Flow Characteristics And Length Reduction Of Curved Annular Duct With Struts

Curved annular ducts with struts are widely existed in airbreathing propulsion systems. Therefore, the investigation on the flow characteristics within a curved annular duct and the quest for effective methods for duct length reduction are meaningful to the advancement of the airbreathing propulsion technology. In the current thesis, with a combined experimental and computional method, the flow characteristics at a typical exit Mach number and the influence of different exit Mach numbers are detailedly investigated. Subsequently, considering that the credibility of the computional approach has been well examined by the experiments, futher exploration is numerically conducted on the duct length reduction.The results show that, at the typical exit Mach number, the airflow accelerates at first and then decelerates within the duct due to the duct area distribution. And two local high-velocity low-pressure areas generate respectively at both leeward sides of the two bends. Moreover, struts impose a great effect on the flow structure in the curved duct, which induce vortex pairs adjacent to both outer and inner walls and wakes right behind the struts. However, the vortex pairs of the two sides have totally different generation mechanism and evolution characteristics. Specifically, the hub-side vortex pair of which the vortex cores travel downstream parallelly evolves from the horseshoe vortex which is induced by the leading edge of the upstream strut, whereas the shroud-side vortex pair originates from the strut trailing edge and the corresponding vortex cores develop in a divergent way. In terms of the wakes, the total pressure distributions at different radial positions present significantly different characteristics. Besides, the exit Mach number exerts a remarkable influence on the inner flowfield. With the increase of the exit Mach number, the adverse pressure gradients near the two bends increase, implying a greater risk of boundary layer separation engenders. Meanwhile, the intensity of the induced vortices gets increased, inducing a higher total pressure loss in the vortex regions and lowering the area-averaged total pressure recovery at the duct exit.On top of that, some conclusions can be made with regard to the investigation on the duct length reduction. The shroud-side boundary layer turns to be more sensitive to the length alteration, prone to separate near the second bend, resulting in a decrease in the area-averaged total pressure recovery and an increase in DC60 on the exit section. To achieve the goal of shortening the curved duct without grievously deteriorating its aerodynamic performance, methods of geometry modulation, vortex generator and bleeding are introduced to restrain the flow separation near the second bend, respectively. However, only bleeding takes effect as expected, which is capable of eliminating the flow separation with a compromise of low bleeding mass flow rate and improving the flowfield performance to the original full-length level.