Local Organizing Committee

Abstract

Present-day engine modeling aims at the development of a simulation tool suitable for application in the design process of reciprocating engines. Computational approaches to the investigation of the fundamental processes governing flow, spray and combustion phenomena in direct injected (DI) diesel engines are particularly suited to study the formation of pollutants, a topic motivated by environmental concerns and the desire to produce cleaner and more efficient engines. Modeling overview [1]: Diesel fuel sprays are frequently modeled as two-phase flows where the gas phase is described by the three-dimensional Favre-Reynolds-averaged conservation equations for species, mass, momentum and energy, in combination with a turbulence model and appropriate initial and boundary conditions. The liquid phase is formulated as a stochastic evolution law of the normalized droplet flux which is governed by various phenomena such as droplet breakups and collisions, evaporation and droplet-air interactions. In order to properly describe these processes, the change in the spray state variables, given by the droplet position, velocity, temperature, size and deformation parameters, is determined by submodels involving the mentioned droplet-droplet and droplet-air interactions, subject to initial and boundary conditions for the liquid state. The coupling between the liquid and the gas phase is achieved by means of appropriate source terms in the gas equations, which are obtained from the spray data by integration of the mass, momentum and energy due to phase changes over all droplets at a given position and time. Solution procedures [2]: The gas-phase solution procedure is based on a Lagrange-Euler method which offers the flexibility to combine computations obtained in a moving frame of reference (updating of spray, chemical reactions and acoustic terms) with solution procedures best suited for a fixed volume approach (convective fluxes to account for a moving mesh). Wide-spread numerical schemes used in this context are variations of the SIMPLE algorithm, an iterative method developed to handle stiff, viscous flow problems. The solution procedure for the spray equation is a discrete particle tracking method, where the normalized droplet flux is approximated by a step function and each step signifies a particle, i.e. a class of droplets of identical states. Liquid jet atomization: The breakup of liquid fuel jets in diesel combustion plays a decisive role in the evolution of the spray and its associated subsequent processes. It is generally agreed that the aerodynamic liquid-gas interaction is the most dominant factor in this disintegration process and, therefore, it is the main phenomenon considered in the modeling of liquid jet breakup. (Other effects due to nozzle shape, fuel properties or the behavior of the fuel supply system are absorbed in model constants.) A unified approach to the breakup classification of stationary liquid jets is presented in [3]. More recent investigations conducted by various researchers, utilizing different experimental techniques, show that transient, high-pressure-driven fuel jets axe broken into liquid fragments of various shape and size at the time they exit the injector nozzle or shortly thereafter. Subsequently, the large liquid blobs are subject to further breakups until they reach a stable state. The fundamental mechanisms which govern the liquid breakup are instabilities occurring on the interface of the two-phase flow. More precisely, these instabilities are either the result of the inertial forces when the denser fluid opposes a system acceleration (Rayleigh-Taylor) or are caused by the viscous forces due to the relative tangential motion at the phase-dividing interface (Kelvin-Helmholtz). The ETAB model [4]: The Enhanced Taylor Analogy Breakup (ETAB) model, discussed in this presentation, imitates the liquid jet disintegration process as a cascade of drop breakups governed by Taylor's linear drop deformation dynamics and the associated drop breakup criterion. In fact, the drop distortion is described by a forced, damped harmonic oscillator, where the forcing term is given by the aerodynamic droplet-gas interaction, the damping is due to the liquid viscosity and the restoring force is supplied by the surface tension. Breakup occurs when the normalized drop distortion exceeds a critical value. The breakup into product droplets is modeled after the experimentally observed bag, stripping or catastrophic breakup mechanisms and the radial velocities of the product droplets are derived from an energy conservation consideration. At the nozzle exit the liquid jet is simulated as a sequence of large, high velocity drops which are very unstable. In order to avoid an immediate breakup, they are equipped with a deformation velocity such that their lifetime is extended to match experimentally observed jet breakup lengths. This computational artifice leads to the simulation of a fragmented liquid core, as is observed experimentally by various research groups. In addition, the initially injected droplets are given a drop size distribution to compensate for the neglect of the surface stripping near the nozzle exit, a phenomenon which is responsible for the fuel-air mixture formation near the nozzle exit and determines the auto-ignition behavior. The breakup model also provides for adjustments of nozzle and injection system specific properties via model constants. The ETAB model has been validated for non-evaporating and evaporating sprays utilizing measurements obtained under controlled conditions from constant volume combustion cells, either reported in the literature or performed at our laboratory. Applications [5]: Computer simulations of the in-cylinder processes of medium and large DI diesel engines have been performed and compared with corresponding experimental data. The focus of these investigations has been on the evolution of the cylinder pressure and the associated rate of heat release, as well as on the spatial distribution of other spray and combustion relevant quantities.