| As a typical rotating machine,axial blower is widely used in forest fire fighting,garden litter cleaning,dust and gravel cleaning,snow cleaning and other fields because of its large air volume,small structure size,good ventilation effect,high efficiency and energy saving.It plays an important role in China’s forestry operations.However,due to the special working environment of the axial flow blower and the complex threedimensional flow inside,especially in the off design condition,the rotating stall occurs in the flow channel,which has a serious impact on the stable operation of the blower,and can not meet the requirements of high efficiency,low noise and stable operation under various working conditions.Therefore,it is of great significance to study the internal flow characteristics of axial blower,especially the unsteady characteristics,for improving the performance and enhancing the stability.In this paper,the design theory and internal unsteady flow characteristics of guide vane axial flow blower are systematically studied through design theory analysis,experimental research,steady numerical simulation and unsteady numerical simulation.The main research work and innovations include:(1)Based on the air characteristics of the isolated airfoil and the Rukovsky law,the radial variation law of the flow parameters under the annular flow pattern was introduced.A small two-stroke gasoline engine driven guide vane axial flow blower was designed and developed,and an axial flow fan performance test-bed was built.The influence of the number of impeller blades and the blade installation angle on the performance was obtained through 9 orthogonal tests;The variance analysis of the influence of the number of blades and blade installation angle on the total pressure and efficiency under rated working conditions was carried out.It was concluded that the blade installation angle has a significant impact on the fan performance,and the number of blades has no significant impact on the fan name.Combined with the interaction diagram of the number of blades and blade installation angle on the total pressure and efficiency,it was concluded that the scheme with the number of blades of 9 and the blade installation angle of 59.5 degrees is the best scheme.(2)The full channel numerical calculation model of the axial flow fan was established.Through the steady numerical simulation of the axial flow fan under different blade installation angles,the distribution of different flow parameters in the impeller and guide vane basin was obtained.The results showed that under the rated working condition,with the increase of blade installation angle,the negative pressure on the suction surface of the blade increases,and the total pressure of the fan increases.The matching of guide vane and impeller affects the conversion of dynamic pressure and static pressure.When the blade installation angle is 59.5 °,the static pressure in the guide vane basin is the largest,and the conversion efficiency of dynamic and static pressure is high.The influence of blade installation angle on the turbulent kinetic energy at the junction of impeller and guide vane is obvious.The increase of blade installation angle leads to the increase of turbulent kinetic energy at the inlet of guide vane and the increase of energy loss.It was concluded that to improve the total pressure efficiency of the impeller,it is necessary to enhance the matching between the guide vane and the impeller,so as to reduce the loss of energy conversion at the interface between the impeller and the guide vane.Through the study of the spatial streamline of the impeller,it was found that the structure of the tip leakage vortex presents obvious spatial structure differences with the change of the installation angle.Under the high flow condition,the small angle is adopted,the tip leakage vortex is cut into two parts by the blade top,and there is only a whole leakage vortex structure at the back of the blade at the large angle.Under stall condition,the three placement angles form a flow structure of recirculation and reentry,which increases the turbulent kinetic energy at the inlet.(3)The effects of different installation angles on pressure pulsation,radial force and aerodynamic noise were studied.The study of pressure fluctuation showed that the frequency spectrum of pressure fluctuation in each channel is concentrated in the rotating frequency of the impeller.Under the design condition,with the increase of the blade installation angle,the frequency spectrum energy at the rotating frequency increases,and more harmonic components are added in the frequency spectrum field,which is more obvious under the stall condition.The study of radial force shows that the period of radial force on the rotor is related to the number of blades of the stator.Under the design condition and stall condition,the average value of radial force on the rotor increases with the increase of blade installation angle.The study of aerodynamic noise shows that with the increase of installation angle,the range of high-intensity oscillation at the inlet narrows,and the range of high-intensity oscillation at the outlet widens.With the increase of blade placement angle,the noise energy also increases under the design condition,but the change of blade placement angle has little effect on the noise energy of axial flow fan under stall condition.(4)The non-uniform flow characteristics and rotating stall characteristics of the fan were studied.The research showed that the rotor region presents obvious non-uniform distribution characteristics under stall conditions,and the flow characteristics on the inlet surface of the fan are consistent with those in the rotor region.The low-speed region of the inlet surface corresponds to the impeller stall channel,and the high-speed region of the inlet surface corresponds to the steady-state channel of the impeller.The research on the generation and development process of stall showed that on the right side of the hump point,the axial fan is only disturbed by the modal wave,and at the hump point,the peak wave replaces the modal wave as the main interference.On the left side of the hump point,the peak disturbance amplitude decreases gradually,and the attenuation evolves into periodic large-scale oscillation.After the peak appears,different sections of the rotor show different degrees of velocity attenuation.With the increase of chord length coefficient,the span position of meridional velocity attenuation decreases.On the inlet surface,the stall disturbance effect is mainly concentrated in the edge region,while on the outlet surface,the velocity decays at 0.34 span.At the hump point,after experiencing a spike,the incident angle of the shunt path gradually rises,exceeds the critical value,and gradually forms a low-speed countercurrent at the edge of the intake surface.The incident angle mode and the distorted inflow mode will propagate with the stall vortex circumferential direction. |
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