The impact of representative inlet conditions on low-emission fuel injector performance

Environmental pollution has been a major point of interest in recent years, and aerospace gas turbine combustion systems must address increasingly more stringent emissions requirements. One potential way of addressing these requirements is the introduction of lean burn combustion technology. However, lean burn fuel injectors are much larger, relative to traditional rich burn injectors. This leads to increased aerodynamic interactions with upstream and downstream components and a highly non-uniform feed to the injector. The impact of this on the combustion process is currently not well understood. Importantly, these effects are not accounted for in typical test facilities used for injector development, as these are generally plenum fed. Hence, the main aim of this work is to study the impact of these aerodynamic interactions on the combustion process in order to obtain the true embedded performance of a lean burn injector and hence improve future low emission fuel injector designs. Using computational fluid dynamics (CFD), validated by particle image velocimetry (PIV), a modified inlet was designed which could be retrofitted into a single sector plenum fed reacting flow facility as typically used in injector development programme. This modified inlet was designed to reproduce the key aerodynamic features associated with the diffuser-injector interaction. Additionally, a new effect associated with the diffuser-injector interaction was identified, which was a mass flow redistribution between the injector passages that clearly has the potential to alter local air-fuel-ratios (AFR). The impact of these aerodynamic interactions on the combustion process was experimentally evaluated using a single sector test facility with both a plenum inlet and the new modified inlet design. Images of the flame and emissions of NOx, CO and UHC all highlighted that inclusion of the modified inlet, and the associated aerodynamic interactions, had a large impact on the combusting performance of the injector. Changes in local AFR distribution, caused by the mass flow redistribution effect, were directly linked to some of the measured changes. However, further data was required to link the diffuser-injector interaction to the remaining measured differences. To allow for a more detailed analysis of the chemical processes a 1-D chemical reacting network model was used. A methodology was developed to build the network based on a predicted flow field thus accounting for realistic residence times and spatial variation in mass flow and temperature. Results from the model showed similar trends to the experimental data and thus a sensitivity analysis was performed. This identified several possible changes to the flow field that influence the emissions in a similar trend to that observed in the experimental data. These include the amount of flow recirculating between the pilot and mains flames and the residence time in the mains flame. Overall, the work has shown the importance of representative inlet conditions on the combusting performance of low emissions lean burn fuel injectors. A methodology for generating these conditions in single-sector test facilities used in injector development has also been developed.