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Numerical Simulation of Diesel Sprays Using an Eulerian-Lagrangian Spray and Atomiza- tion Model
| Content Provider | Semantic Scholar |
|---|---|
| Author | Wang, Y. Wang, B. L. Reitz, Rolf D. |
| Copyright Year | 2011 |
| Abstract | An Eulerian-Lagrangian Spray and Atomization (ELSA) model has been applied in the multi-dimensional Computational Fluid Dynamics (CFD) code KIVA-3V to study diesel engine sprays. The model uses an Eulerian approach for the dense liquid core near the injector nozzle exit, where the complicated phase interactions are hard to be quantitatively measured by experiments. It is assumed that the two-phase flow of liquid fuel and ambient gas can be modeled by a unique turbulent flow with its fuel and gas components being distinct species. An Equation of State (EOS) for the two-phase mixture is implemented and validated using classical benchmark cases. A transport equation for the liquid surface area density is solved to predict the sub-grid liquid droplet size. In the downstream region where the spray is dilute, the Eulerian approach shifts to the conventional Lagrangian particle approach. The liquid fuel is then treated with discrete parcels, and the collision and turbulent dispersion models of KIVA are applied. The criterion to shift from Eulerian to Lagrangian is controlled by the local liquid volume fraction of the fuel. As an integral part of this study, the internal nozzle flow is modeled with a Homogeneous Equilibrium Model (HEM). The model assumes that the local phase changes are fast enough and the fuel liquid and vapor are in equilibrium. The mixture density and pressure are related by a modeled speed of sound as a closure to the conservation equations. The outflow information of the nozzle flow simulation is provided to the ELSA spray simulation as inflow boundary conditions. The entire methodology is applied to study diesel sprays under non-evaporating conditions. Good agreement is found when comparing the CFD predictions with available experimental data of liquid spray tip penetrations and local droplet size distributions. Corresponding author, yuewang05@gmail.com Introduction The processes of fuel injection and atomization determine the outcomes of fuel-air mixing and combustion in a diesel engine. Experimentalists have shown that certain injector nozzle geometry parameters such as length-to-diameter ratio and inlet roundedness are crucial to the internal nozzle flow and the primary atomization near the nozzle exit [1]. The optimization of these parameters has become a prerequisite to optimize the spray, combustion and emission outcomes. Despite the recent development of experimental visualization techniques [2], locally quantitative data inside the injector and in the near-nozzle flow field are still lacking. As an alternative approach, multi-dimensional CFD analysis can be used to provide fully quantitative information that is combined with the prediction of the detailed physical processes. In terms of the CFD methodology for diesel sprays, the best-resolved and the most reliable approach is Direct Numerical Simulation (DNS) [3], but its application to engineering problems is limited by its expensive demand of CPU time and computing facilities. As a compromise, the Large-Eddy simulation (LES) method only resolves motions and structures on the large scale (usually the scale of grid size) and leaves the small (sub-grid) scales to modeling. The sub-grid modeling for two-phase flow problems is very challenging unless the grid is sufficiently refined, but in such cases [4-6] the computational time is also considerable and the distinction between LES and DNS is not that clear. Vallet et al. [7] proposed a model based on the Reynolds-Averaged-Navier-Stokes (RANS) approach. The model assumes large-scale similarity between a fuel-air two-phase mixture and a gas jet with density variance. The RANS-based turbulence models for the gas jet have been well-studied and can be readily applied to two-phase flow problems. A similar methodology was reported in the works of Lagumbay et al [8] and Moreau et al [9]. Since all these models assume a homogeneous mixture of fuel and air, a proper Equation of State (EOS) must be applied as closure to the conservation equations. Vallet [7] and Lagumbay [8] assumed the ideal gas law for the gas phase including fuel vapor and air. Moreau [9] assumed a homogeneous equilibrium EOS for the fuel mixture, and the ideal gas law for the air. Vallet [7] and Demoulin [10] studied these models for a water-air co-flow jet case. However, no quantitative validation of the EOS and its effects on the outcome of engine spray simulations was provided. In this study, an Eulerian-Lagrangian Spray and Atomization (ELSA) model was implemented in the KIVA-3V code [11] and applied to diesel sprays injected from a common-rail high-pressure injector under several typical injection and chamber conditions. The EOS of Vallet [7] and Lagumbay [8] were applied and validated with classical benchmark cases. To take into account the nozzle effects, a Homogeneous Equilibrium Model was applied to model the internal nozzle flow [12]. The transient results at the nozzle exit were then coupled with the external spray simulation as inflow boundary conditions using an interpolation procedure. The simulation results of the ELSA modeling were validated with spray visualization and PDPA droplet data under non-evaporating chamber conditions. Nozzle Flow Simulation The nozzle flow is assumed to be described as a single-component, two-phase fluid. Schmidt et al [13] proposed a Homogeneous Equilibrium Model (HEM), which assumes that the phase change between the liquid and vapor is fast enough that the mixture is homogeneous and in equilibrium. Assuming the flow is isothermal, the definition of sound speed gives: 2 dP a dρ = (1) in which the sound speed of the two-phase mixture is modeled as: ( ) 2 2 1 1 1 v l v v l l a a 2 a α α αρ α ρ ρ ρ ⎡ ⎤ − ⎡ ⎤ = + − + ⎢ ⎥ ⎣ ⎦ ⎣ ⎦ with α being the void fraction of the vapor. Integrating Eq. (1) gives a barotropic EOS for the mixture. Lee and Reitz [12] implemented this EOS into the KIVA-3V code and studied cavitation effects in several typical single-hole and multi-hole fuel injectors. In the present study, a 7-hole, axisymmetric high pressure diesel fuel injector is used to generate the spray. The key dimensions are summarized in Table 1. The inlet of the nozzle is hydro-grounded and the crosssectional area of the nozzle is converging (KS=(DinletDexit)/10μm). The measured high discharge coefficient of 0.88 implies that the exit flow is almost uniform and cavitation may not occur inside the nozzle. The entrance rounding radius was not specified by the manufacturer, so a large rounding radius of 70 μm was assumed (R/D=0.5) to be able to match the measured discharge coefficient. Figure 1 shows the discharge coefficient at two injection pressures, 400 bar and 1800 bar. The agreement between the simulation and measurement is very good. Details of the nozzle flow simulation can be found in [14]. Nozzle hole exit diameter 139 μm Nozzle length (Lnoz) 1.025 mm KS factor 1.5 Discharge Coefficient 0.88 Maximum needle lift 0.7 mm Table 1. Key dimensions of the diesel injector |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://www.ilass.org/2/conferencepapers/ILASS2011-189.PDF |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
