High-pressure fuel spray ignition behavior with hot surface interaction

Fuel-flexible aircraft propulsion systems using compression ignition engines will require novel strategies for reducing the ignition delay of low-reactivity fuels to feasible timescales. Hot surface ignition of fuel sprays has been implemented in some practical situations, but the complex nature of flame formation within the spray structure poses significant challenges. To design next-generation ignition devices, the capacity of hot surface heating elements to promote fuel spray ignition must be investigated. In this study, a rapid compression machine was used to examine the ignition process of a single kerosene-based F-24 jet fuel spray with a cylindrical heating element inserted into the spray periphery. The effects of both heating element surface temperature and insertion depth on the ignition characteristics of the F-24 fuel sprays were investigated under reduced temperature and pressure conditions for compression ignition engines. Experiments were conducted with moderately high injection pressures of 40 MPa. Results showed two modes of ignition governed by surface temperature and insertion depth of the heating element. An optimal position was determined where the heating element tip is located in the fuel vapor cone around the liquid spray. For this configuration, a critical surface temperature was identified (∼1250 K), above which short ignition delays associated with a “spray ignition” mode are consistently achieved. In this case, a local ignition flame kernel propagates downstream to the flame lift-off length before full ignition of the spray. In comparison, below the critical temperature a slower “volumetric” mode was observed. The extended ignition delays associated with this mode may be impractical for compression ignition engines operating at high speeds and increased altitude.