Stochastic and deterministic multiple mapping conditioning for turbulent reacting jets

The work presented in this thesis explores the feasibility of the Multiple Mapping Conditioning (MMC) approach and its closures for real (laboratory) flames. Three different configurations with relatively high Reynolds numbers but without considerable degree of extinction and re-ignition are investigated, and results are compared against experimental measurements of mixing and reactive scalar fields and other commonly used models. MMC combines the probability density function (PDF) approach and the conditioning methods via the application of a generalised mapping function to a prescribed reference space. Stochastic and deterministic formulations of MMC exist. Both formulations have been explored here for the case of one dimensional Gaussian reference space that is associated with the evolution of mixture fraction. The chemically reactive species are implicitly conditioned on mixture fraction, and their fluctuations around the conditional means are neglected for the deterministic approach and modelled for the stochastic approach. Regarding the velocity field evolution, the Reynolds Averaged Navier-Stokes equations are solved with a k-[epsilon]turbulence model. In the deterministic context, this work evaluates the ability 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 [Beta]-PDF. Computed probability distributions agree well with measurements, and a detailed comparison of the modelled conditional and mean scalar dissipation with experimental data and conventional closures demonstrate MMC’s potential. Predictions of reactive species and temperature are in good agreement with experimental data and similar in quality to singly-conditioned, first-order CMC predictions. MMC therefore provides an attractive -since consistent- alternative approach for the modelling of scalar mixing in turbulent reacting flows. In the stochastic context the evolution of the reference space is described by a Markov process that is coupled with a full PDF method for joint scalar evolution. A modified IECM model is applied for the modelling of the mixing operator where the particles mix with their means conditioned on the reference space. The formulation of the closure leads to localness of mixing in the mixture fraction space and consequently localness is expected to be improved in the composition space. Focus is given on the accurate prediction of scattering around the conditional means. Results demonstrate the potential of the method, however some discrepancies are noted in the predictions that can probably be associated with the chemical mechanism and the uncertainties associated with the choice of the minor dissipation time

Assessing the Effect of Differential Diffusion for Stratified Lean Premixed Turbulent Flames with the Use of LES-PDF Framework

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 various efforts have been made in the past for the simulation of the same test case the novelty of this work lies to the fact that it is the first simulation effort that differential diffusion is accounted for given the relatively low Reynolds numbers (13,800) of the configuration. First mean and root mean square velocity simulations are performed for the isothermal cases to assess the effect of the grid resolution and the overall LES flow field solver. Then instantaneous snapshots of the flame are presented to provide insight to the structure of the flame and the effect of stratification. Finally, results for velocities, temperature and mixture fraction are presented and compared with the experimental data. Overall, the results are in very good agreement with experiments

Numerical Treatment of the Interface in Two Phase Flows Using a Compressible Framework in OpenFOAM:Demonstration on a High Velocity Droplet Impact Case

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. The framework incorporates the compressible algebraic Volume-of-Fluid (VoF) method with additional interfacial treatment techniques including volume fraction smoothing and sharpening (for the calculation of the interface geometries and surface tension force, respectively) as well as filtering of the capillary forces. The framework is tested against different benchmarks: A 2D stationary droplet, a high velocity impact droplet case (500 m/s impact velocity) against a dry substrate and, with the same impact conditions, against a liquid film. For the 2D static droplet case, our results are consistent with what is observed in the literature when these strategies are implemented within incompressible frameworks. For the high impact droplet cases we find that accounting for both compressibility and correct representation of the interface is very important in numerical simulations, since pressure waves develop and propagate within the droplet interacting with the interface. While the implemented strategies do not alter the dynamics of the impact and the droplet shape, they have a considerable effect on the lamella formation. Our numerical method, although currently implemented for droplet cases, can also be used for any fast moving interface with or without considering the impact on a surface

Modelling of Turbulent Premixed Stratified Combustion with Multiple Mapping Conditioning Mixing Mode

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 inner flame structure. A multiple mapping conditioning (MMC) mixing model is applied to prevent direct mixing across the flame front. Predicted flame structures are compared with measurements of a stratified premixed laboratory flame yielding good agreement and demonstrating the model’s capability to predict relatively thin flames and to approximate a flamelet-like inner flame structure

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

[EN] 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.This work was supported by the UK’s Engineering and Physical Science Research Council [grants EP/P012744/1 and EP/K020528/1].Vogiatzaki, K.; Crua, C.; Morgan, R.; Heikal, M. (2017). A study of the controlling parameters of fuel air mixture formation for ECN Spray A. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 2-9. https://doi.org/10.4995/ILASS2017.2017.4703OCS2

Assessment of Subgrid-Scale Model Effects on Large Eddy Simulation of a Back-Step Combustor

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 backward step combustor. Overall, the dynamic one equation eddy model behaves better than the WALE and one equation eddy models, in both reproducing main features and statistical quantities of the non-reactive and reactive flow fields. Increasing grid resolution does not necessarily improve the predictions. The results are largely dependent on whether the local flow is turbulence or combustion dominated. This implies that along with an adaptive grid refinement, an adaptive combustion model strategy is needed. In combustion simulation, applying only the first term in the series model is insufficient to well predict the dominating features and statistical quantities of the reacting flows. Thus, we suggest as future work the introduction of additional adaptive terms that will control the variance