Investigation Of Unsteady Flow Characteristics And Mechanism Of Rotating Stall In A Double Suction Centrifugal Fan

The unstable flow phenomena would occur in the centrifugal fan under the low flow rate condition.They not only increase the flow loss,reduce the efficiency of the fan,but also seriously affect the stability and reliability of the fan.In this paper,the methods of numerical calculation and experimental measurement are used to analyze the flow characteristic in the double suction centrifugal fan.Numerical simulation of the steady flow and unsteady flow are performed for the three-dimensional turbulent flow field in the centrifugal fan.The unsteady flow phenomena in the centrifugal fan is analyzed and explained by the energy gradient theory.The generation and development of the rotating stall are discussed in detail.The relationship between the rotating stall and surge is explained.The variation of the internal flow field and the external characteristic curve with the change of flow rate is described detailed.The correlation of the unsteady flow characteristics between the internal flow field and the external characteristic curve in the time domain and frequency domain is established.Based on the analysis of the unsteady flow characteristics in the double suction centrifugal fan,the pressure distribution method to determine the optimal impeller staggered angle is proposed.Then the practical engineering application and experimental verification are carried out.The main research contents and achievements are as follows,(1)In order to make a better comparison with the unsteady flow characteristics,the steady numerical calculation of the internal flow field of the double suction centrifugal fan is carried out at first.It is shown that the numerical results of the characteristics of the fan are basically in agreement with the experimental results.The interior flow field and the energy gradient function K under different axial sections and different flow rate conditions are obtained.It is found that the boundary layer separation first occurs at the trailing edge of the blade suction surface,and then it may occur at the leading edge of the blade pressure surface with the decrease of the flow rate.Before the generation of the boundary layer separation on the blade surfaces,the minimum of the static pressure is located near the leading edge of the blade suction surface,while the maximum of the static pressure is situated near the trailing edge of the blade pressure surface.After the generation of the boundary layer separation on the blade surfaces,the two static pressure curves cross near the leading edge of the blade.With the variation of the axial position and the flow rate,the distribution of the energy gradient function K is consistent with the development of the boundary layer separation and the vortex in the flow field.It is shown that the energy gradient function K can well indicate the location and severity of unstable flow in the centrifugal fan.(2)Numerical simulation of the unsteady flow is performed for the three-dimensional turbulent flow field in a double suction centrifugal fan.The accuracy of the simulation results is verified by the experimental results of the external characteristics.The development and change of the flow field after the occurrence of the rotation stall are analyzed,and the relationship between the rotation stall and the surge is revealed.It is found that low pressure zones are formed at the outlet of the unstable blade channels after the occurrence of the rotating stall.With the rotation of the impeller,the low pressure zones gather in one place and periodically pass by the volute outlet.With the decrease of the flow rate,the low pressure zones periodically appeared at the outlet of the volute becomes larger.In the actual working condition,the low pressure zones may occupy the majority of cross-section of the flow passage at the volute outlet when the flow rate is very low.Thus,the total pressure at the outlet of the fan exhaust pipe is higher than that at the volute outlet at the very low flow rate condition.Under the effects of the pressure difference and disturbance,the high pressure zone at the outlet of the fan exhaust pipe would drive the low pressure zone at the volute outlet to form a reverse flow,therefore the surge would occur.With the drop of the flow rate,the value of the energy gradient function K at the outlet of the unstable blade channels becomes larger.The zone with high K value may promote the development of the disturbance,and then the surge would be stimulated at a certain condition.In summary,the low pressure zones at the impeller outlet caused by the rotating stall increases with the decrease of the flow rate,which is the internal cause of the surge.(3)Numerical simulation and experimental measurement of the pressure fluctuation characteristics are carried out in a double suction centrifugal fan.The dynamic pressure sensors are used to measure the static pressure at sixteen monitoring points on the surface of the volute in the centrifugal fan.The sixteen pressure monitoring points are distributed along the circumferential,radial and axial directions of the fan.The additional three monitoring points at the impeller outlet are added in the numerical calculation of the fan.Both the numerical and experimental results predict the rotating stall phenomenon.The fundamental frequency of the rotating stall cell is 19Hz.The accuracy of the numerical results is verified by the experimental results.By analyzing the pressure fluctuation at the measuring points on the volute wall,it is shown that the surge doesn't occur in the range of the flow rate studied.For the steady operating conditions,the pressure fluctuation at the impeller outlet is mainly affected by the Jet-Wake structure.(4)The unsteady flow behavior in the flow field of the centrifugal fan is analyzed.Three flow states(static stall,weak rotating stall and strong rotating stall)are focused on.The relationship of the unsteady flow characteristics between the internal flow field and the external characteristic curve is discussed.It is found that the pressure fluctuation indicates three stages of evolution with the variation of the flow rate.a)Near the rated flow rate condition,the pressure fluctuation results from the Jet-Wake structure at the outlet of the impeller.b)With the decrease of the flow rate,the pressure fluctuation comes from the combined influence of the Jet-Wake structure and the rotating low pressure cell of high speed at the impeller outlet.c)With the further decrease of the flow rate,the pressure fluctuation is composed of the Jet-Wake structure and the rotating low pressure cell of low speed at the outlet of the impeller.The rotating low pressure cells are caused by the rotating stall.Correspondingly,it is found that the total pressure of the centrifugal fan also indicates three stages of evolution with the drop of the flow rate:a)Near the rated flow rate condition,the total pressure of the centrifugal fan is about a constant value.b)With the decrease of the flow rate,the total pressure of the centrifugal fan fluctuates slightly at the high frequency.c)With the further decrease of the flow rate,the total pressure of the centrifugal fan fluctuates widely at the low frequency.(5)The impeller staggered angle of the double suction centrifugal fan has an important effect on the Jet-Wake structure at the outlet of the impeller and the performance of the fan.The influence of impeller staggered angle on the flow characteristics of the double suction centrifugal fan is studied.The method to determinate the optimal staggered angle of the impeller is explored.It is found that the minimum of the static pressure on one half of the impeller is located at the same circumferential position as that of the maximum of the static pressure on another half of the impeller,then a pressure balance is achieved at the outlet of the impeller.On this foundation,the method of pressure distribution to determinate the optimal impeller staggered angle is proposed.According to the pressure distribution method,the optimal impeller staggered angle of another double suction centrifugal fan is predicted as a=10°.The experiments of the external characteristics of the fan under different impeller staggered angles are carried out.It is shown from the experimental results that the optimal impeller staggered angle is 10°,which is consistent with the numerical results.Thus the correctness of the pressure distribution method is verified.