Rolf Sondergaard

Turbine Separation Control Using Pulsed Vortex Generator Jets

The application of pulsed vortex generator jets to control separation on the suction surface of a low-pressure turbine blade is reported. Blade Reynolds numbers in the experimental, linear turbine cascade match those for high-altitude aircraft engines and aft stages of industrial turbine engines with elevated turbine inlet temperatures. The vortex generator jets have a 30 deg pitch and a 90 deg skew to the free-stream direction. Jet flow oscillations up to 100 Hz are produced using a high-frequency solenoid feed valve. Results are compared to steady blowing at jet blowing ratios less than 4 and at two chordwise positions upstream of the nominal separation zone. Results show that pulsed vortex generator jets produce a bulk flow effect comparable to that of steady jets with an order of magnitude less massflow. Boundary layer traverses and blade static pressure distributions show that separation is almost completely eliminated with the application of unsteady blowing. Reductions of over 50 percent in the wake loss profile of the controlled blade were measured. Experimental evidence suggests that the mechanism for unsteady control lies in the starting and ending transitions of the pulsing cycle rather than the injected jet stream itself. Boundary layer spectra support this conclusion and highlight significant differences between the steady and unsteady control techniques. The pulsed vortex generator jets are effective at both chordwise injection locations tested (45 and 63 percent axial chord) covering a substantial portion of the blade suction surface. This insensitivity to injection location bodes well for practical application of pulsed VGJ control where the separation location may not be accurately known a priori.

The fluid dynamics of LPT blade separation control using pulsed jets

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 deg pitch and 90 deg skew angle) are pulsed over a wide range of frequency at constant amplitude and selected duty cycles. The resulting wake loss coefficient versus 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 percent) 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 percent 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 five to six 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.

Control of Low-Pressure Turbine Separation Using Vortex-Generator Jets

The application of vortex-generator jets to control separation on the suction surface of a low-pressure turbine blade is reported. Blade Reynolds numbers in the experimental, linear turbine cascade match those for high-altitude operation of many aircraft gas-turbine engines, as well as the last stages of industrial ground-based gas turbines. Results are presented for steady blowing at jet blowing ratios from zero to four and at several chordwise positions and two freestream turbulence levels. Findings show that above a minimum blowing ratio, which is dependant on the injection location, the pressure loss in the modified blades wake is reduced by a factor of between two and three. Boundary-layer traverses show that separation is almost completely eliminated with the application of blowing. No significant deleterious effects of vortex-generator jets are observed at higher (nonseparating) Reynolds numbers. The addition of 4% freestream turbulence to the cascade freestream lowers the separation Reynolds number of the turbine blade studied, but does not eliminate the effectiveness of the control technique. The vortex-generator jet control strategy is demonstrated to be a viable technique for low-pressure turbine separation control.

Reducing Low-Pressure Turbine Stage Blade Count Using Vortex Generator Jet Separation Control

An experimental investigation has been conducted into the feasibility of increasing blade spacing (pitch) at constant chord in a linear turbine cascade. Vortex generator jets (VGJs) located on the suction surface of each blade in the cascade are employed to maintain attached boundary layers despite the increasing tendency to separate due to the increased uncovered turning. Tests were performed at low Mach numbers and at blade Reynolds numbers between 25,000 and 75,000 (based on axial chord and inlet velocity). The vortex generator jets (30 degree injection angle and 90 degree skew angle) were operated with steady flow with momentum blowing ratios between zero and five, and from two spanwise rows of holes located at 45% and 63% axial chord. In the absence of control, pitch-averaged wake losses increase up to 600% as the blade pitch is increased from its design value to twice the design value. With the application of VGJs, these losses were driven down to or below the losses at the design pitch. The effectiveness of VGJs was found to increase modestly with increasing Reynolds number up to the highest value tested, Re = 75,000. The fluid phenomenon responsible for this remarkable range of effectiveness is clearly more than a simple boundary layer transition effect, as boundary layer trips installed on the same blades without VGJ blowing had no beneficial effect on blade losses. Also, tests conducted at elevated levels of freestream turbulence (4% at the cascade inlet) where the suction surface boundary layer is generally turbulent, showed wake loss reduction comparable to tests conducted at the nominal 1% freestream turbulence. For all configurations, blowing from the upstream row had the greatest wake influence. These findings open the possibility that future LPT designs could take advantage of active separation control using integrated VGJs to reduce the turbine part count and stage weight without significant increase in pressure losses.© 2002 ASME

Designing Low-Pressure Turbine Blades With Integrated Flow Control

A low pressure turbine blade was designed to produce a 17% increase in blade loading over an industry-standard airfoil using integrated flow control to prevent separation. The design was accomplished using two-dimensional CFD predictions of blade performance coupled with insight gleaned from recently published work in transition modeling and from previous experiments with flow control using vortex generator jets (VGJs). In order to mitigate the Reynolds number lapse in efficiency associated with LPT airfoils, a mid-loaded blade was selected. Also, separation predictions from the computations were used to guide the placement of control actuators on the blade suction surface. Three blades were fabricated using the new design and installed in a two-passage linear cascade facility. Flow velocity and surface pressure measurements taken without activating the VGJs indicate a large separation bubble centered at 68% axial chord on the suction surface. The size of the separation and its growth with decreasing Reynolds number agree well with CFD predictions. The separation bubble reattaches to the blade over a wide range of inlet Reynolds numbers from 150,000 down to below 20,000. This represents a marked improvement in separation resistance compared to the original blade profile which separates without reattachment below a Reynolds number of 40,000. This enhanced performance is achieved by increasing the blade spacing while simultaneously adjusting the blade shape to make it less aft-loaded but with a higher peak cp . This reduces the severity of the adverse pressure gradient in the uncovered portion of the modified blade passage. With the use of pulsed VGJs, the design blade loading was achieved while providing attached flow over the entire range of Re. Detailed phase-locked flow measurements using three-component PIV show the trajectory of the jet and its interaction with the unsteady separation bubble. Results illustrate the importance of integrating flow control into the turbine blade design process and the potential for enhanced turbine performance.Copyright

Control of Separation in Turbine Boundary Layers

The opportunity for the reattachment and control of separated flows occurs in inlets, compressors, transition ducts and turbines. Passive and active control of separated flows has been demonstrated successfully by a number of techniques which employ the introduction of longitudinal or streamwise vortices. The role of these vortices is initially to reenergize the wall boundary layer flow by entraining and redistributing momentum from the primary flow to the wall layer and enhance early transition. A chain of non-linear interactions of these unsteady vortices with large scale unsteady separation vortices and the shed shear layer results in significant alteration of the circulation. The resulting increased circulation allows higher blade loadings, reduced part count, as well as increased performance at low Reynolds numbers. Flow control location has been investigated at chord locations ahead, at, and after separation. Passive dimples with single and multiple rows, varied dimple location and dimple shape have been investigated. Initial investigations of a single row of dimples and their wakes on a high pressure turbine vane ring have been performed at Reynolds numbers down to 13,500 in a full scale matched parameter rig. Properly placed dimples reattach separated flows at all Reynolds numbers investigated. Computations for the dimple geometries with VBI, MISES, and Fluent have been carried out to determine initial separation, compressible implications, reattachment locations, and predicted wake profiles or loss coefficients. Steady and pulsed vortex generator jets with duty cycles down to 1%, have both demonstrated reattachment and reduction of total losses in excess of 40% at Reynolds numbers down to 25,000, without incurring significant additional losses at higher Reynolds numbers. Pulsed vortex generator jets with a duty cycle of 1% have demonstrated blowing coefficients of <10 -4 .

Reynolds Number Effects on the Secondary Flow of Profile Contoured Low Pressure Turbines

Low pressure turbine profiles with high aerodynamic loading can suffer from poor midspan performance at lower Reynolds number. Studies have shown that forward loading can mitigate the low Reynolds number lapse in performance at midspan, but concerns remain about increased secondary loss from front loading profiles. The effect of Reynolds number on low pressure turbine secondary flow losses are considered here. The front loaded L2F profile is studied experimentally in a low speed linear cascade. Several different geometric contours are used to modify the shape of the blade near the endwall, which decreases passage total pressure loss. Profile contouring is accomplished by placing an endwall glove over the blade at the blade to endwall junction. Performance of each profile is compared versus Reynolds number and boundary layer parameters. Translation of dominate secondary loss flow features were tracked using surface oil flow visualization, and their movement in the passage is related to changes in passage total pressure loss. Measurements showed that the lift off-line of the passage vortex moved downstream as Reynolds number decreased, and passage total pressure loss increased.

Unsteady Performance of a Turbine Driven by a Pulse Detonation Engine

American Institute of Aeronautics and Astronautics This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited. Disclaimer: The views expressed in this presentation are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense, or the U.S. Government. Unsteady Performance of a Turbine Driven by a Pulse Detonation Engine

Numerical Investigation of Low-Pressure Turbine Endwall Flows

Highly-loaded low-pressure turbine blades promise overall turbine performance improvements and lower overall costs but suffer from unacceptable endwall losses. The endwall flow physics have to better understood to develop effective flow control strategies aimed at a reduction of the endwall losses. The Air Force Research Laboratory is carrying out highly resolved particle image velocimetry measurements and oil-flow flow visualizations for the L2F airfoil at Re=100,000. To complement the wind tunnel experiments, implicit large-eddy simulations for the same airfoil are being performed for Re=100,000 and 5,000. The simulation for Re=100,000 is in good agreement with the experiment with regard to skin friction line patterns and total pressure loss coefficient. Instantaneous flow visualizations reveal a strong passage vortex that is intermittently loosing its coherence. A linear stability theory analysis of the endwall boundary layer upstream of the cascade indicates cross-flow instability. For Re=5,000 the passage vortex is steady and the suction side separation on the airfoil is much increased. As a result, the total pressure loss coefficient is significantly larger than for Re=100,000.

PIV Investigation of a Highly-Loaded LPT Blade Using a Curved Laser-Sheet

Low Reynolds number flow around a highly loaded LPT blade (the L1A) was investigated by Particle Image Velocimetry (PIV) at the blade midspan. PIV data was acquired in both the blade normal and spanwise direction planes. Measurements in the spanwise direction planes were taken near the airfoil suction surface using a curved lasersheet that closely matched the curvature of the aft portion of the blade profile. Images in the blade normal plane over a range of Reynolds numbers clearly show the shear layer and transition length, while images in the spanwise direction plane show the three-dimensional nature of the transition region. The experiment was performed on a linear cascade of seven blades mounted in the Air Force Research Lab Low Speed Wind Tunnel facility. Images were taken at two different freestream turbulence levels across a Reynolds number range between 25,000 and 125,000.