Turbulence and residual gas effects in pulsed diesel jets
Abstract
Diesel fuel injection strategies which involve multiple injections are currently used to reduce engine emissions. With multiple injections, combustion and pollutant formation may be influenced by interaction between fuel pulses, which is the focus of this study. Specifically, the interest is in two aspects of that interaction: turbulence effects and residual gas effects. Mixing and entrainment characteristics of a fuel pulse can be affected by turbulence generated from prior injections. This impact on mixing would have a corresponding influence on combustion characteristics. Combustion of one fuel pulse can also be affected by interaction with regions of residual product gases from prior pulses. Residual gas effects on combustion in Diesel jets are studied using Reynolds-averaged Navier-Stokes (RANS) and direct numerical simulations (DNS) with multiple-step chemical mechanisms. Findings indicate that dilution and thermal effects of residual gases are dominant, relative to the chemical effects, and can be represented using simple chemistry models. Chemical effects of residual gases are less important, but accurate representation of their influence on ignition characteristics and soot formation and oxidation requires detailed models. Residual gas effects associated with the interaction of multiple fuel pulses in Diesel combustion can improve ignition timing, flame stability, and emissions characteristics in later injection events. Large-eddy simulations (LES) of Diesel jets are used for comparison with RANS models in representing turbulence effects associated with near-field jet interactions. RANS models do not predict mixing enhancement due to turbulence generated by prior injection pulses. Rather, a large decrease in the generation of turbulent kinetic energy in subsequent pulses is observed. This decrease and the mean flow field generated by the prior injection accelerate the jet penetration. LES shows residual turbulence effects on the breakup of the head vortex and subsequent spreading and penetration of the jet. These effects are dependent on the dwell time between pulses. Consequently the effect of dwell time and the impact of pulse interaction on the coherent structures at the leading edge of a jet pulse can be investigated with LES.
Degree
Ph.D.
Advisors
Abraham, Purdue University.
Subject Area
Mechanical engineering
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