Influence of main stage air splits on the ignition performance of TeLESS-II combustor

The centrally staged layout is preferred in the advanced aero-engine combustor to achieve low pollutant emissions as well as stable operation in lean premixed prevaporized combustion. However, because the high-speed main stage airflow prevents the pilot fuel droplets arriving at igniter tip and has a strong convection effect on the initial flame kernel, the application of centrally staged combustor is restricted by its poor ignition and lean blow-out performance. In the centrally staged combustor, the main stage and pilot stage have strong coupled influences on the flow field and fuel distribution. The aim of this paper is to research the impact of the main stage air split on the ignition performance for the baseline case and the comparison case of the main swirler in the TeLESS-II combustor. The main stage air flow rate of the comparison case is about 8 percent less than that of the baseline case. The results of the ignition test at room inlet temperature and pressure indicate that the ignition performance of the comparison case is significantly better than that of the baseline case. The results of the lean blow-out tests show that the main stage air splits do not make the lean blow-out performance worse. To achieve a better understanding of the test results, PLIF technology and CFD analysis were used to measure the fuel distribution and nonreacting flow field. The PLIF and CFD results demonstrate that the most of the fuel spray disperse outward into the main stage cold airflow in the baseline case so that the pilot flame is hard to be established, which leads to poor ignition performance. On the other hand, in the comparison case, the most of the fuel is confined in the recirculation region, which gives a better ignition performance. Compared with the baseline case, the main stage airflow velocity decays faster in the comparison case. It changes the direction of the instantaneous velocity in the spark vicinity, which makes it more likely for the ignition kernel to be captured by the recirculation stream in the comparison case. Therefore, the different fuel distribution and flow field characteristics cause the ignition performance improvement in the comparison case. The improvement is due to the different main stage air flow rates, which is the consequence of the main stage air split.