Konstantina Vogiatzaki

University of Oxford | OX · Department of Engineering Science

Computational Fluid Dynamics (CFD) frameworks of supercritical cryogenic fluids need to employ Real Fluid models such as cubic Equations of State (EoS) to account for thermal and inertial driven mechanisms of fluid evolution and disintegration. Accurate estimation of the non-linear variation in density, thermodynamic and transport properties is req...

In this study, a new three-fluid volume of fluid framework is presented in order to be used as a tool for providing physical insight—in a unified manner—to cavitating sprays and other complex multi-fluid, multiphase fluid flows. The framework accounts for phase change across a sharp interface between two fluids (gas and liquid) including miscibilit...

The impact of a liquid droplet onto a solid surface is a phenomenon present in a wide range of natural processes and technological applications. In this study, we focus on impact conditions characterised by ultra high velocities (up to 500 m/s), to investigate—for the first time—how the impact dynamics change when the compressibility of the liquid...

The work provides novel physical insight into the complex near-field flow dynamics and the primary break-up characteristics of sprays injected through multi-hole injectors (gasoline ECN Spray G). We use a stochastic fields transported-PDF method ( Σ-Y-PDF ) that allows for more accurate modelling of the sub-grid dynamics. Within this framework, the...

In this paper, we investigate the effect of the density ratio on the primary atomization characteristics of a liquid jet injected into a gaseous crossflow. Jets into crossflow with a high density ratio can be challenging for some multiphase models with an interface reconstruction. In this case, it is common practice to artificially reduce the densi...

Advancements in novel high efficiency IC engines, cryogenic rocket engines and cooling technologies have given rise to the need for accurate simulation capabilities of cryogenic jets. The ability to accurately simulate cryogenic jets will be pivotal in controlling the jet penetration and/or heat absorption depending upon the role of cryogenic fluid...

Eulerian Stochastic methods in Large Eddy Simulation context have been successfully used in reactive flows. They have been recently applied to spray atomisation, using surface and liquid volume as independent variables. These methods involve the solutions of Stochastic Partial Differential Equations, which are continuous in space and discontinuous...

Cryogenic supercritical fluids represent an intriguing category of fluids that combine mechanical and thermophysical properties of both ultralow temperature conditions and phenomena taking place well above the critical point. Recent research has demonstrated that it is a common misconception to consider the supercritical state as one homogeneous st...

This paper provides new physical insight into the coupling between flow dynamics and cavitation bubble cloud behaviour at conditions relevant to both cavitation inception and the more complex phenomenon of flow “choking” using a multiphase compressible framework. Understanding the cavitation bubble cloud process and the parameters that determine it...

Flash atomization can occur in rocket thrusters operated at near vaccum conditions just prior to ignition when cryogenic propellants are injected into the reaction chamber. Although the morphology of flashing sprays has been widely studied, empirical observations of the primary breakup mechanisms at the micro-scale are currently not feasible. Still...

In this work air fuel mixing and combustion dynamics in the recuperated split cycle engine (RSCE) are investigated through new theoretical analysis and complementary optical experiments of the flow field. First, a brief introduction to the basic working principles of the RSCE cycle will be presented, followed by recent test bed results relevant to...

Liquid fuel atomisation is largely affected by in and near nozzle phenomena. In this paper we establish a link between the initial droplet distribution at the nozzle exit, used as an input parameter in Lagrangian solvers, and the effect that this selection has on the subsequent predicted spray physics. Three initial PDFs with similar mean but diffe...

The ability to accurately predict the dynamics of fast moving and deforming interfaces is of interest to a number of applications including ink printing, drug delivery and fuel injection. In the current work we present a new compressible framework within OpenFOAM which incorporates mitigation strategies for the well known issue of spurious currents...

In this paper, we study the primary atomization characteristics of liquid jet injected into a gaseous crossflow through direct numerical simulations (DNS) and large eddy simulations (LES). The DNS use a coupled level set volume of fluid (CLSVOF) sharp interface capturing method resolving all relevant scales to predict the drop size distribution (DS...

Correct prediction of the spray in automotive and aerospace engines remains a challenging task. In this work we present a numerical framework to characterize the spray, using both the large scale quantities, like mean liquid volume fractions, as well as small scale structures or liquid droplets. In the limit of high Reynolds and Weber number, the d...

Flash boiling atomization can occur in rocket thrusters operating in the vacuum of space when the cryogenic liquid propellants are injected into the reaction chamber just before ignition. The sudden drop in pressure triggers the nucleation of microscopic vapour bubbles that grow in the superheated liquid, leading to extreme jet expansion and atomiz...

In this paper we provide insight into the thermophysical properties and the dynamics of cryogenic jets. The motivation of the work is to optimise the use of cryogenic fluids in novel ultra low emission engines. For demonstration, we use conditions relevant to an internal combustion engine currently being developed by Dolphin N2 and the University o...

In this paper we present the basic concepts of operations of a hybrid liquid nitrogen and internal combustion engine currently under development by Ricardo Innovations, Dolphin N2 and the University of Brighton, the CryoPower recuperated split cycle engine (RSCE). The engine is based on a new split-cycle combustion concept utilising isothermal comp...

The work presents a numerical investigation of gasoline direct injection and the resulting early development of spray plumes from an eight-hole injector (Engine Combustion Network Spray G). The objective is to evaluate the impact on the droplet size distribution (DSD) statistics from the assumed model physics, particularly for the small scales. Two...

Producing reliable in-cylinder simulations for quick-turnaround engine development for industrial purposes is a challenging task for modern computational fluid dynamics, mostly because of the tuning effort required for the sub-models used in the various frameworks (the Reynolds-averaged Navier–Stokes and large eddy simulation). Tuning is required b...

As the concern for the environment is increasing and the regulations around harmful emissions are becoming more strict, novel Internal Combustion Engine (ICE) concepts are becoming an absolute necessity in order to keep high thermal efficiencies while reducing the pollutant emissions. Moving towards more radical engine designs based on novel thermo...

Flash boiling can occur in rocket thrusters operating in the vacuum of space when cryogenic propellants are injected into the reaction chamber that is initially at low pressure. The dynamics of this process will determine the spray breakup that will then drastically affect the mixing of fuel and oxidizer, the reliability of the ignition and the sub...

The interaction of turbulence and cavitation inside internal nozzle geometries has considerable influence on the subsequent near-nozzle spray formation and break-up, yet it is an ever present challenge to concisely define its role. A numerical framework comprising of three compressible fluid phases with a probabilistic approach to define their surf...

The physics of high-speed liquid jets injected in elevated temperature and pressure conditions are extremely complex due to the multi-scale and multi-phase flow characteristics. Large eddy simulations (LES) are widely applied for simulations of multi-phase flows because large scale mixing of ambient gas with the liquid vapour (when evaporation is o...

In this paper, a new model for closing the sub-grid reaction rate is proposed based on the series expansion of the chemical source term around the filtered value. For validation, large eddy simulations of a bluff-body stabilised premixed flame are performed at three different grid resolutions, and results are compared with experimental data. Simula...

Lean premixed stratified combustion is rapidly growing in importance for modern engine designs. This paper presents large eddy simulations for a new burner design to assess the predictive capability of the probability density function (pdf) approach to flames that propagate through non-homogeneous mixtures in terms of equivalence ratio. Although va...

Producing reliable in-cylinder simulations for quick turnaround engine development for industrial purposes is a challenging task. With the ongoing paradigm shift towards digital engineering, industry is forced to adjust its development and manufacturing processes away from prototyping and reliance on test bed results towards a virtual environment w...

In this study, a combined experimental and Large Eddy Simulation (LES) investigation is performed to identify the vortical structures, their dynamics, and interaction with a turbulent premixed flame in a swirl-stabilized combustor. Our non-reacting flow experiment shows the existence of large scale precessing motion, commonly observed for such flow...

With the ever increasing operating pressures of modern fuel injectors, the spray formation processes are increasingly influenced by the inception and development of cavitation inside the injector orifices and nozzle. The turbulent effects that contribute to the jet formation and subsequent break-up processes are enhanced by the growth and destructi...

An integrated Design of Experiments (DoE) with Reynolds Averaged Navier Stokes (RANS) approach is suggested and implemented to model turbulent spray combustion. In the automotive industry, DoE is often combined with an optimizer and is used to find an optimum set of internal combustion engine calibration parameters for set criteria at reduced exper...

The atomisation process involves a broad range of length and time scales, which make impractical from computation point of view the use of Detail Numerical Simulation to describe atomisation phenomena at all scales. In this work we use Large Eddy Simulations, where the large scales are resolved but the small scales require closure. For the small sc...

Flash boiling can occur in rocket thrusters used for orbital manoeuvring of spacecraft as the cryogenic propellants are injected into the vacuum of space. For reliable ignition, a precise control of the atomization process is required as atomization and mixing of fuel and oxidizer are crucial for the subsequent combustion process. This work focuses...

The physics of high speed liquid jets injected in elevated temperature and pressure conditions are extremely complex due to the multi-scale and multi-phase flow characteristics. Large eddy simulations (LES) are widely applied for simulations of multi-phase flows because large scale mixing of ambient air with the liquid vapour (when evaporation is o...

Phase change phenomena resulting in bubble formation and collapse inside micro channels influence the subsequent jet formation in spray injection processes through their effect on the local flow. Flow turbulence, that contributes to the jet formation and break-up processes can be enhanced by the growth and violent collapse of cavitation bubbles. At...

A numerical framework for modelling micro-scale multiphase flows with sharp interfaces has been developed. The suggested methodology is targeting the efficient and yet rigorous simulation of complex interface motion at capillary dominated flows (low capillary number). Such flows are encountered in various configurations ranging from micro-devices t...

This chapter presents the basic theory and conceptual evolution of the multiple mapping conditioning (MMC) framework, and presents recent applications for turbulent reactive flows. MMC was initially formulated as a method that integrates the probability density function (PDF) and conditional moment closure (CMC) models through a generalisation of m...

Droplet impact on porous media has a broad range of applications such as material processing, drug delivery and ink injection etc. The simulation studies of such processes are rather limited. To represent the spreading and absorption process of the droplet on porous materials, robust numerical schemes capable of accurately representing wettability...

Designing future ultra-high efficiency, ultra-low emission engines requires an in depth understanding of the multi- scale, multi-phase phenomena taking place in the combustion chamber. The performance of the fuel delivery system is key in the air fuel mixture formation and hence the combustion characteristics, however in most spray modelling approa...

Cavitation is a phenomenon affected considerably by the underlying pressure waves that occur on similar timeand length scales as the bubble dynamics. Thus appropriate representation of wave dynamics within numericalframeworks is of paramount importance for the prediction of the phase change process in the nozzle as well as thesubsequent spray forma...

Two principal methods have been used to simulate the evolution of two-phase immiscible flows of liquid and gas separated by an interface. These are the Level-Set (LS) method and the Volume of Fluid (VoF) method. Both methods attempt to represent the very sharp interface between the phases and to deal with the large jumps in physical properties asso...

A hybrid Euler/Lagrange approach is used to model stratified lean premixed combustion in a turbulent flow. Large eddy simulations (LES) are coupled with an artificially thickened flame (ATF) approach for the computation of the reaction progress variable. This approach is combined with a sparse Lagrangian particle method for the modelling of the inn...

Much progress has been made in large-eddy simulation (LES) of turbulent combustion in the last two decades, but a robust and cost-effective LES formulation is still lacking for turbulent combustion in practical configurations. In this paper, we present an assessment of different sgs models and the no sgs approach within the context of LES of a back...

Diesel engines are the most efficient combustion engines today, playing an important role in transport of goods and passengers on land and on sea. The main drivers for the design of future engines is the control of the emissions without sacrificing their advanced fuel economy. These drivers led to important innovations in multiple components of the...

Numerical simulations are often used to understand spray atomisation and estimate the size of the liquid fragments.Several techniques (Level Set, Volume of Fluid, Smooth Particle Hydrodynamics, among others) exist to compute multiphase flows and potentially represent liquid-break-up. However, the complexity of the breakup process and the wide range...

Lean premixed combustion mode under mixture stratification conditions is rapidly growing in importance for modern engine designs. The current work fo-cuses on a numerical framework based on Large Eddy Simulation for a new burner design in order to shed light to the 3D macroscopic characteristics of flames that propagate through non homogeneous mixt...

The ability to accurately predict the dynamics of a droplets impinging on solid and porous surfaces remains of interest due to a number of important applications including material processing, ink printing, drug delivery, fuel injection etc.. In the current work we present the numerical investigation of four cases of such droplets (one based on imp...

A stochastic implementation of the multiple mapping conditioning (MMC) model has been used for the modelling of turbulence–chemistry interactions in a series of turbulent jet diffusion flames with varying degrees of local extinction (Sandia Flames D–F). The mapping function approximates the cumulative probability distribution of mixture fraction an...

In this paper we present a numerical framework in the Large Eddy Simulation context for modelling cavitation dynamics at various back pressures. The focus is on cavitation present in micro-orifices such as modern fuel injectors. The numerical simulation of the two-phase flow is performed following the so called "one-fluid approach". Two different m...

Micro-droplet formation and mobilization is an emerging area of research due to its wide-ranging usage within microfluidics application relevant to tight porous media (Andrew et al., 2015). Physical understanding of this process at the pore scale is helpful in exploring macroscopic phenomena in oil and gas recovery and also forms the basisofmany po...

A stochastic implementation of the Multiple Mapping Conditioning (MMC) approach has been applied to a turbulent piloted jet diffusion flame (Sandia flame F) that is close to extinction. Two classic mixing models (Curl’s and IEM) are introduced in the MMC context to model the turbulent mixing. The suggested model involves the use of a reference spac...

Flow and transport of viscous fluids through media with different porosity is of interest to many sectors of oil and gas industry [1]. Micro models are useful tools to directly visualize the detailed pore scale flow field in saturated porous media of the size of micrometres at geological reservoir conditions. In this study we numerically simulate s...

Modelling fluid flow in rock porous media is a challenging physical problem. Simplified macroscopic flow models, such as the well-known Darcy's law, fail to predict accurately the pressure drop because many flow parameters are not considered while simplifications are made for the multi-scale structure of the rocks. In order to improve the physical...

The implementation of reduced syngas combustion mechanisms in numerical combustion studies has become inevitable in order to reduce the computational cost without compromising the predictions' accuracy. In this regard, the present study evaluates the predictive capabilities of selected detailed, reduced and global syngas chemical mechanisms by comp...