Session Z14: Reacting Flows: Sprays and Multiphase Flow Effects (12:15pm - 1:00pm CST)

The effect of droplet internal circulation on droplet vaporization

The current study investigates the effect of droplet internal circulation on droplet vaporization by using direct numerical simulations. Classical models neglect this effect due to assumptions of quiescent environment and spherical symmetry. However, in applications like gas turbine engines, the density ratio of the droplet to the air is not as great as that in atmospheric conditions. In such conditions, the classical assumptions are invalidated, and changes in droplet vaporization characteristics can be significant. To reveal the relation between vaporization and internal circulation, we will examine the vaporization of a single droplet falling at its terminal velocity. Parameters relevant to internal circulation in both atmospheric and engine conditions will be investigated, by varying the density and the viscosity ratios. In addition, Weber numbers ranged from ~0 to 12 will be investigated, which includes the full range of droplet shapes from spheres to the critical point of breakup. Reynolds numbers less than ~100 will be examined, which cover the range of flows from fully attached flow to the onset of unsteady vortex shedding. An in-house code developed for DNS of vaporizing multiphase flows will be employed for the computation.   

Effect of straining flow and droplet shape on vaporization rate of liquid fuel droplet

This study focuses on the effect of planar straining flow on the vaporization of droplets. This work is motivated by spray combustion in gas turbines where the turbulence inside the combustor leads to the presence of both significant flow strain and droplet deformation. While a small amount of literature exists on the effect of droplet deformation on vaporization, there are no systematic investigations of the effect of flow strain on the vaporization of freely-deforming droplets. Recent theoretical studies on ellipsoidal droplets suggest that deformation enhances the vaporization rate. Additionally, our initial studies [Eastern States of the Combustion Institute, 2020] suggest that flow strain can also impact the vaporization rate. Therefore, a complete understanding of droplet vaporization requires studying the interaction between both droplet deformation and flow strain. This study uses an in-house code for interface-resolved direct numerical simulations of vaporizing multiphase flows. It mimics a freely-deforming droplet falling at its terminal velocity with an imposed strain rate. The influence of the deformation and flow strain on vaporization will be investigated by varying the relevant non-dimensional groups such as the Weber number and the non-dimensional strain rate.   

Detailed Spray Modeling of an Outwardly Opening Injector and its Impact on the Performance of a GDI Engine Operating in a Lean Stratified Combustion Mode

Outwardly opening hollow cone injectors feature a simple design, and with a piezoelectric actuator, can also offer fast and precise actuation. These injectors are suitable for use in gasoline direct injection (GDI) engines, which necessitates high fidelity spray modeling to accurately model the ignition delay and the resultant pressure trace. These can be highly sensitive to spray atomization in engines running with a globally lean, yet stratified mixture, achieved using very late injections in the compression stroke. A modified injection strategy is studied with precise modeling of breakup, collisions, and evaporation of the fuel. Both, the Eulerian - Lagrangian, and the Eulerian -- Eulerian (using the VoF method) models in ConvergeCFD are used for detailed simulation of the hollow cone spray. Further, the effect of cavitation and turbulence-based instabilities on spray breakup are investigated, in addition to aerodynamic instabilities. The results from these models are compared with experimental data at constant temperature and pressure conditions. The model is finally simulated in GDI combustion conditions to assess ignition delay, pressure trace, and soot prediction using the method of moments with interpolative closure (MOMIC).   

Simulation of multi-component transcritical flow using in situ adaptive tabulation of vapor-liquid equilibrium solutions

The studies of trans- and super-critical injection have attracted much interest in the past 30 years. However, most of them were mainly concentrated on the single-component system, whose critical point is a constant value. To capture the thermophysical properties of multi-component, a phase equilibrium solver is needed, which is also called a vapor-liquid equilibrium (VLE) solver. But VLE solver increases the computation cost significantly. Tabulation methods can be used to store the solution to avoids a mass of redundant computation. However, the size of a table increases exponentially with respect to the number of species. When the number of species is greater than 3, the size of a table far exceeds the limit of RAM in today's computers. In this research, an online tabulation method based on In Situ Adaptive Tabulation (ISAT) is developed to accelerate the computation of multi-component fluid. Accuracy and efficiency are analyzed and discussed. The CFD solver used in this research is based on a four-equation two-phase flow model. Peng-Robinson equation of state is used in phase equilibrium.   

Evaporation effect of liquid jet in supersonic crossflow

Due to the longer auto-ignition time with liquid fuels compared with hydrogen, the understanding of interaction of shock wave with the spray and subsequent vapor mixing are significant to design ramjet/scramjet with liquid fuel spray. In this study, an Eulerian-Lagrangian framework with detailed transport models for the Eulerian gas-phase species properties and Lagrangian spray, atomization, evaporation and break-up is developed based on the OpenFOAM platform. In addition, an equilibrium wall function is added to model the near-wall properties. The newly developed solver is used to conduct wall-modeled large eddy simulations (WMLES) on a non-reactive liquid jet in supersonic crossflow (JISCF) with liquid water spray. The strong supersonic flow break up the transverse spray droplet and the droplet evaporation and fuel properties (e.g., heat capacity and enthalpy of evaporation) effects on liquid plume trajectory and penetration length are discussed in this study. It is shown that evaporation effect primarily happens in the temperature field. For n-heptane spray, such impact could be conducted to other properties of the flow field like spray plume size, particle size distribution and volumetric flux, which is caused by the difference in latent heat and heat capacity.   

The terminal edge structure of a transcritical interface between two propellant streams at high pressures

A theory of the transcritical hydrodynamics of propellants in high-pressure combustors is presented in this talk. This theory couples the multicomponent Navier-Stokes conservation equations with an extension of the diffuse-interface theory of van der Waals, and is supplemented with a high-pressure equation of state and appropriate redefinitions of the thermodynamic potentials. This theory predicts the formation of a transcritical interface between the fuel and coflow propellant streams that persists downstream of the injection orifice over a supercriticalization distance of the same order as the characteristic thermal-entrance length of the fuel stream. The transcritical interface vanishes at an edge, where the diffusional critical point plays a fundamental role in the dynamics. A fully supercritical mixing layer emanates from the edge, across which the propellants mix by molecular diffusion as if they were gas-like fluids.   

Parametric Study of Hot Surface Ignition of an Impinging Fuel Spray Using Large-Eddy Simulation

The impingement of a fuel spray on a hot surface is of relevance in fuel-injection systems and for industrial safety. The interaction of a spray with a hot surface may result in the formation of a thin fuel film or the rebounding of impinging droplets. The nature of this interaction is determined by the properties of the phases involved, most importantly by the temperature of the impinged surface and the Weber number of the impinging spray. For sufficiently high surface temperatures, the Leidenfrost effect inhibits direct liquid-solid contact. In all cases, the high temperature of the surface results in evaporation of the fuel spray, and may lead to ignition. In this study, the sensitivity of impinging spray ignition to surface temperature is studied using large-eddy simulations. An impinging n-dodecane spray at ambient pressure is modeled using an Eulerian-Lagrangian formulation, with chemical source terms modeled using finite-rate chemistry. The effect of surface temperature on ignition and subsequent flame propagation is shown, and the effect of fuel film formation is considered parametrically to demonstrate its effect on near-wall composition and chemistry.