NDLI: The Fluid Dynamics of LPT Blade Separation Control Using Pulsed Jets

Content Provider	The American Society of Mechanical Engineers (ASME) Digital Collection
Author	Jeffrey, P. Bons Sondergaard, Rolf Richard, B. Rivir
Copyright Year	2001
Abstract	The effects of pulsed vortex generator jets on a naturally separating low pressure turbine boundary layer have been investigated experimentally. Blade Reynolds numbers in the linear turbine cascade match those for high altitude aircraft engines and industrial turbine engines with elevated turbine inlet temperatures. The vortex generator jets (30 degree pitch and 90 degree skew angle) are pulsed over a wide range of frequency at constant amplitude and selected duty cycles. The resulting wake loss coefficient vs. pulsing frequency data add to previously presented work by the authors documenting the loss dependency on amplitude and duty cycle. As in the previous studies, vortex generator jets are shown to be highly effective in controlling laminar boundary layer separation. This is found to be true at dimensionless forcing frequencies (F+) well below unity and with low (10%) duty cycles. This unexpected low frequency effectiveness is due to the relatively long relaxation time of the boundary layer as it resumes its separated state. Extensive phase-locked velocity measurements taken in the blade wake at an F+ of 0.01 with 50% duty cycle (a condition at which the flow is essentially quasi-steady) document the ejection of bound vorticity associated with a low momentum fluid packet at the beginning of each jet pulse. Once this initial fluid event has swept down the suction surface of the blade, a reduced wake signature indicates the presence of an attached boundary layer until just after the jet termination. The boundary layer subsequently relaxes back to its naturally separated state. This relaxation occurs on a timescale which is 5–6 times longer than the original attachment due to the starting vortex. Phase-locked boundary layer measurements taken at various stations along the blade chord illustrate this slow relaxation phenomenon. This behavior suggests that some economy of jet flow may be possible by optimizing the pulse duty cycle and frequency for a particular application. At higher pulsing frequencies, for which the flow is fully dynamic, the boundary layer is dominated by periodic shedding and separation bubble migration, never recovering its fully separated (uncontrolled) state.
Sponsorship	International Gas Turbine Institute
File Format	PDF
ISBN	9780791878521
DOI	10.1115/2001-GT-0190
Volume Number	Volume 3: Heat Transfer; Electric Power; Industrial and Cogeneration
Conference Proceedings	ASME Turbo Expo 2001: Power for Land, Sea, and Air
Language	English
Publisher Date	2001-06-04
Publisher Place	New Orleans, Louisiana, USA
Access Restriction	Subscribed
Subject Keyword	Suction Cycles Temperature Blades Cascades (fluid dynamics) Aircraft engines Separation (technology) Fluids Bubbles Gas turbines Chords (trusses) Vorticity Wakes Velocity measurement Turbines Economics Relaxation (physics) Fluid dynamics Reynolds number Momentum Generators Flow (dynamics) Pressure Vortices Jets Boundary layers
Content Type	Text
Resource Type	Article

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