Heat transfer and flow visualization experiments of swirling, multi-channel, and conventional impinging jets

Heat transfer and flow visualization experiments were conducted to investigate and compare the performance of swirling and multi-channel impinging jets with that of a conventional impinging jet (CIJ), having the same diameter al the same conditions. Swirling impinging jets (SIJs) employed a 25.4 mm long solid insert at the exit of housing tube to divert the air how through four narrow channels along the surface of the insert, with the desired swirl angle (theta of 15, 30 and 45 degrees). The multi-channel impinging jet (MCIJ) had same dimensions as SIJs, except that the narrow channels in the solid insert were vertical (theta = 0 degrees). The local and surface average Nusselt numbers of MCIJ were generally much higher than those of CIJ. SIJs demonstrated large increases in both Nusselt numbers and significant enhancement in radial uniformity of heat transfer compared to MCIJ and CIJ; best results were for theta = 15 degrees and jet spacing of 50.8 mm. Flow visualization experiments using smoke flow, smoke wires and water jet techniques revealed the mechanisms contributing to the higher and enhanced radial uniformity of heat transfer by SIJs. The smoke flow technique provided images of flow field between jet exit and impinged surface, while smoke wires showed details of flow field at and close to impinged surface. The water jet flow, seeded with tiny air bubbles as tracers, revealed details of flow field and induced mixing on the impinged surface. (C) 1997 Elsevier Science Ltd.