Flow and Heat Transfer Characteristics of Impinging Jet from Expansion Pipe Nozzle with Air Entrainment Holes

Flow and heat transfer characteristics of impinging jet from expansion pipe were experimentally and numerically investigated. The expansion pipe nozzle was drilled on expansion wall for increasing an entrainment of ambient air into a jet flow. The diameter of round pipe nozzle was d=17.2 mm and the diameter of expansion pipe was fixed at D=68.8 mm (=4d). The number of air entrainment holes was varied at 4, 6 and 8 holes, and the expansion pipe length was examined at L= 2d, 4d and 6d. In this study, the expansion pipe exit-to-plate distance was fixed at H=2d and the Reynolds number of jet was studied at Re=20,000. Temperature distribution on the impinged surface was acquired by using an infrared camera. The numerical simulation was carried out to reveal the flow field. The results show that the ambient air enters through the holes and subsequently blocked the entrainment of ambient air into the jet flow. It causes to enhance the heat transfer particularly at stagnation point higher than the case of conventional pipe: 4.68% for 4 holes at L=2d, 6.4% and 6.28% for 4 holes and without holes at L=4d and 5.48% for 8 holes at L=6d.