Investigation of the Effect of Inflow Distortion on the Noise Emitted from an Axial Flow Fan Experiencing Cross Flow

Transcription

1 Investigation of the Effect of Inflow Distortion on the Noise Emitted from an Axial Flow Fan Experiencing Cross Flow J. Tirakala, H.-J. Kaltenbach Lehrstuhl für Aerodynamik und Strömungsmechanik

2 Overview Aim of The Present Investigations Introduction to Fan-in-Plate Configuration Fan-in-Plate Experimental Setup Noise Generation Mechanisms Experimental Results and Discussions Numerical Results Future work outline and open questions Acknowledgement

3 Aim of The Present Investigations To study the effect of massive inflow distortion on sound radiation from a fan operating under strong cross flow conditions To identify the dominant noise generation mechanisms To study the change in noise levels for different inlet-lip geometries To check the predictive capability of hybrid approach (URANS + FW-H) for the present case Strong flow separation at fan inlet

4 Introduction to Fan-in-Plate Configuration S/VTOL Courtesyhttp://history.nasa.gov Courtesy:Eurocopter Fan-in-Wing configuration [1] FIW in BMW aeroacoustic wind tunnel [1].Dr.Nicolas Thouault, Aerodynamic investigations on Generic fan-in-wing configurations

5 U inf To Free stream Fan-in-Wing flow configuration Parameter Fan diameter D f Value 120mm Fan-in-Plate flow configuration Hub-tip ratio 0.42 Rotor blade aspect ratio 0.95 Tip clearance (% D f ) 0.8% Tip mach number Reynolds number,root chord 1.6xe5 Solidity at hub/tip (rotor) 1.1/0.43

6 Fan-in-Plate Experimental Setup Micro Phone array (96 micro phones) Fan-in-Plate Fan-in-Pate configuration is installed in acoustic wind Braunschweig [1] [1] Prof. Jan Delfs, Dr.Thorsten Schwarz, Michael Pott-Pollenske AWB-DLR Braunschweig

7 NdFeB Magnets (ᴓ x L) 2mm x 4mm 4 rotor and 4 stator blades Used in model planes Combinations of 4 X 4 is particularly bad configuration Hall sensor is installed to measure and control the frequency Strain gauges are installed to measure Thrust

8 Different fan inlet configurations Fan diameter=120mm R=0mm R=10mm R=35mm R=43mm

9 Noise generation mechanism Noise generation Mechanism Without cross flow Cross flow Frequency/modes Experiment Numeric { Rotor-stator interaction m=0, nbpf ;n= Inflow distortion Rotor Rotor blade imperfections + + Broaband noise (Turbulence, Tip vortex) Structural vibrations?? Rotor imperfections m=0 at nbpf Harmonics of shaft frequencies Harmonics of shaft frequencies + ++ Broadband Harmonics of shaft frequencies

10 Rotor-stator interaction and Taylor-Sofrin modes m=nb+kv And each modes rotates at nbω/m rps m -lobed rotating mode n-harmonic of BPF B-No. of rotor blades =4 V-No. of stator blades =4 Ω-Angular velocity of rotor Ω c- Critical angular Velocity mode propagates nbω/m > Ω c Harmonic No n K m=nb+kv Ang. Vel. Mode n.b.ω/m Ω c =6Ω (m n) <Ωc Not propagates -1 0 Inf >Ωc propagates (0 1) <Ωc Not propagates <Ωc Not propagates <Ωc Not propagates -2 0 Inf >Ωc propagates (0 2) <Ωc Not propagates <Ωc Not propagates <Ωc Not propagates <Ωc Not propagates -3 0 Inf >Ωc propagates (0 3) <Ωc Not propagates <Ωc Not propagates <Ωc Not propagates

11 Inflow distortion-rotor interaction m=nb+kv And each modes rotates at nbω/m rps B-No. of rotor blades =4 V-No. of stator blades =1 Ω-Angular velocity of rotor Ω c- Critical angular Velocity mode propagates nbω/m > Ω c Harmonic No n K m=nb+kv Ang. Vel. Mode n.b.ω/m Ω c =6Ω (m n) -4 0 inf >Ωc propagates (0 1) <Ωc Not propagates <Ωc Not propagates <Ωc Not propagates <Ωc Not propagates -8 0 inf >Ωc propagates (0 2) >Ωc propagates (1 2) <Ωc Not propagates <Ωc Not propagates <Ωc Not propagates Inf >Ωc propagates (0 3) >Ωc propagates (1 3) >Ωc propagates (2 3) <Ωc Not propagates

12 Experimental Results Validity of the measurements Fan 1 f 2f 3f 4f(1stBPF) 8f(2ndBPF) f 2f 3f 4f(1stBPF) 8f(2ndBPF) f - frequency of rotor Observer Point Located on axis of f ) above the plane of rotation of fan

13 SPL for different inlet-lip radius at different cross flow conditions RPM=26,200 rpm R=0mm Shaft freq. 1st BPF 2nd BPF R=43mm Shaft freq. 1st BPF 2nd BPF Sharper inlet-lip Difference in SPL at Shaft freq. 1.5dB Difference in SPL at 1st BPF- 8dB Smoother inlet-lip Difference in SPL at Shaft freq. 2dB Difference in SPL at 1st BPF- 0.5dB Observer Point Located on axis of f) above the plane of rotation of fan

14 SPL for different inlet-lip radius BPF/4 1st BPF 2nd BPF Observer Point Located on axis of f ) above the plane of rotation of fan

15 Measured thrust value for different inlet-lip at fixed Fan speed Measured Thrust RPM=26,200 rpm Zero Cross flow With Cross flow U=30m/s Thrust With Cross flow U=40m/s Inlet-lip index 1 - R=00mm 2 - R=10mm 3 - R=35mm 4 - R=43mm Inlet-lip Radius index

16 Numerical Setup Computational Methodology: CFD Acoustics Near field/far field Propagation URANS-Solver (Ansys CFXv14) Porous FW-H Integral formulation[1] Parameter Free stream velocity (U ) Fan rotational speed Time step, Δt (CFD) Grid resolution Value 40 m/s 26,200 rpm 8.0 x 10-6 s 5 x 10 6 nodes [1]courtesy : Prof. S Becker and C Scheit of Universität Erlangen-Nürnberg

17 Flow Computations Mesh:Block Structured mesh(icemcfd) Turbulence Model:Shear Stress transport 15 Fan revolution are considered for Acoustic calculations On a 40 CPU node it requires 19 hrs run time for one fan rotation Sliding interface 18

18 Variation of separation bubble at the fan inlet TPR= P T P T P T = mean total pressure P T =free stream mean total pres. R=0mm R=10mm R=35mm R=43mm 19

19 OP Φ=60 CS Difference in SPL at 1st BPF 2dB Difference in SPL at 2nd BPF- 3.5dB Observer Point above the plane of rotation of fan (13.3D f ) 20

20 Sound pressure levels for different fan inlet radius U=40 m/s; RPM=26200 rpm Observer Point Located on axis of above the plane of rotation of fan 21

21 Low frequency oscillations Pressure oscillations at casing close to blade tip Peak at ½ f shaft 1/8thBPF

22 Work Plans and Open questions Quantitive description of inflow distortion ; e.g. circumferential mode decomposition of mean flow for different lip radius Evaluation of sound radiation by FW-H formulation for different control surface in order to distinguish noise creation mechanism (inflow distortion versus rotor/stator interaction) Rotor blades alone Porous FW-H-surface between rotor and stator Stator surface alone Physical cause of low frequency oscillation (at ½ of shaft frequency)? Rotating instabilty? Interlocked frequency w.r.t oscillation of separation bubble Comparison with experiments? Suggestions for other methods to identify relevant source mechanisms? (e.g. modal decomposition of near field?)

23 Different control surface configurations to identify source mechanism CS CS Porous surface above rotor Rotor alone as control surface CS CS CS CS CS Porous surface between rotor and stator Stator surface and part of the fan inlet duct

24 Acknowledgement Prof. Jan Delfs, Dr.Thorsten Schwarz, Michael Pott-Pollenske, for providing the measurement facilities and technical staff support at Acoustic wind DLR Braunschweig. Prof. S Becker and C Scheit of Universität Erlangen-Nürnberg for providing inhouse FW-H code.

25 !!!Thank you for your attention!!!

26 Directivity for different Inlet geometries at Fan speed 26200rpm R=0mm R=10mm 27

27 R=35mm R=43mm 28

28 R=00 mm RPM=26200 rpm;u=0m/s ; Wall pressure over 28 blade pitches i.e 7Revolution 29

29 R=10 mm RPM=26200 rpm;u=0m/s ; Wall pressure over 28 blade pitches i.e 7Revolution 30

30 R=43 mm RPM=26200 rpm;u=0m/s ; Wall pressure over 28 blade pitches i.e 7Revolution 31