J. F. Carrotte

Interaction Between the Acoustic Pressure Fluctuations and the Unsteady Flow Field Through Circular Holes

Gas turbine combustion systems are prone to thermo-acoustic instabilities, and this is particularly the case for new low emission lean burn type systems. The presence of such instabilities is basically a function of the unsteady heat release within the system (i.e., both magnitude and phase) and the amount of damping. This paper is concerned with this latter process and the potential damping provided by perforated liners and other circular apertures found within gas turbine combustion systems. In particular, the paper outlines experimental measurements that characterize the flow field within the near field region of circular apertures when being subjected to incident acoustic pressure fluctuations. In this way the fundamental process by which acoustic energy is converted into kinetic energy of the velocity field can be investigated. Experimental results are presented for a single orifice located in an isothermal duct at ambient test conditions. Attached to the duct are two loudspeakers that provide pressure fluctuations incident onto the orifice. Unsteady pressure measurements enable the acoustic power absorbed by the orifice to be determined. This was undertaken for a range of excitation amplitudes and mean flows through the orifice. In this way regimes where both linear and nonlinear absorption occur along with the transition between these regimes can be investigated. The key to designing efficient passive dampers is to understand the interaction between the unsteady velocity field, generated at the orifice and the acoustic pressure fluctuations. Hence experimental techniques are also presented that enable such detailed measurements of the flow field to be made using particle image velocimetry. These measurements were obtained for conditions at which linear and nonlinear absorption was observed. Furthermore, proper orthogonal decomposition was used as a novel analysis technique for investigating the unsteady coherent structures responsible for the absorption of energy from the acoustic field.

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.

Detailed Measurements on a Modern Combustor Dump Diffuser System

An experimental investigation has been carried out to determine the flow characteristics and aerodynamic performance ofa modern gas turbine combustor dump diffuser. The system comprised a straight walled prediffuser, of area ratio 1.35, which projected into a dump cavity where the flow divided to pass either into the flame tube or surrounding feed annuli. In addition, a limited amount of air was removed to simulate flow used for turbine cooling. The flame tube was relatively deep, having a radial depth 5.5 times that of the passage height at prediffuser inlet, and incorporated burner feed arms, cowl head porosity, cooling rings, and primary ports. Representative inlet conditions to the diffuser system were generated by a single-stage axial flow compressor. Results are presented for the datum configuration, and for a further three geometries in which the distance between prediffuser exit and the head of the flame tube (i.e., dump gap ) was reduced. Relatively high values of stagnation pressure loss were indicated, with further significant increases occurring at smaller dump gaps. These high losses, which suggest a correlation with other published data, are due to the relatively deep flame tube and short diffuser length. Furthermore, the results also focus attention on how the presence of a small degree of diffuser inlet swirl, typical of that which may be found within a gas turbine engine, can result in large swirl angles being generated farther downstream around the flame tube. This is particularly true for flow passing to the inner annulus.

The influence of dilution hole geometry on jet mixing

Measurements have been made on a fully annular test facility, downstream of a row of heated dilution jets injected normally into a confined crossflow at a momentum flux ratio of 4. The investigation concentrated on the consistency of mixing between the jets, as indicated by the regularity of the temperature pattern around the crossflow annulus. When the heated air was supplied from a representative feed annulus, the exit velocity profile across each plunged hole was significantly altered and caused a distortion of the temperature distribution in the ensuing jet. The degree of distortion varies in a random manner, so that each jet has its own mixing characteristics, thereby producing irregularity of the temperature pattern around the annulus. With the same approach and operating conditions some of the plunged dilution holes were modified, and tests on this modified sector indicated a significant improvement in the circumferential regularity of the temperature pattern. Further tests showed that these modifications to the dilution holes had a negligible effect on the values of the discharge coefficients.

Enhanced external aerodynamic performance of a generic combustor using an integrated OGV/prediffuser design technique

In this paper we use experimental measurements to characterize the extent that improved the external aerodynamic performance (reduced total pressure loss, increased flow quality) of a gas-turbine combustion system may be achieved by adopting an integrated OGV/prediffuser technique. Two OGV/prediffuser combinations were tested. The first is a datum design corresponding to a conventional design approach, where the OGV and prediffuser are essentially designed in isolation. The second is an “integrated” design where the OGV blade shape has been modified, following recommendations of earlier CFD work (Final Report No. TT03R01, 2003), to produce a secondary flow/wake structure that allows the prediffuser to operate at a higher area ratio without boundary layer separation. This is demonstrated to increase static pressure recovery and reduce dump losses. Experimental measurements are presented on a fully annular rig. Several traverse planes are used to gather five-hole probe data that allow the flow structure through the OGV, at the inlet and exit of the prediffuser, and in the inner/outer annulus supply ducts to be examined. Both overall performance measures (loss coefficients) and measures of flow uniformity and quality are used to demonstrate that the integrated design is superior.

Detailed performance comparison of a dump and short faired combustor diffuser system

A rectangular model simulating four sectors of a combustion chamber was used to compare the performance of a standard dump diffuser, of overall length 180 mm, with that of a faired design 25.5 mm shorter. The performance of each system was assessed in terms of total pressure loss and static pressure recovery between prediffuser inlet and the annuli surrounding the flame tube. Since the program objective was to test design concepts only, no allowance was made for the presence of burner feed arms or flame tube support pins. In addition, tests were performed with relatively low levels of inlet turbulence and no wake mixing effects from upstream compressor blades. Relative to the dump design, the mass weighted total pressure loss to the outer and inner annuli was reduced by 30 and 40 percent, respectively, for the faired diffuser. Measurements around the flame tube head were used to identify regions of high loss within each system and account for the differences in performance. Within a dump diffuser the flow separates at prediffuser exit resulting in a free surface diffusion around the flame tube head and a recirculating flow in the dump cavity. This source of loss is eliminated in the faired system where the flow remains attached to the casings. Furthermore, the faired system exhibited similar velocity magnitudes and gradients around the combustor head despite its shorter length.

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

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.

Duct Aerodynamics for Intercooled Aero Gas Turbines: Constraints, Concepts and Design Methodology

Economic and environmental concerns are a major driving force behind the development of aero gas turbine technology, with ever more stringent legislation dictating significant reductions in specific fuel consumption and pollutant emissions. Intercooling has long been of interest as it has the potential for lower compressor delivery and turbine cooling air temperatures, together with reduced NOx and higher overall pressure ratios, which enable reduced fuel consumption. However, thus far the technical complexities, both aerodynamic and mechanical, have been prohibitive. For example, improvements in core cycle thermal efficiency could easily be offset by reduced component efficiencies and pressure losses in the intercooler and its associated ducting. This paper describes an intercooled concept typical of those that may be used for a large, high by-pass ratio, high OPR aero-engine. The paper goes on to describe the aerodynamic challenges of designing a duct system to transfer the core air, issuing from the low pressure compressors, into the intercooler modules. A design methodology is developed which includes consideration of: system loss, the inclusion of local constraints such as a radial drive shaft, the need to provide core access for ancillary services, and minimization of aerodynamic interaction with surrounding components. Finally a preliminary duct design is presented for a specific intercooled aero-engine design.Copyright