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Schlieren/shadowgraphy has been adopted in the combustion research as a standard technique for tip penetration analysis of sprays under diesel-like engine conditions. When dealing with schlieren images of reacting sprays, the combustion process and the subsequent light emission from the soot within the flame have revealed both limitations as well as considerations that deserve further investigation. Seeking for answers to such concerns, the current work reports an experimental study with this imaging technique where, besides spatial filtering at the Fourier plane, both short exposure time and chromatic filtering were performed to improve the resulting schlieren image, as well as the reliability of the subsequent tip penetration measurement. The proposed methodology has reduced uncertainties caused by artificial pixel saturation (blooming).

The objective of the paper is the characterization of the macroscopic behavior of Diesel sprays by focusing in at the first instants of the injection process at which the spray is clearly affected by the injector needle dynamic. There are several works dealing with the characterization of Diesel sprays in stationary conditions. Most of them conclude with empirical correlations which predict spray tip penetration as a function of the most important parameters involved in the injection process, such as: injection pressure, gas ambient density, hole diameter and time elapsed from the start of injection. In all these experiments, authors find similar power law dependencies with more or less high level of confidence. Nevertheless, few works have tried to validate or to obtain new correlations for the first instants of the injection process where the spray develops in not stationary conditions because of the influence of injector needle lift.

In the present work a CFD analysis of formation and spray evolution emerging from a Multi-holes-Common Rail Diesel injector under non-evaporative conditions has been carried out. The aim of the work was to set up a tool for fuel spray simulation that could offer reasonable accuracy for the prediction of the spray tip penetration, droplet size and a reduction of CPU time. For this purpose the influence of different primary and secondary break-up models as well as drop interaction has been investigated. Phenomenological relationships have also been implemented in the code in order to enhance the prediction of the stable diameter inside the break-up models, allowing the mean drop size to be better predicted and a reduction of the time necessary to set-up the model.

Starting and operating of diesel engines in cold conditions is a common and important problem. Many factors such as ambient conditions, fuel properties, fuel injection, cranking speed, etc, affect cold engine functionality. In order to improve diesel engine cold start, it is essential to understand better these problems. In this paper the injection development at cold temperatures is studied, since it is an important parameter that affects the fuel interaction with the air, so the future combustion process would also be influenced. In particular, a hydraulic characterization of diesel injection is made, using specialized test rigs that simulate real engine in-cylinder air pressure and density; the fuel is injected from three axi-symmetric convergent nozzles at several injection pressures (30, 50, 80, 120 and 180 MPa), two chamber densities and two temperatures of 255 K (winter) and 298 K (reference).

In this paper, a special technique for visualizing the first 1.5 millimetres of the spray has been applied to examine the link between cavitation phenomenon inside the nozzle and spray behaviour in the near nozzle field. For this purpose, a real Diesel axi-symmetric nozzle has been analyzed. Firstly, the nozzle has been geometrically and hydraulically characterized. Mass flow measurements at stationary conditions have allowed the detection of the pressure conditions for mass flow choking, usually related with cavitation inception in the literature. Nevertheless, with the objective to get a deeper knowledge of cavitation phenomenon, near nozzle field visualization technique has been used to detect cavitation bubbles injected in a pressurized chamber filled with Diesel fuel. Using backlight illumination, the differences in terms of density and refractive index allowed the distinction between vapour and liquid fuel phases.

In the wake of the Turbulent Nonpremixed Flames group (TNF) for atmospheric pressure flames, an open group of laboratories belonging to the Engine Combustion Network (ECN) agreed on a list of boundary conditions -called “Spray A”- to study the free diesel spray under steady-state conditions. Such conditions are relevant of a diesel engine operating at low temperature combustion conditions with moderate EGR, small nozzle and high injection pressure. The objective of this program is to accelerate the understanding of diesel flames, by applying each laboratory's knowledge and skills to a specific set of boundary conditions, in order to give an extensive and reliable experimental database to help spray modeling. In the present work, “Spray A” operating condition has been achieved in a constant pressure, continuous flow vessel. Schlieren high-speed imaging has been conducted to measure the spray penetration under evaporative conditions.

It is known that one of the main parameters that govern the spray penetration development is spray momentum flux. In this paper, a model capable to predict the development of the spray penetration using as an input the temporal variation of the spray momentum flux is presented. The model is based on the division of the momentum flux signal in momentum packets sequentially injected and the tracking of them inside and at the tip of the spray. These packets follow a theoretical equation which relates the penetration with the ambient density, momentum and time. In order to validate the method, measures of momentum flux (impingement force) and macroscopic spray visualization in high density conditions have been performed on several mono-orifice nozzles. High agreement has been obtained between spray penetration prediction from momentum flux measurements and real spray penetration from macroscopic visualization.

In order to study differences between Diesel nozzle holes, four methodologies have been tested. The techniques compared in this paper are: the internal geometry determination, hole to hole mass flow measurement, spray momentum flux and macroscopic spray visualization. The first one is capable of obtaining the internal geometry of each of the orifice of the nozzle; the second one is capable of measuring the mass flow of each nozzle hole in both, continuous and real injections. The third one gives the momentum flux of each orifice, and finally, with the macroscopic spray visualization, the spray penetration and spray cone angle of each hole, are obtained. Generally, all these techniques can be used in order to determine the hole to hole dispersion due to different angle inclination of the holes, different internal geometry of orifices, deposits, nozzle needle off-center, needle deflection, etc.

The temperature of fuel injectors can affect the flow inside nozzles and the subsequent spray and liquid films on the injector tips. These processes are known to impact fuel mixing, combustion and the formation of deposits that can cause engines to go off calibration. However, there is a lack of experimental data for the transient evolution of nozzle temperature throughout engine cycles and the effect of operating conditions on injector tip temperature. Although some measurements of engine surface temperature exist, they have relatively low temporal resolutions and cannot be applied to production injectors due to the requirement for a specialist coating which can interfere with the orifice geometry. To address this knowledge gap, we have developed a high-speed infrared imaging approach to measure the temperature of the nozzle surface inside an optical diesel engine.

Experimental visualization of current gasoline direct injection (GDi) systems are even more complicated especially due to the proximity of spray plumes and the interaction between them. Computational simulations may provide additional information to understand the complex phenomena taking place during the injection process. Nozzle flow simulations with a Volume-of-Fluid (VOF) approach can be used not only to analyze the flow inside the nozzle, but also the first 2-5 mm of the spray. A methodology to obtain plume direction and spray angle from the simulations is presented. Results are compared to experimental data available in the literature. It is shown that plume direction is well captured by the model, whilst the uncertainty of the spray angle measurements does not allow to clearly validate the developed methodology.