Numerical Research On Rotor-stator Interaction And Aerodynamic Noise Of Circumferential Skew Rotor Blades

The viscous flow in turbomachines is highly unsteady and turbulent.Rotor-stator interactions are the main unsteady flows of turbo machinery.The rotor and stator blades, moving relative to each other, interactbecause of the viscous wake and potential effects of the blades. Inmodern high-bypass-ratio turbofan systems, much of noise is generatedby the fan, particularly by the interaction of rotor-wake with downstreamstator. It has been proved that skewed and swept blade technique couldcontrol the distribution of the second flow, to increase aerodynamicefficiency and decrease flow loss. Research on using skewed and sweptblade technique to optimize inner flow structure and reduce rotor-statorinteraction noise, has been one of the more significant trends in theturbomachinery field.In this paper, based on the T35 No.5 rotor blade, three rotor bladeswith different skew direction and skew angle are redesigned. Matchedwith the same stator, these circumferential skewed rotor blades of lowpressure axial fan are as research objects. And then, with experiment andsimulation computation, the effect of rotor blade skew direction and skewangle on aerodynamic performance and unsteady flow structure of rotorstatorturbofan systems is analyzed. Furthermore, the possibility ofskewed and swept blade technique on controlling rotor-stator interactionnoise is also discussed.Main research contents and conclusions of this paper are as follows:1) According to the demand of research, circumferential forward,backward skewed and radial rotor are designed and manufactured. Theresults of aerodynamic performance tests show that the forward rotorblade has better aerodynamic than the radial rotor blade. The three dimensional steady and unsteady flow fields are simulated withNUMECA computation fluid dynamic software. Compared to theexperimental results, the accuracy of the computation results is validated.2) The effect of blade circumferential skew on flow field structure ofaxial fan is analyzed. The radial distributions of aerodynamic parametersshow that compared to the radial blade, there are a higher flow rate andan improved flow in mid-span region of the forward-skewed blade.Underlying physical mechanisms is extracted. Based on full radialequilibrium equation, because of radial component of body force ofcircumferential skewed rotor blades, there are controlled radialdistributions of pressure in flow passage. Circumferential forward skewrotor could decrease passage second flow and the accumulation ofboundary layer in blade tip region, and increase aerodynamicperformance.3) Propagation of wakes through inter-stage clearance is presented inthis paper. It is turned out that rotor circumferential skewing, which has apositive influence on wake mixing process, can accelerate decayingprocess of total pressure fluctuation. RMS pressure has been defined toexpress the unsteady surface pressures on the stator blades. Skewing canalso change the radial shape of the centerline of the rotor wake(wakeprofile)directly. At the same time, because of the spanwise difference ofthe relative exit flow angle, the radial shape of the centerline of the rotorwake is twisted. The angle between the wake shape and the leading edgeof stators lead to a radial phase lag. This phase lag has an importantinfluence on interaction between rotor wake and stators.4) Low speed fan noise generally consists of discrete frequencynoise and broadband noise. A hybrid prediction method, based on ahybrid method coupling FW-H equation and three dimensional unsteadyCFD simulations, is adopted to predict the noise radiated from the statorrotorsystem. The shedding vortex noise model developed by Fukano isadopted to predict broadband noise. The results show that sheddingvortex noise of rotor blades are main noise sources. The broadband noiseof the forward skewed rotor has a lowest sound pressure level. And forthe interaction noise between stator wake and rotor, the point force model for discrete frequency noise developed by Lowson is adopted. The noiseprediction results show that, both forward skewed rotor blade andbackward skewed blade could reduce discrete frequency noise radiatedfrom downstream stator.