Tip Noise Passive Control in Low Mach Number Axial Fan

Transcription

1 Tip Noise Passive Control in Low Mach Number Axial Fan Stefano BIANCHI XI Israeli Symposium of Jet Engines and Gas Turbines Haifa, Israel, October

2 Credits The present research was done in the context of the contract FW-DMA10-11, between Fläkt Woods Ltd and Dipartimento di Ingegneria Meccanica e Aerospaziale Sapienza University of Roma Alessandro CORSINI Anthony G. SHEARD DMA-URLS Fläkt Woods Ltd

3 Outlines of civil aircraft issues

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5 Outlines on Military aircrafts ssues Introduction - Launch crews, on aircraft carriers, are exposed to excessive noise levels during takeoffs and landings leading to costly and escalating hearing loss compensation programs. Pictures Marine Nationale - The Secretary of the US Navy, has estimated a total potential liability of $350 million should litigants prevail in lawsuits involving jet noise.

6 Introduction - Typically launch/recovery support personnel can be exposed to brutal acoustic loads of up to 150 dba. Each launch typically involves a 30 sec average mil power exposure (occasionally full AB). - The carrier deck personnel may experience up to 200 launches/recoveries per 12 hr duty shift. Picture US Navy The goal of engines noise reduction is considered of high importance.

7 - For high by pass ratio engines turbine noise account only during the Landing operation, as the TO thrust is exerted mainly by the fan. - Low bpr engines are more senstive to turbine noise, even for TO operation. NASA report: FS GRC - In low bpr engines turbine noise is comparable with the rear component of compressor noise: directed roughly to the engine side. Rolls-Royce, 2006

8 Techniques and concepts for tip treatment, a first review (ii) During the last decade have appeared several passive noise control concepts based on the blade tip modifications by means of anti-vortex appendages anti-vortex devices as end-plates at the tip originally inspired by the technique developed for the control of tip vortex and induced drag reduction in aircraft wings, also used as anti-vortex devices for catamaran hulls Karman end-plate Boeing, blended winglets F2005, rear winglets

9 Axial fan family Test model and demonstrator based on AC90/6 fan. unswept rotor fan in ducted configuration in service experiences demonstrated good acoustics performance AC90/6 blade profiles modified ARA-D Design concept Arbitrary vortex with radially increasing work distribution U tip about 50 m/s

10 Conventional end-plate design Base-line solution for the tip treatments, Corsini et al. (2006, 2007), defined a blade configuration with use of tip appendages to implement a control on leakage flow rotor AC90/6/TF tip blade section modified by an end-plate on pressure surface with squaretail trailing edge TF end-plate tip blade section is locally thickened of a factor 3:1 wrt maximum thickness at the tip of datum blade According to the theory behind the end-plate design, this dimension was chosen as the reference radial dimension of leakage vortex to be controlled approx blade span, (Inoue et al., 1986) (Corsini et al., 2004).

11 Analysis on inner workings of TF end-plates, tip leakage vortex bursting This finding was in accordance with the existence of a vortex breakdown Criterion for VB detection based on vortex swirl analysis an interpretive criterion to detect the appearance of vortex breakdown makes use of the Rossby number (or inverse swirl parameter) (Ito et al., 1985) Ro = V /(r W) Ro design point for a confined vortex the scales are: r, radial distance from the vortex axis at which the swirl velocity was at its maximum in accord with characteristic viscous length scale Leibovich (1984) V = watlv, was taken to be the axial velocity W, be the rotation rate in the wing-tip vortices due to the solid body rotation-like structure (estimated near the vortex centre) fraction of chord breakdown region

12 End-plates for tip leakage vortex swirl control, design goals The work is dedicated to sketch new concepts for the shaping (design) of the anti-vortex tip end-plate The rationale is to designate a variation of end-plate thickness, in order to implement a manipulation of the chord-wise evolution of the leakage vortex swirl Visualization and Analytics Center for Enabling Technologies (VACET) DOE, USA Aircraft Vortex Manipulation, Manipulation of the blade vortex interaction of helicopters with nonplanar blade tips on the main rotor Institute of Aeronautics and Astronautics, Aachen, Germany The aim is to promote an enhancement or a detriment of near-axis swirl by influencing the momentum transfer from the leakage flow and by inducing some waviness into the leakage vortex trajectory, as attempted in delta-wing platform design Srigrarom and Kurosaka (2000)

13 End-plate for Multiple Vortex Breakdown MVB concept rationale Corsini A, Rispoli F & Sheard AG, A Meridional Fan, Patent granted No. GB , July (2009) Rossby number definition for a confined vortex TLV rotational frequency and length measures given by Rain (1954) and Lakshminarayana (1970) Leakage flow modelled as a two-dimensional flow orthogonal to the chord line AC90/6 fans datum TF end-plate The TLV Rossby number could be expressed along the blade chord as a function of: - gap geometry, (height & width) TFvte end-plate - kinetic energy of the leakage flow MVB end-plate - friction factor which is itself a function of leakage flow Reynolds number, Regap - local blade load conditions Ro(xc)=f [tip gap, blade load, leakage flow, end-plate thickness]

14 II generation of end-plate for Multiple Vortex Breakdown MVB, design goals Corsini A, Rispoli F & Sheard AG, A Meridional Fan, Patent granted No. GB , July (2009) The idea is to obtain an end-plate geometric able to combine the acoustic pay-off of TF related to the mixing enhancement effect induced by the vortex bursting the aerodynamic performance benefit of TFvte consequence of the healthier status of the flow in the annulus and in the wake The rationale was to profile the end-plate at the blade tip according to a prescribed chordwise distribution of the TLV Rossby number The design Rossby number distribution would drive the TLV to a sequence of critical condition able to induce the onset of vortex bursting fraction of chord

15 CFD survey details datum The authors solved the Reynolds-Averaged Navier-Stokes equations by an original parallel Multi-Grid Finite Element flow solver, using C++ technology and libmesh libraries TF The physics involved in the fluid dynamics of incompressible 3D turbulent flows in a rotating frame of reference is modelled with a non-linear k-e model, here in its topology-free low-reynolds variant. The Finite Element flow solver uses second-order accurate approximations in space for primary-turbulent variables. TFvte Concerning the solution strategy, the authors solved the Navier-Stokes and turbulence equations fully coupled. The linear solver uses a SOR preconditioned GMRES(5) technique. The mesh was built using a non-orthogonal body fitted coordinate system with block-structured topology. The mesh consists of about 0.6 million linear hexahedral elements. Standard boundary condition settings were adopted, as previously used in studies on high-performance fans

16 datum MVB tip leakage flow survey TLV detection helicity criterion (i) MVB TF TFvte Identical TLV onset point wrt to datum and TF Vortex core path deformation according to the enhancement/detriment of swirl

17 TLV helicity & 3D streamlines MVB tip leakage flow survey TLV streamlines (ii) datum TF TFvte swirl enhancement No evidences of helicity inversion swirl detriment No evidences of separation cores, TLV goes through the VB onset without actually bursting MVB

18 MVB tip leakage flow survey, TKE & n t (iii) Turbulent kinetic energy iso-surfaces MVB datum rotor AC90/6/TF AC90/6/TFvte Normalized turbulent viscosity nt iso-surfaces nt_2: 10 2 nmol; nt_1: 5 10 nmol n t _1 n t _1 n t _1 MVB n t _2 n t _2 n t _1 n t _2 datum rotor AC90/6/TF AC90/6/TFvte

19 Turbulent kinetic energy iso-surfaces MVB Lw db(a) frequency (Hz) Normalized turbulent viscosity nt iso-surfaces The noise has a broadband characteristic MVB Tones are due to the tip leakage flow in the pressure side Boundary layer separation appears in the suction side at the trailing edge FLAKT WOODS Ltd

20 operating point 8 m3/s narrow band (3 Hz) sound pressure level spectra MVB rotor performance assessment aeroacoustics (ii) sound power spectra in one-third octave band BPF motor signature

21 MVB rotor performance assessment aeroacoustics (iii)

22 Experimental techniques, rig (i) Fan 8 6 m 3 /s Anechoic chamber BS848 p2 Far-field microphone Noise farfield: Span traversed microphone (when operating in the rig) distance = 2 rotor D from the fan section six azimuthal positions = 0 = 90 = 75 = 60 = 45 = 30 Pressure nearfield: distance d=15 mm from the trailing edge 2% of the chord spanwise traversing microphone in 14 positions (10% span each)

23 Experimental techniques, data post-processing (ii) Near-field Far-field Hypotesis: - H(t,r) Linear throug the anechoic space - H(t,r) depends only from geometries tip S(t,1) H(t,1) Cross-correlation, between near- and far-field Auto&cross spectra G SS G NN G SN S(t,r) H(t,r) S(t,0) H(t,0) N(t) Coherence: Co 2 G G SS 2 SN G NN hub S(t,r) H sound (t,r) H psdsound (t,r) H(t,r) D(t,r) N(t,r) To far-field

24 datum Norm Freq (ʄ = f/bpf) ʄ = 1-7 MVB rotor performance assessment Near-field aeroacoustic source dissection (ii) Source characterization Span wise Coherence 30 MVB Norm Freq (ʄ = f/bpf) ʄ = 1-7 TF TFvte AC90/6/TFMVB

25 Conclusions The passive control technique explored in the study was based on blade tips that had been modified by the addition of anti-vortex appendages as end-plates. The end-plate configuration aimed to control the chord-wise evolution of the leakage vortex swirl level by preventing the occurrence of tip-leakage vortex bursting by the enhancement of near-axis swirl. The leakage-flow survey has shown that the variable-thickness configuration MVB was able to exert control over near-axis leakage vortex swirl. As a result, although the critical Rossby number region is reached, the end-plate configuration provides effective control over leakage vortex bursting. The comparative aerodynamic and aeroacoustic assessment, demonstrated the overall gain for the investigated end-plates was found to be a consequence of the implemented passive flow control concept acting on the tip leakage flow

26 Thank you! (Corsini A, Rispoli F & Sheard AG, A Meridional Fan, Patent granted No. GB , July 2009)

27 backup slide MVB rotor performance assessment aerodynamics (i)

28 backup slide MVB design concept, sensitivity analyses sensitivity on tip gap sensitivity on blade tip