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Experimental Investigation of Impeller-Diffuser Interaction Rita Patel, Eric Savory and Robert Martinuzzi Outline • Background • Motivation • Current Work – Design of experimental rig – CFD analysis • Results and discussion • Conclusions • Future Work Terminology Cumpsty (1978) Types of Impellers and Diffusers Impellers • Radially ending • Backswept • Pre-swirl • Above w/splitter blades Diffusers • Vaneless • Vaned – Radial – Wedge • Discrete-passage Pictures courtesy of Compressor Branch NASA Glenn Research Center Radial Impeller Discharge • Increasing BL on shroud-suction side due to curvature Jet Wake PS SS – Separation – Wake on shroudsuction side – Jet displaced to hubpressure side Dean and Senoo (1960), Eckardt (1976) & Krain (1981) Diffuser Inlet • Large inlet distortions due to impeller wake – Angle and velocity fluctuations • Distortions have least effect in passage diffusers than vaned, and most in vaneless • Mixing-out of jet-wake stimulated by presence of vanes Impeller-Diffuser Interaction • Vanes – Stationary vanes produce unsteady pressure disturbances to rotating impeller, Gallus et al. (2003) – Velocity fluctuations of 17-20% in vaneless space, Krain (1981) – Cause of backflow to impeller, Cui (2003) – Decrease traveled distance of impeller discharge distortion, Ghiglione et al. (1998) Impeller-Diffuser Interaction (cont’d) • Radial Gap – Too small = increase backflow, Cumpsty and Inoue (1984) – Too large = less mixing-out of jet-wake Gallus et al. (2003) Motivation Why study impeller-diffuser interaction when numerous studies have been done? • All configurations are different – This project will lead into the study of a tandem-bladed impeller coupled with a fishtail diffuser – Study the magnitude and effect of pressure disturbances in vaneless space – Validate previously obtained CFD results Picture courtesy of Douglas Roberts (P&WC) Current Work • Design a test facility (SCR*) that simulates a typical radial impeller exit flow field in steady state through a nonrotating cascade configuration – 5 stationary radial impeller blades – Diffuser with 5 flat plate splitters – Pipe to provide required inlet flow • Obtain LDV data of flow field *Stationary Cascade Rig Purpose of SCR • CFD – Experimental validation of results obtained on SCR • Seeding flow distribution, flow patterns, etc… • Better understanding of how to apply LDV technique to a full-scale rig – Test use of very small optical access ports – Type of seeding for this specific flow Picture courtesy of Douglas Roberts (P&WC) SCR Impeller + Diffuser Close-up of impeller Optical Access 10mm diameter 15mm diameter Upstream Impeller Blades Blade passages ‘hub side’ Seeding Ports Six Ports SCR Specifications Ttip Thub hblade Ablade A5 blades Ageometrical A5 passages m alpha c2 cr2 cx1 Inlet 2.51 2.62 23.37 59.95 299.75 2849.67 2549.92 0.288 0.00 n/a n/a 94.84 Outlet 1.04 4.27 15.75 41.80 209.00 2998.19 2789.19 0.288 72.00 291.69 90.14 n/a [mm] [mm] [mm] [mm 2] [mm 2] [mm 2] [mm 2] [kg/s] deg [m/s] [m/s] [m/s] • • • • Outlet Ma: 0.85 Total length: 2.0 [m] Total height: 1.4 [m] Similar physical dimensions of fullscale rig CFD Analysis • ICEM CFD 10.0 with CFX 10.0 Mesh: 1.1 million tetrahedral + 0.2 million prism element mesh • SST k-ω model Boundary Conditions: Inlet: Ptotal = 172.4 [kPa] Ttotal = 288.15 [K] Outlet: Pstatic = 101.3 [kPa] mexpected= 0.245 kg/s mCFX= 0.242 kg/s Mach Number Contour Plot - Impeller-diffuser only - Region of high velocity in left most passage - Obtaining close to desired Ma of 0.85 at impeller exit 50% blade height Flow Behaviour • Shock wave at trailing edge of each blade • Passage width increasing, while height decreasing • Greater shock wave in left passage as result of diffuser sidewall Flow Behaviour Blade Passages Flow behaviour similar in passages up to outlet Migration of high velocity region to shroud suctionside and vice versa Pressure Contour Plot -impellerdiffuser only - Typical blade suction/pressure behaviour Suction side Pressure side - Corresponding region of low pressure in left most passage Conclusions • From CFX results – Presence of separation in diffuser – No separation in impeller – Good flow pattern agreement between blade passages • Close agreement between theoretically calculated and CFX values at boundaries • SCR will provide a good understanding of how to apply LDV technique in a high-speed, highlyconfined, compressible flow Future Work • Experimental – Measurements in SCRF – Compare with current CFD results • Computational – Track seeding particles • Apply LDV technique and CFD model on fullscale rig at P&WC Acknowledgements • Advanced Fluid Mechanics Research Group – http://www.eng.uwo.ca/research/afm/default.htm • Kevin Barker and Doug Phillips – University Machine Shop • Rofiqul Islam – University of Calgary • Suresh Kacker, Douglas Roberts, Feng Shi and Peter Townsend – Pratt and Whitney Canada Thank You Questions?