Edward Vlasic

Influences of Inlet Swirl Distributions on an Inter-Turbine Duct: Part II—Hub Swirl Variation

The inter-turbine transition duct (ITD) of a gas turbine engine has significant potential for engine weight reduction and/or aerodynamic performance improvement. This potential arises because very little is understood of the flow behavior in the duct in relation to the hub and casing shapes and the flow entering the duct (e.g., swirl angle, turbulence intensity, periodic unsteadiness and blade tip vortices from upstream HP turbine blade rows). In this study, the flow development in an ITD with different inlet swirl distributions was investigated experimentally and numerically. The current paper, which is the second part of a two-part paper, presents the investigations of the influences of the hub swirl variations on the flow physics of ITD. The results show that the radial movement of the low momentum hub boundary layer and wake flow induces a pair of hub counter-rotating vortices. This pair of counter-rotating vortices merges with the upstream vorticity, forming a pair of stronger vortices, which persist until ITD exit. Due to the hub streamwise adverse pressure gradient, the hub 3D separation occurs at the exit of the ITD. The hub counter-rotating vortices are strongest with the highest inlet swirl gradient. The hub boundary layer thickness is thickest with the largest inlet hub swirl angle. The hub 3D separation is reduced by the increased hub swirl angle. Based on the studies in both parts of this paper, a detailed loss mechanism has been described. The total pressure coefficient shows that the loss increases gradually at the first bend, and then increases more rapidly at the second bend. The total pressure coefficients within the ITD are significantly redistributed by the casing and hub counter-rotating vortices.© 2011 ASME

Aerodynamic Performance of Suction-Side Gill Region Film Cooling

The performance of suction-side gill region film cooling is investigated using the University of Utah transonic wind tunnel and a simulated turbine vane in a two-dimensional cascade. The effects of film cooling hole orientation, shape, and number of rows, and their resulting effects on the aerodynamic losses, are considered for four different hole configurations: round axial (RA), shaped axial (SA), round radial (RR), and round compound (RC). The mainstream Reynolds number based on axial chord is 500,000, exit Mach number is 0.35, and the tests are conducted using the first row of holes, or both rows of holes at blowing ratios of 0.6 and 1.2. Carbon dioxide is used as the injectant to achieve density ratios of 1.77-1.99 similar to values present in operating gas turbine engines. Presented are the local distributions of total pressure loss coefficient, local normalized exit Mach number and local normalized exit kinetic energy. Integrated aerodynamic losses (IAL) increase anywhere from 4% to 45% compared with a smooth blade with no film injection. The performance of each hole type depends on the airfoil configuration, film cooling configuration, mainstream flow Mach number number of rows of holes, density ratio, and blowing ratio, but the general trend is an increase in IAL as either the blowing ratio or the number of rows of holes increase. In general, the largest total pressure loss coefficient C p magnitudes and the largest IAL are generally present at any particular wake location for the RR or SA configurations, regardless of the film cooling blowing ratio and number of holes. The SA holes also generally produce the highest local peak C p magnitudes. IAL magnitudes are generally lowest with the RA hole configuration. A one-dimensional mixing loss correlation for normalized IAL values is also presented, which matches most of the both rows data for RA, SA, RR, and RC hole configurations. The equation also provides good representation of the RA, RC, and RR first row data sets.

Experimental and Numerical Study on an Inter-Turbine Duct

The inter-turbine transition duct (ITD) between the high-pressure (HP) and low-pressure (LP) turbines of a gas turbine has the potential for significant length reduction and therefore engine weight reduction and/or aerodynamic performance improvement. This potential arises because very little is understood of the flow behavior in the duct in relation to the hub and casing shapes, and the flow entering the duct (e.g., swirl angle, turbulence intensity, periodic unsteadiness and blade tip vortices from upstream HP turbine blade rows). Moreover, it is unclear how well CFD is able to predict the complex flow-field in these ducts. This paper presents the results of a detailed experimental and computational study of an ITD, which is representative of a modern engine design. The experiments were conducted in a low-speed annular test rig where the effects of inlet free-stream turbulence intensities and swirl angle were investigated. Numerical studies were performed using commercial CFD software. The capability of different turbulence models, including the B-L, S-A, k-e and SST models, have been explored. The predicted results are compared with the experimental data. Both experimental and numerical results are analyzed in detail to investigate the flow development both inside the ITD and along the end-walls.Copyright

Effects of Area Ratio and Mean Rise Angle on the Aerodynamics of Inter-Turbine Ducts

The present work, a continuation of a series of investigations on the aerodynamics of aggressive inter-turbine ducts (ITD), is aimed at providing detailed understanding of the flow physics and loss mechanisms in four different ITD geometries. A systematic experimental and computational study was carried out for varying duct mean rise angles and outlet-to-inlet area ratio while keeping the duct length-to-inlet height ratio, Reynolds number and inlet swirl constant in all four geometries. The flow structures within the ITDs were found to be dominated by the counter-rotating vortices and boundary layer separation in both the casing and hub regions. The duct mean rise angle determined the severity of adverse pressure gradient in the casing’s first bend whereas the duct area ratio mainly governed the second bend’s static pressure rise. The combination of upstream wake flow and the first bend’s adverse pressure gradient caused the boundary layer to separate and intensify the strength of counter-rotating vortices. At high mean rise angle, the separation became stronger at the casing’s first bend and moved farther upstream. At high area ratios, a 2-D separation appeared on the casing. Pressure loss penalties increased significantly with increasing duct mean rise angle and area ratio.Copyright

Geometric Optimization of Aggressive Inter-Turbine Ducts

To reduce the harmful effects of aviation on the environment, aircraft gas turbine manufacturers continue to focus on producing engines with lower specific fuel consumption and weight. To address the engine weight challenges, R&D efforts continue to center around extending aerodynamic design limits, thus enabling reduced airfoil/stage count, reducing engine length or some combination thereof. The inter-turbine transition duct (ITD), located between the high-pressure (HP) and low-pressure (LP) turbines, is one of the components for which potentially significant weight reduction can be achieved through aggressive aerodynamic designs. Such ducts could have larger HP-to-LP radial offset and/or shorter length resulting in Aggressive Inter-Turbine Ducts (AITD). This paper presents a geometry optimization process to design AITD with minimum total pressure losses. Geometry optimizations were performed using the built-in optimization process in NUMECA Fine/Turbo 8.7. To evaluate the optimization process, one baseline ITD geometry was first generated with the same inlet and outlet coordinates as an existing ITD. The performance of the optimized ITD was studied numerically in comparison with the existing ITD. After the evaluation study, a second ITD geometry with more aggressive parameters, equivalent to increasing mean rising angle by 25% was optimized. Based on the studies of those two optimized geometries, a generic design rule of ITD with mild parameters was developed and the third ITD geometry with increased 20% area ratio (AR) was designed. The performance of designed ITDs was investigated numerically and the results are discussed in the paper.Copyright

Flow Control in an Aggressive Inter-Turbine Duct Using Low Profile Vortex Generators

This paper presents the experimental investigation of the flow in an aggressive inter-turbine duct (AITD). The goal is to improve the understanding of the flow mechanisms within the AITD and of the underlying physics of low-profile vortex generators (LPVGs).The flow structures in the AITD are dominated by counter-rotating vortices and boundary layer separations in both the casing and hub regions. At the first bend of the AITD, the casing boundary layer separates in a 3D mode because of the upstream wakes; this is followed by a massive 2D boundary layer separation. Due to the effect of the radial pressure gradient at the first bend, the streamwise vorticity generated by the casing 3D separation stays close to the casing endwall, and later mixes with the casing counter-rotating vortices formed at the second bend.By using LPVGs with different configurations installed on the casing, the casing boundary layer separation is significantly reduced. The streamwise vortices generated by the LPVGs have the potential to generate another pair of counter-rotating vortices at the AITD second bend, which help to delay/prevent the boundary layer separation. Therefore, the total pressure loss in the AITD was significantly reduced.Copyright

Rapid Airfoil Design for Uncooled High Pressure Turbine Blades

The aero-engine design process is highly iterative, multidisciplinary in nature and complex. The success of any engine design depends on best exploiting and considering the interactions among the numerous traditional engineering disciplines such as aerodynamics and structures. More emphasis has been placed lately on system integration, cross disciplines leveraging of tools and multi-disciplinary-optimization at the preliminary design phase. This paper investigates the automation of the airfoil generation process, referred to as Rapid Airfoil 3D (RAF-3D), for uncooled high pressure turbine blades at the preliminary design phase. This approach uses the TAML (Turbine Aero Mean Line) program in parallel with a database of previously designed P&WC airfoils, in-house design rules and best practices to define a pre-detailed airfoil shape which can be fed back to other analytical groups for pre-detail analyses, such as for structural integrity and vibrations. Resulting airfoil shapes have been aerodynamically validated using an in-house three dimensional Reynolds averaged Navier-Stokes code. RAF-3D will shorten the turnaround time for Pratt & Whitney turbine aerodynamics group to provide a preliminary 3D airfoil shape to turbine structures group by up to a factor of ten. Additionally, the preliminary assessments of stress and vibration specialists will be more accurate as their assessments will be based on an airfoil that has had inputs from all functional groups even though it is “first pass” design.