Effect Of Tip Clearance On Unsteady Flow And Aerodynamic Noise Characteristics Inside Axial Flow

Axial flow fan is a kind of fluid machinery widely used in production and life.With the rapid development of the fan industry,the market has higher and higher requirements for axial flow fans,and the aerodynamic performance and noise are more and more strict.Expanding the high efficiency interval,increasing stability and reducing noise have become the hot and difficult points in the research of axial flow fan.Because of its simple structure,easy production,small space,convenient maintenance and other advantages,axial flow fan has huge development space and development potential in the future.In this paper,the axial flow fan is taken as the research object,and a variety of conditions such as different tip clearance,fixed clearance ratio model and circumferential groove processor case of axial flow fans are numerically calculated.The internal flow and aerodynamic noise characteristics of axial flow fans are analyzed according to the steady and unsteady calculation results.Firstly,the influence of tip clearance on unsteady flow in axial fan under different working conditions is studied.The full flow of axial fan with different tip clearance is calculated numerically.The results show that there are complex tip leakage flow,separation flow and secondary flow in axial flow fan.With the decrease of flow rate of axial flow fan,the pressure gradient at tip clearance increases,leakage flow is enhanced and strong disturbance is generated,which is easy to induce unstable flow such as rotary stall.The increase of tip clearance accelerates the formation rate of leakage vortex,and the increase of leakage vortex increases the flow loss,thus reducing the aerodynamic performance of axial flow fan.Tip clearance has a great influence on the aerodynamic performance of axial flow fan.Under the design flow of 18038 m~3/h,tip clearance decreases by 2 mm,static pressure efficiency increases by 5.08%,and static pressure increases by 13.23%.When the tip clearance is reduced by 2 mm,the static pressure efficiency and static pressure are increased by 2.58%and 2.97%respectively.The Dr Of the fan model with fixed clearance ratio increased from 0.3125 to 0.5208,and the static pressure efficiency at the optimum operating point increased by 1.20%,the static pressure value increased by 6.15 Pa,and the static pressure increased by 5.01%.When Dr Increases from 0.5208 to 1,the static pressure efficiency at the optimum operating point increases by 5.55%,the static pressure value increases by 25.60 Pa,and the static pressure increases by 19.86%.Secondly,the influence of the circumferential groove processing box on the unsteady flow in the axial flow fan is studied.Based on the circumferential groove processing box method and the study on the internal flow of axial flow fan with different tip clearance,the optimum groove depth of 5 mm and the optimum groove width of 15 mm were determined according to the numerical results.The leakage flow increases with the increase of circumferential groove depth,the pressure gradient decreases with the increase of groove depth,and the circumferential groove inlet velocity increases with the increase of groove depth.The leakage vortex is greatly affected by different slot widths and circumferential slots.As the width of the circumferential groove increases,the pressure gradient at the tip clearance decreases,the pressure difference between the two sides of the blade decreases,the accelerated leakage vortex breaks,the interference to the main stream increases,and the aerodynamic performance of the axial flow fan is reduced.The influence of three circumferential grooves on leakage flow of axial flow fans is studied.Compared with fans without grooves,circumferential grooves can effectively reduce the pressure gradient at tip clearance and increase the stability of axial flow fans.The aerodynamic efficiency of axial flow fan of circumferential trough casing was increased by 3.27%.Finally,the aerodynamic noise characteristics of axial flow fan are analyzed.On the basis of studying the unsteady flow field inside the axial flow fan,monitoring points were set up in the leading edge and trailing edge of the blade,the clearance of the blade tip and the fan outlet,and the internal and wake noises of the axial flow fan were quantitatively analyzed.The numerical results show that the leading edge noise of the axial flow fan blade is the largest,and the main noise is 20-1000 Hz.The sound pressure level of noise near blade tip clearance is 112 dB,and that of wake noise is 69 dB.The aerodynamic noise caused by leakage vortex rupture increases with the decrease of flow rate,and the wake noise of axial flow fan increases with the increase of tip clearance.The numerical results show that the aerodynamic noise in the axial flow air and the aerodynamic noise in the wake are reduced.The noise suppression at tip clearance of axial flow fan by different groove depths and widths of circumferential grooves is studied.Aerodynamic noise caused by leakage vortexes can be reduced by appropriately reducing the groove depth of circumferential grooves,and the peak value of aerodynamic noise at tip clearance can be reduced by increasing the groove width.The circumferential trough fan can effectively suppress the aerodynamic noise in the tip clearance.In conclusion,by studying different tip clearance,fixed clearance ratio model and circumferential groove treatment of box axial flow fans,this paper comprehensively studies the influence of tip clearance on unsteady flow and aerodynamic noise characteristics in axial flow fans,which provides a basis for diffusing efficiency and optimizing noise reduction of axial flow fans.