A study of the controlling parameters of fuel air mixture formation for ECN Spray A

Abstract

Designing future ultra-high efficiency, ultra-low emission engines requires an in depth understanding of the multiscale, multi-phase phenomena taking place in the combustion chamber. The performance of the fuel delivery system is key in the air fuel mixture formation and hence the combustion characteristics, however in most spray modelling approaches is not considered directly. Thus, it is important to understand how the selection of models that mimic injection process affect predictions. In this paper we present an Eulerian-Lagrangian framework based on OpenFOAM libraries to model spray injection dynamics. The framework accounts for primary droplet formation (based on a parcel method with predefined initial droplet size distribution), secondary droplet breakup, evaporation and heat transfer. In order to account for the interaction of droplets with turbulence, simulations were performed within the LES context with two different turbulence models. A systematic variation of the key injection parameters (parcel number, parcel size distribution) of the parcel method as well as the grid size was considered. Varying the parcel number affects the initial droplet size distribution which in turn, depending on the selection of the turbulence and the evaporation sub-models, affects: spray dispersion; spray penetration; and subsequent droplet size distribution. Results were validated against the baseline experimental data for evaporating ECN Spray A with n-dodecane chosen as a surrogate for Diesel fuel. Keywords ECN Spray A, Eulerian-Lagrangian, LES, OpenFOAM IntroductionIn both Diesel and spark ignition engines fuel is injected into the combustion chamber at elevated pressures. Nowadays realistic injection pressures for common-rail Diesel reach up to 3500 bars. The reason behind the need for such extreme pressures is to promote primary and secondary atomisation until a combustible mixture is formed. The higher the injection pressure the higher the shear between the static air within the combustion cylinder and the liquid. At extreme ambient pressures and temperatures the fuel can even exhibit supercritical behaviour that causes a reduction in its surface tension. Combustion occurs in a lifted, turbulent diffusion flame mode. Numerous studies indicate that the combustion and emissions in such engines are strongly influenced by the lifted flame characteristics, which are in turn determined by fuel and air mixing in the upstream region of the lifted flame, and consequently by the liquid breakup and spray development processes [1,19,14,5,6]. These processes clearly play a critical role in determining the engine combustion and emission characteristics.

Assessment of LES-STRIP approach for modeling of droplet dispersion in diesel-like sprays

“…An Euler scheme for temporal discretion and a second order scheme for spatial discretization are used in the simulation. The injection and breakup model constants have been selected based on [39,40], where influence of these parameters on the spray results have been reported in detail.…”