Dynamic Mode Analysis Of Unsteady Flow Inside Turbomachinery

Turbomachinery is a general term for rotary machinery that realizes the effective conversion of mechanical work and fluid energy.It is widely used not only in various production sectors of the national economy,but also in the field of advanced technologies,such as aerospace.The three-dimensional complex flow inside the turbomachinery has obvious unsteady characteristics.Extracting the characteristic frequency of the internal flow field of turbomachinery and its corresponding flow structure is very significant for analyzing the flow phenomenon,understanding the flow mechanism and even ensuring the safe and efficient operation of the machine.With the development of computer performance and computing methods,dynamic mode decomposition(DMD),a data-driven method,is applied to the field of fluid mechanics.In this paper,this method is applied to the unsteady flow of the last stage of steam turbine under small mass flow condition,the flow inside axial compressor under rotating instability condition and the internal flow of the centrifugal compressor volute under unstable condition.The characteristic frequency and its mode of unsteady flow are extracted,which provides a new method for further understanding of the unsteady flow phenomena.The main work of this paper is as follows:1.DMD method is used to analyze the dynamic characteristics of the inlet flow field in the last stage of steam turbine at 25% and 17% mass flow conditions.The results show that DMD method is capable of capturing the main characteristic frequencies and corresponding disturbance structures in the flow field.Besides,under the 17% mass flow condition,the typical characteristics of rotating instability phenomenon are captured,and the variation of the disturbance with time is visually showed by the reconstruction of DMD mode.2.The flow field of the axial rotor Rotor35 under small mass flow conditions is analyzed by DMD method.The flow modes corresponding to different frequencies in the broadband hump structure and the number of circumferential disturbance modes are identified.In addition,the circumferential propagation characteristics of the disturbance in different axial position are visually displayed.3.DMD is applied to analyze the internal flow of volute under the unstable operation condition of centrifugal compressor.The characteristic frequencies,relating to the blade rotation and unstable condition,and the corresponding DMD modes are captured.Through the reconstruction of DMD mode,the disturbance structure liking a standing wave at the four o'clock region inside the volute is visually displayed.4.Compressed dynamic mode decomposition(CDMD)is used to improve the analysis efficiency of three-dimensional flow field of the volute.The effects of compression ratio on the identification of characteristic frequencies and modes are analyzed.This work provides guidance for the determination of compression ratio in the application of CDMD method.