Investigation Of Unsteady Characteristic Analysis And Design Method In A Counter-rotating Fan Based On Harmonic Balance Method

Currently,most matured compressor/fan aerodynamic design system adopt assumption of time-independent.This assumption neglects the unsteady components and resulting in uncertainties of compressor/fan aerodynamic design and flowfeild prediction.Especially,nowadays,the higher load and more compact structure of compressor/fan enhance the unsteadiness between rows.It means the time-independent hypothesis would lead to magnification of uncertainties,and restrict the excavation of compressor/fan aerodynamic performance.Additionally,some advanced design concept(e.g.integral aircraft/engine aerodynamic design)would induce the inlet distortion to compressor/fan,and lead to the unsteadiness of the flowfield,which means the steady physical model is not applicable.All of above indicate that it is necessary to introduce unsteady effects into the compressor/fan aerodynamic design system.This paper aims at systematically investigating the unsteady effects caused by row-to-row interaction and inlet distortion,studying the law and mechanism of unsteady characteristic along the variation of operation condition,furthermore,exploring the unsteady compressor/fan aerodynamic optimization design technique and provide the reference and idea for the related researchers.The achievement of above research requires high efficient unsteady numerical simulation tools.Based on the periodic characteristic of flowfield in the turbomachinery,this article introduces "Harmonic Balance Method",deriving the unsteady harmonic balance governing equations,implicit-linearizing the discrete equations,implementing efficient and stable unsteady numerical simulation of turbomachinery.Then Based on throughflow calculation,the initial counter-rotating fan is designed.In order to lay the foundation for the later studies,taking this fan as the research object,the efficiency and stability of turbo3 d on steady and unsteady numerical simulation are validated.All of the unsteady studies in this article are conducted on "Harmonic Balance Method".Firstly,the steady and unsteady flowfield of counter-rotating fan under 100%?90% and 80% design rotating speed is simulated numerically.By comparing the steady and unsteady aerodynamic characteristics,the unsteady effects and its variation law with working condition.The results show that the unsteady interaction between rows would induce the remarkable change of aerodynamic performance near endwall region.Specifically,due to the row-to-row interaction,near design point,the loss near the blade tip and root of both rotors usually increases while,near stall point,it often decreases.Furthermore,the analyses into detailed time-averaging and instantaneous flowfields show that: for the front rotor,it is hardly affected directly by the the unsteady disturbance,the change of aerodynamic performance of blade tip and root mainly result from the variation of the endwall flow characteristic of rear rotor induced by unsteady interactions;for the rear rotor,its flow regime is mainly influenced by upstream wakes,which make the tip clearance vortex shape change periodically,enlarge the scope of tip clearance loss and decrease the intensity of tip clearance loss;additionally,upstream wakes could lead to strong vorticity vibration and augment the boundary layer friction loss,while it also increases the turbulence intensity of boundary layer which could suppresses the flow separation;lastly,upstream wakes would also bring about "negative jet" phenomenon when it interact with main flow,which makes the flow of lowkinetic energy in the boundary layer on the suction side move toward regions of higher mass flux and enhance the energy exchange between main flow and boundary layer,as a result,it can restrain the flow separation and this effect is more apparent at near stall working condition.Secondly,the unsteady characteristic induced by inlet distortion under 100% design rotating speed is investigated.Two types of sinusoidalcircumferential total pressure distortion are imposed at inlet.The time-averaging results show the nouniformity of inlet total pressure at circumferential direction would lead to performance deterioration,and the effect from low-frequency sinusoidal distortion is more significant.Furthermore,by analyzing the instantaneous flowfield corresponding to the different inlet circumferential location,it is found that the flow characteristics of rotors are influenced in two ways-magnitude and circumferential gradient of inlet total pressure.Specifically,the inlet boundary with a lower total pressure would make the working point of the counter-rotating fan closer to stall condition,and augment the blade load,lead to higher adverse pressure gradient,thereby contributing to more serious hub corner separation and tip clearance and vice versa.Yet,the circumferential favorabletotal pressure gradient would raise the angle of attack of inflow,accelerate the corner separation and tip leakage flow and exacerbate the flow separation caused by the interaction between shock wave and boundary layer and vice versa.Then a typical inlet total pressure distortion originated from boundary layer ingestion(BLI)propulsion system is investigated.The results display that the law and mechanism in the study of sinusoidaldistortion are applicable in BLI distortion,which,to some extent,indicates the related theory is reliable and universal.Lastly,based on the idea of "edge-matching",the unsteady optimization design platform is constructed,and the unsteady aerodynamic design is completed at near design point.The results show that the peak-efficiency of optimized counter-rotating fan increases by 0.4% and the total pressure ratio augment for the whole working range,the numerical stall margin is enlarged apparently.The analysis of flowfield demonstrates that "edge-matching" contains "bow-matching" which belongs to the steady/time-averaging effect and "circumferential phase-matching" which is correspond to the unsteady traits.Moreover,the augment of efficiency brought by the bowed effects afford the main improvement of total efficiency of counter-rotating fan while the phase-matching has less influence.The unsteady perturbation of optimized counter-rotating fan across the blade span decline at different degree,and the reduction of unsteady disturbance in the downstream blade passage mainly results from the descent of the intensity of upstream wakes and tip leakage flow.