P. Pelloni

Modeling Atomization of High-Pressure Diesel Sprays

This paper deals with a numerical and experimental characterization of a high-pressure diesel spray injected by a common-rail injection system. The experiments considered a free non-evaporating spray and they were performed in a vessel reproducing the practical density that characterizes a D.I. diesel engine at injection time. The fuel was supplied at high pressure by a common-rail injection system with a single hole tip. The computations have been carried out by using both the TAB model and a hybrid model that allows one to describe both liquid jet atomization and droplet breakup. In order to validate the breakup model, an extensive comparison between data and numerical predictions has been carried out in terms of spray penetration, Sauter mean diameter, near and far spray cone angles, and spray structure.

Advanced Modelling of a New Diesel Fast Solenoid Injector and Comparison with Experiments

Upcoming Euro 4 and Euro 5 emission standards are increasing efforts on injection system developments in order to improve mixture quality and combustion efficiency. The target features of advanced injection systems are related to their capability of operating multiple injection with a precise control of the amount of injected fuel, low cycle-by-cycle variability and life drift, within flexible strategies. In order to accomplish this task, injector performance must be optimised by acting on: optimisation of electronic, driving circuit, detailed investigation of different nozzle hole diameter configurations, assessment of the influence of manufacturing errors on hole diameter and inlet rounding on injector performance.

Numerical Analysis of High-Pressure Fast-Response Common Rail Injector Dynamics

Managing the injection rate profile is a powerful tool to control engine performance and emission levels. In particular, Common Rail (C.R.) injection systems allow an almost completely flexible fuel injection event in DI-diesel engines by permitting a free mapping of the start of injection, injection pressure, rate of injection and, in the near future, multiple injections. This research deals with the development of a network-based numerical tool for understanding operating condition limits of the Common Rail injector. The models simulate the electro-fluid-mechanical behavior of the injector accounting for cavitation in the nozzle holes. Validation against experiments has been performed. The model has been used to provide insight into the operating conditions of the injector and in order to highlight the application to injection system design.

A Quasi-Direct 3D Simulation of the Atomization of High-Speed Liquid Jets

In this paper a quasi-direct solution of transient three-dimensional CFD calculations based on a finite volume approach has been adopted to simulate the atomization process of high velocity liquid jets issuing an injector-like nozzle. An accurate Volume-of-Fluid (VOF) method is used to reconstruct and advect the interface between the liquid and gas phases. An extended mesh which includes the injector nozzle and the upstream plenum has been considered in order to investigate accurately the effect of nozzle flow conditions on the liquid jet atomization. Cavitation modeling has not been included in the present computations. Two different mean injection velocities, 150 m/s and 270 m/s, respectively, have been considered in the calculations as representative of semi-turbulent and fully-turbulent nozzle flow conditions. The liquid-to-gas density ratio is kept fixed at 57. The calculations show that atomisation is directly linked to the temporally and spatially correlated turbulence of the liquid jet. The bulk flow perturbation and the relaxation of the boundary layer have been found to be the basic mechanisms that generate surface perturbations of the liquid jet.© 2005 ASME

Modeling of Wall Film Formed by Impinging Spray Using a Fully Explicit Integration Method

A wall film model has been implemented in a customized version of KIVA-3 code developed at University of Bologna. The model simulates the dynamics of a liquid wall film generated by impinging sprays by solving the mass, momentum and energy equations of a two-dimensional liquid flow over a three-dimensional surface under the basic hypothesis of a ‘thin laminar flow’. The major phenomena taken into account in the present model are: wall film formation by impinging spray; body forces, such as gravity or acceleration of the wall; shear stress at the interface with the gas and no slip condition on the wall; momentum contribution and dynamic pressure generated by the tangential and normal component of the impinging drops; film evaporation by heat exchange with wall and surrounding gas. The governing equation have been integrated in space by using a finite volume approach with a first order upwind differencing scheme and they have been integrated in time with a fully explicit method. Particular care has been taken in numerical implementation of the model. Two different test cases reproducing PFI gasoline and DI Diesel engine wall film conditions have been simulated. The comparisons with experimental data show that the present wall film model well reproduces the evolution in time and the spatial distribution of the liquid film thickness in both cases.© 2005 ASME

On Non-Equilibrium Turbulence Corrections in Multidimensional HSDI Diesel Engine Computations

The introduction of high-pressure injection systems in D.I. diesel engines has highlighted already known drawbacks of in-cylinder turbulence modeling. In particular, the well known equilibrium hypothesis is far from being valid even during the compression stroke and moreover during the spray injection and combustion processes when turbulence energy transfer between scales occurs under non-equilibrium conditions. The present paper focuses on modeling in-cylinder engine turbulent flows. Turbulence is accounted for by using the RNG k-e model which is based on equilibrium turbulence assumptions. By using a modified version of the Kiva-3 code, different mathematically based corrections to the computed macro length scale are proposed in order to account for non-equilibrium effects. These new approaches are applied to a simulation of a recent generation HSDI Diesel engine at both full load and partial load conditions representative of the emission EUDC cycle. The numerical results show that the proposed corrections improve the physical behavior of the combustion model by a self-scaling of the eddy-turnover time depending on the engine operating conditions. The overall achievement is the extension of modeling reliability over a wider range of operating conditions and in particular over those that are of interest in the European emission test cycle. INTRODUCTION Complying with automotive emission standards is a very complicated task since a reduction of NOx and soot engine-out levels is continuously required to limit air pollution. Engine performance and emissions are controlled by a great number of parameters related to the injection system, combustion chamber geometry, boost pressure and EGR percentage. Due to the expense of experimental investigations on engines, the development process must be supported by CFD simulations in order to reduce the cost and time required to bring a new engine into the market. In order to be useful in engine design, CFD has to be reliable. Models must describe correctly the in-cylinder processes and they must require very limited tuning of the empirical model constants. Despite the fact the many efforts have been spent in order to provide reliable predictions over a wide range of engine operating conditions, spray combustion simulations have often followed experimental development of engine combustion chambers and injection systems due to shortcomings in the models used and the limited accuracy of experimental data used as input parameters [1]. As it is well known, good NOx and soot engineout level predictions are strictly linked not only to the models themselves but also to the accuracy in determining local equivalence ratio and turbulence distribution. The available models for NOx and soot have proved to be able to capture at least the trade-off if local conditions are correctly predicted [2,3]. Unfortunately, as pointed out earlier, in almost all practical cases a proper 2001-01-0997 On Non-Equilibrium Turbulence Corrections in Multidimensional HSDI Diesel Engine Computations G. M. Bianchi and P. Pelloni DIEM University of Bologna G.-S. Zhu and R. Reitz University of Wisconsin Madison Copyright

The Role of Simulation in the Development of a Fast-Actuation Solenoid C.R. Injection System

Upcoming Euro 4 and Euro 5 emission standards are increasing efforts on injection system developments in order to improve mixture quality and combustion efficiency. The target features of advanced injection system are related to their capability of operating multiple injection with a precise control of amount of fuel injected, low cycle-by-cycle variability and life drift, within flexible strategies. In order to accomplish this task, performance must be optimised since injection system concept development by acting on. The extensive use of numerical approach has been identified as a necessary integration to experiments in order to put on the market high quality injection system accomplishing strict engine control strategies. The modelling approach allows focusing the experimental campaign only on critical issues saving time and costs, furthermore it is possible to deeply understand inner phenomena that cannot be measured. The lump/ID model of the whole system built into the AMESim® code was presented in previous works: particular attention was devoted in the simulation of the electromagnetic circuits, actual fluid-dynamic forces acting on needle surfaces and discharge coefficients, evaluated by means 3D-CFD simulations. In order to assess new injection system dynamic response under multiple injection strategies reproducing actual engine operating conditions it is necessary to find to proper model settings. In this work the integration between the injector and the system model, which comprehends the pump, the pressure regulator, the rail and the connecting-pipes, will be presented. For reproducing the dynamic response of he whole system will be followed a step-by-step approach in order to prevent modelling inaccuracies. Firstly will be presented the linear analysis results performed in order to find injection system own natural frequencies. Secondly based on linear analysis results will be found proper injection system model settings for predicting dynamic response to external excitations, such as pump perturbations, pressure regulator dynamics and injection pulses. Thirdly experimental results in terms of instantaneous flow rate and integrated injected volume for different operating conditions will be presented in order to highlight the capability of the modelling methodology in addressing the new injection system design.Copyright

A 2D Simulation Method for Computing Droplet Size Spectrum During the Atomization of High-Speed Liquid Jets

Based on both experimental observations and available numerical methods, an innovative 2D approach for determining droplet size during the atomization process has been developed. Based on experimental evidences (see [1] and [2]) atomization of turbulent high speed jets is assumed to occur in a two stage process: ligaments detachment and droplets formation. The simulation method here proposed wants to take the advantages typical of the two most effective methods in spray investigation. It joins LES (i.e Large Eddy Simulations) approach and Linear Stability Analysis: the first one is used to solve the liquid-air fluid dynamics interaction and in particular the instabilities leading to ligament formation. The second one is finally adopted to compute the droplet size spectrum from ligament break-up. Therefore dynamics of ligament formation is directly computed while droplet formation is modelled by using a Linear Stability Analysis. The numerical simulation adopts a VOF (i.e. Volume of Fluid) method to track liquid-gas interface. Turbulence effects on liquid surface are accounted for by adding a turbulent flow field at the nozzle exit which represents a part of the boundary condition of the computational domain. A physical criterion is then applied to detach ligaments from liquid jet surface which will reduce in diameter during simulation. The droplet formation is then computed by applying the linear stability analysis to the ligaments, assumed being circular and subject to circulation. An extensive validation and sensitivity analysis has been carried out in order to assess method advantages and limits. The experimental results of Wu et al. [3] and Horoyasu et al. [4] were used as test cases. A sensitivity analysis has been performed under typical HSDI Diesel engine injection conditions. The method proved to exhibit promising attitude in the reconstruction of the droplet size spectrum depending on injection parameter or conditions.Copyright

Definition of a LES Numerical Methodology for the Simulation of Engine Flows on Fixed Grid

To improve the overall engine performance, it is necessary to clearly understand the main unsteady phenomena that occur inside an IC engine. Since experimental technique can provide only lump parameters, the CFD numerical approach has been identified as a valid alternative tool to perform detailed investigations on the fluid dynamics behaviours. The numerical analysis of engine flows is commonly performed by using RANS approach. Adopting a RANS methodology only the mean flow variable distributions could be obtained because the time average of the generic flow variable fluctuation is zero by definition. To perform an effective analysis about the unsteady characteristic of a generic flow and, in particular, of an engine flow it is necessary to improve the numerical solution level adopting the LES (Large Eddy Simulation) approach. LES solves directly the large scales of motion (responsible for the main energy transport inside the flow) while only the small scales are modelled using a Sub-Grid Scale model. Moreover, the LES approach could also be used as test bench case to properly define and understand how it is possible to improve the solution accuracy of RANS simulation. This paper regards the LES analysis of a steady non-reactive wall-bounded flow over a test bench engine geometry. In particular, two LES models, i.e., the Wall Adaptive Local Eddy-Viscosity (WALE) [25] model and the one-equation Dynamic Model by Kim and Menon [23, 24, 29] have been tested. The numerical set-up has been defined performing a preliminary parametric CFD simulations on a basic flow over a backward facing step case. In particular, a bounded second order central differencing scheme was adopted and a discussion of the kinetic energy conservation attitude of such a scheme is performed. LES results have been compared to available experimental LDA measurements of mean and rms fluctuations of both axial and tangential velocity components and with numerical predictions obtained by an optimized RANS simulation of the same case. This paper shows the advantages and the limits of the LES simulation approach applied to IC engine flows.Copyright