Investigation Of S-shaped Inlet Distortion Control And Its Impact On The Flow Field Of The Rear Transonic Fan-stage

As we know, along with the demand of missile and military aircraft survivability increases since the middle of last century, the S-shaped inlet receive widespread attention thanks to their simple structure, small frontal area and favorable stealthy performance. However, the inlet distortion which is harmful to the rear aerodynamic components is usually serious, especially the half flush-mounted and the submerged S-shaped inlet with large boundary layer ingestion. So the deep study of the internal flow characteristic, the exit distortion formation mechanism of S-shaped inlet and its impact on subsequent components’ aerodynamic performance and flow field, is of great significance to expand the S-shaped inlet application scope, to improve the flow field structure, and even to enhance the whole engine’s dynamic performance. Now most of design philosophies tend to treat the S-shaped inlet as an isolated component, and few work taking it and the rear compressor as a whole has been reported.In this paper, for giving a real exit boundary, and also for investigating the impact of S-shaped air intake exit distortion on the rear fan stage performance as well as its evolution process in the rear components, a half flush-mounted airintake has been designed first for the fan-stage of an small turbofan engine. In view of improvement of the airintake aerodynamic performance is limited after optimization, further study of airintake internal flow control such as blowing or suction has been conducted. Due to high energy fluid injected into, the ability of low-energy fluid in the boundary layer to overcome inverse pressure gradient increases, the separation point moves backward if the high energy fluid is injected before the original separation point. More injected mass flow, more move back quantity until the flow separation disappeared at last. While blowing in the original separation region, small scale flow separation only appears in the region between the original separation point and the blowing location, and the separated flow that behind the blowing location reattached again. The location selected for blowing has significant influence on the final effect. Blowing at the first curved bottom can inhibit the generation of the vortex embryonic as well as the flow separation, so undoubtedly the 1st bend that just before the separation point is the best blowing location. For avoiding the high-energy air from pipes mix with mainstream in the airintake, the blowing mass flow should not be too large, and little blowing angle should be selected for ensuring the blown fluid is always in the boundary layer. When suction proceeds at the throat location, a wide range of backward flow field will be profited, simultaneously, formation of the vortex embryo and the flow separation can be restrained. Considering this two aspects, throat is the optimal suction location. And the suction angle design need to consider the boundary layer fluid movement direction, the mainstream movement direction and the boundary layer thickness. Combinational flow control effect is not the simple superposition of the single blowing control effect and the single suction control effect, but is the largest one among the three.Taking into account the airintake exit distortion is serious and very detrimental to the fan stage. Numerical investigation of the whole structure of the airintake that before and after optimization or flow control and the fan-stage has been conducted, for investigating the impact of S-shaped air intake exit distortion on the rear fan stage performance as well as its evolution process in the rear components. Giving a real exit boundary or not has significant impact on the airintake numerical simulation results, especially to the circumferential total pressure distor tion and the swirl distortion. The total pressure ratio as well as the efficiency of the fan stage decreases dramatically due to substantial boundary layer ingesting, after optimization, the stable operation range is extended significantly. With flow contr ol, the overall performance of the fan-stage further improves significantly. The maximum isentropic efficiency of the fan-stage increase about 0.57% with suction control while increase about 2.26% with blowing control. As to the combinational flow control, the maximum isentropic efficiency and the maximum total pressure ratio of the fan-stage increase about 2.70% and 3.13% respectively. Under effect of rotating rotor, the significant low energy region at bottom of the inlet exit decreases ceaselessly when it developing backward to front-edge of the rotor blade, moreover, the high-energy fluid mixes with the low-energy fluid. This phenomenon always exists no matter the optimization or the suction control proceeded or not, while disappears under the best blowing scheme or the combination scheme. The distorted low energy fluid covers several blade passages when it goes through the fan-stage. The relative total pressure and velocity of the fluid in these distorted blade passages is obviously less than that in other passages, which has a little increase after optimization or flow control. Meanwhile, after airintake optimization and internal flow control, the region covered by this distorted fluid has a little decrease. Serious flow separation occurs in the stator blade suction sides. But this flow separation mainly occurs at 50% stator blade span and below, and weakens after optimization or flow control has been carried out.Finally, based on the above study, the flow control with bleeding pipe as flow control device and high energy fluid in the stator cascade as bleeding air source has been researched. Bleeding pipe structure, installation position and angle are all very crucial to the flow control effect. More bleeding mass flow does not mean better, and quality of the bleeding air jet is the key. After bleed air from the stator hub for blowing control, the whole performance of the fan-stage improves a lot when it works under the condition that between the maximum efficiency point and the stall margin, due to the low energy fluid in the stator cascade being partly sucked and then blowing in the airintake to improve the airintake exit air quality. The stable operation range extends significantly as bleed air conducted at the stator shroud and the bleed air pipe diameter equal to each other. The control effect of this scheme is much better when the fan-stage working near the surge margin than when it working at the maximum efficiency point or near the blocking margin. So, if the bleed air system can work like a bleed valve rather than always in work state, the stable operation range as well as its aerodynamic performance improves substantially.