Effects of in-cylinder flow simplifications on turbulent mixing at varying injection timings in a piston bowl PPC engine 2019-01-0220
In computational fluid dynamic simulations of partially pre-mixed combustion engines it is common to find simplifi-cations of the in cylinder flow conditions in order to savecomputational cost. One common simplification is to startthe simulation at the moment of intake valve closing withan assumed initial flow condition, rather than making afull scavenging simulation. Another common simplifica-tion is the periodic sector assumption, limiting all sectorcuts of the full cylinder to be identical periodic copies ofeach other. This work studies how such flow simplifica-tions affect the spray injection and in turn the fuel/air mix-ing at different injection timings. Focus is put on the strat-ification of fuel concentration and gas temperature dueto interaction of the spray, turbulence and piston geom-etry. The investigated engine setup consists of a lightduty engine with a piston bowl and a five-hole injector.The simulations are performed under non-reacting con-ditions and utilize the large eddy simulation turbulencemodel. Both full cylinder mesh and sector mesh simula-tions are carried out to evaluate the effects of (a) turbulentvs non-turbulent initial conditions at intake valve closing,(b) sector periodicity assumption on the fuel/air mixing inthe compression stroke under different PPC injection tim-ings. The unresolved scales are modeled using a trans-ported one-equation sub-grid scale (SGS) turbulence clo-sure and the fuel spray is modelled using Lagrangian par-ticle tracking (LPT). The simulations show that simple lam-inar initial conditions produce similar levels of mixing asadvanced initial conditions with tilted swirl and anisotropicturbulence. The influence of sector periodicity restrictioncompared to a full cylinder simulation shows slightly in-creased levels of stratification in terms of temperature andfuel concentration. This difference is shown to depend onturbulent scales at the center of the cylinder inside thepiston bowl and was strongest for the advanced injectiontimings but could also be observed for the earliest injec-tions.
Christian Ibron, Mehdi Jangi, Xue-Song Bai