Experimental Investigation of Recursive Sequential Combustion: Early Experimental Demonstration, Emission Analysis, and CFD Insights

Abstract The Recursive Sequential Combustion (RSC) concept introduces an innovative approach to exhaust gas recirculation for lean combustion processes in aero-engines, utilizing the flow dynamics driven by a special combustor design. This modified form of sequential combustion arranges the flames consecutively in a closed loop, solely through its flow design. The primary objective is to maximise the interaction between the exhaust gases of one flame and the fresh reactants of the subsequent flame along the annular path. This creates a consistent circulation of a defined amount of hot exhaust gases continuously refreshed by each flame, acting as a thermal buffer. This technology is intended to provide stable lean combustion, augmenting the safety margin against lean blow-out, and reducing soot and NOx emissions, by comparison to conventional aero gas turbine combustors. This study reports on early investigations under atmospheric conditions on the RSC concept through the discrete burner approach. Each burner represents a single, discrete section, incorporating all necessary flow and combustion components (inlets, outlet, mixer, splitter, ignition) to enable either a conventional combustion mode by using a single burner, or the RSC mode by using at least two burners in a closed loop arrangement. This approach allows for experimental demonstration of the RSC process while facilitating comparisons with the conventional operating mode using the same burner. Additive manufacturing (AM) technologies permitted the production of a sophisticated and functional burner geometry made from the high-temperature alloy Inconel 718. As part of the investigation, the study examined the combustion stability of a propane flame near the lean blow-out (LBO) limit, the pollutant emissions in terms of NO and CO, and the flame behaviour under forced thermo-acoustic excitation in both RSC and conventional mode. RANS CFD simulations were also performed to provide insights into the flow field and mixing of fresh reactants and recirculated exhaust gases. This paper presents early experimental results that validate the RSC concept using a discrete burner approach. Based on the experimental data obtained, the proof of concept was positively evaluated: the RSC technology provides a robust lean combustion with an improved safety margin against LBO, and good emission performance.