| The stall problem has always been a complicated problem that must be faced in the process of compressor design and use.After a long-term research,academia has gained a specific understanding of the stall phenomenon in the conventional rotor-stator alternate layout compressor.However,as an unconventional rotor-rotor aerodynamic layout,the mechanism of unsteady flow in contra-rotating compressors has not been fully understood.In this paper,a low-speed axial flow contra-rotating compressor test bench with complete measurement and control functions is constructed firstly.Then,a time-frequency analysis method suitable for the non-stationary and nonlinear signals in the compressor stall process was proposed.Subsequently,the experimental study on the dynamic stall process of the contra-rotating compressor when the speeds of the front rotor and the rear rotor are the same were carried out.Finally,the distribution law of the compressor characteristic curve under different speed ratios(the ratio of front rotor speed to rear rotor speed)and the influence of speed ratio on the dynamic process of stall initiation-development-recovery are studied.The main work of the dissertation includes:1)Design and construction of the measurement and control system for the contra-rotating compressor.The overall goal of the research on the compressor stall is to realize the compressor’s characteristic measurement and stall process capture.The hardware system is subdivided into six subsystems to achieve,including speed control,flow adjustment,pressure measurement,acquisition phase lock,shaft power measurement,and gas relief.A combined proportional-integral-differential and Kalman filter algorithm is developed to achieve the control target that the relative deviation of speed is less than 0.1%.A blade tip fiber-motor shaft encoder combined phase-lock system is designed to measure the unsteady tip clearance flow with the sparsed sensor arrays.The high-precision phase-locking function is realized in the full speed range.In response to the demand for the measurement and control of the compressor stall research,the use of multi-threaded programming technology has developed six sub-modules including parameter configuration,pressure oscilloscope,speed control,flow velocity vector monitoring,data acquisition card monitoring and main measurement module.The unified integration of various instruments and equipment under the same software system through data communication has laid a solid foundation for the smooth progress of subsequent experiments.2)The development of time-frequency analysis methods for non-stationary and nonlinear signals during compressor stall.Aiming at the advantages and disadvantages of the Hilbert-Huang transform and local mean decomposition algorithm in processing non-stationary and nonlinear signals,they are split and reorganized,then the joint empirical mode decomposition-local mean decomposition method is developed.The effectiveness of this method in solving the instantaneous frequency is proved by using the Duffing wave signal.The robustness of the method is verified by applying the method to gravitational wave signals and compressor stall signals.Studies have shown that this method not only inherits the advantages of the robustness of the empirical mode decomposition algorithm but also has the characteristic that the local mean decomposition algorithm has an exact physical meaning in solving the instantaneous frequency,that is,it solves the negative frequency problem in the Hilbert-Huang transform.Therefore,it is more suitable for complex time series signals in nature and industry,such as the typical compressor stall signal.3)Research on the dynamic stall process of the contra-rotating compressor when the speeds of the front rotor and the rear rotor are the same.The study included the compressor running at low speed(800rpm)and design speed(2400rpm).The dynamic process of the casing wall pressure of the spontaneous stall event is captured by the blade tip fiber-motor shaft encoder combined phase-locking system.The results show that the stall disturbance is significantly different from the conventional spike inception and modal inception.It has a low circumferential rotation speed(8.3% shaft speed at low speed and 6.2% shaft speed at design speed)and a wide coverage(135°),in which the leading edge of the rear rotor stalls first and quickly expands to the entire front/rear rotor.When the flow rate increases to a certain extent,the compressor spontaneously enters the stall recovery process,and the front rotor first resumes stable flow.Before the recovery of the stall,the circumferential dimension of the stall disturbance collapses rapidly,and the rotation speed increases rapidly(from 7.5% to 47%shaft speed at low speed,and from 6% to 18.1% shaft speed at design speed).4)The distribution law of the contra-rotating compressor characteristic curve under different speed ratios.The experiment is divided into three groups,and respectively,the speed ratio is less than 1(including 0.75,0.857,0.875,0.889),equal to 1 and greater than 1(including 1.125,1.167,1.333).There are different speed configurations under the same speed ratio.The experiment is divided into 19 speed configurations,covering the whole range of low speed-medium speed-high speed.Studies have shown that under the premise that the geometric mean of the front and rear rotor speeds are equal,compared with the case where the speed ratio is less than 1,the compressor is more likely to stall when the speed ratio is greater than 1,and the supercharging capacity is stronger,that is,stability and supercharging ability check and balance each other.In the method of dividing the stall boundary of the contra-rotating compressor,the stall boundary is independent of each other at different speed ratios.The single stall boundary in a conventional compressor is a subset of the speed ratio of1 in the contra-rotating compressor(not considering the difference in the direction of rotation).The dimensionless characteristics(flow coefficient-total to static pressure rise coefficient)under the same speed ratio almost coincide,and they are independent of each other at different speed ratios.5)The influence of speed ratio on the stall initiation-development-recovery process.The experimental data is obtained by the high-frequency response pressure sensor array located on the top of the compressor rotor casing,and the compressor is also running at the above 19 speed configurations.The results show a critical speed ratio in the contra-rotating compressor(in this paper,the value is between 1.125 and 1.167).When the speed ratio is less than this value,the compressor stall initial disturbance is similar to spike disturbance,the perturbation propagates in the circumferential direction in the direction of rotation of the rear rotor.When the rotation speed ratio is greater than the critical value,the stall disturbance is located at a fixed position in the circumferential range of the compressor casing and does not move in the circumferential direction.When a fixed disturbance occurs,it can evolve into rotating stall by adjusting the outlet area block ratio.However,at this time,the direction of the disturbance propagating in the circumferential direction is consistent with the front rotor,which is significantly different from the conventional compressor. |
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