Unsteady Characteristics Of Double-suction Backward Centrifugal Fan At Low Flow-rate Conditions

The steady and unsteady flow characteristics of the internal flow in the doublesuction backward centrifugal fan at low flow rates are studied using the computational fluid dynamics method.The study’s main aim is to discover the characteristics of flow instability mechanisms at low flow rates.The double-suction rearward centrifugal fan is modeled and numerically measured in this paper at various flow speeds.The flow characteristic changing with time,the velocity contour,the pressure contour,the streamline,and the streamline changing with time are all investigated.Large-scale vortices are studied,including secondary flow on the meridian surface,separation flow on the revolution surface,and the volute’s outlet.The vortex’s creation location,generating factors,and development law are discussed.According to numerical equations,as the flow rate decreases,more and more vortices fill the impeller’s interior.At low flow rates,a lot of backflow and separation flows occur near the volute’s outlet,and the separation vortex blocks the flow within the impeller.The gas is constantly driven by the impeller rotation,and the evolution of these different vortexes over time has a direct impact on the blade pressure surface.Small vortexes emerge progressively as the individual vortexes connect with the blade surface.As the flow is decreased in a certain range,the pressure and velocity fluctuation range progressively increase.At 0.3Qn-Qn,the unsteady pulsation characteristics acting on the volute tongue steadily improve,whereas at 0.15 Qn,the unsteady pulsation characteristics acting on the volute tongue weaken.The unsteady characteristics of the revolving stall and surge of the double-suction backward centrifugal fan are explained by these findings.The principles of turbulent kinetic energy and turbulent kinetic energy dissipation rate are introduced,as well as the importance of studying suction surface and blade wake flow at low flow rates.Near the suction surface and blade wake,the velocity distribution characteristics,pulsation characteristics,and turbulent velocity range are investigated.Near the suction surface,the temporal and spatial association of velocity pulsation is studied.The findings show that the main effect of a decrease in flow rate on the blade’s wake and suction surface is as follows: the wake velocity attenuates faster along the flow path,and the hub boundary layer has a significant impact on the axial distribution.Multiple flow separation and reattachment phenomena occur near the suction surface as the flow rate decreases.At low flow speeds,the frequency band width of the inertial subregion decreases while the frequency band width of the dissipative sub-region increases,according to the characteristics of the turbulent velocity continuum.It is clear that as the flow rate is reduced,the viscous dissipation increases,but the inertial conveying capability is clearly inadequate.The characteristics of velocity pulsation’s space-time correlation show that the correlation is strong in a small time and space scale,and conforms to He’s "elliptical model" The association between time and space decreases exponentially as the time and space scales are increased.The author created a centrifugal fan test platform.The centrifugal fan test platform’s aerodynamic efficiency was evaluated,and the test platform’s reliability was confirmed when compared to the initial centrifugal fan performance parameters.The speed pulsation at the inlet and outlet of the fan was also planned and monitored in this paper,and the data was analyzed in the time domain and frequency domain to verify the periodic contact between the impeller and the air.