Outlet Guide Vane Research Articles

The Influence of Dump Gap on External Combustor Aerodynamics at High Fuel Injector Flow Rates

The increasing demand to reduce fuel burn, hence CO2 emissions, from the gas turbine requires efficient diffusion to reduce the system pressure loss in the combustor. However, interactions between prediffuser and combustor can have a significant effect on diffuser performance. For example, the consequence of increased fuel injector flow at a dump gap set using conventional design guidelines has been shown (Walker, A. D., Carrotte, J. F., and McGuirk, J. J., 2007. “Compressor∕Diffuser∕Combustor Aerodynamic Interactions in Lean Module Combustors,” ASME Turbo Expo 2007—Power for Land Sea and Air, Paper No. GT2007-27872) to introduce a destabilizing interaction between fuel injector and upstream components. The present paper concentrates on examining the effects of increased dump gap. Dump gap ratios of 0.8, 1.2, and 1.6 were employed, with each test utilizing the same inlet guide vane, compressor rotor, integrated outlet guide vane (OGV)∕prediffuser, and dump geometry. The flow fraction of compressor efflux entering the combustor cowl was set to be representative of lean combustors (50–70%). Measurements were made on a fully annular rig using a generic flame tube with metered cowl and inner∕outer annulus flows. The results demonstrate that, with fixed cowl flow, as the dump gap increases, component interactions decrease. At a dump gap ratio of 0.8, the proximity of the flame tube influences the prediffuser providing a beneficial blockage effect. However, if increased to 1.2, this beneficial effect is weakened and the prediffuser flow deteriorates. With further increase to 1.6, the prediffuser shows strong evidence of separation. Hence, at the dump gaps probably required for lean module injectors, it is unlikely the prediffuser will be influenced beneficially by the flame tube blockage; this must be taken into account in the design. Furthermore, with small dump gaps and high cowl flow fraction, the circumferential variation in cowl flow can feed upstream and cause OGV∕rotor forcing. At larger dump gaps, the circumferential variation does not penetrate upstream to the OGV, and the rotor is unaffected. The optimum dump gap and prediffuser design for best overall aerodynamic system performance from rotor through to feed annuli is a compromise between taking maximum advantage of upstream blockage effects and minimizing any 3D upstream forcing.

Compressor/Diffuser/Combustor Aerodynamic Interactions in Lean Module Combustors

The paper reports an experimental investigation into the possibility of increased interactions between combustor external aerodynamics and upstream components, e.g., prediffuser, compressor outlet guide vane (OGV), and even the compressor rotor, caused by the trend in lean module fuel injectors to larger mass flows entering the combustor cowl. To explore these component interaction effects, measurements were made on a fully annular rig comprising a single stage compressor, an advanced integrated OGV/prediffuser, followed by a dump diffuser and a generic combustor flametube with metered cowl and inner/outer annulus flows. The flow split entering the cowl was increased from 30% to 70%. The results demonstrate that, with fixed geometry, as the injector flow increases, the performance of the prediffuser and feed annuli suffer. Prediffuser losses increase and at high injector flow rates, the diffuser moves close to separation. The substantial circumferential variation in cowl flow can feed upstream and cause rotor forcing. Notable differences in performance were observed inline and between injectors at the OGV exit, suggesting that geometry changes such as an increased dump gap or nonaxisymmetric prediffuser designs may be beneficial.

Effect of fan outlet guide vane on the acoustic treatment design in aeroengine nacelle

The main objective of this study is to clarify the effect of the outlet guide vane (OGV) on the acoustic treatment design in aeroengine nacelle, which received less attention previously. A model of sound propagation through a lining section and a blade row is developed to investigate the interaction between sound sources of blade rows and liners in a channel of parallel walls containing uniform mean flow. The present method makes it possible to evaluate the performance of liner while a blade row is inserted in the channel and the sound attenuation in a duct with both liner section and cascade. Various numerical results show that the effect of the cascade may have diverse effects on sound attenuation of the liner under different conditions, but the existence of the OGV always enhances the total sound attenuation in the duct due to energy dissipation caused by vortex shedding from the tailing edge of the OGV. To pursue a better design of acoustic liner in aeroengine nacelle, it is thus necessary to include the effect of OGV on the sound attenuation.

Advanced High-Turning Compressor Airfoils for Low Reynolds Number Condition—Part II: Experimental and Numerical Analysis

Part I of this paper describes the design and optimization of two high turning subsonic compressor cascades operating as an outlet guide vane (OGV) behind a single stage low pressure turbine at low Reynolds number condition Re=1.3×105. In the numerical optimization algorithm, the design point and off-design performance has been considered in an objective function to achieve a wide low loss incidence range. The objective of the present paper is to examine some of the characteristics describing the new airfoils as well as to prove the reliability of the design process and the applied flow solver. Some aerodynamic characteristics for the two new airfoils and a conventional controlled diffusion airfoil (CDA), have been extensively investigated in the cascade wind tunnel of DLR Cologne. For an inlet Mach number of 0.6 the effect of Reynolds number and incidence angle on each airfoil performance is discussed, based on experimental and numerical results. For an interpretation of the airfoil boundary layer behavior, results of some boundary layer calculations are compared to oil flow visualization pictures. The design goal of an increased low loss incidence range at low Reynolds number condition could be confirmed without having a negative effect on the high Reynolds number region.

Advanced High Turning Compressor Airfoils for Low Reynolds Number Condition—Part I: Design and Optimization

High performance compressor airfoils at a low Reynolds number condition at Re=1.3×105 have been developed using evolutionary algorithms in order to improve the performance of the outlet guide vane (OGV), used in a single low pressure turbine (LPT) of a small turbofan engine for business jet aircrafts. Two different numerical optimization methods, the evolution strategy (ES) and the multi-objective genetic algorithm (MOGA), were adopted for the design process to minimize the total pressure loss and the deviation angle at the design point at low Reynolds number condition. Especially, with respect to the MOGA, robustness against changes of the incidence angle is considered. The optimization process includes the representation of the blade geometry, the generation of a numerical grid and a blade-to-blade analysis using a quasi-three-dimensional Navier-Stokes solver with a k-ω turbulence model including a newly implemented transition model to evaluate the performance. Overall aerodynamic performance and boundary layer properties for the two optimized blades are discussed numerically. The superior performance of the two optimized airfoils is demonstrated by a comparison with conventional controlled diffusion airfoils (CDA). The advantage in performance has been confirmed by detailed experimental investigations, which are presented in Part II of this paper.

Influence of Compressor Exit Conditions on Combustor Annular Diffusers Part II: Flow Redistribution

In gas-turbine engines the velocity of air, issuing from the compressor, must be reduced in order to permit effective operation of the downstream combustor. This is partly achieved by locating an annular diffuser behind the compressor outlet guide vanes (OGVs) and, in modern systems, the inlet of this diffuser is usually located at the trailing edge of the blade row. This paper is concerned with some of the interactions that occur between these components and, in particular, the e ow redistribution that occurs along the diffuser length due to the e ows generatedbytheOGVbladepassageandupstreamrotor.Amainlyexperimentalinvestigationhasbeenundertaken, on a fully annular facility, which incorporates a single-stage axial e ow compressor and simulated e ame tube. In addition, immediately downstream of the OGV row a constant-area passage, ordiffusers of area ratio 1.45 or 1.60, can be incorporated. The OGV blade row produces a proe le that, as a result mainly of the blade wakes, contains an excess of kinetic energy relative to that of a uniform proe le. The mixing out of these wakes therefore enhances the pressure rise within the downstream diffuser. Measured mean velocity data are used to determine the path of streamlinesalongeachdiffuserandindicateregionswherehigh-energye uidisbeingconvected,towardeachcasing, and low-energy boundary layere uidisbeing removed.Thisisbecauseoftheremnantsofthee owsgeneratedwithin each OGV passage. The mean momentum equation along each diffuser is then used to indicate that such e ows signie cantly offset the changes in momentum, within each boundary layer, that are associated with the applied pressure gradient. Such effects are therefore thought signie cant in terms of reducing the boundary-layer growth and delaying e ow separation from the casings.

Influence of Compressor Exit Conditions on Combustor Annular Diffusers, Part 1: Diffuser Performance

In gas-turbine engines, the velocity of air issuing from the compressor must be reduced to permit effective operation of the downstream combustor. This is partly achieved by locating an annular diffuserbehind the compressor outlet guide vanes (OGV) and, in modern systems, the inlet of this diffuser is usually located at the trailing edge of the blade row. The interactions that occur between these components and, in particular, the impact on the measured diffuser performance are studied. A mainly experimental investigation has been undertaken in a fully annular facility that incorporates a single-stage axial-e ow compressor and simulated e ame tube. In addition, a constant-area passage, or diffusers of area ratio 1.45 or 1.60, can be incorporated immediately downstream of the OGV row. The results indicate that, within experimental error, the diffusers have little effect on the e ow within the OGVbladepassages. However,theOGV blade rowproducesa proe lethat, duemainly to thebladewakes,contains a relatively large amount of kinetic energy. Hence, even within the downstream constant area passage a signie cant pressure rise is observed as these wakes mix out. Additional pressure forces are introduced with the downstream diffusers present, but analysis of the experimental data indicates these have a limited effect on the wake mixing process, both in terms of stagnation pressure loss and static pressure rise. Hence, the overall static pressure rise measured, between the inlet and exit of each diffuser, is greater than that predicted using design charts obtained using more conventional axisymmetric inlet conditions. This cone rms previous work where it was thought that wake mixing can enhance diffuser performance.

Design and Testing of Swept and Leaned Outlet Guide Vanes to Reduce Stator–Strut–Splitter Aerodynamic Flow Interactions

Large circumferentially varying pressure levels produced by aerodynamic flow interactions between downstream stators and struts present a potential noise and stability margin liability in a compression component. These interactions are presently controlled by tailoring the camber and/or stagger angles of vanes neighboring the fan frame struts. This paper reports on the design and testing of a unique set of swept and leaned fan outlet guide vanes (OGVs) that do not require this local tailoring even though the OGVs are closely coupled with the fan frame struts and splitter to reduce engine length. The swept and leaned OGVs not only reduce core-duct diffusion, but they also reduce the potential flow interaction between the stator and the strut relative to that produced by conventional radial OGVs. First, the design of the outlet guide vanes using a single blade row three-dimensional viscous flow analysis is outlined. Next, a two-dimensional potential flow analysis was used for the coupled OGV–frame system to obtain a circumferentially nonuniform stator stagger angle distribution to reduce the upstream static pressure disturbance further. Recognizing the limitations of the two-dimensional potential flow analysis for this highly three-dimensional set of leaned OGVs, as a final evaluation of the OGV–strut system design, a full three-dimensional viscous analysis of a periodic circumferential sector of the OGVs, including the fan frame struts and splitter, was performed. The computer model was derived from a NASA-developed code used in simulating the flow field for external aerodynamic applications with complex geometries. The three-dimensional coupled OGV–frame analysis included the uniformly staggered OGV configuration and the variably staggered OGV configuration determined by the two-dimensional potential flow analysis. Contrary to the two-dimensional calculations, the three-dimensional analysis revealed significant flow problems with the variably staggered OGV configuration and showed less upstream flow nonuniformity with the uniformly staggered OGV configuration. The flow redistribution in both the radial and tangential directions, captured fully only in the three-dimensional analysis, was identified as the prime contributor to the lower flow nonuniformity with the uniformly staggered OGV configuration. The coupled three-dimensional analysis was also used to validate the design at off-design conditions. Engine test performance and stability measurements with both uniformly and variably staggered OGV configurations with and without the presence of inlet distortion confirmed the conclusions from the three-dimensional analysis.

Compressor Exit Conditions and Their Impact on Flame Tube Injector Flows

Within a gas turbine engine the flow field issuing from the compression system is nonuniform containing, for example, circumferential and radial variations in the flow field due to wakes from the upstream compressor outlet guide vanes (OGVs). In addition, variations can arise due to the presence of radial load bearing struts within the pre-diffuser. This paper is concerned with the characterization of this nonuniform flow field, prior to the combustion system, and the subsequent effect on the flame tube fuel injector flows and hence combustion processes. A mainly experimental investigation has been undertaken using a fully annular test facility which incorporates a single stage axial flow compressor, diffuser, and flame tube. Measurements have been made of the flow field, and its frequency content, within the dump cavity. Furthermore, the stagnation pressure presented to the core, outer and dome swirler passages of a fuel injector has been obtained for different circumferential positions of the upstream OGV/pre-diffuser assembly. These pressure variations, amounting to as much as 20 percent of the pressure drop across the fuel injector, also affect the flow field immediately downstream of the injector. In addition, general variations in pressure around the fuel injector have also been observed due to, for example, the fuel injector position relative to pre-diffuser exit and the flame tube cowl.

Measurements of the Flow Field Within a Compressor Outlet Guide Vane Passage

A series of tests have been carried out to investigate the flow in a Compressor Outlet Guide Vane (OGV) blade row downstream of a single-stage rotor. The subsequent flow field that developed within an OGV passage was measured, at intervals of 10 percent axial chord, using a novel design of miniature five-hole pressure probe. In addition to indicating overall pressure levels and the growth of regions containing low-energy fluid, secondary flow features were identified from calculated axial vorticity contours and flow vectors. Close to each casing the development of classical secondary flow was observed, but toward the center of the annulus large well-defined regions of opposite rotation were measured. These latter flows were due to the streamwise vorticity at inlet to the blade row associated with the skewed inlet profile. Surface static pressures were also measured and used to obtain the blade pressure force at three spanwise locations. These values were compared with the local changes in flow momentum calculated from the measured velocity distributions. With the exception of the flow close to the outer casing, which is affected by rotor tip leakage, good agreement was found between these quantities indicating relatively weak radial mixing.