M. Carreres

Large-eddy simulation analysis of the influence of the needle lift on the cavitation in diesel injector nozzles:

The cavitation phenomenon has a strong influence on the internal flow and spray development in diesel injector nozzles. Despite its importance, there are many aspects which still remain unclear, especially for partial needle lifts when the injector is in the opening and closing phases. For that reason, the current paper is focused on the influence of the needle lift on the internal flow in a diesel nozzle. This study was carried out with three-dimensional simulations at a high injection pressure (160 MPa) using a homogeneous equilibrium model implemented in OpenFOAM to model the cavitation phenomenon. The nozzle was simulated with large-eddy simulation methods at six different needle lifts (10 μm, 30 μm, 50 μm, 75 μm, 100 μm and 250 μm), providing relevant information about the evolution of the internal flow, the turbulence development (the vorticity, the turbulence–cavitation interaction and the turbulent structures) and the flow characteristics in the nozzle outlet (the mass flow, the momentum flux and the effective velocity) with the needle position.

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.

On the relation between the external structure and the internal characteristics in the near-nozzle field of diesel sprays

In this paper, a high-resolution visualization technique was used in combination with an extensively validated zero-dimensional model in order to relate the external structure of a diesel spray to the internal properties in the vicinity of the nozzle. For this purpose, three single-hole convergent nozzles with different diameters were tested for several pressure conditions. The analysis of the obtained images shows that the spray width significantly changes along the first few millimetres of the spray. From the high-resolution images obtained, two parameters were evaluated. The first is the external non-perturbed length, where droplet detachment was not observed. The second is a transitional length, which is defined as the axial position where the spray width increases linearly after transient behaviour, making it possible to establish a spray cone angle definition. Furthermore, the internal liquid core length was estimated for these nozzles using an extensively validated zero-dimensional model. The liquid core length proved to be correlated with both the transitional length and the non-perturbed length with a very high degree of reliability. In the case of the transitional length, a quadratic correlation was observed, whereas a linear relationship was confirmed between the liquid core length and the non-perturbed length. The results presented here may help to shed light on better understanding of such a complex process as atomization.

Determination of critical operating and geometrical parameters in diesel injectors through one dimensional modelling, design of experiments and an analysis of variance

In this paper, a design of experiments and a statistical analysis of variance (ANOVA) are performed to determine the parameters that have more influence on the mass flow rate profile in diesel injectors. The study has been carried out using a one dimensional model previously implemented by the authors. The investigation is split into two different parts. First, the analysis is focused on functional parameters such as the injection and discharge pressures, the energizing time and the fuel temperature. In the second part, the influence of 37 geometrical parameters, such as the diameters of hydraulic lines, calibrated orifices and internal volumes, among others, are analysed. The objective of the study is to quantify the impact of small variations in the nominal value of these parameters on the injection rate profile for a given injector operating condition. In the case of the functional parameters, these small variations may be attributed to possible undesired fluctuations in the conditions that the injector is submitted to. As far as the geometrical and flow parameters are concerned, the small variations studied are representative of manufacturing tolerances that could influence the injected mass flow rate. As a result, it has been noticed that the configuration of the inlet and outlet orifices of the control volume, together with the discharge coefficient of the inlet orifice, among a few others, play a remarkable role in the injector performance. The reason resides in the fact that they are in charge of controlling the behaviour of the pressure in the control volume, which importantly influences injector dynamics and therefore the injection process. Variations of only 5% in the diameter of these orifices strongly modify the shape of the rate of injection curve, influencing both the injection delay and the duration of the injection process, consequently changing the total mass delivered.

Numerical analysis of flow characteristics in diesel injector nozzles with convergent-divergent orifices

The geometry of diesel injector nozzles is known to significantly affect the characteristic spray behavior and emissions formation. In this paper, a novel nozzle concept, consisting of orifices with a convergent–divergent shape, is investigated through Computational Fluid Dynamics techniques. Three of these nozzles, characterized by different degrees of conicity, are compared to a nozzle with cylindrical orifices, which acts as a baseline. A homogeneous equilibrium model, validated against experimental data in previous works by the authors, is used to calculate the eventual cavitation formation inside these orifices. Additionally, the characteristics of the flow at the orifice outlet are analyzed for the four aforementioned nozzles in terms of their steady-state mass flow, effective outlet velocity and area coefficient. The results show that convergent-divergent nozzles exhibit a high cavitation intensity, located in the transition between the convergent and the divergent sections. This high cavitation intensity tends to compensate for the expected velocity decrease induced by the divergent shape, producing effective velocity values similar to those achieved by the cylindrical nozzle in many of the simulated conditions. The characteristics of the flow, together with the higher spray opening angles expected due to the divergent section of the nozzle, may improve atomization and fuel-air mixing processes.

Effect of the nozzle holder on injected fuel temperature for experimental test rigs and its influence on diesel sprays

One of the test boundary conditions whose control is necessary for both experimental and numerical studies about the automotive engines research field is the temperature of the fuel inside the injector body during the injection. However, it is a difficult parameter to be directly measured in a non-intrusive way due to the injector architecture and the nature of the standard experiments that are done to characterize sprays. An experimental analysis is performed in this work employing a continuous flow test chamber, normally used in the optical diagnosis of diesel sprays, in order to compare and characterize two different designs for the nozzle holder of the test rig. The first one consisted in an aluminum holder that coats the nozzle until its tip, while the second one is made of steel and only supports the nozzle without covering it from the environment. The employed methodology was to set the test rig at a wide range of thermodynamic conditions inside the vessel such as ambient temperature and density and also the coolant temperature at the outlet of the injector casing for both cooling pieces. In this case, a dummy injector provided with a thermocouple was used to measure the tip temperature. In this way, correlations are obtained to estimate the injector body temperature out of the measurement points. Further experiments with a real single-hole diesel injector, controlling its tip temperature according to the previous results, are also discussed in this article in order to analyze the effect of this parameter on the spray evaporation process. Liquid length at inert conditions and both ignition delay and lift-off length at reactive ones were measured employing the same test vessel. All those parameters showed to be shortened with an increment of injected fuel temperature, and the lower the ambient temperature, the stronger this influence is.

Comparison of Different Techniques for Characterizing the Diesel Injector Internal Dimensions

The geometry of certain parts of diesel injectors is key to the injection, atomization and fuel-air mixing phenomena. Small variations on the geometrical parameters may have a strong influence on the aforementioned processes. Thus, OEMs need to assess their manufacturing tolerances, whereas researchers in the field (both experimentalists and modelers) rely on the accuracy of a certain metrology technique for their studies. In the current paper, an investigation of the capability of different experimental techniques to determine the geometry of a modern diesel fuel injector has been performed. For this purpose, three main elements of the injector have been evaluated: the control volume inlet and outlet orifices, together with the nozzle orifices. While the direct observation of the samples through an optical microscope is only possible for the simplest pieces, both Computed Tomography Scanning and the visualization of silicone molds technique have proven their ability to characterize the most complex internal shapes corresponding to the internal injector elements. Indeed, results indicate that the differences observed among these methodologies for the determination of the control volume inlet orifice diameter and the nozzle orifice dimensions are smaller than the uncertainties related to the experimental techniques, showing that they are both equally accurate. This implies that the choice of a given technique for the particular application of determining the geometry of diesel injectors can be done on the basis of availability, intrusion and costs, rather than on its accuracy.