Experimental Studies on Droplet-Size Distribution in a Reverse-Flow Combustor of a Two-Staged Fuel Injector

The ultracompact combustion chamber is a promising propulsion system for low-altitude, miniaturized aircraft, with fuel injection characteristics directly influencing the flame stability limit, a critical factor for optimizing combustion performance. This study innovatively measured the fuel droplet distribution characteristics under flame stability critical conditions for a two-stage injector in a closed, reverse-flow combustion chamber and explored the impact of fuel atomization on the stability limit. The experiment was conducted at standard temperature and pressure using a laser particle size analyzer to measure droplet distribution at three points downstream of the nozzle, with the nozzle supply pressure drop corresponding to the ignition and lean blowout limits. Results showed that as the inlet velocity ranged from 9.4 m/s to 42.1 m/s, the average sauter mean diameter (SMD) of fuel droplets near the spark plug was between 23.34 and 63.86 μm. As the inlet velocity increased, the fuel supply pressure difference at the flameout limit also increased. The fuel particle size distribution near the ignition electrode expanded from 20–50 μm to 29–144 μm, and then decreased to 16–95 μm. The main fuel particle diameter first increased from 34 μm to 87 μm, then decreased to 51 μm. When the inlet velocity exceeded 28.9 m/s, the droplet size near the spark plug became smaller and more uniform, suggesting that fuel atomization was not the primary cause of the increased pressure drop at the flameout limit. These findings provide valuable insights for extending the flame stability limit by adjusting fuel droplet distribution.