Combustion LES of CESAR Multi-Burner Annular Combustor

Abstract

In this study we present results from simulations of a single sector and a fully annular multiburner aero engine combustor. The objectives are to facilitate the understanding of the flow, mixing and combustion to help improve the design process and the combustor design as well as to demonstrate that it is now feasible to perform high-fidelity reacting flow simulations of fully annular gas turbine combustors. The simulations are executed by a combustion Large Eddy Simulation (LES) model in which all flow features larger than the grid are resolved whereas the effects of the small scale flow features on the large ones are modeled. The LES method used is validated against a range of non-reacting and reacting flow configurations a few of which are reported here. The aero engine combustor of interest is here modeled both using a conventional single sector configuration and a fully annular multi-burner configuration and the Jet-A combustion chemistry is modeled by a global three-step mechanism. The single-sector and fully annular multi-burner predictions are similar but with the fully annular multi-burner configuration showing a different combustion dynamics and thus also somewhat different time-averaged profiles of the dependent variables. For the fully annular multi-burner configuration azimuthal pressure fluctuations are observed, resulting in successive reattachment-detachment of the flames in the azimuthal direction. For civilian and military aeropropulsion, including turboshaft engines for helicopters and small aircrafts, turbofans for large aircraft, and afterburning turbojets and turbofans for combat aircraft there is no realistic substitute for gas turbine engines. Modern aeropropulsion gas turbine engines usually have an annular combustion system with multiple burners sharing a common fuel supply line. Constraints on such gas turbines include velocity and temperature profiles delivered to the turbine (affecting turbine life), pressure drop across the combustor (affecting thermal efficiency), the ability to withstand flame extinction, blow-out and pressure oscillations, the ability to relight at high altitudes, unsteady thermal loads and mechanical vibrations as well as emission regulations on CO, CO2, NO, unburned hydrocarbons and smoke. Although the current trend in gas turbine design is towards fuel lean and premixed conditions to reduce emissions, most aeropropulsion gas turbines makes use of spray flames. The fuel spray is usually created by passing the liquid fuel through an air-blast atomizer in which the liquid fuel film is broken up into droplets that are sprayed into the combustor together with air under swirling conditions. Neighboring spray flames in an annular configuration interact with each other, and with the acoustic pressure field in the combustion chamber, and are further influenced by the air-flow through the mixing, dilution and film cooling holes, resulting in multiple complex flow phenomena. Accurate observations and quantitative measurements of these processes in real engines are difficult and expensive, and are thus in short supply. To reach the future ACARE goals on emissions, fuel flexibility and efficiency, improved qualitative and quantitative understanding of the combustion processes are sorely needed. This can only be achieved by combining laser-based diagnostic methods (typically based on fiber optics) with high