Effect Of Impeller Offset On The Aerodynamic Performance And Internal Unsteady Flow Of A Centrifugal Fan

The double-suction backward centrifugal fan is a type of ventilation equipment that possesses superior energy efficiency and aerodynamic performance.As one of the most critical components,the impeller’s relative positioning with respect to the volute is of paramount importance as it is closely tied to the overall effectiveness and efficacy of the entire system.However,due to substandard installation quality and the deleterious effects of prolonged operational use,including bearing overheating and wear and tear on other component parts,it is challenging to ensure the impeller is consistently located in the center position of the fan,resulting in operational discrepancies,such as radial offset or skew of the impeller.This impeller displacement not only alters the gap between the collector and impeller inlet,but also impacts the compatibility between the volute and impeller,resulting in a degradation of the fan’s aerodynamic performance and an internal flow field that is nonuniform,thereby negatively impacting the normal operations of the fan.For the impact of radial deviation error of the impeller,this thesis only moves the radial position of the impeller to conduct numerical simulation while ensuring the geometric shape of the left and right impellers remains unchanged.It also explores the reasons why the radial deviation of the impeller causes uneven distribution of the internal flow field in the fan,ultimately leading to a decrease in the aerodynamic performance of the fan.Numerical results show that the offset impeller changes the size of the gap between the impeller and collector in the radial direction,which produces a non-uniform distribution of static pressure and even reversed flow at the inlet of the impeller.As the impeller is offset away from the baffle of the volute,reversed flow back into the volute is observed,and strong circulating vortices form around the baffle,which weakens the diffusing capability of the volute.However,as the impeller is offset close to the baffle,the internal flow of the volute generally improves the flow patterns within the impeller,while the boundary layer separates from the pressure surface of the blades near the front disc as the flow angle varies;the maximum velocity of reversed flow could reach 20m/s which lowers the efficiency of the fan.For the influence of impeller deflection error,this thesis keeps the position of the impeller center unchanged,and deflects the impeller in different directions to simulate the deflection of the impeller due to the wear of the bearing or the lateral and longitudinal deflection of the bearing seat during the long-term operation of the fan.The results show that the deviation of the impeller axis from the center line of the fan will affect the whole process of gas diffusion and flow inside the fan,and the degree is mainly related to the radial distance of the deflection.When the radial degree of impeller deflection is small,the flow field inside the fan is relatively stable,and even the impurity frequency component and amplitude of pressure fluctuation in the inlet of impeller flow channel are reduced.The lateral deflection improves the boosting effect on the left side of the fan by 6.9Pa.When the deflection degree is too large,the flow field distribution inside the fan becomes extremely uneven,and this negative effect enlarges continuously during the process of gas flow through various components,resulting in a significant decrease in aerodynamic performance,which is mainly manifested in: the axial and radial clearances at the inlet of the impeller are not uniform in circumference due to deflection,and the pressure fluctuation on the wall of the collector increases sharply at the maximum change of the clearance,and the axial speed of the gas entering the impeller decreases by about 75%.Due to the decrease of axial velocity at the inlet of the impeller,the flow distribution is not uniform in each blade passage in the impeller,and the radial return flow occurs at the lower flow,resulting in a decrease of the functional force on the pressure surface of the blade.The deflection of the impeller changes the direction of the outlet face of the impeller,which makes the pressure on the spiral case wall asymmetrically distributed.At the same time,it is affected by the flow reduction at the outlet of the impeller at various circumferential positions,resulting in uneven circumferential pressure expansion and weakening the expansion effect of the spiral case.At the outlet of the volute,excessive lateral deflection of the impeller will make the fluid on the right side of the volute easier to recirculate,and excessive longitudinal deflection of the impeller will cause more secondary flow losses,which will cause insufficient kinetic energy of the fluid flowing out of the volute and then flow back to the volute again.