Heat transfer characteristics of unexpanded jet impingement in piccolo hot air anti-icing chamber

Abstract

Jet impingement are widely used in the piccolo hot air anti-icing chambers of aircraft. The high pressure at the nozzle and low pressure in the chamber lead to a relatively large nozzle pressure ratio, triggering an unexpanded jet state. The unexpanded jet impingement significantly differs from the subsonic jet impingement in flow and heat transfer. Therefore, this study conducts numerical investigations on the flow and heat transfer characteristics of unexpanded jet impingement. In addition, the effects of nozzle pressure ratio (NPR) and dimensionless impinging distance (L/d) are considered. According to the numerical results, the impinging air cannot be fully expanded in the free jet zone, and will continue to expand after entering the wall jet zone, triggering an abrupt drop in the hot air temperature, under a small dimensionless impinging distance (L/d = 2.5 and L/d = 5.0). In this state, the hot air will not heat the surface effectively, as the surface will heat the air reversely. Furthermore, it is also determined that the ambient temperature should be included as an influencing factor in the analysis of jet impinging heat transfer. For high-speed jet impingement, especially unexpanded jet state, the ambient air will influence the total temperature of jet boundary due to the large difference of static temperature between free jet zone and chamber environment.