ECN GDi Spray G : Coupled LES Jet Primary Breakup-Lagrangian Spray Simulation and Comparison with Data

Computational fluid dynamic (CFD) simulation of in-cylinder mixture preparation is an important component of the gasoline direct injection (GDi) engine spray pattern (or targeting) optimization process. A major area of shortcoming in CFD Lagrangian stochastic simulation of GDi spray is the proper account of the jet primary breakup (with regards to the initial droplet size velocity distribution function) due to the substantial influence of nozzle geometry on the primary atomization process. The objective of this study is to assess the predictive capability of the volumeof-fluid large-eddy-simulation (VOF-LES) method for quantitative analysis of the spray primary breakup, so to enable a fully predictive analysis of the complete GDi spray processes. The paper presents results from a VOF-LES analysis of the ECN spray G seat flow and the near-field primary atomization coupled to a Lagrangian stochastics simulation method adopting the discrete droplet model (DDM). The analysis is carried out for a vaporizing n-Heptane spray injection into the atmospheric ambient. The distinction of this case, compared with previous application of the method, is the notable interaction of spray with the counter-bore walls. Hence, the interest is whether the VOF-LES method properly captures the interaction effects on the spray plume primary atomization. The injector internal flow and jet primary breakup simulation is performed with the Open-FOAM software suite. The simulation of the spray processes propagation, secondary atomization, and the droplet-air exchanges are carried out using the AVL-FIRE commercial CFD code. The accuracy of the VOF-LES primary atomization data is inferred from the predictive accuracy of the simulated far-field spray plume trajectory, cone angle, droplet-size distribution and the transient spray-tip penetration versus the experimental data. The comparison shows encouraging agreement. * Corresponding author: bizhan.befrui@delphi.com ILASS Americas 28th Annual Conference on Liquid Atomization and Spray Systems, Dearborn, MI, May 2016 Introduction A major area of complexity in the Lagrangian discrete-droplet simulation of the spray processes is the jet primary atomization and description of the spray initial droplet size-velocity distribution functions [1]. There is increasing recognition of the substantial influence of the injector seat-nozzle geometry – and its fluid dynamic features on the jet primary breakup structure and, thereof, the spray morphology and macroscale parameters [2-4]. In general, mathematical modelling of the processes that contribute to the primary atomization is complex. The current models rely on heuristic and phenomenological concepts [5] whose predictive accuracy cannot be verified. An approach pursued by Delphi is to utilize the VOF-LES method in order to extract quantitative jet primary atomization droplet size-velocity joint probability density functions (pdf) for description of the spray initial conditions in Lagrangian stochastic spray simulations. Assessment of the method for a purposebuilt three-hole GDi seat geometry has shown excellent prediction of the spray temporal propagation and the far-field characteristics [6]. The method requires calibration of the secondary atomization model parameters, with the aid of the spray far-field droplet-size distribution pdf. So, currently the method is not fully ‘predictive’; however the expectation is that, with resolving the primary atomization process, a universal secondary atomization model may emerge. This paper presents application of the method to the Engine Combustion Network (ECN) GDi spray G geometry [7]. What distinguishes this case from the previous application of the method [6] is the geometry of the stepped-nozzle (counter-bored) ECN Spray G seat design. Specifically, the counter-bore diameter is reduced, in order to package the eight stepped-hole nozzles into the seat geometry. As will be shown in this paper, this geometry promotes notable interaction of the atomizing jet – issuing from the flow-hole with the counter-bore walls, which affects the subsequent jet breakup and near-field plume structure (cone angle, etc.). Therefore, the objective of this study is to assess if the VOF-LES method provides quantitatively reliable prediction of the effect of this interaction on the spray primary breakup. In the absence of nozzle near-field measurements, this is inferred from comparison of the predicted spray temporal development and the far-field droplet-size distribution with the experimental data. The VOF-LES study of the injector internal flow and the nozzle near-field jet primary breakup, performed on a contiguous computational domain, is carried out with the Open-FOAM software suite [8]. The Lagrangian spray simulation is performed with the AVL-FIRE commercial CFD code [9]. A WAVE type model [10] is adopted for the droplet secondary atomization, and all spray-air exchanges for a vaporizing spray are accounted for in the Lagrangian simulation. The injection conditions are: n-Heptane fuel (at 20°C), fuel pressure of 100bar and the atmospheric ambient (back-pressure) conditions. The Computational Method A detailed description of the method employed in this study is available in [6], therefore only a brief overview of the method is provided here. The Eulerian VOF-LES Method The VOF-LES mathematical model and the numerical solution method are formally identical to the VOFbased liquid interface tracking method in Open-FOAM [8, 11]. The mathematical model is composed of the governing equations for the conservation of mass and momentum of a two-phase flow system, comprised of two immiscible, incompressible Newtonian fluids. The set of conservation equations that describe the flow of the two-phase mixture are: 0 ) ( = ⋅ ∇ + ∂ ∂ U t ρ ρ (1) ∫ − ⋅ ⋅ + ⋅ ∇ + −∇ = ⊗ ⋅ ∇ + ∂ ∂