Jeffrey P. Bons

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.

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

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 .

The Effect of Unsteadiness on Film Cooling Effectiveness.

Abstract : A unique feature of turbine rotor blade film cooling is the main flow unsteadiness caused by the upstream stator vanes. The combined effect of the vane inviscid flow field and the trailing edge wake results in a rapidly changing external pressure at the film cooled blade surface. Because the film flow through the cooling holes is usually unchoked, the varying external pressure results in a modulation of the mass flow through the holes. This study examined the effect of coolant flow modulation on the film effectiveness and the heat transfer downstream of a row of film cooling holes. Coolant oscillation frequencies and amplitudes were selected to match typical modern gas turbine engine conditions when represented in the appropriate non-dimensional forms. Time average blowing rate (the ratio of coolant mass flux to free stream mass flux) was varied from 0.6 to 1.5. Measurements were made of the flow velocity and temperature fields, the adiabatic film effectiveness, and the film cooling heat transfer using a constant flux heat transfer surface. Both the axial (streamwise) and lateral (cross stream) distributions of these quantities were measured from a single row of five circular holes angled at 60 degrees with respect to the surface normal. Frequency spectra taken from measurements of the fluctuating velocity were used to find the extent of the influence of the driving frequency downstream. The observed effect of the coolant flow oscillation was to decrease effectiveness in the streamwise direction, while having little or no influence on effectiveness in the cross stream direction. The rate of decrease of streamwise effectiveness is, however, a strong function of blowing rate, frequency, and amplitude of fluctuations.

The Many Faces of Turbine Surface Roughness

Results are presented for contact stylus measurements of surface roughness on in-service turbine blades and vanes. Nearly 100 turbine components were assembled from four land-based turbine manufacturers. Both coated and uncoated, cooled and uncooled components were measured, with part sizes varying from 2 to 20cm. Spanwise and chordwise 2D roughness profiles were taken on both pressure and suction surfaces. Statistical computations were performed on each trace to determine centerline averaged roughness, rms roughness, and peak to valley height. In addition, skewness and kurtosis were calculated as well as the autocorrelation length and dominant harmonics in each trace. Extensive 3D surface maps made of deposits, pitting, erosion, and coating spallation expose unique features for each roughness type. Significant spatial variations are evidenced and transitions from rough to smooth surface conditions are shown to be remarkably abrupt in some cases. Film cooling sites are shown to be particularly prone to surface degradation.© 2001 ASME

Complementary Velocity and Heat Transfer Measurements in a Rotating Cooling Passage With Smooth Walls

An experimental investigation was conducted on the internal flowfield of a simulated smooth-wall turbine blade cooling passage. The square cross-sectioned passage was manufactured from quartz for optical accessibility. Velocity measurements were taken using Particle Image Velocimetry for both heated and non-heated cases. Thin film resistive heaters on all four exterior walls of the passage allowed heat to be added to the coolant flow without obstructing laser access. Under the same conditions, an infrared detector with associated optics collected wall temperature data for use in calculating local Nusselt number. The test section was operated with radial outward flow and at values of Reynolds number and Rotation number typical of a small turbine blade. The density ratio was 0.27. Velocity data for the non-heated case document the evolution of the coriolis-induced double vortex. The vortex has the effect of disproportionately increasing the leading side boundary layer thickness. Also, the streamwise component of the coriolis acceleration creates a considerably thinned side wall boundary layer. Additionally, these data reveal a highly unsteady, turbulent flowfield in the cooling passage. Velocity data for the heated case show a strongly distorted streamwise profile indicative of a buoyancy effect on the leading side. The coriolis vortex is the mechanism for the accumulation of stagnant flow on the leading side of the passage. Heat transfer data show a maximum factor of two difference in the Nusselt number from trailing side to leading side. A first-order estimate of this heat transfer disparity based on the measured boundary layer edge velocity yields approximately the same factor of two. A momentum integral model was developed for data interpretation which accounts for coriolis and buoyancy effects. Calculated streamwise profiles and secondary flows match the experimental data well. The model, the velocity data, and the heat transfer data combine to strongly suggest the presence of separated flow on the leading wall starting at about five hydraulic diameters from the channel inlet for the conditions studied.© 1998 ASME

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 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.© 2001 ASME

Comparison of Computational Velocity and Heat Transfer Predictions to Experimental Measurements in a Rotating Cooling Passage With Smooth Walls

Numerical predictions of the turbulent velocity field and wall heat transfer for a simulated turbine blade cooling passage are presented. The square cross-sectioned, smooth-walled passage is identical to one for which velocity and heat transfer data are available for comparison. Reynolds number (8000), rotation number (0.2), and buoyancy numbers (0 and 0.49) are typical of gas turbine applications. Predictions are presented for three turbulence models: standard k-e, Renormalization Group k-e, and Reynolds Stress. In addition, two wall treatments are evaluated: wall functions and a two-layer zonal model. Results from the three models are comparable, however the two-layer zonal wall treatment provides the best match to both the experimental flowfield data and the Nusselt distribution. Wall functions are shown to be unsuitable for this flowfield. General flow features in the passage are adequately captured by the zonal model including the Coriolis-induced double vortex and the distorted streamwise velocity profile due to the buoyancy effect. Agreement between the calculated and measured streamwise velocity profile (from leading to trailing wall) is particularly remarkable and contributes to an impressive leading and trailing Nu match with the data. This agreement suggests that the model adequately accounts for the buoyancy effect on the bulk flow without any buoyancy terms in the k or e conservation equations. The model is less effective, however, at capturing the specific vortex position and strength. Specifically, the model vortex has only half the measured vortex maximum velocity and is located forward or aft of the passage centerline (depending on the density ratio of the flow).Copyright