Study of soot production for double injections of n-dodecane in CI engine-like conditions

Abstract

Soot production mechanism in multiple injections is complex since it involves its dependence on turbulent interactions of constituting injections and their combustion progress. A concise study was performed in a constant-volume combustion vessel by considering a double injection scheme of 0.3ms pilot injection, 0.5ms dwell time and 1.2ms main injection (nomenclature: 0.3/0.5/1.2ms) with n-dodecane as fuel and replicating the thermodynamic operating condition of a compression ignition (CI) engine. Experimental ambient temperature variations of 900K and 800K were performed at 15% ambient oxygen level. Simultaneous planar laser-induced fluorescence (PLIF) of formaldehyde and schlieren imaging techniques were employed to analyze the ignition and flame characteristics experimentally. These studies revealed almost similar heat release rates for a double injection at 900K and 800K ambient gas temperatures due to combustion of a longer main injection which is enhanced by pilot combustion event. A lower soot production for 800K ambient condition over 900K case was observed, which was concluded to be due to its higher lift-off length which would allow for a leaner combustion of fuel-air mixtures. Numerical simulations were performed using a Large Eddy Simulation (LES) approach by extensively validating the 900K double injection condition with respect to non-reacting vapor penetration profiles of both injections, reacting jet heat release rate and spatial as well as temporal (qualitative) soot production. As part of LES work, a dwell time variation of 0.65ms (0.3/0.65/1.2ms) was performed to reveal the sensitivity of soot production to variations in dwell time. It was observed numerically that marginally higher quasi-steady lift-off length of the 0.3/0.65/1.2ms injection causes increased entrainment of surrounding oxygen into the flame region. This leads to combustion of slightly leaner fuel-air mixture and hence relatively less soot when compared to a 0.3/0.5/1.2ms injection.