Experimental and computational study of hybrid diffusers for gas turbine combustors

Diffusers are essential in gas turbine combustors, decelerating the compressor efflux prior to the combustion chamber to reduce total pressure losses. Modern, low emission, radially staged combustors require even more diffusion due to the increased flame tube depth of this type of combustor. Furthermore, these high rates of deceleration are accompanied by large adverse pressure gradients and an associated risk of flow separation. Previous studies have shown that hybrid diffusers can achieve high rates of efficient diffusion in far shorter lengths than conventional faired diffusers or dump diffuser systems, representing a potential performance gain and weight saving. Hybrid diffusers consist of a wide angle diffuser immediately downstream of a sudden expansion, with flow separation prevented by bleeding off a small amount of the mainstream flow. However, previous studies have not provided a conclusive understanding of the associated flow mechanisms leading to hybrid diffusers currently being considered high risk. Additionally definitive data does not exist on the influence of bleed gap geometry and therefore hybrid diffusers cannot, currently, be optimised for use in a modern gas turbine. Further issues also not addressed by earlier studies, but concerning the use of hybrid diffuser in gas turbine combustors, are the effect of representative inlet conditions incorporating vane wakes at diffuser inlet, the quality of the bleed air and its potential for use for component cooling, the effect of radial struts within a hybrid diffuser and the quality of the flow delivered to the combustor feed annuli (total pressure losses). Therefore, a predominately experimental study, coupled with CFD predictions, was undertaken to investigate the controlling flow mechanisms of hybrid diffusers and address the questions necessary to evaluate the suitability of hybrid diffusers for use in modern, low emission, radially staged combustion systems. An existing isothermal test facility was used comprising a fully annular, staged combustor downstream of a single stage axial compressor incorporating engine representative outlet guide vanes. Initial experimental work led to rig modifications which allowed a range of hybrid diffusers to be studied. To act as a benchmark the performance of a conventional single-passage, dump diffuser system was first studied. A hybrid diffuser demonstrated a 53% increase in area ratio within the same axial length as the conventional diffuser. Results showed that this hybrid diffuser achieved a 13% increase in static pressure recovery which, in turn, improved the feed to the combustor feed annuli and decreased total pressure loses by 25%. Notably this brought the annulus losses within accepted target values; something the conventional diffuser system was unable to do. Additionally, it was clearly shown, in contradiction to previous studies, that bleeding air via a vortex chamber was not necessary. Bleeding air via a simple duct arrangement achieved the same results without altering the governing flow mechanisms. To provide a better understanding of these flow mechanisms, a computational investigation was also undertaken. A commercial CFD code, Fluent, was used to solve the Reynolds averaged Navier-Stokes equations for an incompressible flow regime, employing a blended second order upwind/central differencing scheme and the SIMPLE pressure correction algorithm. The turbulence was modelled using the k-ε model in conjunction with a standard wall function. Several generic two-dimensional hybrid diffusers were studied in order to reveal the controlling flow mechanisms and enable optimisation of the bleed gap geometry. Importantly, this revealed that many features previously thought to contribute to the flow mechanisms were, in fact, unnecessary. 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 much more complex interaction between the accelerating bleed flow and the diffusing mainstream flow. Firstly, momentum is transferred from the accelerating bleed flow to the diffusing mainstream flow, enabling a fresh boundary layer to be formed on the diffuser wall which is sufficiently energetic to overcome the high rates of diffusion and high adverse pressure gradient. Secondly, the radial pressure gradient created by the bleed causes deflection of the mainstream flow which also transports higher momentum fluid into the boundary layer. Understanding this resulted in a greatly simplified design for the hybrid diffuser not only potentially reducing weight but also reducing bleed flow total pressure losses. Predictions for a three-dimensional representation of the experimental facility displayed many similarities in the flow field and similar performance trends to the experimental data. However, predicted values of total pressure loss and static pressure recovery differed from experimental data and it was thought that this was due to an incomplete description of the turbulence (k and ε) at inlet and/or known problems the k-ε turbulence model has with predicting some unconfined flows. Nonetheless, three-dimensional predictions revealed an interaction between the OGV wake fluid and bleed flow causing localised, but small, modification of the flow mechanisms. Furthermore, it was shown that without the levels of turbulence produced downstream of an axial compressor the hybrid diffuser under study would, in fact, stall. Overall, experimental and computational results obtained in the current research suggest that the performance of hybrid diffusers is more than satisfactory for use within lowemission, staged, gas turbine combustion systems. An understanding of the governing flow mechanisms and the effect of features such as OGV wakes or radial struts has lead to a more practical design of hybrid diffuser, simplifying the geometry and reducing bleed flow total pressure losses (increasing the possibility of this air being used for component cooling)

Experimental and computational study of the flow around a stationary and rotating isolated wheel and the influence of a moving ground plane

This study investigates the aerodynamic behavior of the flow around a rotating and stationary 60% scale isolated wheel, with and without the use of a moving ground plane. The aim of this research was to improve the understanding of the fundamental aerodynamic flow features around a wheel and to examine how rotation and moving ground planes modify these and affect the production of drag. A bespoke rotating wheel rig was designed and wind tunnel tests were performed over a range of pre to post critical Reynolds numbers. Force coefficients were obtained using balance measurements and flow field data were obtained using Particle Image Velocimetry (PIV). The unsteady flow field data generated was used to validate unsteady CFD predictions. These were performed using STARCCM+ and a k- SST Improved Delayed Detached Eddy Simulation (IDDES) turbulence model. This was seen to outperform other models by capturing an increased amount of finer detailed, high frequency vortical structures. The CFD showed good agreement with the experimental results providing, for the first time, a validated numerical methodology. Comparing stationary and rotating wheels the CFD and experimental data both illustrated large scale structural differences in the surrounding flow due to changes in separation and wake structure. The rotating model also exhibited a lower drag at post critical Reynolds numbers, which is corroborated by existing literature. Importantly, the CFD showed minimal difference between a stationary and moving ground plane simulation with a rotating wheel. This is evidence that, provided the wheel is rotating, valid experiments can be performed without the complexity of a moving ground plan

Investigation of wheelhouse flow interaction and the influence of lateral wheel displacement

The aim of this research was to improve the understanding of the complex flow features found around a wheel and wheelhouse and to examine how the lateral displacement of the wheel affects these features and the production of exhibited pressures and forces. A bespoke rotating wheel rig and accompanying wheelhouse with a fully-pressure-tapped wheel arch was designed and manufactured at Loughborough University. Wind tunnel tests were performed where force and pressure measurements and Particle Image Velocimetry (PIV), data were obtained. The experimental data were used to validate unsteady CFD predictions where a k-ω SST Improved Delayed Detached Eddy Simulation (IDDES) turbulence model was used in STAR-CCM+. The CFD showed good agreement with all trends of the experimental results providing a validated numerical methodology. For both methodologies, a lower amount of wheelhouse drag was found generated when the wheel was rotating. However, the CFD showed that whilst this was the case, total configuration drag had increased. This was attributed to an increase of the wheel and axle drag, illustrated by the change in separation over the wheel itself when located within a wheelhouse and so overcompensating the reduction in body and stand drag. Differences in vortex locations when comparing to previously-attained results were due to differences in housing geometry, such as blockage in the cavity or housing dimensions. Experimental and computational results showed that up until a 10-mm displacement outboard of the housing, overall drag decreased. The reduction in housing drag was credited to a reduction in the size of outboard longitudinal vortex structures. This led to the lateral width of the shear layer across the housing side being narrower. Overall, this study identified that there were potential benefits to be gained when offsetting a wheel outboard of the longitudinal edge of a model housing

An investigation of flush off-takes for use in a cooled cooling air system

The design and evaluation of off-takes has traditionally focused on increasing ram pressure recovery with little consideration given to flow uniformity. Preliminary studies on a proposed cooled cooling air system for a large aero gas turbine indicated that the offtake represented a weak point in the design with the non-uniformities it generated negatively affecting system performance. High levels of diffusion and a uniform flow are required to minimise loss and to maximise the effectiveness of the downstream heat exchanger. This paper presents a numerical and experimental parametric study of parallel wall flush off-takes with focus placed on the quality of the downstream flow and its uniformity. A realisable k-omega turbulence closure was employed with a standard wall function to examine the pressure recovery and uniformity of flush off-takes. The performance of the off-take was investigated with different inflow boundary layer thicknesses in conjunction with changes in various design parameters. The current investigation highlights that there exists a direct trade-off between the diffusion and uniformity that can be achieved by a flush offtake. Nevertheless, the work provides an improved understanding of how each performance parameters can be maximised with respect to uniformity and this knowledge is currently being applied to the development of an optimal off-take design

Analysis of single hole simulated battle damage on a wing using particle image velocimetry

Particle Image Velocimetry (PIV) has been used to map the complex flow field generated by simulated battle damage to a two-dimensional wing. Previous studies have relied on surface flow visualisation techniques to study the flow but here PIV data has enabled the flow field away from the surface to be analysed for the first time. Damage was simulated by a single hole with a diameter equal to 20% of the chord, located at mid-chord. Wind tunnel tests were conducted at a Reynolds number of 500,000 over a range of incidences from 0-10 with two-component PIV measurements made on three span-wise planes; on the damage centre line and o set by 0.5 and 1.0 hole radii. The PIV data was seen to be in good agreement with existing surface flow visualisation showing strong evidence of the formation of a horse shoe vortex, a counter-rotating vortex pair and reverse flow regions. Large variations in the flow structure were observed over the range of incidences tested as the jet transitioned from weak at lower angles to strong at higher angles. The data also revealed some significant differences in the flow compared to classic Jets In Cross-Flow (JICF) behaviour. Notably in the case of battle damage the jet never fully occupies the hole and jet velocity pro le is highly skewed towards the rear of the hole. Additionally, the measured velocity ratios are much less than would be expected for typical JICF. For example, strong jet behaviour is observed at a velocity ratio as low as 0.22 whereas JICF studies would suggest a much higher ratio (> 2) is required. Increasing velocity ratio has been related to a reduction in lift and an increase in drag. At the highest incidence tested (10 ) the velocity ratio of 0.32 resulted in a reduction of the lift coe fficient by 0.18 and an increase in the drag coeffi cient of 0.035

Improved modelling capabilities of the airflow within turbine case cooling systems using smart porous media

Impingement cooling is commonly employed in gas turbines to control the turbine tip clearance. During the design phase, Computational Fluid Dynamics is an effective way of evaluating such systems but for most Turbine Case Cooling (TCC) systems resolving the small scale and large number of cooling holes is impractical at the preliminary design phase. This paper presents an alternative approach for predicting aerodynamic performance of TCC systems using a “smart” porous media to replace regions of cooling holes. Numerically (CFD) defined correlations have been developed, which account for geometry and local flow field, to define the porous media loss coefficient. These are coded as a user defined function allowing the loss to vary, within the calculation, as a function of the predicted flow and hence produce a spatial variation of mass flow matching that of the cooling holes. The methodology has been tested on various geometrical configurations representative of current TCC systems and compared to full cooling hole models. The method was shown to achieve good overall agreement whilst significantly reducing both the mesh count and the computational time to a practical level

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%

The application of porous media to simulate the upstream effects of gas turbine injector swirl vanes

Numerical simulations are an invaluable means of evaluating design solutions. This is especially true in the initial design phase of a project where several simulations may be required as part of an optimisation study. The design of aircraft gas turbine combustor external aerodynamics frequently calls upon the services of numerical methods to visualise the existing flow field, and develop architectures which improve the performance of the system. Many of these performance improvements are driven by the desire to reduce fuel burn and cut emissions lowering the environmental impact of aviation. The gas turbine combustion chamber is, however, reasonably complex geometrically and requires a high fidelity model to resolve small geometric details. The fuel injector is the most geometrically complex component, requiring around 20% of the mesh cells of the entire domain. This makes it expensive to model in terms of both requisite computational resource and run time. Most modern aircraft gas turbines utilise swirling flow fields to stabilise the flame front in the combustion liner. The swirl cone is generally generated using fixed angle vane rows within the injector. It is these small features that are responsible for the requisite high mesh cell count. This paper presents a numerical method for replacing the injector swirl vane passages with mathematically porous volumes which replicate the required pressure drop. Modelling using porous media is preferential to modelling the fully featured injector as it allows a significant reduction in the size of the computational domain and number of cells. Additionally the simplification makes the geometry easier to change, scale and re-mesh during development. This in turn allows significant time savings which serve ultimately to expedite the design process. This method has been rigorously tested through a range of approach conditions and flow conditions to ensure that it is robust enough for use in the design process. The loss in accuracy owing to the simplification has been demonstrated to be less than 4.4%, for all tested flow fields. This error is dependent on the flow conditions and is generally much less for passages fed with representative levels of upstream distortion

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

Investigation of the flowfield induced by simulated battle damage

Particle Image Velocimetry (PIV) has been used to study the complex flowfield created by simulated battle damage to a two-dimensional wing. Computational Fluid Dynamics (CFD) predictions have also been used for validation of internal cavity flow. Two damage cases were selected for the study; both cases were simulated using a single hole with diameters equal to 20% and 40% of the chord, located at the wing half-chord. Wind tunnel tests were conducted at a Reynolds number of 500,000 over a range of incidences from 0 to 10◦ with two-component PIV measurements made on three chordwise and three spanwise planes. The PIV data were analysed and compared to CFD data of the same damage cases. The PIV data have shown lower velocity ratios and lower vorticity in the jet compared to past Jet in Cross-Flow experiments and CFD was used to describe the flow features inside the cavity of the wing. It was seen that the wing cavity has large effects on the external flow features, particularly for the 20% damage case. Finally, the flow field data have been related to force balance data. At higher incidence angles, the larger force coefficient increments in both lift and drag can be attributed to the larger wakes and higher jet strengths