Konstantina Vogiatzaki

Modeling of scalar mixing in turbulent jet flames by multiple mapping conditioning

Multiple mapping conditioning (MMC) combines the probability density function (PDF) and the conditional moment closure (CMC) methods via the application of a generalized mapping function to a prescribed reference space. Stochastic and deterministic formulations of MMC exist, and the deterministic implementation has been applied here to a piloted jet diffusion flame (Sandia Flame D). This paper focuses on the feasibility of MMC and its closures for real (laboratory) flames and a relatively simple one-dimensional reference space that represents mixture fraction has been used. The remaining chemically reactive species are implicitly conditioned on mixture fraction and their fluctuations around the conditional mean are neglected. This work primarily evaluates the ability of the deterministic form of MMC to provide accurate and consistent closures for the mixture fraction PDF and the conditional scalar dissipation which do not rely on presumed shape functions for the PDF such as the commonly used β-PDF. Comput...

The effect of timescale variation in multiple mapping conditioning mixing of PDF calculations for Sandia Flame series D–F

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 and the corresponding variance can be controlled by a standard implementation of the scalar mixing timescale. The conditional fluctuations are controlled by a minor dissipation timescale, τmin. The results show a clear dependence of the conditional fluctuations on the choice of the minor timescale, and the appropriate value for turbulent jet flames is similar to values determined in related direct numerical simulation (DNS) studies of homogeneous turbulent reacting flows. The predictions of means and variances of temperature and species mass fractions are very good for all flames, indicating an appropriate modelling of the conditional variances. Further sensitivity studies with respect to particle number density demonstrate a relative insensitivity of the results to the particle number in the numerical solution procedure. Good results can be obtained with as few as 10 particles per cell, allowing for a computationally inexpensive implementation of a Monte Carlo/probability density function (PDF) method.

Evaluation of the Accuracy of Selected Syngas Chemical Mechanisms

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 comparing the numerical results with experimental laminar flame speed values of lean premixed syngas flames. The comparisons are carried out at varying equivalence ratios, syngas compositions, operating pressures, and preheat temperatures to represent a range of operating conditions of modern fuel flexible combustion systems. NOX emissions predicted by the detailed mechanism, GRI-Mech. 3.0, are also used to study the accuracy of the selected mechanisms under these operating conditions. Moreover, the selected mechanisms’ accuracy in predicting the laminar flame thickness, species concentrations of the reactants, and OH profiles at different equivalence ratios and syngas compositions are investigated as well. The laminar flame speed is generally observed to increase with increasing equivalence ratio, hydrogen content in the syngas, and preheat temperature, while it is decreased with increasing operating pressure. This trend is followed by all mechanisms understudy. The global mechanisms of Watanabe-Otaka and Jones-Lindstedt for syngas are consistently observed to over-predict and under-predict the laminar flame speed up to an average of 60% and 80%, respectively. The reduced mechanism of Slavinskaya has an average error of less than 20% which is comparable to the average error of the GRI-Mech. 3.0. It however overpredicts the flame thickness by up to 30% when compared to GRI-Mech. 3.0. The NO prediction by Li mechanism and the reduced mechanisms are observed to be within 10% prediction range of the GRI-Mech. 3.0 at intermediate equivalence ratio (φ = 0.7) up to stoichiometry. Moving towards more lean conditions, there is significant difference between the GRI-Mech. 3.0 NO prediction and those of the reduced mechanisms due to relative importance of the prompt NOX at lower temperature compared to thermal NOX that is only accounted for by the GRI-Mech. 3.0.

Evaluation of two-phase flow solvers using Level Set and Volume of Fluid methods

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 associated with it. Both methods have their own strengths and weaknesses. For example, the VoF method is known to be prone to excessive numerical diffusion, while the basic LS method has some difficulty in conserving mass. Major progress has been made in remedying these deficiencies, and both methods have now reached a high level of physical accuracy. Nevertheless, there remains an issue, in that each of these methods has been developed by different research groups, using different codes and most importantly the implementations have been fine tuned to tackle different applications. Thus, it remains unclear what are the remaining advantages and drawbacks of each method relative to the other, and what might be the optimal way to unify them. In this paper, we address this gap by performing a direct comparison of two current state-of-the-art variations of these methods (LS: RCLSFoam and VoF: interPore) and implemented in the same code (OpenFoam). We subject both methods to a pair of benchmark test cases while using the same numerical meshes to examine a) the accuracy of curvature representation, b) the effect of tuning parameters, c) the ability to minimise spurious velocities and d) the ability to tackle fluids with very different densities. For each method, one of the test cases is chosen to be fairly benign while the other test case is expected to present a greater challenge. The results indicate that both methods can be made to work well on both test cases, while displaying different sensitivity to the relevant parameters.

Validation study of large-eddy simulations of wake stabilized reacting flows using artificial flame thickening approaches

Wake flows are the preferred mode of flame stabilization in lean premixed combustion in gas turbine engines, low NOx burners, afterburners etc. These flows exhibit inherent unsteadiness and for their numerical modeling and simulations, large eddy simulation (LES) techniques with an appropriate combustion model and reaction mechanism afford a balance between computational complexity and predictive accuracy. Before using them in practical systems, these techniques must be validated against experimental measurements in a number of canonical cases. In this work, results from LES of non-reacting and reacting flows are compared to data from a number of experiments, corresponding to the following configurations: a triangular bluff body in a rectangular duct, a backward facing step, and a cylindrical sudden expansion with swirl. The artificial flame thickening approach is applied for modeling turbulence-combustion interactions at small scales. Algebraic and equationbased efficiency function models are implemented, along with an appropriate reduced chemistry mechanism. A novel dynamic formulation for the efficiency function based on the flame-wrinkling equation that explicitly incorporates the influence of strain and time-history effects is proposed, and a detailed combustion chemistry mechanism is also used. Results show that the approaches are effective in simulating turbulent premixed combustion.

Simulation of droplet spreading on micro-CT reconstructed 3D real porous media using the volume-of-fluid method

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 as well as capillary effects need to be established. The current work, presents one of the first studies of droplet impact on a real porous media geometry model extracted from a micro-CT scan. The process involves processing of CT image and subsequent threshold based on the structures segmentation. The porous geometry is extracted in the form of a STL (STereoLithography) model, which, with the aid of dedicated software like ANSA and SnappyHexMesh, is converted to an unstructured mesh for successful discretization of the flow domain. The solution algorithm is developed within the open source CFD toolbox OpenFOAM. The numerical framework to track the droplet interface during the impact and the absorption phases is based on previous work. The volume-of-fluid (VOF) method is used to capture the location of the interface, combined with additional sharpening and smoothing algorithms to minimise spurious velocities developed at the capillary dominated part of the phenomenon (droplet recession and penetration). A systematic variation of the main factors that affect this process are considered, i.e. wettability, porous size, impact velocity. To investigate the influence of porous structures on droplet spreading, the average porosity of the media is varied between 18.5% and 23.3% . From these numerical experiments, we can conclude that the droplet imbibition mainly depends on the porous wettability and secondly that the recoiling phase can be observed in the hydrophobic case but not in the hydrophilic case.

Theory and Application of Multiple Mapping Conditioning for Turbulent Reactive Flows

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 mapping closure. MMC models utilise a reference space, whose PDF is prescribed a priori or which is simulated by some means such as a Markov diffusion process. The turbulent fluctuations of all scalars in this method are divided into major and minor groups, and the former are associated with the reference space via a mapping function. The reference space describes a low-dimensional manifold which can fluctuate in any given way, while the fluctuations of the (real) scalars are fully or partially confined relative to that reference space. The dimensionality of the reference space is usually small. For example, in non-premixed combustion a reference space emulating the mixture fraction usually suffices. There are both conditional and probabilistic conceptualisations of MMC and both deterministic and stochastic mathematical formulations. In the past decade, an extension of probabilistic MMC has emerged that is known as generalised MMC that removes some of the formality of the original formulation and extends the type and usage of the reference variables. Generalised MMC is commonly associated, although not exclusively, with large eddy simulations (LES). This chapter reviews the conceptual and theoretical advances in MMC since its original formulation and also reviews some of the recently published applications of MMC in turbulent reactive flows.

Detailed simulation of air-assisted spray atomization: effect of numerical scheme at intermediate Weber number

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 of scales prevents the use of an unified approach to simulate the complete spray. Numerical techniques face different challenges depending on the spray characteristics. The incorrect representation of surface forces in capillary dominated flows, creates large parasitic currents that distort and in some cases destroy the interface. Methods that perform well in the capillary regime aim to capture the interface directly and the surface radius cur- vature is therefore larger than the mesh size. However, this creates large constrains on the mesh resolution and limits its applications to low Weber number flows, when there is no extensive atomization. Methods that simulate large Weber number flows (typical of industrial injectors) do not resolve the interface directly and the mesh is larger than the smallest radius of curvature. These models often have numerical or artificial diffusion that destroys small scale structures and alters the break-up. However, even at large Weber flows, the spray formation can be affected by errors due to the local imbalance between pressure and surface tension forces and interface curvature errors. Numerical schemes work around these problems by adjusting the amount of numerical diffusion of the scheme depending on the spray application. Intermediate Weber number sprays are well suited to study the performance of numerical methods as they exhibit hybrid behaviour between capillary flows and full atomization. In the present work an intermediate gas Weber of a laboratory air-blast atomiser is investigated using a volume of fluid approach. The amount of numerical diffusion is controlled by a compressive factor in the volume of fluid transport equation. The effect of the compressive term on spray atomization and droplet size distribution is explored. The results suggest that the optimal amount of diffusion depends on the local Weber number.

A study of the controlling parameters of fuel air mixture formation for ECN Spray A

Designing future ultra-high efficiency, ultra-low emission engines requires an in depth understanding of the multiscale, 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 approaches is not considered directly. Thus, it is important to understand how the selection of models that mimic injection process affect predictions. In this paper we present an Eulerian-Lagrangian framework based on OpenFOAM libraries to model spray injection dynamics. The framework accounts for primary droplet formation (based on a parcel method with predefined initial droplet size distribution), secondary droplet breakup, evaporation and heat transfer. In order to account for the interaction of droplets with turbulence, simulations were performed within the LES context with two different turbulence models. A systematic variation of the key injection parameters (parcel number, parcel size distribution) of the parcel method as well as the grid size was considered. Varying the parcel number affects the initial droplet size distribution which in turn, depending on the selection of the turbulence and the evaporation sub-models, affects: spray dispersion; spray penetration; and subsequent droplet size distribution. Results were validated against the baseline experimental data for evaporating ECN Spray A with n-dodecane chosen as a surrogate for Diesel fuel.