Research On Spatiotemporal Characteristics Of Internal Stall Flow In A Centrifugal Refrigeration Compressor

Centrifugal refrigeration compressors are widely used in industrial,aerospace,and construction equipment et al energy fields due to their excellent aerodynamic performance and structural forms.However,in practical operation,due to the complex and variable working environment and the presence of unsteady inflow in the pipe network,centrifugal compressors often operate near off-design operating points,leading to a degradation of overall performance and internal flow field stability.In order to design compressors with higher aerodynamic efficiency and broaden their operating range for stable operation,it is of great significance to conduct in-depth analysis of the spatiotemporal correlation characteristics of turbulent structures in various components of centrifugal compressors under off-design conditions,as well as the mechanisms of flow losses.Qualitative and quantitative summaries of the variations in aerodynamic parameters such as pressure and velocity in the flow field are essential.This manuscript focuses on a centrifugal compressors with vaneless diffusers as the research object,conducting steady and unsteady numerical simulations in the entire flow passage.The unstable flow structures inside the centrifugal compressor under off-design conditions with larger and smaller flow rates at the design operating point are investigated.By comparing the internal flow field parameters such as pressure field,velocity field,and streamline between the two operating conditions,two different degradation mechanisms of aerodynamic performance and efficiency for centrifugal compressors under larger and smaller flow rate conditions are explored.Due to the presence of more complex internal flow losses and flow characteristics under smaller flow rate conditions,further investigations are conducted on the internal flow mechanisms of centrifugal compressors under small flow rate conditions.Based on the trend of pressure ratio and isentropic efficiency decreasing on the performance map,the development characteristics of internal instability structures are explored from the perspectives of energy cascade theory and turbulent development at different stages.Monitoring points are set at multiple locations in the flow field for transient data acquisition,and the rotating stall mechanism and spatiotemporal evolution status under small flow rate conditions are quantitatively analyzed.The research contents of this paper can be summarized into the following parts.Firstly,based on computational fluid dynamics(CFD)methods,the spatiotemporal evolution and fluctuation characteristics of flow field parameters of centrifugal compressors under larger and smaller flow rates at off-design conditions are investigated,and the blocking mechanisms of the two types of off-design conditions are summarized.The differences in characteristic frequencies on the pressure fluctuation spectrum are found,and the spatiotemporal development status of pressure fluctuations in the vaneless diffuser of the centrifugal compressor is explored through data acquisition and analysis at monitoring points along the circumference of the vaneless diffuser and volute.Secondly,three operating points of different stall stages are selected on the external characteristic curve of centrifugal compressor and the internal unsteady flow mechanism is explored.The dimensionless mass flow rate(M~*)is defined to reflect the compression rate of the whole machine under each stall condition.It is found that there is a hump-like pulse in M~*under deep stall condition,and its pulsation amplitude is an order of magnitude different from that under near stall and stall conditions.From the perspective of turbulent kinetic energy and static entropy,the decomposition and evolution of vortices convert the mainstream energy into turbulent kinetic energy and viscous heat dissipation.Through the space-time tracking of the vortex in the period of the convection channel,it is found that under different stall degrees,the space-time evolution characteristics of the vortex under the unstable and uneven inverse pressure gradient have certain differences.Further quantitative analysis of the vortex structure of the whole machine based on the Q criterion shows that the vortex structure has different evolution periods under different stall conditions.Comparing the static pressure distribution along the blade profile at 50%blade height under different working conditions,it is found that the fitting curve of static pressure distribution along the blade profile is more divergent under deep stall condition.Further,the flow difference of each working condition is quantitatively analyzed from the perspective of time domain and frequency domain.By collecting dynamic data at different positions of the flow field and performing fast Fourier transform(FFT),the pulsation and spectrum characteristics of flow field parameters under different stall conditions are obtained.It is found that there is a close spatial-temporal correlation between the characteristic frequency of the flow field and the unstable flow structure.The time domain and frequency domain analysis of the pressure fluctuation on the inner flow surface of the impeller found two different modes of stall signals,and explored the difference between the characteristic frequency and the dominant frequency in the flow field under different degrees of stall conditions.Finally,the temporal and spatial correlation between the evolution of the stall cell in the impeller domain and the existence of the vaneless diffuser and volute is explored under the mode of fluid changing from axial flow to circumferential flow in the impeller domain.Due to the effect of the backflow vortex in the vaneless region,the blade passing frequency signal can also be monitored in the flow channel,which is mutually verified with the countercurrent phenomenon of negative radial velocity on the meridian plane of the vaneless diffuser,and the qualitative and quantitative results are well matched.At the same time,based on the pressure pulsation and its RMSE value,the stall mechanism of the circumferential flow in the vaneless zone of the centrifugal compressor is explored from both time domain and frequency domain.