Numerical Benchmarking of Tip Vortex Breakdown in Axial Turbines Eunice Allen-Bradley April 22, 2009

TVB Cascade Tip Vortex – Overview & Introduction Tip leakage losses have been studied since the 1950s: –Rains (1954) was the first to experimentally measure tip vortex in compressor cascade test; –Further studies focused on tip leakage losses in compressor and fan cascades in the 1960s, 1970s, & 1980s; Lakshminarayana et al. (1962), Lewis et al. (1977), Pandya et al. (1983), Inoue et al. (1989). –Tip leakage loss studies in turbine cascades conducted in 1980s,1990s, & 2000s; Booth et al. (1982), Sjolander et al. (1987), Moore et al. (1988), Morphis et al. (1988), Yamamoto (1989), Dishart et al. (1990), Yaras et al. (1992), Chan et al. (1994), Govardhan et al. (1994), Sondak et al. (1999) –Tip desensitization studies in turbine & compressor cascades conducted in 1990s & 2000s. Hamik et al. (2000), Schabowski et al. (2007), Shavalikul et al. (2008), Van Ness et al. (2008). Tip vortex breakdown studies (published) have been limited to external body applications: –General definition given by Hall (1972) as any abrupt change in vortex core behavior. –Delta wing tip vortex formation, unsteady effects, far field wake effects: Sarpkaya (1971), El-Ramly (1972). Studies are lacking in which the event of tip vortex breakdown occurs in turbomachines: –Is it possible to adequately predict tip vortex breakdown in turbomachines with the current computational tools available? –Current study will focus on prediction capability in axial turbine simulations, using RANS CFD.

TVB Tip Vortex Methodology & Procedure – Design of Experiments Using the geometry and boundary conditions of an existing cascade facility, model tip leakage with RANS CFD. Alter boundary conditions until tip vortex breakdown is predicted; –Tip clearance, exit Mach number, inlet flow angle. Confirm results with several turbulence models for benchmarking and possible cascade testing for validation. TVB Cascade Design Conditions Cascade Span (H)2.4” Axial Chord (B X )1.0” Inlet flow angle (  1 ) 63 o Exit flow angle (  2 ) 26 o Exit Mach number (M 2 )0.8

TVB Tip Vortex Results & Discussion – Tip Clearance Effect Sjolander and Cao (1994) varied the size of the tip gap in their tip leakage flow study, and showed that cascade loss increased with an increase in tip clearance. These results are also consistent with the results of the Govardhan et al. (1994) study. The CFD predicted tip vortex breakdown for the 0.004” & 0.006” tip gap at design conditions. The size of the tip vortex grows with the increase in tip gap.

TVB Tip Vortex Results & Discussion – Inlet Boundary Layer Effect Chan et al. (1994) concluded in their turbine cascade tip leakage study that the inlet boundary layer has no effect on the cascade performance. The CFD results suggest, however, that this may also be a function of the measurement location. The CFD did not predict tip vortex breakdown for any of the inlet boundary layer thickness for design conditions and tip gap of 0.010”. The size of the tip vortex did not change, but the intensity of the core does.

TVB Tip Vortex Results & Discussion – Exit Mach Number Effect The tip vortex core size and subsequent losses can be directly linked to the discharge coefficient (the ratio of actual gap mass flow rate to the mass flow rate at 1D flow conditions). The CFD did not predict tip vortex breakdown with a tip gap of 0.010” at the design inlet flow angle. The size of the tip vortex grows with the increase in exit Mach number.

TVB Tip Vortex Results & Discussion – Inlet Flow Angle effect As Willinger and Haselbacher (2004) showed in their tip leakage flow study, the cascade performance loss increases as the inlet flow angle is set to more positive incidences. However, as tip vortex breakdown was predicted in the simulations for the similar incidence change, they did not report the event of vortex breakdown in their study. The CFD predicted tip vortex breakdown for  1 = 43 o &  1 = 53 o at all exit Mach number and tip gap conditions.

TVB Cascade Tip Vortex Results & Discussion – Benchmarking Conditions ON - Tip CLR = 0.010”; M 2 = 0.8;  1 = 53 o OFF - Tip CLR = 0.010”; M 2 = 0.8;  1 = 63 o Recall that the axial chord of the TVB cascade is 1.0”: –The size of potential measurement probes may be larger than the core of the predicted tip vortex; –Furthermore, the presence of potential measurement probes may artificially induce tip vortex to breakdown. –An alternate method for confirmation of tip vortex breakdown is needed.

The suction side streamlines of the TVB cascade serve as further visual confirmation of predicted tip vortex breakdown. ON - Tip CLR = 0.010”; M 2 = 0.8;  1 = 53 o OFF - Tip CLR = 0.010”; M 2 = 0.8;  1 = 63 o Direction of Flow These are the result from the Baldwin-Lomax fully turbulent prediction

TVB Cascade Tip Vortex Contour of Total Pressure Axial Location = 1.50*Bx ONOFF Spanwise Loss Plot* *Mass averaged results from Baldwin-Lomax fully turbulent model

The suction side streamlines of the TVB cascade serve as further visual confirmation of predicted tip vortex breakdown. ON - Tip CLR = 0.010”; M 2 = 0.8;  1 = 53 o OFF - Tip CLR = 0.010”; M 2 = 0.8;  1 = 63 o Direction of Flow These are the result from the k  fully turbulent prediction

TVB Cascade Tip Vortex Contour of Total Pressure Axial Location = 1.50*Bx ONOFF Spanwise Loss Plot* *Mass averaged results from k  fully turbulent model

The suction side streamlines of the TVB cascade serve as further visual confirmation of predicted tip vortex breakdown. ON - Tip CLR = 0.010”; M 2 = 0.8;  1 = 53 o OFF - Tip CLR = 0.010”; M 2 = 0.8;  1 = 63 o Direction of Flow These are the result from the k  transitional prediction

TVB Cascade Tip Vortex Contour of Total Pressure Axial Location = 1.50*Bx ONOFF Spanwise Loss Plot* *Mass averaged results from k  transitional model

The suction side streamlines of the TVB cascade show NO visual confirmation of predicted tip vortex breakdown. ON - Tip CLR = 0.010”; M 2 = 0.8;  1 = 53 o OFF - Tip CLR = 0.010”; M 2 = 0.8;  1 = 63 o Direction of Flow These are the result from the SST k  fully turbulent prediction

TVB Cascade Tip Vortex Contour of Total Pressure Axial Location = 1.50*Bx ONOFF Spanwise Loss Plot* *Mass averaged results from SST k  fully turbulent model

The  Loss generation plots between four different models show the same trend through the cascade passage. ON - Tip CLR = 0.010”; M 2 = 0.8;  1 = 53 o OFF - Tip CLR = 0.010”; M 2 = 0.8;  1 = 63 o Baldwin-Lomax fully turbulent k  transitional k  fully turbulent SST k  fully turbulent

The performance comparison of  loss for the various models run to date suggest solid confirmation of tip vortex breakdown prediction. Average  Loss between 4 models Tip vortex breakdown is predicted for three out of the four models shown above. 0.44%

Conclusions 3D steady CFD models are trend accurate to performance, and capture many of flow features that have been measured experimentally. Confirmation of the vortex breakdown prediction is demonstrated with several different turbulence models: –The CFD calculations predict an average delta cascade loss of 0.44% between the models used to simulate the cascade. Results suggest that resulting vortex breakdown phenomenon is not the driving cause of the increased loss through the cascade: –This is supported by the accompanying spanwise loss plots. Future Work: –Cascade measurements taken based on benchmarking conditions; –Explore influence of relative rotation of outer endwall.

Back up slides

TVB Cascade Tip Vortex: Contour of Total Pressure Baldwin Lomax fully turbulent model ON - Tip CLR = 0.010”; M 2 = 0.8;  1 = 53 o OFF -- Tip CLR = 0.010”; M 2 = 0.8;  1 = 63 o Axial Location = 0.75*Bx Axial Location = 0.95*Bx Axial Location = 1.05*Bx

TVB Cascade Tip Vortex: Contour of Total Pressure kw fully turbulent results ON - Tip CLR = 0.010”; M 2 = 0.8;  1 = 53 o OFF -- Tip CLR = 0.010”; M 2 = 0.8;  1 = 63 o Axial Location = 0.75*Bx Axial Location = 0.95*Bx Axial Location = 1.05*Bx

TVB Cascade Tip Vortex: Contour of Total Pressure kw transitional results ON - Tip CLR = 0.010”; M 2 = 0.8;  1 = 53 o OFF -- Tip CLR = 0.010”; M 2 = 0.8;  1 = 63 o Axial Location = 0.75*Bx Axial Location = 0.95*Bx Axial Location = 1.05*Bx

TVB Cascade Tip Vortex: Contour of Total Pressure SST kw fully turbulent results ON - Tip CLR = 0.010”; M 2 = 0.8;  1 = 53 o OFF -- Tip CLR = 0.010”; M 2 = 0.8;  1 = 63 o Axial Location = 0.75*Bx Axial Location = 0.95*Bx Axial Location = 1.05*Bx