Experimental Investigation of Impeller-Diffuser

InteractionRita 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 – Separation

– Wake on shroud-suction side

– Jet displaced to hub-pressure side

Dean and Senoo (1960), Eckardt (1976) & Krain (1981)

Jet

Wake

SS

PS

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

– 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

Why study impeller-diffuser interaction when numerous studies have been done?

• All configurations are different

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 non-rotating 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

• 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

• CFD– Experimental validation of results obtained on SCR

• Seeding flow distribution, flow patterns, etc…

Picture courtesy of Douglas Roberts (P&WC)

SCR

Impeller + Diffuser

Close-up of impeller

Upstream Impeller Blades Blade passages ‘hub side’

Optical Access

10mm diameter

15mm diameter

Seeding Ports

Six Ports

SCR Specifications

• Outlet Ma: 0.85• Total length: 2.0 [m]• Total height: 1.4 [m]• Similar physical

dimensions of full-scale rig

Inlet Outlet

Ttip 2.51 1.04 [mm]

Thub 2.62 4.27 [mm]

hblade 23.37 15.75 [mm]

Ablade 59.95 41.80 [mm2]

A5 blades 299.75 209.00 [mm2]

Ageometrical 2849.67 2998.19 [mm2]

A5 passages 2549.92 2789.19 [mm2]

m 0.288 0.288 [kg/s]alpha 0.00 72.00 degc2 n/a 291.69 [m/s]

cr2 n/a 90.14 [m/s]cx1 94.84 n/a [m/s]

CFD Analysis• ICEM CFD 10.0 with CFX 10.0

Mesh:1.1 million tetrahedral + 0.2 million prism element mesh

Boundary Conditions:Inlet: Ptotal = 172.4 [kPa] Ttotal = 288.15 [K]Outlet: Pstatic = 101.3 [kPa]

• SST k-ω model

mexpected= 0.245 kg/s

mCFX= 0.242 kg/s

- Impeller-diffuser only

- Region of high velocity in left most passage

- Obtaining close to desired Ma of 0.85 at impeller exit

Mach Number Contour Plot

50% blade height

• 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

Flow Behaviour

Blade Passages

Flow behaviour similar in passages up to outlet

Migration of high velocity region to shroud suction-

side and vice versa

Pressure side

Suction side

-impeller-diffuser only

- Typical blade suction/pressure behaviour

- Corresponding region of low pressure in left most passage

Pressure Contour Plot

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, highly-confined, compressible flow

Future Work

• Experimental– Measurements in SCRF– Compare with current CFD results

• Computational– Track seeding particles

• Apply LDV technique and CFD model on full-scale 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?