Flow Improvement in Rectangular Air Intake by Submerged Vortex Generators

Rectangular S-duct diffusers are widely used in air-intake system of several military aircrafts. A well-designed diffusing duct should efficiently decelerate the incoming flow, over a wide range of incoming conditions, without the occurrence of streamwise separation. A short duct is desired because of space constraint and aircraft weight consideration, however this results in the formation of a secondary flow to the fluid within the boundary layer. The axial development of these secondary flows, in the form of counter rotating vortices at the duct exit is responsible for flow non-uniformity and flow separation at the engine face. Investigation on S-shaped diffusers reveals that the flow at the exit plane of diffusers is not uniform and hence offers an uneven impact loading to the downstream components of diffuser. Experiments are conducted with an S-shaped diffuser of rectangular cross-section at Re = 1.34u10 5 to find out the effects of the corners (i.e. sharp 90o, 45o chamfered etc.) on its exit flow pattern. A ‘fishtail’ shaped submerged vortex generators (VG) are designed and introduced at different locations inside the diffusers in multiple numbers to control the secondary flow, thereby improving the exit flow pattern. It is found that the locations of the VG have a better influence on the flow pattern rather than the number of the VG used. The best combination examined in this study is a 45q chamfered duct with 3u3 VG fixed at the top and bottom of the duct inflexion plane. The results exhibit a marked improvement in the performance of S-duct diffusers. Coefficient of static pressure recovery (CSP) and coefficient of total pressure loss ( CTL) for the best configuration are reported as 48.57% and 3.54% respectively. With the best configuration of VG, the distortion coefficient (DC60) is also reduced from 0.168 (in case of bare duct) to 0.141.