Effect of hot probe temperature on ignition of alcohol-to-jet (Atj) fuel spray under aircraft propulsion system conditions

The effect of ignition assistant probe temperature on ignition delay was investigated to achieve reliable compression ignition engines in aircraft systems using numerical simulations of alcohol-to-jet (ATJ) fuel injected into a combustion chamber with a hot surface probe. The main goal of this study is to improve understanding of the interaction between low cetane number fuel jet and hot surface probe under the aircraft engine conditions and suggest a baseline for optimized designs of the ignition assistant probe for maximum ignition probability. The numerical simulations provide transient temperature and equivalence ratio distributions, as well as the ignition process. The typical process of probe-enhanced ignition shows two different major mechanisms. The process starts from local direct ignition by the probe due to the surface tension of the attached fuel spray, followed by global autoignition with limited heating from the probe. The direct ignition and autoignition correspond to pressure recovery and maximum pressure rise rate ignition delays, respectively. The autoignition has two modes of ignition enhancement depending on the probe temperature and corresponding ignition locations: typical autoignition and spray-combustion for low and high probe temperature ranges, respectively. From the ignition delay breakdown analysis, a simple estimation method of spray ignition delay with a hot probe was suggested. The spray breakup and evaporation time scales were estimated by the Kelvin-Helmholtz Rayleigh-Taylor model and tracking the total number of parcels with time marching, respectively. The chemical ignition delay was calculated by a zero-dimensional solver with a proper effective temperature estimation from ambient air, spray droplet, and probe temperatures. The estimation method can predict the total ignition delay of ATJ fuel spray with hot surface probe well, and can help to define the effectiveness of probe temperature.