Autoignition of Liquid hydrocarbon droplets in lean, high pressure natural gas mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase fuel efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are constrained by onset of uncontrolled fast combustion (i.e. engine knock) or incomplete combustion, which results in high unburned hydrocarbon emissions. Detailed computational engine modeling has been shown to capture these effects with reduced chemical kinetic mechanisms but little data are available to validate the chemistry at enginelike conditions. To better validate dual fuel mechanisms, experiments are being conducted in a rapid compression machine in which single n-heptane droplets are suspended and ignited via compression-ignition in a quiescent, high-pressure, high-temperature, lean methane/air environment. Computational modeling has been performed with CONVERGE™ using an 86-species dual-fuel chemical kinetic mechanism developed previously. The simulations capture the ignition event in the vicinity of a spherical n-heptane droplet, which bifurcates into a propagating, premixed methane/air flame and stationary n-heptane/air diffusion flame. Comparisons against experimental measurements of droplet gasification rate, premixed flame propagation speed, and non-premixed flame position will be used to develop revised dual-fuel chemical kinetic mechanisms.