Rolls Royce UTCin Combustion System Aerothermal Processes

Research and Facilities

The Loughborough UTC studies all areas relating to the aerothermal processes occurring within gas turbine combustion systems. This is a significant challenge as the combustion system consists of many highly complex components and contains a wide range of complex flow physics.

 

The primary remit of the UTC is to carry out a coordinated programme of computational and experimental research related to current and future gas-turbine combustion systems (and related components). A particular focus is placed upon design and performance aspects which are influenced by aerodynamic and aerothermal processes. This includes studying the fundamental aerodynamic flows found within various gas turbine components (diffusing flows, wake mixing separated flows, streamline curvature, highly swirling flows, vorticity etc.). It also encompasses the study of flows within more complex engine representative environments considering, for example, the aerodynamic interfaces between the combustion system and its neighboring engine components. Research is also extended to include multi-disciplinary areas where the aerodynamic flow field strongly interacts with, and influences, other processes such as in two-phase flows in the fuel injector, aero acoustic features of the combustion system, combustion instabilities and heat transfer and component cooling.

The main areas of activity  are highlighted below:

Combustion System Aerodynamics

Historically, combustion system aerodynamics represents the core research activity of the Loughborough UTC. With the continual drive for reduced fuel burn and reduced emissions it remains a key activity. The UTC invests significant time and effort into the experimental and numerical study of fundamental aero-thermal processes for individual components but it is also essential to understand the whole combustion system where components and processes interact.

The main areas of research currently include:

  • Rich and lean burn system aerodynamics,
  • Reduction of combustion system aerodynamic loss,
  • OGV/pre-diffuser and dump gap aerodynamics,
  • Fuel injector airflow feed and the feed to various cooling features,
  • Novel and blue-sky combustor architectures, and
  • Direct support for Rolls-Royce engine projects.

Several state of-the-art fully annular test facilities are employed to investigate combustion system aerodynamics. These include engine representative geometries and replicate the system from compressor exit to turbine entry. Importantly they include a bespoke 1½ stage axial compressor which generates representative inlet conditions. These facilities replicate individual component performance and importantly any potential component interactions in a system wide simulation. Aerodynamic measurements are typically made using a combination of pneumatic probes, hot-wire anemometry, PIV and CO2 gas tracing. The resultant data, supported by CFD predictions, are used to study the overall flow field distribution and pressure losses throughout the combustion system and thereby develop and validate combustor aerodynamic designs. Importantly this provides total pressure loss data which is invaluable to the design process in order to estimate the system performance at an early stage before complex and expensive hot, high pressure tests are conducted (by Rolls-Royce).

Combustor – Compressor/Turbine Interactions

The aerodynamic interfaces between the combustion system and the adjacent compressor and turbine must be carefully managed in order to ensure optimal performance. The Loughborough UTC conducts experimental and computational research aimed at improving both these interfaces. This has led to the development of new, novel and integrated designs with particular focus on the changing combustor architectures required to meet future low-emission and low fuel-burn targets.

For example, work on the development of an integrated compressor outlet guide vane (OGV) and pre-diffuser concept received a Best Paper award from the ASME Combustion and Fuels Committee in 2006. After initial development at the University this concept was advanced to a higher TRL by Rolls-Royce and adopted in their design for the Trent XWB engine which powers the Airbus A350 XWB. This is one of the enabling technologies that make the Trent XWB today's most efficient large aero engine.

Fuel Injector and Two-Phase Flows

Gas turbine fuel injection is a notoriously challenging problem with large scale mixing induced by poorly understood aerodynamic phenomenon, and two-phase fluid mechanics, all occurring within a highly turbulent swirling environment. Fuel injector research in the Loughborough UTC spans a broad spectrum ranging from fundamental to more focused research reflecting the close integration between the University and Rolls-Royce. The UTC approach is to simultaneously develop numerical and experimental techniques and to apply these techniques in as representative an environment as is practicable. This includes:

  • 2D and 3D PIV measurements of the flow field downstream of various injectors,
  • Refractive index matched PIV to measure the fuel injector internal airflow,
  • PDA and PLIF to measure droplet size and fuel distribution,
  • Establishing best practice RANS, uRANS and LES CFD methods, and
  • Development of multiphase CFD capability including primary and secondary break-up models.

Aeroacoustics

The interaction of the unsteady aerodynamic flow field with pressure fluctuations generated by unsteady heat release is important as it can give rise to damaging aerothermal instabilities. A range of experimental and numerical projects are being undertaken to investigate the aero-acoustic phenomena that are relevant to current and future gas turbine combustion systems.

Experimental facilities include a unique electro-pneumatic driven 165 dB aero-acoustic noise generator to simulate combustion generated pressure fluctuations. Various test sections can be placed downstream of the noise generator including simple holes, various types of acoustic damping systems, individual fuel injectors (with or without fuel) or more representative combustor sectors. Data acquisition is performed using multi-channel Kulites (acoustics), PIV, PLIF, PDA and hotwire anemometry which provide the flow field (single and two-phase) response, impedance, acoustic energy transmission and reflection.

Studies include fundamental analysis of the mechanisms by which acoustic energy can be absorbed into the flow field generated by various geometrical features followed by the design and testing of less acoustically sensitive systems. New analysis techniques have been developed to enable the acoustically generated flow field structures to be isolated from the unsteady aerodynamics flows. The improved capture of the aero-acoustic processes facilitates understanding of the flow physics and peak absorption. These techniques are now employed to assess the effects of acoustic waves on the unsteady flow field characteristics associated with fuel injectors for both single and two-phase flows.

Experimental data have also enabled the development and validation of new analytical models which simulate acoustic absorption. These models can predict the performance of new configurations much more rapidly than traditional experiments and are therefore invaluable to the design process. Additionally, CFD based methods are also being developed and validated which incorporate the correct aero acoustic boundary conditions necessary to model these complex systems.

Heat Transfer and Cooling

Three experimental facilities are used to investigate advanced and novel combustor film cooling geometries. In conjunction with computational predictions this provides increased understanding and ultimately design guidance for improved cooling geometries. Current research activities are focused on the development of novel combustor effusion cooling designs which exploit new manufacturing techniques such as direct laser deposition (DLD). Typical tests involve creating large scale segments of the combustor liner using materials with appropriate thermal properties. This enables accurate scaling of both thermal and aerodynamic conditions (Re, Nu, Biot, etc.) in a low temperature laboratory environment allowing high fidelity measurements to be made. Overall cooling effectiveness is assessed using time averaged IR surface measurements to establish a thermal ranking of different designs. The most promising schemes are then investigated further on a high turbulence film cooling test facility, which enables an in-depth analysis of coolant film development using a combination of IR thermography, thermally sensitive liquid crystal coatings and temperature traverse data.

High pressure/temperature tests are also performed with multiple HP air supply lines at 14 bar. The UTC's high pressure test facility is highly reconfigurable; a small gas turbine combustor can be used to heat air to 1000K and a chiller system can be installed to cool air, these operations can be run simultaneously on differing streams, providing a wide range of application possibilities. The facility can be used for aerodynamic assessment of propulsion exhaust nozzles and associated cooling studies. Data acquisition techniques include pneumatic probes, LDA, thermal cameras, constant current and constant temperature anemometry. These provide details of flow field velocity, turbulence measurements, heat transfer surface film effectiveness and heat transfer coefficients. The excellent uniformity of the delivered air is also exploited to perform high accuracy calibration of engine gas-path instrumentation.

CFD Methods Development and Validation

Computational Fluid Dynamics is used across the Loughborough UTC as an integral part of the research activities. Often this is the routine use of established CFD methods to support the design and analysis of experimental work. However, it also encompasses the development of new modelling techniques to allow the accurate simulation of the complex aerodynamic phenomena present in the combustion system. This is a significant challenge; a single modelling approach is not applicable across the broad range of flow physics. Consequently hybrid techniques are being developed which allow coupling of different turbulence models and flow solvers across different engine components.

Commercial and proprietary in-house codes are used for grid generation, analysis and post-processing, with a range of grids and turbulence models providing steady state or time dependent solutions. A 1956-core high performance PC cluster provides a parallel computing platform within the Department of Aeronautical and Automotive Engineering, and a larger 3000-core platform is available through the EPSRC University High Performance Computing (Midlands) service. The available computing power is therefore sufficient for large LES time dependent solutions to be run within reasonable time scales.

Methods have been developed within the UTC to simulate the primary breakup of liquid fuel. A hybrid Coupled Level Set Volume of Fluid (CLSVOF) method has been established which combines the liquid surface tracking accuracy of level set methods with the mass conservation of VOF. This allows accurate simulation of two-phase flows while reducing the number of computational cells.

Combustion models are also being developed which will allow Large Eddy Simulations (LES) to capture the complex combination of turbulence and chemistry which takes place in gas turbine combustors. Unlike many existing models these are being developed with the aim that they can be used with complex real-world, rather than idealised, geometry.

Other CFD work has studied the effect of numerically forcing combustion calculations in order to extract ‘flame describing functions’ and help understand unsteady heat release and combustion instabilities. Further work to understand and model the unsteady flow in the combustor has used pressure based compressible unsteady RANS (uRANS) techniques. These have the advantage of relatively low computational cost (compared to density based or LES codes) for this type of problem allowing them to be applied to realistic problems. Additionally, by developing appropriate acoustic boundary conditions it has been possible to obtain excellent agreement with experiments for the unsteady aero-acoustic response of both simple and complex geometries.