A. Duncan Walker

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.

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

Intercooled Aero-Gas-Turbine Duct Aerodynamics: Core Air Delivery Ducts

The development of radical new aero engine technologies will be key to delivering the step-changes in aircraft environmental performance required to meet future emissions legislation. Intercooling has the potential for higher overall pressure ratios, enabling reduced fuel consumption, and/or lower compressor delivery air temperatures and therefore reduced NOx. This paper considers the aerodynamics associated with the complex ducting system that would be required to transfer flow from the core engine path to the heat exchanger system. The cycle benefits associated with intercooling could be offset by the pressure losses within this ducting system and/or any detrimental effect the system has on the surrounding components. A suitable branched S-shaped duct system has been numerically developed which diffuses and delivers the flow from the engine core to discrete intercooler modules. A novel swirling duct concept was used to locally open larger spacing between certain duct branches in order to provide engine core access whilst hiding the resultant pressure field from the upstream turbomachinery. The candidate duct system was experimentally evaluated on a bespoke low speed, fully annular isothermal test facility. Aerodynamic measurements demonstrated the ability of the design to meet the stringent aerodynamic and geometric constraints.

Annular Diffusers with Large Downstream Blockage Effects for Gas Turbine Combustion Applications

In engineering applications, diffuser performance is significantly affected by its boundary conditions. In a gas turbine combustion system, the space envelope is limited, the inlet conditions are generated by upstream turbomachinery, and the downstream geometry is complex and in close proximity. Published work discusses the impact of compressor-generated inlet conditions, but little work has been undertaken on designing diffusers to accommodate a complex downstream geometry. This paper considers the design of an annular diffuser in the presence of a large downstream blockage. This is most applicable in the combustion system of a low-emission landbased aero-derivative gas turbine, where immediately downstream of the diffuser approximately 85% of the flow moves outboard and 15% moves inboard to supply the various flame-tube and turbine-cooling features. Several diffuser concepts are numerically developed and demonstrate 1) the interaction between the diffuser and downstream geometry and 2) how this varies with changes in diffuser geometry. A preferred concept is experimentally evaluated on a low-speed facility that simulates the combustion system and provides compressorgenerated inlet conditions. A conventionally designed aero-derivative diffuser system is also evaluated and, with reference to this datum, the system total pressure losses are reduced by between 20 and 35%.

Enhanced External Aerodynamic Performance of a Generic Combustor Using an Integrated OGV/Pre-Diffuser Design Technique

The paper uses experimental measurements to characterise the extent that improved external aerodynamic performance (reduced total pressure loss, increased flow quality) of a gas-turbine combustion system may be achieved by adopting an integrated OGV/pre-diffuser technique. Two OGV/pre-diffuser combinations were tested. The first is a datum design corresponding to a conventional design approach, where OGV and pre-diffuser 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 ([14]), to produce a secondary flow/wake structure that allows the pre-diffuser 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 5-hole probe data which allow the flow structure through the OGV, at inlet and exit of the pre-diffuser, 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.Copyright

Impact of a Cooled Cooling Air System on the External Aerodynamics of a Gas Turbine Combustion System

The trend for higher overall pressure ratios means that turbine entry temperatures are continually increasing. Furthermore, the development of lean, low-emission combustion systems reduces the availability of cooling air and is accompanied by new problems at the combustor/turbine interface. For example, the exit temperature traverse differs from that found in traditional rich-burn combustors with increased swirl and a much flatter profile. Effectively cooling the turbine components is becoming increasingly difficult. One solution is to employ cooled cooling air (CCA) where some of the compressor efflux is diverted for additional cooling in a heat exchanger located in the by-pass duct. An example CCA system is presented which includes an off-take within the dump cavity and the addition of radial struts within the pre-diffuser through which the cooled air is returned to the engine core. This paper addresses the impact this CCA system has on the combustion system external aerodynamics. This included the development of a fully annular, isothermal test facility which incorporated a bespoke 1.5 stage axial compressor, engine relevant outlet guide vanes, pre-diffuser and combustor geometry. A datum aerodynamic performance was established for a non-CCA configuration with a clean, un-strutted pre-diffuser. Results for this baseline CCA system demonstrated that inclusion of a bleed in the dump cavity had limited effect on the overall flow field. However, the inclusion of struts within the pre-diffuser caused a reduction in area ratio and a notable increase in system loss. Consequently an alternative pre-diffuser was designed (using CFD) with the aim of increasing the area ratio back to that of the un-strutted datum. A so-called hybrid diffuser was designed in which the CCA bleed was moved to the pre-diffuser outer wall. The bleed was then used to re-energize the boundary layer, preventing flow separation, enabling the area ratio to be increased close to the datum value. The mechanisms of the hybrid diffuser are complex; the geometry of the off-take and its location with respect to the OGV and strut leading edge were seen to be critical. Experimental evaluation of the final design demonstrated the effective operation the hybrid diffuser with the result that the system loss returned to a level close to that of the datum. Only small differences were seen in the overall flow field.© 2015 ASME

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 pre-diffuser 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 [2] to introduce a destabilising 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 utilising the same IGV, compressor rotor, integrated OGV/pre-diffuser 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 flametube 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 flametube influences the pre-diffuser providing a beneficial blockage effect. However, if increased to 1.2, this beneficial effect is weakened and the pre-diffuser flow deteriorates. With further increase to 1.6 the pre-diffuser shows strong evidence of separation. Hence, at the dump gaps probably required for lean module injectors it is unlikely the pre-diffuser will be influenced beneficially by the flametube blockage; this must be taken into account in the design. Further, 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 pre-diffuser design for best overall aerodynamic system performance from rotor through to feed annuli is a compromise between taking maximum advantage of upstream blockage effects, whilst minimizing any 3D upstream forcing.Copyright

Hybrid Diffusers for Radially Staged Combustion Systems

Modern, low-emission, radially staged combustors present the diffuser with a flametube of increasing radial depth. However, conventional diffuser systems limit the amount of flow diffusion and deflection that can be achieved in a given length, and, therefore, unconventional configurations such as hybrid or bled diffusers must be considered. This paper reports on the design and development of a hybrid diffuser for use with a radially staged combustor and compares its performance with that of a conventional bifurcated diffuser system. A simplified computational model was employed in conjunction with a fully annular isothermal test facility incorporating engine representative outlet guide vane wakes and a radially staged combustor. The hybrid diffuser was shown to operate well in excess of conventional design limits, and good agreement was seen between predicted and measured velocity profiles at diffuser exit. Moreover, the increase in area ratio offered by the hybrid system was realized without deterioration in the diffuser effectiveness, and, as a result, a 25% reduction in the total pressure loss to the combustor feed annuli was achieved.

Experimental and Computational Study of Hybrid Diffusers for Gas Turbine Combustors

The increasing radial depth of modern combustors poses a particularly difficult aerodynamic challenge for the prediffuser. Conventional diffuser systems have a finite limit to the diffusion that can be achieved in a given length and it is, therefore, necessary for designers to consider more radical and unconventional diffuser configurations. This paper will report on one such unconventional diffuser; the hybrid diffuser which, under the action of bleed, has been shown to achieve high rates of diffusion in relatively short lengths. However, previous studies have not been conducted under representative conditions and have failed to provide a complete description of the relevant flow mechanisms making optimisation difficult. Utilising an isothermal representation of a modern gas turbine combustor an experimental investigation was undertaken to study the performance of a hybrid diffuser compared to that of a conventional, single passage, dump diffuser system. The hybrid diffuser achieved a 53% increase in area ratio within the same axial length generating a 13% increase in the pre-diffuser static pressure recovery coefficient which, in turn, produced a 25% reduction in the combustor feed annulus total pressure loss coefficient. A computational investigation was also undertaken in order to investigate the governing flow mechanisms. A detailed examination of the flow field, including an analysis of the terms within the momentum equation, demonstrated that the controlling flow mechanisms were not simply a boundary layer bleed but involve a more complex interaction between the accelerating bleed flow and the diffusing mainstream flow. A greater understanding of these mechanisms enabled a more practical design of hybrid diffuser to be developed that not only simplified the geometry but also improved the quality of the bleed air making it more attractive for use in component cooling.Copyright