Heat transfer visualization of co/counter-dual swirling impinging jets by thermochromic liquid crystal method

The objective of the experimental study is to investigate heat transfer of co/counter-dual swirling impinging jets (Co-DSIJs/C-DSIJs) on the impingement surfaces under uniform wall heat flux boundary condition. Two twisted-tapes were inserted into pipe nozzles with different arrangements: (1) each tape was twisted in the same direction as the co-dual tapes for producing co dual swirls and (2) each tape was twisted in the different directions as the counter-dual tapes for inducing counter dual swirls. The effect of the Co-DSIJs/C-DSIJs with baffles placed between the tapes for jet confinement on heat transfer was also investigated. The effects of jet Reynolds number (5000 ⩽ Re ⩽ 20,000), jet-to-plate spacing (1 < L/D < 8) and tape twist ratio (y/W = 3, 4, 5 and 6) were examined. In addition, the experiments using a single swirling impinging jet (SIJ) and a conventional impinging jet (CIJ) were also carried out, for comparison. The temperature distributions on impinged surfaces were recorded via a thermochromic liquid crystal (TLC) sheet and then Nusselt number distributions were obtained by a liquid crystal thermography technique. The experimental results revealed that for all Co-DSIJs and C-DSIJs, heat transfer increased with decreasing nozzle-to-plate spacing (L/D) and increasing Reynolds number. For the Co-DSIJs and C-DSIJs without baffle, heat transfer increased with increasing twist ratio (decreasing swirl number) while the opposite trend was found for the Co-DSIJs and C-DSIJs with baffle. At similar conditions, the Co-DSIJs and C-DSIJs with baffles offered higher heat transfer than the ones without baffle, owing to the combined effect of swirling flow and jet confinement. For small jet-to-plate spacings (L/D = 1 and 2), all swirling jets possessed considerably higher average Nusselt numbers than the conventional jets, at similar conditions. At large jet-to-plate spacings (L/D = 4, 6 and 8), the average Nusselt numbers of the conventional and swirling jets became comparable. For the studied range, the maximum average Nusselt number of 110 was achieved by using C-DSIJs with baffle at L/D = 1, y/W = 3 and Re = 20,000.