J. Gimeno

Measurements of Spray Momentum for the Study of Cavitation in Diesel Injection Nozzles

In Diesel injection Systems, cavitation often appears in the injection nozzle holes. This paper analyses how cavitation affects the Diesel spray behavior. For this purpose two spray parameters, mass flux and momentum flux, have been measured at different pressure. We know that cavitation brings about the mass flux choke, but there are few studies about how the cavitation affects the momentum and the outlet velocity. The key of this study is just the measurement of the spray momentum under cavitation conditions.

Determination of Diesel sprays characteristics in real engine in-cylinder air density and pressure conditions

The present paper centers on the establishment of a quantified relationship between the macroscopic visual parameters of a Diesel spray and its most influential factors. The factors considered are the ambient gas density, as an external condition relative to the injection system, and nozzle hole diameter and injection pressure as internal ones. The main purpose of this work is to validate and extend the different correlations available in the literature to the present state of the Diesel engine, i.e. high injection pressure, small nozzle holes, severe cavitating conditions, etc. Five mono-orifice, axi-symmetrical nozzles with different diameters have been studied in two different test rigs from which one can reproduce solely the real engine in-cylinder air density, and the other, both the density and the pressure. A parametric study was carried out and it enabled the spray tip penetration to be expressed as a function of nozzle hole diameter, injection pressure and environment gas density. The temporal synchronization of the penetration and injection rate data revealed a possible explanation for the discontinuity observed as well by other authors in the spray’s penetration law. The experimental results obtained from both test rigs have shown good agreement with the theoretical analysis. There have been observed small but consistent differences between the two test rigs regarding the spray penetration and cone angle, and thus an analysis of the possible causes for these differences has also been included.

A Study of the Relation Between Nozzle Geometry, Internal flow and Sprays Characteristics in Diesel Fuel Injection Systems

This study examines the influence of geometry on the internal flow and macroscopic behavior of the spray in Diesel nozzles. For this investigation, two bi-orifice nozzles were employed : one cylindrical and one conical. The first step is to use a non-destructive characterization method which is based on the production of silicone moulds so that the precise internal geometry of the two nozzles can be measured. At this stage the nozzles have been characterized dimensionally and therefore the internal flow can be studied using CFD calculations. The results gained from this experiment make it possible also to ascertain the critical cavitation conditions. Once the critical cavitation conditions have been identified, the macroscopic parameters of the spray can be studied in both cavitating and non-cavitating conditions using a test rig pressurized with nitrogen and with the help of a image acquisition system and image processing software. Consequently, research can be carried out to determine the influence that cavitation has on macroscopic spray behavior. From the point of view of the spray macroscopic behavior, the main conclusion of the paper is that cavitation leads to an increment of the spray cone angle. On the other hand, from the point of view of the internal flow, the hole outlet velocity increases when cavitation appears. This phenomenon can be explained by the reduction in the cross section of the liquid phase in the outlet section of the hole.

Flow regime effects over non-cavitating diesel injection nozzles

The research conducted and explained in this paper aims to explore and understand the influence of flow regime (laminar, transition or turbulent) inside diesel injector nozzles. For this purpose, an experimental study based on mass flow rate and momentum flux measurements on three convergent nozzles has been carried out. The combination of both types of measurement has been helpful to obtain information about the nature of the flow and its consequences on important variables, such as injection effective velocity and effective area of nozzle outlet orifices. As a main result of the investigation, and depending of the flow regime, a differentiated behaviour has been observed which was clearly reflected in the non-dimensional flow parameters defined and used through the study.

Effect of fuel properties on diesel spray development in extreme cold conditions

Abstract This paper presents an experimental and theoretical study with the purpose of quantifying and predicting the influence of fuel properties in extreme cold conditions on macroscopic diesel spray behaviour. The parameters studied were the fuel density and viscosity at different temperatures. Previous correlations available in the literature were analysed and extended in order to introduce the viscous term. A specific test rig that simulates the real engine in-cylinder air pressure and density was used. The fuel was injected from a convergent nozzle with eight holes at two injection pressures of 30MPa and 120MPa, two chamber densities of 23kg/m3 and 56kg/m3, and two temperatures of 255K (winter) and 298K (reference). In order to achieve cold conditions, the facility was introduced into a specialized climatic room with optical access, where the temperature and humidity were carefully controlled. The experimental results enabled the theoretical analysis that related the viscosity effect with the spray-tip penetration to be validated, obtaining high accuracy. The main conclusion of the paper is that fuel properties change significantly with the temperature in cold conditions, which affects the spray development, producing a shorter penetration in the transition stage. This phenomenon can be explained by a decrease in the fluid discharge coefficient in the outlet section of the hole as a result of the change in the fluid properties with the temperature.

On the dependence of spray momentum flux in spray penetration: Momentum flux packets penetration model

Momentum flux is a very important parameter for predicting the mixing potential of injection processes. Important factors such as spray penetration, spray cone angle, and air entrainment depend largely on spray momentum. In this article, a model is obtained which is able to predict the spray tip penetration using as an input the spray momentum flux signal. The model is based on the division of the momentum flux signal into momentum packets (fuel parcels) sequentially injected, and the tracking of them along the spray. These packets follow a theoretical equation which relates the penetration with the ambient density, momentum, spray cone angle 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.

The potential of Large Eddy Simulation (LES) code for the modeling of flow in diesel injectors

This work deals with numerical simulations of the internal flow in diesel injectors, evaluating the skills of the Large Eddy Simulation (LES) code for capturing the turbulent patterns present in the flow. In order to evaluate the potential of the LES code, the results of the simulations have been compared to standard numerical RANS method results, and simultaneously validated with experimental values and DNS data. The experiments were made using special diesel injector geometry, reaching a high injection pressure condition and so ensuring a turbulent regime. In general, results show good agreement with the experimental values and DNS data. It was found that LES is capable of reproducing the turbulent structures in diesel nozzles, and as expected is more accurate than RANS, mainly in the boundary layer.

Complete modelling of a piezo actuator last-generation injector for diesel injection systems:

An experimental and computational study of an increasingly used third-generation common-rail injection system with a piezo actuator has been carried out. A complete characterization of the different elements of the system, both geometrically and hydraulically, has been performed in order to describe its behaviour. The information obtained through the characterization has been used to create a one-dimensional model that has been implemented in the commercial software AMESim and extensively validated against experimental data. The results of the validation demonstrate the model ability to predict the injection rate of the injector with a high level of accuracy, therefore, constituting a powerful tool in order to carry out further studies of this type of injection system.

Comparison between different hole to hole measurement techniques in a diesel injection nozzle

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. Moreover, the results obtained from theses techniques are a very useful tool in order to study the injection process. Special attention deserves the new capability developed to carry out the hole to hole mass flow measurement. In fact, although Diesel nozzle mass flow measurement, either injection rate or continuous mass flow, is a technique widely used, it has the disadvantage that the complete nozzle mass flow is characterized without a distinction between the mass flow from each of the orifices. Here, a new test rig is presented in order to achieve this kind of measurements.

Experimental analysis on the influence of nozzle geometry over the dispersion of liquid n-dodecane sprays

Understanding and controlling mixing and combustion processes is fundamental in order to face the challenges set by the ever more demanding pollutant regulations and fuel consumption standards of direct injection diesel engines. The fundamentals of these processes haven been long studied by the diesel spray community from both experimental and numerical perspectives. However, certain topics such as the influence of nozzle geometry over the spray atomization, mixing and combustion process are still not completely well understood and predicted by numerical models. The present study seeks to contribute to the current understanding of this subject, by performing state-of-the-art optical diagnostics to liquid sprays injected through two singe-hole nozzles of different conicity. The experiments were carried out in a nitrogen-filled constant-pressure-flow facility. Back pressures were set to produce the desired engine-like density conditions in the chamber, at room temperature. The experimental setup consists in a diffused back illumination setup with a fast pulsed LED light source and a high-speed camera. The diagnostics focused on detecting the liquid spray contour and evaluating the influence of nozzle geometry over the time-resolved and quasi-steady response of the spray dispersion, at similar injection conditions. Results show a clear influence of nozzle geometry on spray contour fluctuations, where the cylindrical nozzle seems to produce larger dispersion in both time-resolved fluctuations and quasi-steady values, when compared to the conical nozzle. This evidences that the turbulence and radial velocity profiles originated at the cylindrical nozzle geometry are able to affect not only the microscopic scales inside the nozzle, but also macroscopic scales such as the steady spray. Observations from this study indicate that the effects of the flow characteristics within the nozzle are carried on to the first millimeters of the spray, in which the rest of the spray formation downstream is pre-defined.