Frederic Ravet

Evaluation of the Eulerian–Lagrangian Spray Atomization (ELSA) model in spray simulations: 2D cases

Abstract The aim of this paper is the evaluation and validation of the Eulerian–Lagrangian Spray Atomization (ELSA) model implemented in a CFD code by Renault. ELSA is an integrated model for capturing the whole spray evolution, in particular including primary break-up and secondary atomization. Two-dimensional simulations have been performed during the study, which is in fact enough to capture some of the main features of the spray, such as the spray penetration and the axial velocity. A mesh independence study has also been carried out in order to characterize the lowest mesh size that can be used to correctly characterize the spray. Furthermore, the two-equation k – e turbulence model has been adjusted by changing some of the parameters of the dissipation rate transport equation in order to accurately characterize the spray. Finally some analyses of the results obtained, in terms of penetration, liquid mass fraction and droplet number and size, are presented in the last section of the paper.

Evaluation of the Eulerian-Lagrangian spray atomisation (ELSA) in spray simulations

Many approaches have been used to simulate the spray structure especially in modelling fuel sprays, i.e., Eulerian, Lagrangian, Lagrangian- Eulerian, Eulerian-Eulerian and Eulerian-Lagrangian approaches. The present study uses an Eulerian-Lagrangian spray atomisation (ELSA) method which is an integrated model for capturing the whole spray evolution starting directly from injector nozzle still the end. Our goal in this study is to evaluate the ELSA model which is implementing into the commercial software Star-CD, for numerical modelling of diesel sprays. There are two key studies in these validations, at first we examine the turbulent parameters through the three different scenarios and then we study mesh dependency. The results show in form of liquid penetrations, droplet velocity, and axial velocity profiles. All numerical results are compared with experimental data from our research institute, CMT-Motores Termicos.

Experimental and Numerical Analysis of Diluted Combustion in a Direct Injection CNG Engine Featuring Post- Euro-VI Fuel Consumption Targets

The present paper is concerned with part of the work performed by Renault, IFPEN and Politecnico di Torino within a research project founded by the European Commission. The project has been focused on the development of a dedicated CNG engine featuring a 25% decrease in fuel consumption with respect to an equivalent Diesel engine with the same performance targets. To that end, different technologies were implemented and optimized in the engine, namely, direct injection, variable valve timing, LP EGR with advanced turbocharging, and diluted combustion. With specific reference to diluted combustion, it is rather well established for gasoline engines whereas it still poses several critical issues for CNG ones, mainly due to the lower exhaust temperatures. Moreover, dilution is accompanied by a decrease in the laminar burning speed of the unburned mixture and this generally leads to a detriment in combustion efficiency and stability. The optimization of in-cylinder turbulence plays a fundamental role in compensating this trend. The present paper is specifically focused on the characterization of the diluted combustion in the direct injection engine. The results of an experimental activity have been presented, aimed at characterizing the in-cylinder combustion process and the exhaust temperatures at 2000 rpm and variable load, both without dilution and with 20% of external EGR. At the same time, a 3-D numerical model for the incylinder turbulence and combustion simulation has been developed in Converge. The model embeds a user-specified laminar-flame speed submodel, which was derived from a 1-D combustion simulation model with detailed chemistry. The model has been calibrated against experimental data and then used to characterize the heat release dependence on the dilution. The experimental activity has evidenced the potential of EGR to increase the engine efficiency, by allowing to increase the boost level at full load and by reducing pumping losses at partial load. As far as the maximum allowed EGR rate is concerned, the CFD activity showed that the limit can be detected on the basis of a threshold value of the MFB0-50 interval. At 2000 rpm and medium load the maximum EGR rate ranged around 35% and showed an increasing trend versus load. It also demonstrated a decreasing trend against the engine speed. Introduction and present work Natural gas (NG) is considered one of the most promising alternative fuels, which can effectively cope with the reduction of the carbon footprint and emissions of vehicles, with respect to conventional IC engines [1,2]. As a matter of fact, methane is the primary constituent of NG. It has the highest molecular hydrogen-to-carbon ratio of all hydrocarbons, which allows a potential of 20% CO2 reduction as compared to the long-chained molecules found in Diesel oil and gasoline. Another attractive chemical property of methane is its research octane number of 130 which is considerably higher than that of gasoline. This indicates high knock resistance as a promising enabling factor for extracting more power from a given engine displacement and therefore increasing engine specific power. NG engines operating with high compression ratio and boost pressure level as well as lean-burn approach or heavy exhaust gas recirculation could be equivalent to their gasoline counterparts in terms of torque and power, allowing also a substantial reduction in pollutant emissions and improvement in engine thermal efficiency [1,3,4]. The objective of the project Gason (“Gas-Only internal combustion engines”, H2020 innovation action of the European Commission) is to design an engine for passenger cars or small duty vehicles applications and to demonstrate the potential of CNG fuels in a dedicated design [5]. The work presented in this paper was carried out by Renault, IFPEN and Politecnico di Torino within the Gason WP4, “Charge dilution (lean burn, internal and / or external EGR) and exhaust-gas temperature management”. Within this context the activity was aimed at studying the impact of the charge dilution with EGR on the combustion process. The attractiveness of diluted combustion is connected to its potential to improve the global efficiency of the engine [6]. Although this technology is relatively new for CNG engines, a few interesting works can be found in the literature [7-11]. Zoldak and Naber [7] performed an extensive experimental analysis of diluted combustion in a HD CNG engine. They found a good combustion stability up to EGR=20%, and a maximum indicated gross thermal efficiency of 45% was achieved. McTaggart-Cowan et al. [8] analyzed the influence of fuel composition on EGR tolerance and found that the influence of the percentage range of the lower hydrocarbon molecules is relatively small. Reduction in ignition delay at constant combustion phasing leads to lower peak heat release rates. However, these changes are not large enough to significantly impair engine operations. Morteza Fathi et al. [9] investigated the HCCI combustion of a n-eptane/CNG blend and used EGR rate to control autoignition time, flame-propagation speed and peak-pressure rise rate. These findings are useful also in view of EGR adoption in a more conventional spark-ignited engine. Despite the already mentioned high resistance of CNG to abnormal combustion, the current limitation to the achievable boost level is the maximum

Three-dimensional computational fluid dynamics engine knock prediction and evaluation based on detailed chemistry and detonation theory:

Engine knock is an important phenomenon that needs consideration in the development of gasoline-fueled engines. In our days, this development is supported using numerical simulation tools to further understand and predict in-cylinder processes. In this work, a model tool chain which uses a detailed chemical reaction scheme is proposed to predict the auto-ignition behavior of fuels with different octane ratings and to evaluate the transition from harmless auto-ignitive deflagration to knocking combustion. In our method, the auto-ignition characteristics and the emissions are calculated using a gasoline surrogate reaction scheme containing pathways for oxidation of ethanol, toluene, n-heptane, iso-octane and their mixtures. The combustion is predicted using a combination of the G-equation based flame propagation model utilizing tabulated laminar flame speeds and well-stirred reactors in the burned and unburned zone in three-dimensional Reynolds-averaged Navier–Stokes. Based on the detonation theory by Bradley et al., the character and the severity of the auto-ignition event are evaluated. Using the suggested tool chain, the impact of fuel properties can be efficiently studied, the transition from harmless deflagration to knocking combustion can be illustrated and the severity of the auto-ignition event can be quantified.

Grid Size Optimization for Diesel Injection Spray Nozzle Using CFD Analysis

In this chapter an experimental and numerical simulation were carried out for diesel spray to investigate its characteristics over a wide range of pressure. It would be helpful to predict the spray behavior such as penetration length, atomization, droplet distribution and spray angle for different pressure differences, covering low to high injection pressure. By knowing the behavior of injector over a pressure range gives a way to discover its applicability in a particular engine. The effect of various spray breakup models is studied and its influencing parameters like breakup length, breakup size constants were optimized iteratively. These parameters have a great influence in deciding the way in which the primary and secondary breakup occurring. Grid independency is done to have an optimized grid size by making a trade-off between accuracy and simulation time. Also the effect of discrete phase models (DPM), turbulence models and its constants, total parcels and maximum cells used for adaptive meshing were studied and compared with the test bench results. From the numerical simulation results, the optimum parameters were decided to predict the spray behavior closer to the test bench results.

A RECENT EULERIAN–LAGRANGIAN CFD METHODOLOGY FOR MODELING DIRECT INJECTION DIESEL SPRAYS

The global objective of this work is to show the capabilities of the Eulerian–Lagrangian spray atomization (ELSA) model for the simulation of Diesel sprays in cold starting conditions. Our main topic is to focus in the analysis of spray formation and its evolution at low temperature 255 K (-18°C) and nonevaporative conditions. Spray behavior and several macroscopic properties, included the liquid spray penetration, and cone angle are also characterized. This study has been carried out using different ambient temperature and chamber pressure conditions. Additionally, the variations of several technical quantities, as the area coefficient and effective diameter are also studied. The results are compared with the latest experimental results in this field obtained in our institute. In the meantime, we also compare with the normal ambient temperature at 298 K (25°C) where the numerical validation of the model has shown a good agreement.