Heat transfer enhancement on a surface of impinging jet by increasing entrainment using air-augmented duct

Flow and heat transfer characteristics of impinging jet from pipe nozzle with air-augmented duct were experimentally and numerically investigated. The effects of air-augmented duct geometry on heat transfer enhancement were concerned. The experimental parameters included a diameter (D) and a length (L) of air-augmented duct in the range of D = 2d, 3.3d, 6d, and L = 2d, 4d, 6d where d was the inner diameter of main pipe nozzle at 17.2 mm. The distance from air-augmented duct outlet to impingement surface (S) at S = 2d, 4d and 6d were considered. The conventional impinging jet was also studied to compare the results with the case of an air-augmented duct. The result comparison was based on constant jet mass flow rate by fixing the jet Reynolds number of conventional pipe at Re = 20,000. The temperature distributions on the impingement surface were measured by using a thermal infrared camera, and profiles of velocity and turbulence intensity of the jet were measured by using hot-wire anemometer. The 3-D numerical simulation with SST k-ω turbulence model was also applied to reveal the flow characteristics. The results show that the heat transfer rate on the impingement surface for the case of an air-augmented duct in conditions of 2d ≤ D ≤ 4d and L = 2d is noticeably higher than the case of conventional impinging jets due to increasing air entrainment. The heat transfer rate for the case of D = 6d, L = 2d at S = 2d, is the largest by getting 25.42% higher compared to a conventional impinging jet.