Investigation On Separation Structure And Its Steady&Unsteady Control In Super Highly Loaded Compressor Cascade

Continuously aspiring after the thrust-weight ratio of aero-engine resulted in the dilemma of axial compressor field. It exists nonnegotiable contradiction between the increasing load and exacerbating flow separation. How to upgrade the load in large range with high efficiency is the target strived by researchers in turbo machinery. Engine aerodynamic research center of HIT developed a new method of improving the load which was named low reaction high load compressor. Based on this idea, the rotor is designed with low reaction by increasing the inlet pre-rotation to reduce the demand of pressurize, that insure the rotor can efficiently get large range addition of total pressure ratio. While the transformation from total pressure ratio to static pressure ratio is summed up in the stator with large deflection angle. So this idea has broken apart the traditional diffusion model depending on the flow passage. Instead it builds up a new one which is based on flow control. The conversion of compressing cognition makes up of the main contents of this dissertation.Before the study of flow separation control, meticulous analysis of separation structure in low reaction high load compressor cascade is carried out firstly. The contents include from two dimensional cascade to three dimensional rectangular cascade, from steady flow to unsteady flow. During the investigation of two dimensional cascade, it is detected that the concomitance of trailing edge shedding vortex and suction surface separation vortex is the remarkable character compared with flow field of conventional cascade. What will derive from this character when the two dimensional flow converts to three dimensional flow is difficult to forecast. Therefore the study of separation structure in conventional cascade is performed as the base of analysis in low reaction high load cascade. The research indicates that the flow field of conventional cascade undergoes three separation shapes named corner separation, corner stall and cascade stall. On the whole, the vortex structures in flow field can be distinguished by quasi column vortexes and bedded vortexes. The representative quasi column vortexes are horse shoe vortex, passage vortex and corner vortex, while the representative bedded vortex is suction surface separation vortex. The pressure side branch of horse shoe vortex will not consolidate in the passage vortex inevitably as well as the passage vortex isn’t derived from the pressure side branch of horse shoe vortex. The corner vortex is also not the inducement of passage vortex. The passage vortex and corner vortex have their own generation mechanism. The suction surface separation vortex is a complicated curved face with some thickness which is latched-up itself. It mainly determines the loss distribution of flow field, but the effect caused by quasi column vortexes are not obvious. The flow field in three dimensional low reaction high load cascade maintains cascade stall pattern all the while. The suction surface separation vortex breaks apart the original balance mechanism by reason of dynamic instability indigenous to the flow field and the asymmetric flow evolved finally. As a result, the suction surface vortex grows rapidly in space scale and turns into the key factor that determines the separation structure and loss distribution in flow field.Based on the perspicuity of idiographic separation structure, the boundary layer suction and blow are used in low reaction high load cascade to control separation flow. The investigation indicates that effectual control of suction surface separation vortex plays a crucial role in low reaction high load cascade flow control. The control succeeds with the process of transformation from cascade stall to corner stall or corner separation. Boundary layer blow works by the momentum exchange between jet flow and low energy flow of the boundary layer. This process takes on distinct vector character, consequently the jet angle influences the outcome in fundamentality. However the boundary layer suction regulates the separation structure by sucking of low energy flow to stave off the boundary layer development as well as to exalt the ability of counteract inverse pressure gradient. But its outcome is especially sensitive to location and mass flow in the suction, resulting in low adaptability of variation in work condition.To improve the deficiency of steady flow control technique, a classical unsteady skill named synthetic jet is introduced into low reaction high load cascade to control the separation flow. The study shows that synthetic jet regulates boundary layer using momentum exchange, on the other hand it makes use of the wave vortex interaction caused by its unsteady excitation effect to disperse suction surface vortex and precipitate it coiling and shedding. All these operations lead to outstanding outcome of flow control.On the foundation of deep analysis of boundary layer suction and synthetic jet, a new concept named boundary layer oscillation suction is proposed in this dissertation. To certify its validity, numerical simulations are performed in low reaction high load cascade as well as conventional high load cascade. The result shows BLOS has the same process of wave vortex interaction, which makes BLOS can improve the control result remarkably compared with BLS. In addition, it can upgrade the adaptability of variation in work condition in a wide range.