S. A. Sjolander

Measurements of Secondary Losses in a Turbine Cascade With the Implementation of Nonaxisymmetric Endwall Contouring

An approach to endwall contouring has been developed with the goal of reducing secondary losses in highly loaded axial flow turbines. The present paper describes an experimental assessment of the performance of the contouring approach implemented in a low-speed linear cascade test facility. The study examines the secondary flows of a cascade composed of Pratt & Whitney PAKB airfoils. This airfoil has been used extensively in low-pressure turbine research, and the present work adds intrapassage pressure and velocity measurements to the existing database. The cascade was tested at design incidence and at an inlet Reynolds number of 126,000 based on inlet midspan velocity and axial chord. Quantitative results include seven-hole pneumatic probe pressure measurements downstream of the cascade to assess blade row losses and detailed seven-hole probe measurements within the blade passage to track the progression of flow structures. Qualitative results take the form of oil surface flow visualization on the endwall and blade suction surface. The application of endwall contouring resulted in lower secondary losses and a reduction in secondary kinetic energy associated with pitchwise flow near the endwall and spanwise flow up the suction surface within the blade passage. The mechanism of loss reduction is discussed in regard to the reduction in secondary kinetic energy.

Application of Nonaxisymmetric Endwall Contouring to Conventional and High-Lift Turbine Airfoils

Here, we report on the application of nonaxisymmetric endwall contouring to mitigate the endwall losses of one conventional and two high-lift low-pressure turbine airfoil designs. The design methodology presented combines a gradient-based optimization algorithm with a three-dimensional computational fluid dynamics (CFD) flow solver to systematically vary a free-form parameterization of the endwall. The ability of the CFD solver employed in this work to predict endwall loss modifications resulting from nonaxisymmetric contouring is demonstrated with previously published data. Based on the validated trend accuracy of the solver for predicting the effects of endwall contouring, the magnitude of predicted viscous losses forms the objective function for the endwall design methodology. This system has subsequently been employed to optimize contours for the conventional-lift Pack B and high-lift Pack D-F and Pack D-A low-pressure turbine airfoil designs. Comparisons between the predicted and measured loss benefits associated with the contouring for Pack D-F design are shown to be in reasonable agreement. Additionally, the predictions and data demonstrate that the Pack D-F endwall contour is effective at reducing losses primarily associated with the passage vortex. However, some deficiencies in predictive capabilities demonstrated here highlight the need for a better understanding of the physics of endwall loss-generation and improved predictive capabilities.

An Empirical Prediction Method for Secondary Losses in Turbines: Part I — A New Loss Breakdown Scheme and Penetration Depth Correlation

Despite its wide use in meanline analyses, the conventional loss breakdown scheme is based on a number of assumptions that are known to be physically unsatisfactory. One of these assumptions states that the loss generated in the airfoil surface boundary layers is uniform across the span. The loss results at high positive incidence presented in a previous paper (Benner et al. [1]) indicate that this assumption causes the conventional scheme to produce erroneous values of the secondary loss component. A new empirical prediction method for secondary losses in turbines has been developed, and it is based on a new loss breakdown scheme. In the first part of this two-part paper, the new loss breakdown scheme is presented. Using data from the current authors’ off-design cascade loss measurements (Benner et al. [1]), it is shown that the secondary losses obtained with the new scheme produce a trend with incidence that is physically more reasonable. Unlike the conventional loss breakdown scheme, the new scheme requires a correlation for the spanwise penetration depth of the passage vortex separation line at the trailing edge. One such correlation exists (Sharma and Butler [2]); however, it was based on a small database. An improved correlation for penetration distance has been developed from a considerably larger database, and it is detailed in this paper.Copyright

Aerodynamic Performance of a Transonic Turbine Cascade at Off-Design Conditions

The paper presents detailed measurements of the midspan aerodynamic performance of a transonic turbine cascade at off-design conditions. The measurements were conducted for exit Mach numbers ranging from 0.5 to 1.2, and for Reynolds numbers from 4 × 10 5 to 10 6 . The profile losses were measured for incidence values of +14.5 deg, +10 deg, +4.5 deg, 0 deg, and -10 deg relative to design. To aid in understanding the loss behavior and to provide other insights into the flow physics, measurements of blade loading, exit flow angles, trailing-edge base pressures, and the axial velocity density ratio (AVDR) were also made. It was found that the profile losses at transonic Mach numbers can be closely related to the base pressure behavior. The losses were also affected by the AVDR.

Influence of Leading-Edge Geometry on Profile Losses in Turbines at Off-Design Incidence: Experimental Results and an Improved Correlation

The most recent correlations for turbine profile losses at off-design incidence include the leading-edge diameter as the only aspect of the leading-edge geometry that influences the losses. Cascade measurements are presented for two turbine blades that differ primarily in their leading-edge geometries. The incidence was varied over a range of {+-}20 deg and the results show significant discrepancies between the observed profile losses and those predicted by the available correlations. Using data from the present experiments, as well as cases from the literature for which sufficient geometric data are given, a revised correlation has been developed. The new correlation is a function of both the leading-edge diameter and the wedge angle, and it is significantly more successful than the existing correlations. It is argued that the off-design loss behavior of the blade is influenced by the magnitude of the discontinuity in curvature at the points where the leading-edge circle meets the rest of the blade profile. The wedge angle appears to be an approximate and convenient measure of the discontinuity in curvature at these blend points.

Active Flow Control Using Steady Blowing for a Low-Pressure Turbine Cascade

The paper presents mid-span measurements for a turbine cascade with active flow control. Steady blowing through an inclined plane wall jet has been used to control the separation characteristics of a high-lift low-pressure turbine airfoil at low Reynolds numbers. Measurements were made at design incidence for blowing ratios from approximately 0.25 to 2.0 (ratio of jet-to-local freestream velocity), for Reynolds numbers of 25,000 and 50,000 (based on axial chord and inlet velocity), and for freestream turbulence intensities of 0.4% and 4%. Detailed flow field measurements were made downstream of the cascade using a three-hole pressure probe, static pressure distributions were measured on the airfoil suction surface, and hot-wire measurements were made to characterize the interaction between the wall jet and boundary layer. The primary focus of the study is on the low-Reynolds number and low-freestream turbulence intensity crises, where the baseline airfoil stalls and high profile losses result. For low freestream turbulence (0.4%), the examined method of flow control was effective at preventing stall and reducing the profile lasses. At a Reynolds number of 25,000, a blowing ratio greater than 1.0 was required to suppress stall. At a Reynolds number of 50,000, a closed separation bubble formed at a very low blowing ratio (0.25) resulting in a significant redaction in the profile lass. For high freestream turbulence intensity (4%), where the baseline airfoil has a closed separation bubble and low profile losses, blowing ratios below 1.0 resulted in a larger separation bubble and higher losses. The mechanism by which the wall jet affects the separation characteristics of the airfoil is examined through hot-wire traverse measurements in the vicinity of the slot.

Comparative Investigation of Three Highly Loaded LP Turbine Airfoils: Part II — Measured Profile and Secondary Losses at Off-Design Incidence

At the 2006 ASME-IGTI Turbo-Expo, low-speed cascade results were presented for the midspan aerodynamic behaviour of a family of three highly loaded low-pressure (LP) turbine airfoils operating over a wide range of Reynolds numbers (25,000 to 150,000 based on the axial chord and inlet velocity), and for values of freestream turbulence intensity of 1.5% and 4%. All three airfoils have the same design inlet and outlet flow angles. The baseline cascade has a Zweifel coefficient of 1.08 and the two additional blade rows have values of 1.37. The new, more highly-loaded blade rows differ mainly in their loading distributions: one is front-loaded while the other is aft-loaded. The new front-loaded airfoil was found to have particularly attractive profile performance. Despite its exceptionally high value of Zweifel coefficient, it was found to be free of a separation bubble on its suction side at Reynolds numbers as low as 50,000, and this was reflected in very good profile loss behaviour. However, it was also noted in the earlier paper that the choice of a particular loading level and loading distribution would be influenced by more than its profile performance at design incidence. The present two-part paper extends the midspan aerodynamic comparison of the three airfoils to the secondary flow performance. The first part of the paper discusses both the profile and secondary flow performance of the three cascades at their design Reynolds number of 80,000 (or ∼ 125,000 based on exit velocity) for two freestream turbulence intensities of 1.5% and 4%. The secondary flow behaviour was determined from detailed flowfield measurements made at 40% axial chord downstream of the trailing edge using a seven-hole pressure probe. In addition to providing total pressure losses, the seven-hole probe measurements were also processed to give the downstream vorticity distributions. As has been found in other secondary flow investigations in turbine cascades, the present front-loaded airfoil showed higher secondary losses than the aft-loaded airfoil with the same value of Zweifel coefficient.Copyright

Measurements of Losses and Reynolds Stresses in the Secondary Flow Downstream of a Low-Speed Linear Turbine Cascade

Experimental measurements of the mean and turbulent flow field were preformed downstream of a low-speed linear turbine cascade. The influence of turbulence on the production of secondary losses is examined. Steady pressure measurements were collected using a seven-hole pressure probe and the turbulent flow quantities were measured using a rotatable x-type hotwire probe. Each probe was traversed downstream of the cascade along planes positioned at three axial locations: 100%, 120%, and 140% of the axial chord (Cx ) downstream of the leading edge. The seven-hole pressure probe was used to determine the local total and static pressure as well as the three mean velocity components. The rotatable x-type hotwire probe, in addition to the mean velocity components, provided the local Reynolds stresses and the turbulent kinetic energy. The axial development of the secondary losses is examined in relation to the rate at which mean kinetic energy is transferred to turbulent kinetic energy. In general, losses are generated as a result of the mean flow dissipating kinetic energy through the action of viscosity. The production of turbulence can be considered a preliminary step in this process. The measured total pressure contours from the three axial locations (1.00, 1.20, and 1.40Cx ) demonstrate the development of the secondary losses. The peak loss core in each plane consists mainly of low momentum fluid that originates from the inlet endwall boundary layer. There are, however, additional losses generated as the flow mixes with downstream distance. These losses have been found to relate to the turbulent Reynolds stresses. An examination of the turbulent deformation work term demonstrates a mechanism of loss generation in the secondary flow region. The importance of the Reynolds shear stresses to this process is explored in detail.

The influence of leading-edge geometry on secondary losses in a turbine cascade at the design incidence

The paper presents detailed experimental results of the secondary flows from two large-scale, low-speed linear turbine cascades. The aerofoils for the two cascades were designed for the same inlet and outlet conditions and differ mainly in their leading-edge geometries. Detailed flow field measurements were made upstream and downstream of the cascades using three and seven-hole pressure probes and static pressure distributions were measured on the aerofoil surfaces. All measurements were made exclusively at the design incidence. The results from this experiment suggest that the strength of the passage vortex plays an important role in the downstream flow field and loss behavior It was concluded that the aerofoil loading distribution has a significant influence on the strength of this vortex. In contrast, the leading-edge geometry appears to have only a minor influence on the secondary flow field, at least for the design incidence.

Aerodynamics of a Low-Pressure Turbine Airfoil at Low-Reynolds Numbers: Part 1 — Steady Flow Measurements

This two-part paper presents a detailed experimental investigation of the laminar separation and transition phenomena on the suction surface of a high-lift low-pressure (LP) turbine airfoil, PakB. The first part describes the influence of Reynolds number, freestream turbulence intensity and turbulence length scale on the PakB airfoil under steady inflow conditions. The present measurements are distinctive in that a closely-spaced array of hot-film sensors has allowed a very detailed examination to be made of both the steady and unsteady behaviour of the suction surface boundary layer. In addition, this paper presents a technique for interpreting the transition process in steady, and periodically unsteady, separated flows based on dynamic and statistical properties of the hot-film measurements. Measurements were made at Reynolds number varying from 25,000 to 150,000 and for freestream turbulence intensities of 0.4%, 2% and 4%. Two separate grids were used to generate turbulence intensity of 4% with integral length scales of about 10% and 40% of the airfoil axial chord length. The first is comparable with the turbulence length scales expected in the engine and the second is considerably larger. While the higher levels of freestream turbulence intensity promoted earlier transition and a shorter separation bubble, the varying turbulence length scale did not have a noticeable effect on the transition process. The size of the separation bubble increased with decreasing Reynolds number, and under low freestream turbulence levels the bubble failed to reattach at low Reynolds numbers. As expected, the losses increased with the length of the separation bubble on the suction side of the airfoil, and increased significantly when the bubble failed to reattach.Copyright