Lean burn combustor systems are a key technology to reduce NOx emissions for future engines. The ability to maintain the desired combustor metal temperature is critical to achieving acceptable durability. The levels of fuel-air premixing inherent in lean burn designs makes them susceptible to thermo-acoustics instabilities which will have a drastic impact on the durability of the combustor. The overall aim of this project was to develop validated methodologies for the prediction of combustor temperature and thermo-acoustics instabilities to allow confident design of the combustion system of a demonstrator engine at TRL6.
The first work package focused on cooling and radiative heat transfer. It used Computational Fluid Dynamics to highly resolve the combustor liner geometric features so that a cheaper model may be obtained for design purposes. In addition, the sensitivity of radiative heat transfer to the choice of physics models was assessed. The resulting models were validated against existing experimental data from Loughborough University and the industrial partner. The second work package developed a smart system for combustor design by bringing together a variety of analysis techniques and creating software that can directly drive CAD software. A response surface supported by multi-fidelity; multi-objective robust design approaches were used to deliver a world class combustor design process. Thermoacoustics were considered by using CFD to study the response of a fuel injector to acoustic plane waves and by modelling a complete annular combustion system in order to resolve circumferential modes. The thermoacoustic results were validated against existing experimental data available at Loughborough and Cambridge University.
Lean burn combustor systems were a key technology to reduce NOx emissions for future aero engine gas turbines. The ability to maintain the desired combustor metal temperature was critical to achieving acceptable durability. The levels of fuel-air premixing inherent in lean burn designs made them susceptible to thermo-acoustics instabilities which had a drastic impact on the durability of the combustor.
The overall aim of this project was to develop validated methodologies for the prediction of combustor temperature and thermo-acoustics instabilities to allow confident design of the combustion system of a demonstrator engine at Technology Readiness Level 6.
The first work package focused on cooling and radiative heat transfer. It used Computational Fluid Dynamics to highly resolve the flow in two types of complex combustor liner. These were then used to create and calibrate two sub-grid scale models for the CFD that allowed the effect of pedestals and impingement-effusion to be represented without explicitly resolving the geometrical details. This allowed the simulation of the cooling tiles with significantly less computational resource requirements making it suitable for rapid design calculations. Radiative heat transfer to the combustor is strongly influenced by the soot concentrations close to the fuel injector, and simulations were carried out to test the sensitivity of this radiative load to the choice of models used to represent combustion and soot production. This was also used with a conjugate heat transfer analysis to compute the metal temperature of part of the fuel injector. The overall results were found to be relatively insensitive to the modelling approach chosen.
The second work package further developed a design tool for the automated analysis of a combustor design. This used links to CAD software to automatically create the fluid volume, generate the CFD mesh, apply network determined boundary conditions and finally calculate the fluid flow and post-process the data. The main focus here was the development of coupling to a FEA system for mechanical stress and thermals in order to also automate these tasks. The accurate knowledge of metal temperature in the combustor is the key to determining durability. Also incorporated was an automatic link to an external network model representing the combustor cooling tiles. The overall process was demonstrated on several combustor geometries.
The final work package focussed on the thermoacoustics behaviour of the combustor. This had two strands: the first looked at an isolated, isothermal single fuel injector and its aerodynamic response to a plane wave. This was validated against experimental data and compared to previous simulations using a different CFD code. The work was then extended by considering how the spray droplet size and velocity varies due to the varying flow speed in the fuel injector. This took an existing droplet model and modified and calibrated with experimental data. A real combustor contains of multiple fuel injectors and includes reacting flow. This second strand looked at different modelling approaches for this problem, in particular comparing Unsteady RANS and Large Eddy Simulation for forced and unforced annular simulations. LES was found to be preferable as it fully captured the unsteadiness, particularly of the flame.