Simona Tonini

Modelling of internal and near-nozzle flow of a pintle-type outwards-opening gasoline piezo-injector

Abstract Computational fluid dynamics (CFD) methods have been used to investigate the internal nozzle flow of an outwards-opening piezo-driven pintle injector designed for spray-guided direct injection gasoline engines. The internal nozzle flow has been investigated for various nozzle designs, with emphasis placed on the effect of manufacturing tolerances on the internal and near-nozzle flow characteristics. The methods employed include features such as moving wall boundaries and time-dependent pressure or flowrate inlet conditions, cavitation as well as Eulerian and Lagrangian near-nozzle and spray models. The results reveal that not only the nozzle internal geometric details and manufacturing tolerances influence significantly the flow conditions at the exit of the pintle injector, but the actual spray characteristics are significantly influenced by the external geometry of the nozzle housing and the pintle shape. The atomization process of the liquid emerging from the pintle nozzle seems to be different from that realized in other nozzle designs used in direct injection gasoline engines; a so-called string-type of spray is formed at the nozzle exit, as confirmed by near-nozzle CCD spray images. The mechanism of string formation is attributed to the limited liquid volume passing through the needle seat area and partly occupying the available volume, while the details of the geometry in this location may enhance local air entrainment. The velocity differences along the circumference of the nozzle exit magnify those flow instabilities and have a predictable effect on the dispersion of the liquid droplets near the nozzle exit.

Prediction of Liquid and Vapor Penetration of High Pressure Diesel Sprays

A dense-particle Eulerian-Lagrangian stochastic methodology, able to resolve the dense spray formed at the nozzle exit has been applied to the simulation of evaporating diesel sprays. Local grid refinement at the area where the spray evolves allows use of cells having sizes from 0.6 down to 0.075mm. Mass, momentum and energy source terms between the two phases are spatially distributed to cells found within a distance from the droplet centre; this has allowed for grid-independent interaction between the Eulerian and the Lagrangian phases to be reached. Additionally, various models simulating the physical processes taking place during the development of sprays are considered. The cavitating nozzle flow is used to estimate the injection velocity of the liquid while its effect on the spray formation is considered through an atomisation model predicting the initial droplet size. Various vaporisation models have been tested, including high-pressure and non-equilibrium effects and chemical composition change. Different droplet break-up and droplet aerodynamic drag models are used to assess the predicted results. In particular, the increased surface area of the droplets associated with their fragmentation process is found to play a major role on the exchange of heat and mass between the evaporating liquid and the surrounding air. This has allowed for calculation of liquid penetration length independent of the injection pressure. The model has been successfully validated against experimental data for the liquid and vapour penetration under a variety of injection pressure, back pressure and temperature, injection hole diameter and fuel initial temperature and composition.

Multi-component fuel vaporization modelling and its effect on spray development in gasoline direct injection engines

Abstract A multi-component fuel vaporization model has been developed and implemented into an in-house multi-phase computational fluid dynamics flow solver simulating the flow, spray, and air-fuel mixing processes taking place in gasoline direct injection (GDI) engines. Multi-component fuel properties are modelled assuming a specified composition of pure hydrocarbons. High-pressure and -temperature effects, as well as gas solubility and compressibility, are considered. Remote droplet vaporization is initially investigated in order to quantify and validate the most appropriate vaporization model for conditions relevant to those realized with GDI engines. Phenomena related to the fuel injection system and pressure-swirl atomizer flow as well as the subsequent spray development are considered using an one-dimensional fuel injection equipment model predicting the wave dynamics inside the injection system, a Eulerian volume of fluid-based two-phase flow model simulating the liquid film formation process inside the injection hole of the swirl atomizer and a Lagrangian spray model simulating the subsequent spray development, respectively. The computational results are validated against experimental data obtained in an optical engine and include laser Doppler velocimetry measurements of the charge air motion in the absence of spray injection and charge coupled device images of the fuel spray injected during the induction stroke. The results confirm that fuel composition affects the overall fuel spray vaporization rate, but not significantly relative to other flow and heat transfer processes taking place during the engine operation.

DNS Investigation of the Primary Breakup in a Conical Swirled Jet

The mechanisms of primary break-up of a jet produced by a Pressure Swirl Atomizer for aircraft use is investigated. A grid study has been done together with the inlet boundary adopted to setup the simulation. The simulations are undertaken using the in-house multiphase code FS3D based on the volume of Fluid (VOF) method. All simulations were carried out on the Cray XC40 at the High Performance Computing Center Stuttgart (HLRS).

Moving Boundary Problem for Heating and Evaporation of a Spherical Drop

The process of heat and mass transfer from a spherical liquid drop evaporating into a gas environment is investigated relaxing the commonly used quasi-steady approximation and accounting for the inherent unsteadiness caused by the sudden immersion of a liquid drop in a gaseous environment. The drop radius shrinking due to evaporation settles a moving boundary problem, which is changed to a fixed boundary one by a proper coordinates transformation. The heat and evaporation rates and the drop diameter evolution is quantified by numerical solution of the species and energy conservation equations and the overall mass and energy balances over the drop for different species (water, n-octane, n-dodecane, ethanol). The paper extends the analysis previously reported by the same authors by considering the coupling between the species and energy equations.

Numerical Simulations of Internal Flow in an Aircraft Engine Pressure Swirl Atomizer

The internal flow and the liquid structure at the nozzle exit are investigated in a typical pressure swirl atomizer for aeroengine applications under an isothermal nonreacting environment using a two-phase flow modeling according to the volume-of-fluid numerical methodology. A parametrical analysis is performed to investigate the effect of operating conditions on the injector performances and the characteristics of the liquid jet exiting the nozzle. The liquid film thickness at the nozzle exit and the injector discharge coefficient predicted by the simulations are compared against commonly used correlations available in the literature. The influence of the turbulence model on the injector performances is also analyzed using three different models: the renormalization group k-ϵ model, the Reynolds stress model, and the large-eddy simulation model.