Exploiting the potential of large eddy simulations (LES) for ducted fuel injection investigation in non-reacting conditions

The diesel combustion research is increasingly focused on ducted fuel injection (DFI), a promising concept to abate engine-out soot emissions in compression-ignition engines. A large set of experiments carried out in constant volume vessel and numerical simulations, at medium-low computational cost, showed that the duct adoption in front of the injector nozzle activates several soot mitigation mechanisms, leading to quasi-zero soot formation in several engine-like operating conditions. However, although the simplified CFD modelling so far played a crucial role for the preliminary understanding of DFI technology, a more accurate turbulence description approach, combined with a large set of numerical experiments for statistical purposes, is of paramount importance for a robust knowledge of the DFI physical behaviour. In this context, the present work exploits the potential of large eddy simulations (LES) to analyse the non-reacting spray of DFI configuration compared with the unconstrained spray. For this purpose, a previously developed spray model, calibrated and validated in the RANS framework against an extensive amount of experimental data related to both free spray and DFI, has been employed. The tests have been carried out considering a single-hole injector in an optical accessible constant volume vessel, properly replicated in the simulation environment. This high-fidelity simulation model has been adapted for LES, firstly selecting the best grid settings, and then carrying out several numerical experiments for both spray configurations until achieving a satisfying statistical convergence. With this aim, the number of independent samples for the averaging procedure has been increased exploiting the axial symmetry characteristics of the present case study. Thanks to this approach, a detailed description of the main DFI-enabled soot mitigation mechanisms has been achieved, shrinking the knowledge gap in the physical understanding of the impact of spray-duct interaction

Gridless particle technique for the Vlasov-Poisson system in problems with high degree of symmetry

In the paper, gridless particle techniques are presented in order to solve problems involving electrostatic, collisionless plasmas. The method makes use of computational particles having the shape of spherical shells or of rings, and can be used to study cases in which the plasma has spherical or axial symmetry, respectively. As a computational grid is absent, the technique is particularly suitable when the plasma occupies a rapidly changing space region

Ensemble average method for runtime saving in Large Eddy Simulation of free and Ducted Fuel Injection (DFI) sprays

Computational Fluid Dynamics (CFD) with Large Eddy Simulation (LES) turbulence model is a valuable tool to investigate complex problems. However, for high Reynolds number problems, the associated huge computational cost often leads researchers to the use of more simplified and less accurate approaches, especially if statistics is needed for the generalization of the results and comparison against experimental data. Therefore, the introduction of innovative methodologies to reduce the computational cost maintaining results reliability would be of paramount importance for LES-based investigation. In this context, the aim of this work is to assess a runtime saving methodology to ensemble average several axial symmetric spray simulations obtained with LES. In particular, the number of independent samples for the average procedure has been increased by exploiting the axial symmetry characteristics of a diesel spray case study, extracting more realizations from a single simulation. This ensemble average approach was compared with the standard one, based on one realization per simulation, at equal statistical sample size. Main spray physical quantities and turbulence characteristics were examined, both globally and locally. The same procedure was also applied to a different diesel-relevant spray configuration, known as ducted fuel injection. The reliability of this ensemble average methodology has been herein proven for both spray configurations, highlighting a dramatic runtime saving without any worsening of the accuracy level. In particular, this approach, as applied in this work, guaranteed a computational cost reduction of 50–75%. Thereby, the present methodological assessment could motivate researchers involved in the investigation of spray processes to undertake the path of statistically significant LES analysis

N-body simulation of nanoplasmas

The irradiation of atomic or molecular clusters with ultraintense lasers can induce the formation of hot completely ionized nanoplasmas, which rapidly expand into a vacuum, as predicted by Dawson. The physics of the expansion of these nanoplasmas, composed by 102 104 positive ions and electrons, has been extensively studied under a variety of conditions, using different analytical and numerical approaches, based upon fluid or kinetic models. A rigorous analysis of the dynamics of nanoplasmas is presented here by using the N-body simulation method and comparing the results with reference solutions for the collisionless kinetic equations, obtained using the shell model [1], which is a gridless, particle-based algorithm. The analysis is carried out comparing ensemble averages in order to take into account different initial conditions of the system. Two test cases, the electron dynamics in the initial phase of the expansion and the formation of shock shells during Coulomb explosion have been considered. For the electron dynamics in the initial phase of the expansion, the results indicate that the collisionless kinetic model is in good agreement with the N-body simulation, as far as mean values are considered; however, in a single experiment the calculated values may differ significantly from the average. Larger differences are observed studying the formation of shock shells during Coulomb explosions

An Engine Parameters Sensitivity Analysis on Ducted Fuel Injection in Constant-Volume Vessel Using Numerical Modeling

The use of Ducted Fuel Injection (DFI) for attenuating soot formation throughout mixing-controlled diesel combustion has been demonstrated impressively effective both experimentally and numerically. However, the last research studies have highlighted the need for tailored engine calibration and duct geometry optimization for the full exploitation of the technology potential. Nevertheless, the research gap on the response of DFI combustion to the main engine operating parameters has still to be fully covered. Previous research analysis has been focused on numerical soot-targeted duct geometry optimization in constant-volume vessel conditions. Starting from the optimized duct design, the herein study aims to analyze the influence of several engine operating parameters (i.e. rail pressure, air density, oxygen concentration) on DFI combustion, having free spray results as a reference. Furthermore, the duct wall temperature influence is investigated in order to preliminary explore the needs in terms of duct thermal management. The impact of the above-mentioned parameters on combustion and emissions formation processes is assessed, highlighting the soot mitigation mechanisms enabled by DFI operation. The optimized duct design led to a strong soot reduction for most of the operating conditions tested, thus confirming the robustness of the proposed geometry. This preliminary understanding step via numerical simulation of DFI calibration requirements paves the way to future studies on duct-equipped engine applications

Assessment of Flow Noise Mitigation Potential of a Complex Aftertreatment System through a Hybrid Computational Aeroacoustics Methodology

Flow noise produced by the turbulent motion of the exhaust gases is one of the main contributions to the noise generation for a heavy-duty vehicle. The exhaust system has therefore to be optimized since the early stages of the design to improve the engine's Noise Vibration Harshness (NVH) performance and to comply with legislation noise limits. In this context, the availability of reliable Computational Aero-Acoustics (CAA) methodologies is crucial to assess the noise mitigation potential of different exhaust system designs. In the present work, a characterization of the sound generation in a heavy-duty exhaust system was carried out evaluating the noise attenuation potential of a design modification, by means of a hybrid CAA methodology. In a first step, a steady state 3D-CFD simulation of the exhaust system in its baseline configuration was carried out with a RANS approach, to gather an analysis of the flow inside the diffusor and to obtain the turbulence intensity distribution necessary to localize and quantify the noise sources. Then, in a second step, the Stochastic Noise Generation and Radiation (SNGR) method was employed to synthetize the noise sources for the subsequent computation of the radiated acoustic field. A sensitivity analysis on the far field noise to the main method parameters was also performed, especially on the noise source region extension. Moreover, the baseline design of the exhaust system was also studied with a Direct Noise Calculation (DNC) approach, providing absolute flow noise levels to be compared with the results obtained by the means of the hybrid CAA approach. Then, a modified version of the exhaust diffusor was analysed with the proposed hybrid CAA methodology, highlighting the impressive potential in terms of noise attenuation of the new design configuration. The adoption of proposed hybrid CAA methodology was therefore demonstrated to allow a dramatic downscaling of the computational cost compared to DNC simulations, being fully compatible with the limited time available for the development of a new product in the automotive industry

Ducted Fuel Injection: Experimental and numerical investigation on fuel spray characteristics, air/fuel mixing and soot mitigation potential

Enhancing mixture preparation upstream of the premixed autoignition zone is a solution to reduce soot emission formation in compression ignition engines. With this aim, in recent years, Ducted Fuel Injection (DFI) concept has been developed: DFI is based on the idea of injecting the fuel spray through a small cylindrical pipe within the combustion chamber at a certain distance from the nozzle injector hole. Recent research studies have highlighted the high potential of this innovative concept for soot mitigation in both constant volume vessel and engine-like operating conditions. However, the mechanisms driving the soot reduction have not yet been fully understood. The aim of this research work is to further investigate the DFI concept, evaluating its impact on the spray characteristics, on the air/fuel mixing and, therefore, on the soot formation phenomena. Firstly, an experimental activity was carried out by means of a constant volume vessel test bench with optical accesses to compare the spray evolution and sizing with and without duct adoption, over a wide range of vessel thermodynamic conditions and injection pressures. After that, a simulation setup was defined in the commercially available 3D-CFD software CONVERGE reproducing the experimental test bench, calibrating and validating the spray model. Firstly, the spray model was calibrated and validated considering the same non-reacting conditions exploited in the experimental analysis. Then, the calibrated spray model was used as a virtual tool to investigate the air entrainment process. As a results, the DFI adoption increases the air entrainment in the near-nozzle region caused by the high velocity spray that generates a pumping effect at the duct inlet. Moreover, mixing process is also enhanced by the turbulence distribution at the duct exit resulting in a narrower distribution of equivalence ratio at the ignition. As a consequence of the more effective air entrainment and improved turbulent mixing, DFI remarkably mitigates soot emissions, with a reduction up to 80% with respect to free spray configuration in all tested operating conditions

Development of a Numerical Methodology for the Assessment of Flow Noise in Complex Engine Exhaust Systems

Worldwide regulations concerning noise emissions of road vehicles are constantly demanding further reductions of acoustic emissions, which are considered a major environmental health concern in several countries. Among the different sources contributing to noise generation in vehicles equipped with internal combustion engines, exhaust flow noise is one of the most significant, being generated by turbulence development in the exhaust gases, and robust and reliable numerical methodologies for its prediction in early design phases are currently still needed. To this extent, Computational Aero-Acoustics (CAA) can be considered a valuable approach to characterize the physical mechanisms leading to flow noise generation and its propagation, and it could therefore be used to support exhaust system development prior to the execution of experimental testing campaigns. This paper describes the development of a CAA methodology suitable for automotive applications that can be used to support the design of new exhaust system components in their early phases. In particular, the work focuses on the flow noise generated in a complex heavy-duty exhaust system, featuring three tailpipes located next to the ground. Firstly, the near-field acoustic field is obtained with a Direct Noise Computation (DNC) approach from an unsteady compressible 3D Computational Fluid Dynamics (3D-CFD) simulation, carried out by means of the commercially available 3D-CFD software STAR-CCM+. A Detached Eddy Simulation (DES) technique is implemented to reduce the high computational cost of the DNC approach and it has been initialized by a steady-state converged solution. The steady-state simulation has been also exploited to extract qualitative predictive indexes, as a preliminary characterization of the system in terms of mean flow and broadband noise generation. Finally, predicted near-field noise levels are evaluated to obtain an assessment of engine exhaust system acoustic performance