Method And Mechanism Investigation Of Reducing Flow Separation In Large Diffusion Compressors By Pulsed Jet

High thrust-weight ratio is an important index of modern high performance aeroengine. For the purpose of realizingthis aim, the stage pressure ratio of compressor should been improved significantly.Under the premise of not to increase rotation linear velocity obviously, the flow separation phenomenon will appear with the increase of stage pressure ratio, so the introduction of flow separation control technology to suppress the separation flow has become a focus of current research. Based on the flow separation characteristic in a compressor, a micro pulsed aspirated jet concept is presented, and the mechanism of this unsteady control method is conducted.Themainsteps areas following:Firstly, on the basis of the unsteady separation flow and pulsed-jet control characteristics, an unsteady model has been established, and the maximum Lyapunov exponent has been introduced to evaluate overall control effect. According to the different frequency, intensity and phase control results, it should be noted that when the ratio between pulsed jet frequency and system characteristics frequency is in half octave, one octave, two octave and three octave states, the maximum Lyapunov exponent decreased significantly, and the system transform from the chaotic state to steady or unsteady periodic state, the best control frequency is equal to the system characteristics frequency. Furthermore, the motion characteristics of most fluid particle tends to straight flow mode in the effective excitation states, there is a obvious self-synchronization of unsteady flow separation with periodic excitation, and with the increasing of excitation intensity, the effective controlled fluid particle ratio will be increased, this flow phenomenon presents more orderly in macroscopic state.Secondly, built on the characteristic of micro pulsed aspirated jet control method, a primaryelectromagnetic pulsed jet device is designed. And in terms of experimental and numerical results, there is a phenomenonthat the instantaneous outlet total pressure may be less than inlet total pressure,the physical mechanism of this phenomenon has been investigated. In addition, the velocity waveform in different rotation speed and pressure gap is obtained, and the flow characteristic in the pulsed jet is performed.Thirdly, combined with this new unsteady control method, an imitation cascade channel is established. The steady and dynamic pressure characteristics in the no-control state are obtained, and the characteristic frequency of the separated vortex is found to be about 266 Hz, the corresponding Strouhal number 0.18. Upon the foundation of the numerical results, the proper orthogonal decomposition(POD) method is introduced to analyze the unsteady control mechanism.In the light of the POD results, the first mode represents the structure of average flow, and the other modes represent different vortex structures. The actual flow can be regarded as a mix of all modes in a certain weight, and based on the decoupling temporal structure; it will besimplerto analyze the complex unsteady separated flow characteristics.Fourth, on the principle of the experimental and numerical results about the unsteady frequency in typical control state, when the frequency of pulsed jet is close to the characteristic frequency of separation vortex, the control effects are more obvious. With the combination of the POD analysis results, it should be noted that the main effect of unsteady pulsed jet is based on the nonlinear effect, through reallocating the energy of each mode, and selectively strengthening or weakening the certain modes; the effect of steady jet is overall reducing the energy of high order modes and linear changing the flow characteristic. Based on the reasonable control parameters of pulsed jet, the energy in higher modes will be transferred to the average flow mode, and the translation of modal energy comes from the reconstructing of spatial flow structures and the ordering of modal evolution characteristic.Fifth, the control effects in different location, angle and intensity has been carried out. When the location is far form the separation point, the control effect will be decreased; the best pulsed jet location is nearby the separation point, and the energy of each mode will be transferred to the average flow modeobviously in the best control location state. The best control angle is about 15°in the research angle range, in the best control angle state, the penetration effect and promoting effect is under the best coupling status. What is more, under the proper intensity conditions and within a certain frequency range, the flow structure will leave the on-control periodic orbit and stabilize in the new periodic orbit which is determined by the pulsed jet, the intercoupling flow structure and pulsed jet achieve spatiotemporal synchronization.Finally, combined with cascade tunnel, thevalidation of pulsed control effect and POD analysis has been carried out. Results show that when the frequency of the pulsed jet is in the range from 85% to 120% characteristic frequency of the separated vortex, the control effect is more obvious. Compared with blade slot treatment and the other steady control method, the suction mass flow of this pulsed control method is few, and the influence of suction phenomena to the flow characteristics on pressure side and overall performance has been decreased significantly, this new unsteady control method is a promising control method in compressors.