Jet-jet interaction in multiple injections: A large-eddy simulation study

Abstract

This paper reports on studies of multiple-injection strategies of gaseous fuel in a model combustion chamber and the role of jet-jet interactions on the mixing processes in the chamber using large-eddy simulation (LES). A highpressure non-reacting gas flow injected through a jet with a nozzle diameter of 1.35 mm into a quiescent inert air environment is considered. First, we validate the method and our computational setup by comparing the simulation results of a single injection case with available experimental data. It is shown that the transient ensemble-averaged LES results agree well with the experimental measurements. Second, we simulate and compare fourteen injection strategies in order to understand the effect of the main and the post-injections duration, the dwell time and the mass flow rate of post-injection on the mixing, jet penetration, and near-nozzle mixture. The contribution of each injection in the local mixture composition is quantified by solving transport equations for the mixture fraction of each injection.The results show that the turbulence generated in the main injection is enhanced when the post-injection flow into the main injection flow. The increase of the local turbulence intensity is in favor of increasing the scalar dissipation rate and enhancing the mixing rate. However, the penetration of the post-injection flow into the main injection flow and the level of the gas flow from the interaction of two injections depend on the dwell time and the momentum of the post-injection.The results also show that the post-injection modifies the near-nozzle mixture. The comparison of cases with different mass flow rates in the post-injection indicates that the momentum of the post-injection can be optimized either to push away the near-nozzle remaining gas from the main injection and reduce the near-nozzle residue by more than 25% or enrich this fuel-lean region and increase the near-nozzle gasses by more than 43%. These results are very interesting for optimization of the post-injection to reduce engine-out emissions. Highlights• A high-momentum short post-injection reduces the near-nozzle residue gases.• A low-momentum short post-injection enriches the near-nozzle over-lean mixture.• The longer the dwell time, the lower the peak mass of the fuel-rich mixture.• The cases with shorter dwell time and longer post-injection have more significant jet-jet interaction.

Impact of a split injection strategy on mixing, ignition and combustion behavior in Premixed charge compression ignition combustion

“…1. Previous experimental and numerical studies of closely coupled double injections under constant ambient conditions showed an increased penetration rate of the subsequent fuel spray caused by the persistent momentum of the preceding jet [20,21,47]. Under transient RCEM conditions, however, the aerodynamic gain from the preceding injection is counteracted by the density rise during the compression stroke, eventually leading to shorter penetrations of the consecutive jet.…”

The role of a split injection strategy in the mixture formation and combustion of diesel spray: A large-eddy simulation

“…The role of timing and other parameters on fuel/air mixing is studied in detail in Refs. [11,12]. Figure 5: The solid lines are results from the double injection case and the dash-dotted lines are from the single injection case.…”

“…In lights of this, the focus of the current study is on the understanding of mechanisms by which a split injection changes the combustion of a diesel spray. The suggested explanations for the effects of split injection in the literature can be categorized into three main mechanims [8]: (i) the split injection enhances the mixing and it reduces the local equivalence ratio and improves premixing before the start of high-temperature combustion [11][12][13][14][15]. In this work, we call this mechanism the equivalence ratio effects, and it will act in favor of reducing the rate of soot formation.…”

LES study of diesel flame/wall interaction and mixing mechanisms at different wall distances

“…Diesel engines have been widely studied during the last two decades in order to lower the emissions of soot, while improving efficiency. Apart from the after-treatment systems, the reduction of soot emissions in diesel engines can be achieved using a number of in-cylinder methods, e.g., high injection pressure, multiple injection strategies, or exhaust gas recirculation (EGR) [1][2][3]. These techniques can reduce the soot emissions either by improving the mixing process and thus enhancing the soot oxidation, or by altering the combustion temperature and therefore the soot formation.…”