Evolution Mechanism And Backflow Suppression Of Multi-blade Centrifugal Fan Asymmetric Internal Flow

The experimental measurement,numerical calculation on the complex threedimensional(3D)unsteady viscous flow in turbo-machinery and the flow control of the related separation are the main research directions of this discipline.Expanding the machine's stable operating range,making it quietly and efficiently run under a restricted environment or non-design working conditions is the hot issue in the design and optimization of turbomachinery related products in recent years.An in-depth understanding of the impeller flow status and its evolution among the decreasing flow rate is the key to solving this problem.Aims at exploring the low-pressure centrifugal fans' inner flow separation characteristics affect the aerodynamic performance and their instability process with working conditions.Multi-blade centrifugal fans widely used in high-end home appliance products were selected as the research objects in the present research.Advanced measurement technology combined with computational fluid dynamics(CFD)was induced to measure and analyze the instantaneous flow field under different working conditions.The corresponding modification design and flow control research on different flow-through components was then carried out.The main work and research results are as follows:1.A particle image velocimetry(PIV)platform was constructed to measure the flow field in the test fan.The reversely flow trend near the volute tongue was captured,and there exists an unsteady vortex in the middle of the inlet chamber;the intensity of both flow structures are significantly enhanced under the low-flow condition.Comparing and combining the CFD results with the PIV flow field,the characteristics of separation,backflow and vortex in the fan and related evolution process with operating conditions were given as:part of the airflow passes through the blade tunnels reversely driven by the pressure gradient near the volute tongue and returns to the impeller's upstream.This part of reverse flow further flows across the impeller and blocks some of the blade passages,resulting in the leakage of the impeller gap and the airflow cannot return to the impeller in time and gather near the collector.Under the low-flow operating conditions,the cross-impeller flow will lose stability and entrain the leakage flow into the fan inlet chamber reversely.2.Large eddy simulation calculations were carried out on the instantaneous flow in the studied multi-blade centrifugal fan under various operating conditions,based on a simplified model for the r-? section.The change of the flow field and the vortex motion on the section were detailed analyzed.The formation mechanism of the blade tunnel reverse-flow near the volute tongue and the unsteady state of the cross-impeller flow were also revealed.By analyzing the pressure signal of the monitor points,the time domain pulsation of the fan total pressure and the spatiotemporal motion map of the reverse-flow along the blade cascade under various conditions,the evolution processing of the fan's internal flow was clarified as:from the steady at the large-flow condition to the periodic pulsating state at best efficiency point,and loss instability completely with the fan flow rate continues decreasing.Through LES,the low-frequency pulsation under each condition was identified,the process of the leading edge separation vortex and its shedding frequency was also captured.3.Various of modified method for different components are proposed to suppress the aggravated backflow of the fan under low-flow conditions,and the corresponding optimal design parameters were given based on the experimental and numerical results.A dual-arc blade profile was optimized using a modified NSGA-? algorithm.It is found that the long narrow blade passage with a slow tail curvature not only inhibits the secondary separation of the suction surface separated flows but also prevents the airflow from penetrating the passage reversely under low-flow conditions.The fan pressure under various operating conditions is increased by 2-5%,and the efficiency is increased by about 1 percent;Cutting the volute profile 90°downstream the tongue with a secondary clearance of 0.02D₂ can change the re-entering position of the cross-impeller airflow.The increase in the impeller working capacity under low-flow is higher than the additional pressure loss caused by volute cutting,the fan pressure was,therefore,increased by nearly 7%;An asymmetric D-type collector was designed with l_D/R_nozzle=0.7 and installed along the direction of 150°,which maintained the cross-impeller flow and shroud clearance leakage inside the fan,prevented the occurrence of reverse-flow,and weakened the inlet chamber vortex.Experimental results show that the fan pressure under zero flow-rate condition was increased by about 6%.