this experimental test, the effect of crossflow was investigated for three exit flow directions, each with a jet Reynolds number ranging from 2500 to 7700. Heat transfer was enhanced near the edge of grooves, whereas the heat transfer was degraded inside grooves. For the half-smooth, half-rough surface, the sudden change in surface geometry broke the flow development and caused intensified flow mixing in the impingement flow channel. Compared with traditional fully roughened surfaces, the half-rough surface is more effective for heat transfer, and an enhancement of more than 50% was achieved for the longitudinal grooves. The idea of partially roughened surfaces may be further extended to the other internal flow channels with different roughness elements.