Evaporation and Autoignition Studies of Liquid n-Alkane Droplets in Lean, High Pressure Methane/Air Mixtures in a Rapid Compression Machine

摘要

The combustion of two fuels with disparate reactivity (e.g. natural gas/diesel) in conventional diesel engines has been demonstrated as a means to increase fuel efficiency, reduce fuel costs and reduce pollutant formation compared to standard diesel operation. To maximize the economic benefits, it is desirable to maximize the natural gas substitution rate. However, the natural gas substation rate is constrained by onset of uncontrolled fast combustion (i.e. engine knock) and/or incomplete combustion, which results in high unburned hydrocarbon emissions. Previous detailed computational engine modeling with reduced chemical kinetics and simplified spray models have captured these effects but little data are available to validate chemistry and spray models at these engine-like conditions. In this study, a rapid compression machine (RCM) was used as a platform to provide a high-temperature/high-pressure environment to better understand the thermodynamic, transport and chemical kinetic phenomena of dual-fuel combustion. The RCM was modified to perform evaporation and combustion experiments on single n-alkane fuel droplets in gaseous inert, O_2/inert and O_2/CH_4/inert environments. Droplet evaporation experiments were performed on C_5 to C_(12) n-alkane droplets in inert gas to measure droplet evaporation rates at near supercritical and supercritical conditions (18 bar < P < 35 bar; 450 K < T < 850 K). Droplet ignition experiments were conducted to measure ignition delay as a function of temperature, pressure and ambient CH_4 concentration.