Evaporation of Liquid Wall Film in Direct Injection Spark Ignition Engine-like Conditions

The liquid wall film formed by the spray impingement in Direct Injection Spark Ignition (DISI) engines can directly produce a large amount of Particle Matter (PM) emissions. The PM emissions can be tremendously reduced if all the liquid wall film can evaporate completely before flame propagates to the wall surface and the combustion of ‘pool fire’ fed by the evaporating liquid wall film can be totally eliminated. Evaporation models are widely used to predict the evaporation of liquid wall film in engines, but requiring accurate mass transfer correlations. However, it is challenging to experimentally determine the accurate mass transfer correlations of the liquid wall film in engines; since the evaporation time of the thin liquid wall film in engines is quite short and the thickness of the liquid wall film is extremely thin. Thus, numerical simulation has become a useful tool to provide insight into the underlying transient evaporation characteristics of liquid wall film in DISI engine-like conditions and to derive the mass transfer correlations. In this thesis research, numerical study has been conducted for a two-dimensional, two-phase, transient, non-isothermal and species transport problem representing the evaporation of liquid wall film in DISI engine-like conditions. The unique features of the numerical models are the inclusion of the transient motion and heating of the liquid phase, the blowing effects caused by evaporation, and the variation of thermo-physical properties. The governing equations which mathematically describe the transient evaporation process of liquid wall film in DISI engines, are discretized and solved using a Finite Volume Method (FVM) based software, Fluent, with its capability of User Defined Function (UDF) programming. The numerical evaporation models are validated with existing analytical and experimental data; and good agreements are observed. Subsequently, the validated models are used for the numerical study of the evaporating liquid wall film in DISI engine-like conditions to investigate its transient evaporation characteristics and determine its mass transfer correlations. The results show that the evaporation rate of liquid wall film, characterized by mass transfer coefficient, is non-uniform along the wall film, which is consistent with the development of species boundary layer and the decline of species concentration gradient within the boundary layer. The transient evaporation of liquid wall film in DISI engine-like conditions is mainly determined by the gas/liquid interfacial temperature, which can be directly affected by the transient heating of the liquid phase. The newly developed mass transfer correlations taking into account the blowing effects and effects caused by convection and the variation of thermo-physical properties during the transient evaporation process of the liquid wall film can predict their evaporation rate much more accurately than the existing correlations available in literature

Multicomponent Flow-Transport-Reaction Modeling of Trickle Bed Reactors: Application to Unsteady State Liquid Flow Modulation

A One-Dimensional Reactor and Catalyst Pellet Scale Flow-Transport-Reaction Model Utilizing the Multicomponent Stefan-Maxwell Formulation for Inter- and Intraphase Transport is Developed to Simulate Unsteady State Operation in Trickle Bed Reactors. the Governing Equations and Method of Solution Are Discussed. Results Are Presented for a Model Reaction System (Hydrogenation of A-Methylstyrene) under Gas Reactant Limiting Conditions, for Liquid Flow Modulation as a Test Case of Unsteady State Operation. Model Simulations Predict that Periodic Liquid Flow Modulation Can Alter the Supply of Liquid and Gaseous Reactants to the Catalyst and Result in Reactor Performance Enhancement above that Achieved in Steady State Operation. the Effects of Key Modulation Parameters Such as the Total Cycle Period, Cycle Split, and Liquid Mass Velocity Are Simulated, and Model Predictions Are Found to Be in Agreement with Experimentally Observed Trends in the Literature. © 2005 American Chemical Society

A seven-equation diffused interface method for resolved multiphase flows

The seven-equation model is a compressible multiphase formulation that allows for phasic velocity and pressure disequilibrium. These equations are solved using a diffused interface method that models resolved multiphase flows. Novel extensions are proposed for including the effects of surface tension, viscosity, multi-species, and reactions. The allowed non-equilibrium of pressure in the seven-equation model provides numerical stability in strong shocks and allows for arbitrary and independent equations of states. A discrete equations method (DEM) models the fluxes. We show that even though stiff pressure- and velocity-relaxation solvers have been used, they are not needed for the DEM because the non-conservative fluxes are accurately modeled. An interface compression scheme controls the numerical diffusion of the interface, and its effects on the solution are discussed. Test cases are used to validate the computational method and demonstrate its applicability. They include multiphase shock tubes, shock propagation through a material interface, a surface-tension-driven oscillating droplet, an accelerating droplet in a viscous medium, and shock-detonation interacting with a deforming droplet. Simulation results are compared against exact solutions and experiments when possible

Predicting the evaporation rate of stationary droplets with the VOF methodology for a wide range of ambient temperature conditions

This paper presents CFD predictions for the evaporation of nearly spherical suspended droplets for ambient temperatures in the range 0.56 up to 1.62 of the critical fuel temperature, under atmospheric pressures. The model solves the Navier-Stokes equations along with the energy conservation equation and the species transport equations; the Volume of Fluid (VOF) methodology has been utilized to capture the liquid-gas interface using an adaptive local grid refinement technique aiming to minimize the computational cost and achieve high resolution at the liquid-gas interface region. A local evaporation rate model independent of the interface shape is further utilized by using the local vapor concentration gradient on the droplet-gas interface and assuming saturation thermodynamic conditions. The model results are compared against experimental data for suspended droplet evaporation at ambient air cross flow including single- and multi-component droplets as well as experiments for non-convective conditions. It is proved that the detailed evaporation process under atmospheric pressure conditions can be accurately predicted for the wide range of ambient temperature conditions investigated

A CFD study of biomass pyrolysis in a downer reactor equipped with a novel gas-solid separator-II thermochemical performance and products

A Eulerian-Eulerian CFD model was used to investigate the fast pyrolysis of biomass in a downer reactor equipped with a novel gas-solid separation mechanism. The highly endothermic pyrolysis reaction was assumed to be entirely driven by an inert solid heat carrier (sand). A one-step global pyrolysis reaction, along with the equations describing the biomass drying and heat transfer, was implemented in the hydrodynamic model presented in part I of this study (Fuel Processing Technology, V126, 366-382). The predictions of the gas-solid separation efficiency, temperature distribution, residence time and the pyrolysis product yield are presented and discussed. For the operating conditions considered, the devolatilisation efficiency was found to be above 60% and the yield composition in mass fraction was 56.85% bio-oil, 37.87% bio-char and 5.28% non-condensable gas (NCG). This has been found to agree reasonably well with recent relevant published experimental data. The novel gas-solid separation mechanism allowed achieving greater than 99.9% separation efficiency and < 2 s pyrolysis gas residence time. The model has been found to be robust and fast in terms of computational time, thus has the great potential to aid in future design and optimisation of the biomass fast pyrolysis process

Aerodynamic breakup of an n-decane droplet in a high temperature gas environment

The aerodynamic droplet breakup under the influence of heating and evaporation is studied numerically by solving the Navier-Stokes, energy and transport of species conservation equations; the VOF methodology is utilized in order to capture the liquid-air interphase. The conditions examined refer to an n-decane droplet with Weber numbers in the range 15–90 and gas phase temperatures in the range 600–1000 K at atmospheric pressure. To assess the effect of heating, the same cases are also examined under isothermal conditions and assuming constant physical properties of the liquid and surrounding air. Under non-isothermal conditions, the surface tension coefficient decreases due to the droplet heat-up and promotes breakup. This is more evident for the cases of lower Weber number and higher gas phase temperature. The present results are also compared against previously published ones for a more volatile n-heptane droplet and reveal that fuels with a lower volatility are more prone to breakup. A 0-D model accounting for the temporal variation of the heat/mass transfer numbers is proposed, able to predict with sufficient accuracy the thermal behavior of the deformed droplet