The Eulerian-Lagrangian Spray Atomization (ELSA) Model of the Jet Atomization in CFD Simulations: Evaluation and Validation

Abstract

Fuel sprays play a major role in order to achieve the required combustion characteristics and pollutant emissions reduction on internal combustion engines, and thus, an accurate prediction of its behavior is required to perform reliable engine combustion and pollutant simulations. A great effort both on experimental and theoretical studies of spray atomization and dispersion has been performed in the latest years. As a result, ComputationalFluid Dynamics (CFD) calculations have become a standard tool not only for spray physics understanding but also for design and optimization of engine spray systems.However, spray modeling in its different uses in the Internal Combustion Engine (ICE) context is still nowadays a challenging task due to the complex interrelated phenomena taking place, some of them still not fully understood.Primary atomization and secondary breakup, droplet collision, coalescence and vaporization, turbulent interactions between phases have to be solved under high Reynolds (so they are turbulent) and Weber numbers conditions due to the high speed (~500 m/s) and small nozzle diameter (~100 µm) imposed by current engine injection systems technologies. Moreover, Taylor numbers cover a wide range, according to the composition of the injected liquid. Those conditions make experimental observations quite challenging and probably insufficient, especially in the very near nozzle region, where primary atomization takes place.Most of the CFD spray models are currently based on the Discrete Droplet Method. The continuous liquid jet is discretized into 'blobs' or 'parcels', which consists in a number of droplets with the same characteristics. ALagrangian method is applied to track the liquid phase parcels, which are subject to breakup according to atomization models mainly based on the linear instability theory proposed by Reitz and later extended by Huh and Gosman for liquid turbulence effects to be considered. This approach has been successfully Abstract ii applied because it is computationally efficient and at the same time is able to provide reasonable agreements with experimental measurements. However, it requires extensive calibration, being this requirement its main drawback, which is caused, on the one hand, by the semi-empirical nature of the liquid phase sub-models, and, on the other hand, for both physical and numerical limitations on the near nozzle dense sprays region description.In According to the previous statements, the first purpose of this PhD thesis has been to perform a validation of the ELSA spray model implemented in the Star-CD code and check it against CMT-Motores Térmicos Institute database. This validation has been made for several engine conditions (evaporative or non-evaporative, cavitating nozzles, cold starting chamber, etc).In all cases, the ELSA model gave accurate and reasonable results, once a proper mesh is made.After this validation, an extensive evaluation of the model in 3D has been made. Probably, the most important result of the thesis is the coupling between a flow co...

Development of an evaporation model for the dense spray region in Eulerian-Eulerian multiphase flow simulations

“…The axial and radial extensions of such domain are smaller than the experimental chamber ( i.e. 108mm x 108mm in experiments against 100mm x 20mm in present calculations), but it has been shown [15,20] that it should not impact the jet evolution and the atomization process. Furthermore, considering that the focus of the present work is on evaporation modelling, the injector duct has not been included and the diameter has been reduced based on the area contraction coefficient experimentally measured (see Table I).…”

“…Such assumption allows us to neglect the cavitation inside the nozzle that would affect the real velocity profile and to concentrate the study just on the downstream region. As shown in [15,20], where ELSA formulation is used to model the liquid/gas interface for the same test case, a mesh sizing with 10 elements along the injector diameter is able to properly reproduce the main features of the flow-field together with the liquid-air mixing. As suggested in [20], the aspect ratio of the cells comprised in the inlet patch is kept close to 1.0, while they are stretched in radial and axial direction.…”

“…As shown in [15,20], where ELSA formulation is used to model the liquid/gas interface for the same test case, a mesh sizing with 10 elements along the injector diameter is able to properly reproduce the main features of the flow-field together with the liquid-air mixing. As suggested in [20], the aspect ratio of the cells comprised in the inlet patch is kept close to 1.0, while they are stretched in radial and axial direction. Therefore, a structured mesh counting 12500 cells with a size of 0.008 mm at the injector exit was generated.…”

“…Strong attention should be paid on the definition of phase penetration. In technical literature about numerical simulations of the ECN data, several and different definitions on this topic can be found and in the present study, following the work of [20] and [21], the liquid penetration has been evaluated using iso-contour of α l = 0.1%. In this way it has been possible to evaluate a liquid jet penetration of 10.2 mm which nearly corresponds to the experimental one (i.e.…”