Foundamental Study Of Mechanisms Of Fuel Spary Atomization Near The Nozzle Exit In DI Diesel Enignes

Direct injecting (DI) diesel engines are promising for light-duty and heavy-duty vehicle applications due to their higher thermal efficiency and lower carbon dioxide emissions. For diesel engnes, atomization and air-fule mixing is the most important processes. Spray atomization has a crucial influence on the performance of DI diesel engine. Although the mechanisms by which high speed jet distintegrate have been under investigation for many years, the jet flow is too complex to be understood completely, especially the primary breakup near the orifice exit. The study of primary breakup mechanisms in DI diesel engines is of help to deepening the fuel spray atomization mechanism's understanding, enriching and developing the related fundamental theories of fuel spray atomization, therefore has important theoretic signification and engineering practical value.In this thesis large eddy simulation (LES) methodologies was used to study the primary breakup mechanism of spray atomization. The effect of injector configuration parameters on the internel flow of injector nozzle and initial perturbations has been analyzed. Results show that the spray configuration simulated by multiphase LES methodologies was considerable similar to the experimental results, if the injector configurations such as needle,sac and nozzle were considered. The spray atomization near the nozzle exit greatly depends on the internal flow and the initial flow condition near the orifice exit. The boundary layer instability and turbulence are primary factors in perturbations, as the liquid jet leaves the nozzle. While internal flow keeps laminar, the wave length of initial perturbations is determined by the boundary layer instability. While internal flow turbulent, the configurations of initial perturbations are mainly related to turbulence. Moreover, whether the initial perturbations and spray are of axisymmetric depends on injector configuration. The initial perturbations are axisymmetric, when injector configuration is axisymmetric. Otherwise, the initial perturbations and spray are asymmetric. In addition, the effect of the configuration of surface waves on spray atomization has been investigated based on LES methodologies. Results show that initial surface waves with different wave length can superimpose each other, and finally has great inflence on spray atomization. The larger initial swing of surface waves, the faster liquid core disintegration rate and the stronger spray atomization.Based on the analysis of the present spray atomization hypothesis and theories, the experimental conclusions in recent years and conclusions drew by author using LES methodologies in this thesis, a novel understanding of primary breakup mechanisms is brought forward. Then Huh-Gosman model and WAVE model is improved. It suggested that boundary layer instability, turbulence and cavitation are three factors in initial perturbations. Spray atomization processes is divided into primary and sondary breakup. The primary breakup of liquid jet is found to result from growth of initial perturbations induced by above three factors through the Kelvin-Helmholtz instability mechanism, which is driven by the inter-phase aerodynamic interaction. As a result, the crests of these waves break up to form ligaments and droplets, leading to the erosion of liquid core and the formantion of a surrounding droplet cloud which deifnes the spray angle. The sondary breakup of ligaments and droplets formed by primary breakup maybe occur. The effect of injector configuration, injecting pressure and ambient back pressure on internal flow is modeled using Lichtarowicz's model. Lichtarowicz's model can provide several important initial conditions for the primary breakup model like nozzle effective exit flow area, effective exit velocity, discharge coefficient and the radius of an equivalent bubble. In the Huh-Gosman model, the Kelvin-Helmholtz instability theory on an infinite plane for an inviscid liquid is used to calculate the growth time scale of surface waves. The growth time scale of surface waves is derived from the Kelvin-Helmholtz instability theory on an infinite column for an inviscid liquid. Of the two the latter can consider influence of jet diameter on jet instabilities. The rate of change in the liquid core and product droplets is calculated based on energy conservation, which can consider the influence of boundary layer instability, turbulence and cavitation. Because intact liquid core is more similar continuous column compared with sphere, a new method simulating the primary breakup processes based on DDM is put forward in this thesis. Comparisons with experimental results were also performed. Results show that the spray near nozzle region (< 1.0mm) can well predicted using new method. Inclusion of the Rayleigh-Taylor accelerative instabilities in competition with Kelvin-Holmholtz is used to model droplets sondary breakup. Therefore, the hybrid breakup model in this thesis can consider the effect of injector configurations on spary atomization.The fuels spray experiments in constant volume chamber under different ambient back pressure and injecting pressure condition are performed by high-speed photography technology. Based on the spray images, the spray tip penetration, spray near angle (micro-angle) and spray far angle (macro-angle) results were measured. After the whole spray breakup model was imple mented in KIVA3V code, the multi-dimensional numerical simulation of spray atomization has been performed. Analysis of the experimental results the atomization mechanism of this thesis, and comparison between the calculation and experiment has a good agreement. The results show that the theoretical investigation, correlative results and breakup model are reasonable.