Paul A. Denman

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.

Turbulence Levels are High at the Combustor-Turbine Interface

Turbulence measurements are made in a novel gas turbine rig facility recently used to study combustor-turbine interactions in jet engines [1]. The rig is capable of numerous area traverses surrounding engine turbine nozzle guide vanes (NGVs). The rig is unique in that complete engine hardware of the annular combustion subsystem is used to simulate the upstream flow entering the turbine. The rig operates at cold, near-atmospheric conditions. The turbulence measurements include both the turbulence intensities and lengthscales and span an area over a single combustor sector. Axial measurement planes include locations both upstream and downstream of the real engine hardware NGVs. The upstream plane corresponds to a conventional combustor-turbine interface plane. In [1], pressure, velocity, and passive scalar mixing measurements were presented along with RANS CFD predictions. Here, in addition to the newly measured turbulence quantities, large-eddy simulations (LES) are performed for the complete, coupled combustor-turbine system.Good agreement between rig data and CFD is seen at the combustor-turbine interface, with LES yielding improved predictions over RANS. For the flow through the NGV passages, vortex visualizations of the simulated flowfields show significant differences to the classic, commonly accepted picture of Langston [2] and others [3]. The difference is attributed to the high turbulence levels created by the combustor. The impact of the limitations of the combustor-turbine rig on these findings is discussed.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%.

Aerodynamic Evaluation of Double Annular Combustion Systems

Legislation controlling the permitted levels of pollutant emissions from aircraft gas turbines has been an increasingly important design driver for the combustion system for some time, particularly with respect to oxides of nitrogen. This has lead to many suggestions for radical departures from the geometry of the classical combustor configuration involving, for example, lean premixed module technology, or staging (axially or radially) of combustor pilot and main zones. The optimum operation of any combustor also requires, however, appropriate and efficient distribution of compressor delivery air to the various flametube features (fuel injectors, dilution ports, for cooling and for air bleed purposes). Radial staging, leading to double annular combustor configurations, poses a particularly difficult challenge. The radial depth of the combustor increases to a level where the external aerodynamics of the combustor involves large flow turning after the pre-diffuser. Careful design is then needed to achieve acceptable levels of loss coefficient in the outer annulus. If these aspects are not properly addressed then inadequate penetration and mixing in the combustor interior can result, rendering low emissions performance impossible. This paper will report on the design, instrumentation and operation of a fully annular isothermal test facility, which has been developed specifically to enable this important issue of external flow quality in double annular combustor systems to be assessed. Representative inlet conditions to the combustion system are generated using a single stage axial compressor; modular construction enables quick and inexpensive changes to components of the combustor (pre-diffuser, cowl shape, liner port locations and geometrical details). Computerised rig control and data acquisition allows the collection of large amounts of high quality data. In addition to the calculation of overall system performance, it is then possible to identify flow mechanisms and loss-producing features in various zones and suggest appropriate modifications.Copyright

Detailed Performance Comparison of a Dump and Short Faired Combustor Diffuser System

A rectangular model simulating 4 sectors of a combustion chamber was used to compare the performance of a standard dump diffuser, of overall length 180mm, with that of a faired design 25.5mm shorter. The performance of each system was assessed in terms of total pressure loss and static pressure recovery between pre-diffuser 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% 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 pre-diffuser 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 inner casing. Furthermore, the faired system exhibited similar velocity magnitudes and gradients around the combustor head despite its shorter length.Copyright

Experimental and numerical investigation of the effect of compressor OGV profile on combustor exit measurements using an isothermal, non-reacting tracer

Turbine subsystem cooling design depends on the profile of the non-dimensional temperature dis-tribution function (TDF), measured at a traverse plane upstream of the nozzle guide vane (NGV). To date, the compressor discharge OGV profile was thought to have an insignificant effect on the resulting combustor exit traverse, hence a generic OGV geometry has been used for such tests, which typically remained unchanged between varying combustor designs. The present study how-ever shows that the wake profile of the OGV has a significant influence on the measured combustor exit traverse profile. Experiments were performed at Loughborough University with varying OGV geometries to simulate the aerodynamic field surrounding the combustor. Corresponding numer-ical analyses were performed using an in-house combustion analysis code with a passive scalar technique to model the CO2 tracer gas injection and mixing. Analysis of the experimental and nu-merical simulations confirm that the pressure and velocity profiles presented to the system by an axial flow compressor influence both the mass flow and pressure distributions within the combus-tor feed annuli. This in turn affects the ratio of the mass flow rates entering the flame tube through the dilutions ports located around the inner and outer annuli. The flow through these ports con-trols the bulk mixing within the flame tube, resulting in a change in mixture concentration profile measured near the combustor exit. Hence, reproducing engine-representative OGV wake struc-tures for a given engine together with an accurate representation of the combustor configuration is of key importance to reproducing the temperature profiles that inform turbine cooling design.

Measurement and Prediction of Adiabatic Film Effectiveness of Combustor Representative Effusion Arrays

Thermal protection of gas turbine combustors relies heavily upon the delivery of a carefully managed film of coolant air to the hot-side of the combustor liner. Furthermore, improvements in engine sfc and the trend to ever more aggressive engine cycles means greater emphasis is being placed upon more efficient use of the proportion of combustion system air made available for cooling. As a result, there is a requirement to better understand the development of cooling films deposited onto the hot-side of the liner through complex effusion arrays. This study, therefore, is concerned with the prediction and measurement of adiabatic film effectiveness of a number of engine-representative designs. A RANS based CFD approach is used to predict film effectiveness in which computational cost is minimised by solving first for a single coolant passage to provide high fidelity, near-exit boundary conditions to the effusion arrays. Equivalent measurements are made for each test case using a Pressure Sensitive Paint (PSP) technique in which the oxygen-quenched fluorescence properties of the paint are employed together with a Nitrogen gas cooling simulant to determine adiabatic film effectiveness. This study demonstrates that whist the model under-predicts the mixing of the coolant with the main-stream flow, and hence the film development over the surface, the approach works well at quantifying the relative performance of each design.Copyright

Measurement of Flow and Scalar Distribution at Gas Turbine Inlet Section

The goal of paper is to investigate the flow and scalar distribution through the HP Nozzle Guide Vane (NGV) passage. Flow and scalar distribution measurement are conducted by using 5-hole pressure probe and tracing technique, respectively. Three different experimental cases are considered depending on cooling flow condition. The result shows that the vortical secondary flow patterns are observed clearly and these flow characteristics maintain through the NGV passage regardless of cooling flow injection. Compared to center region, the high axial velocity flow is observed near wall region due to cooling flow injection. Without cooling flow, the (scalar) distribution becomes to be uniform quickly due to the strong flow mixing phenomenon. However, in cases of cooling flow, scalar distribution is significantly non-uniform.

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.