Browsing by Author "Ukai, Satoshi"

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    Conditional moment closure modelling of turbulent spray flames
    (2014) Ukai, Satoshi; Kronenburg, Andreas (Prof. Dr.)
    The scope of this thesis is to develop an improved methodology for the simulation of turbulent spray flames. Spray combustion is a typical multi-scale problem. It is practically impossible to resolve all physical scales, and appropriate models need to be used for the subgrid scales. Here, large eddy simulation (LES) for the computation of the flow field, conditional moment closure (CMC) for the modelling of turbulence-chemistry interactions, and a Lagrangian particle tracking approach with stochastic droplet modelling for transport and evaporation of the droplets are combined to form a comprehensive spray combustion model. The LES flow solver and the liquid phase models have been validated by comparison with experimental data from an evaporating spray jet. The numerical predictions show good agreement with the measurements. The influence of the stochastic particle dispersion and evaporation models is assessed. The stochastic dispersion does not have large effects on the droplet dispersion statistics probably due to low levels of turbulence in the cases investigated here. However, higher evaporation rates are seen when the stochastic evaporation model is used. In a further step, the effects of additional terms in the CMC formulation that arise due to the presence of the evaporating droplets are investigated by comparison with experiments from a series of turbulent ethanol spray flames. Overall, the numerical predictions show good agreement with measurements, but large discrepancies of centerline temperature are found in downstream regions of the flow. The reasons can be found in the rather simplistic conventional boundary conditions used in this first study. The simplistic boundary treatment may suffice for simple gaseous flames or spray flames without pre-evaporation. However, it is not applicable to the spray flames under investigation here, and a new boundary treatment of the upper limit in mixture fraction space is necessary. Therefore, two novel approaches are proposed and developed for the consistent CMC modelling of spray flames with pre-evaporation. The first model is a two-conditional moment approach. It solves for two sets of conditional moments. The first set is conditioned on a fully conserved mixture fraction that does not take droplet evaporation into account. The second set is conditioned on a mixture fraction that is based on the fuel originating from the pre-evaporated droplets plus the fuel evaporated within the combustion chamber. The LES solution can be found by using the weighted average of these two conditional moments and integration across mixture fraction space. The two-conditional moment approach is applied to simulate the turbulent spray flames and the accuracy of the numerical predictions is markedly improved when compared to the conventional approach. The second model is based on a CMC approach coupled with tabulated chemistry. CMC can solve for unsteady and inhomogeneous conditional moments, whereas tabulated chemistry is pre-processed and it is usually not a function of space or time. On the other hand, tabulated chemistry can be constructed over multiple sampling spaces, while CMC is typically conditioned on only one characteristic quantity. Therefore, CMC with tabulated chemistry is developed to couple the advantages of the two approaches. The numerical simulations have again been validated by comparison with experiments, and overall good agreement with all available experimental data are obtained. In conclusion, a new mixture fraction boundary treatment and two novel CMC approaches have been developed that expand the applicability of CMC to spray flames with partial pre-evaporation of the fuel. The new models are validated by comparison with measurements from a spray flame series conducted at the University of Sydney. Future work will seek the extension of the current approaches to more complex flame regimes such as partially premixed spray flames.