Ignition and flame stabilisation of primary reference fuel sprays at engine-relevant conditions

摘要

This study aims to investigate the underlying processes governing ignition and flame stabilisation incompression-ignition (CI) engine-relevant conditions. The experiments feature a canonical configurationwith a single fuel jet injected into a constant-volume combustion chamber. Primary reference fuels(PRFs), including PRF100 (neat iso -octane, a gasoline surrogate), PRF80 (a blend of 80 vol.% iso -octaneand 20 vol.% n -heptane) and PRF0 (neat n -heptane, a diesel surrogate), were tested to simulate changesin fuel ignition quality inside a quiescent steady environment with an ambient density of 22.8 kg/m~3 andan O_2 concentration of 15 vol.%. The ambient gas temperatures were controlled at 1150 K (PRF100), 1120K (PRF80) and 900 K (PRF0), in order to adapt to the fuel reactivity so that a constant ignition delayof 1.15 ms can be achieved for all blends. This approach was employed in order to substantially reducethe effect of fuel-oxidiser mixing prior to ignition while highlighting the effect of fuel chemistry on theignition process and flame evolution. Under the test conditions of this study, optical imaging reveals thatthe blends with higher iso -octane content exhibit a faster spreading of combustion after ignition and establisha steady lifted flame that is closer to the nozzle. Imaging by CH_2O-PLIF indicates that blends withhigher iso -octane content produce CH_2O that is distributed across larger portions of the jet at an earliertiming when compared to neat n -heptane that shows a propagating first-stage ignition through thefuel jet. Supporting unsteady flamelet calculations are presented to investigate the effect of chemistryand turbulent mixing. The flamelet calculations agree qualitatively in several respects to the experiments,especially in the spatial and temporal trends for CH_2O production and consumption. Synthesis of theflamelet and experimental results suggests that for the iso -octane-containing fuels, CH_2O is formed viasingle-stage ignition reactions rather than exhibiting the typical two-stage ignition behaviour which isfound in the pure n -heptane fuel case. Furthermore, the flamelet calculations suggest high-temperatureignition occurs first in lean mixtures in the iso -octane-containing fuel cases, but in rich mixtures for thePRF0 case. If autoignition is the mode of flame stabilisation, this provides an explanation for why thePRF100 and PRF80 cases stabilise further upstream, since lean mixtures have longer residence times, experiencelower scalar dissipation rate, and may be more likely to be exposed to a supporting peripheralreservoir of hot products, should one exist. Overall, this study provides insights into the roles of fuelchemistry and turbulent mixing on the ignition and combustion behaviour of PRFs under engine-relevantconditions.