Study On Rotating Stall Flow Characteristics Of Multi-blade Centrifugal Fan

Multi-blade centrifugal fans are important energy conversion equipment,and are widely used in air-conditioning ventilation and other fields because of their low cost,large pressure coefficient,small size and strong anti-noise ability.With the continuous improvement of production and living needs,people have put forward higher requirements for the operation stability of multi-blade centrifugal fans.When the actual operating flow of the multi-blade centrifugal fan is small to a certain extent,the flow separation phenomenon will occur in the impeller flow channel,which will lead to rotating stall.Rotating stall will not only reduce the operating efficiency of the fan,but also affect the stability of the fan’s operation,bringing various safety hazards.At present,most researches in the field of rotating stall focus on axial flow compressors and centrifugal compressors.There are few studies on the flow instability phenomenon of multi-blade centrifugal fans,and most of them are based on the premise of uniform flow.Therefore,this paper takes the multi-blade centrifugal fan as the research object,and conducts a numerical simulation of the flow characteristics of the multi-blade centrifugal fan under the rotating stall condition,and considers the effect of non-uniform incoming flow on the rotating stall.The main research contents and results are as follows:1.The numerical calculation model of the multi-blade centrifugal fan was established,and the external characteristic test of the multi-blade centrifugal fan was carried out at the same time.The total pressure of the test was highly consistent with the numerical simulation,which proved the accuracy of the simulation method of the multi-blade centrifugal fan,and focused on the research of the multi-blade centrifugal fan.The steady flow characteristics under different flow conditions are as follows:(1)When the gas flows from the impeller inlet to the blades,the flow direction will change,and the flow angle at the front cover of the impeller changes greatly,the flow is unstable,and the flow along the axis going deeper,the flow change angle is small,and the flow field stability is improved.The flow angle change is the smallest at the rear cover plate,but the flow field stability is slightly worse due to the impact of the airflow on the wall surface of the rear cover plate.(2)As the flow rate decreases,the flow stability of the fan decreases.The unstable flow area is mainly concentrated near the inlet of the impeller and the separation tongue.The unstable characteristics are mainly characterized by extremely uneven streamline,flow velocity and pressure distribution.The back flow in the unstable area is intensified,the vortex structure increases,and the turbulent kinetic energy increases.2.Through the calculation of unsteady constant value,the flow rate of the multi-blade centrifugal fan when rotating stall were determined,and the mechanism of the fan rotating stall and the transient change characteristics of the flow field during the rotating stall were studied.The conclusions are as follows:(1)The multi-blade centrifugal fan appears rotating stall when the flow rate of Q/Q_n=0.6,and the degree of rotating stall is aggravated when the flow rate of Q/Q_n=0.3.When the flow rate of Q/Q_n=0.6,the state of the rotating stall flow field is relatively fixed,and there is no significant change with the flow time,the flow rate continues to decrease,the degree of rotating stall intensifies,and the flow structure fluctuates greatly with time.(2)Rotating stall first occurs in the flow channel near the separation tongue.With the intensification of the degree of rotating stall,the range of the stall area gradually becomes larger,and the number of stall clusters increases;Back flow occurs in the areas where the stall is most severe.3.Different circumferential,radial and axial monitoring points were set at the outlet of the impeller,and the internal pressure pulsation law of the fan under the rotating stall condition was analyzed.The conclusions are as follows:(1)The pressure coefficient of the monitoring point at the separation tongue is the largest,and as the flow rate decreases,the intensity of pressure pulsation increases;the outlet of the impeller is affected by the dynamic and static interference between the impeller and the volute,and the amplitude of the pressure coefficient is the largest.The radial movement of the volute wall reduces the degree of pressure pulsation;the pressure pulsation law is less affected by the change of the axial position.(2)The change of the degree of rotating stall will cause the change of pressure pulsation.The stronger the rotating stall,the more severe the pressure pulsation,the wider the range of low-frequency pulsation,and the higher the stall frequency amplitude.4.Two working conditions of circumferential non-uniform inflow and radial non-uniform inflow were introduced,and the effects of different inflows on the rotating stall of the multi-blade centrifugal fan were studied.The conclusions are as follows:(1)Under the condition of Q/Q_n=0.6 flow rate,affected by the radial non-uniform inflow,the degree of back flow near the separation tongue increases,the rotational stall intensifies,and the low-frequency pulsation increases,while the circumferential non-uniform inflow has a greater impact on its stall characteristics.Small;when Q/Q_n=0.3 flow rate,the circumferential non-uniform flow will cause backflow at the outlet of the volute,which will cause more stall clusters to appear at the inlet of the blade flow channel under the tongue,which will seriously affect the impeller inflow state and lead to stall frequency.Amplitude increases.(2)The non-uniform flow at the rated flow will change the inflow state in the impeller channel,which will cause the radial force on the impeller to fluctuate.The radial force increases,and the radial non-uniform inflow will improve the velocity distribution at the inlet of the blade runner and reduce the radial force on the impeller;during the rotating stall,the radial force on the impeller increases greatly,and the impeller inlet flow is not Stable,non-uniform flow reduces the influence of radial force on the impeller.