Drag Reduction Mechanism Analysis And Application Of Bionic Riblets On Low Speed Fan Airfoil

The complex flow phenomenon inside the impeller and the resulting flow separation are the main sources of losses in the blade passage.Inhibiting the blade flow separation and clarifying the internal flow characteristics and separation mechanism are the important way to further improve the mechanical performance and enhance its operation stability.In recent years,the surface treatment technology based on bionic method has achieved certain results in improving flow separation,and has been preliminarily applied in aeronautics and astronautics and other fields.However,further discussion is needed on how to arrange it on the low pressure fan and improve the blade separation flow.Advanced measurement technology combined with computational fluid dynamics and experiment were induced to explore the drag reduction law of the riblets perpendicular to the flow direction on the airfoil of the turbomachinery.The fan optimization and flow control research were also carried out by arranging riblets on centrifugal fan blade.The main work and research results are as follows:(1)The numerical model for the small-scale flow of the riblets perpendicular to the flow direction is established,and the airfoil boundary layer flow is detailly analyzed.The lift and drag of airfoils with different riblets schemes under multiple angles of attack are compared,and the relationship between riblets parameters and drag reduction is quantitatively explained.It is found that the drag reduction effect of the riblet schemes with longer riblet and smaller groove is better.Meanwhile,the influence of riblets on the flow in the airfoil boundary layer is qualitatively revealed,and their drag reduction mechanism is also clarified.(2)The internal relationship between the wake vortex shedding model and the unsteady aerodynamic characteristics of the airfoil at large angle of attack is carried out,and the effects of different riblets parameters on the development of wake vortex shedding are compared.By analyzing the transient flow fields of different schemes,it is found that the riblets with appropriate size inhibit the formation and development of secondary vortices on the airfoil surface and expand the airfoil’s stable operation.The transient flow structure in the boundary layer also demonstrated that the drag reduced riblet scheme weaken the fluid pulsation at the boundary layer bottom and the Reynolds stress caused by turbulence bursts.(3)Based on the quasi three dimensional computational model,the large eddy simulation of the airfoil flow is carried out,and the variation of transient flow field and the instantaneous development rule of vortex structure are revealed.According to the friction coefficient distribution,the suction surface of the original can be mainly divided into three sections: the rapidly decreasing friction domain,the stable friction domain and the fluctuating friction domain.By comparing the airfoil friction coefficient,pressure coefficient and near wall velocity distributions,it is found that the response sensitivity of the stable friction domain is higher than the rapidly decreasing friction domain.In addition to the achievement of the riblets drag reduction law and the groove size with positive drag reduction effect,it can be seen that the key to reduce the airfoil drag is the suppression of the vortex structure and the characteristic parameters improvement near the wall.(4)Referring to the flow phenomenon research and theoretical analysis results of the airfoil riblets,different riblets are designed on the blade surface of a backward centrifugal fan.Their effects on the fan aerodynamic characteristics and the internal flow of impeller are also evaluated.The numerical results show that it is better to arrange the riblets at the position where the friction coefficient decreases rapidly near the leading edge,while the optimal riblets effectively suppresses the flow separation and end zone effect under some working conditions.The experiment results show that the riblets located at 0.1 to 0.2 times the chord length with a size of 0.5mm improves the fan total pressure and efficiency by 4.2% and 4.5%,respectively,and successfully improve the fan aerodynamic performance under low flow conditions.