Numerical Prediction of Turbulent Spray Flame Characteristics Using the Filtered Eulerian Stochastic Field Approach Coupled to Tabulated Chemistry

The Eulerian stochastic fields (ESF) method, which is based on the transport equation of the joint subgrid scalar probability density function, is applied to Large Eddy Simulation of a turbulent dilute spray flame. The approach is coupled with a tabulated chemistry approach to represent the subgrid turbulence–chemistry interaction. Following a two-way coupled Eulerian–Lagrangian procedure, the spray is treated as a multitude of computational parcels described in a Lagrangian manner, each representing a heap of real spray droplets. The present contribution has two objectives: First, the predictive capabilities of the modeling framework are evaluated by comparing simulation results using 8, 16, and 32 stochastic fields with available experimental data. At the same time, the results are compared to previous studies, where the artificially thickened flame (ATF) model was applied to the investigated configuration. The results suggest that the ESF method can reproduce the experimental measurements reasonably well. Comparisons with the ATF approach indicate that the ESF results better describe the flame entrainment into the cold spray core of the flame. Secondly, the dynamics of the subgrid scalar contributions are investigated and the reconstructed probability density distributions are compared to common presumed shapes qualitatively and quantitatively in the context of spray combustion. It is demonstrated that the ESF method can be a valuable tool to evaluate approaches relying on a pre-integration of the thermochemical lookup-table

Turbulent ethanol spray flame simulations with k-E turbulence model.

Diversos equipamentos industriais utilizam processos de combustão com sprays. As principais vantagens deste processo estão relacionadas ao aumento do controle da chama e à maior segurança na logística do combustível líquido. Atualmente, o interesse na utilização de bio-combustíveis como alternativa para a redução na emissão de dióxido de carbono é crescente. Entre os tipos de bio-combustíveis o etanol se destaca por ser utilizado em vários países misturado à gasolina no setor de transportes. Partindo deste panorama, o presente trabalho apresenta a modelagem e simulação de uma chama turbulenta de spray de etanol. Os resultados das simulações realizadas são comparados com dados experimentais da literatura. O modelo resultante baseia-se no método dos volumes finitos para escoamentos com baixo número de Mach e em regime permanente. O spray foi calculado com a aproximação de escoamentos separados com uma formulação Euler-Lagrange, em que a fase dispersante é modelada com a abordagem Euleriana, enquanto que a fase dispersa é modelada com a abordagem Lagrangeana. As duas fases foram completamente acopladas nos dois sentidos. O modelo de turbulência k- Padrão foi utilizado na fase dispersante. A evaporação de gotículas foi considerada, em que o modelo de condutividade infinita foi utilizado para a fase líquida. Dessa forma, a distribuição de temperaturas no interior da gotícula é uniforme, porém varia conforme ela se move no spray. Para reproduzir os efeitos do resfriamento evaporativo, a combustão foi modelada com um modelo de folha de chama modificado que considerou uma função joint -PDF de fração de mistura e entalpia. Transferências de calor por radiação foram negligenciadas neste trabalho. Aproximações razoáveis foram obtidas entre os perfis medidos e calculados de temperatura média da fase gasosa e de distribuições de tamanhos de gotículas. Algumas discrepâncias foram observadas nas comparações entre os perfis do componente axial de velocidade média da fase gasosa, que foram atribuídas à difusão superestimada das quantidades médias transportadas pela fase gasosa nas simulações.Several industrial equipments use combustion processes with sprays. The main advantages of this process are related to the increase in the flame control and in the safety of liquid fuel logistics. Currently, the interest on the utilization of biofuels as an alternative to the reduction of carbon dioxide emissions is increasing. Among the types of biofuels the ethanol stands out by being used blended with gasoline in the transport sector of several countries. From this overview, this work presents the modeling and simulation of an ethanol turbulent spray flame. The results of the simulations were compared with experimental data from the literature. The resulting model was based on the finite volume method for low Mach number and steady state flows. The spray was calculated using the Separated Flow method (SF) with an Euler-Lagrange model, where the gaseous phase was described by an Eulerian model and the liquid phase by a Lagrangian particle method. Both phases were fully coupled in order to account for shared effects. The turbulence model k- Standard was used to determine the dispersant phase. Evaporation of droplets was calculated with the assumption of the infinite-liquid-conductivity model, where the droplet inner temperature is uniform, but varies with the mass and heat transfer within the dispersant phase. To reproduce the effects of the evaporative cooling the combustion was modeled with a modified flamesheet model which regarded a jointed mixture fraction-enthalpy -PDF. Radiactive heat transfer was not accounted for in this work. Reasonable agreement between measured and computed mean profiles of temperature of the gas phase and droplet size distributions was achieved. Some deviations were observed in the mean velocity profiles comparisons between experimental data and simulations, which were assigned to the over predicted diffusion of the mean quantities transported by the gas phase

A new robust modeling strategy for multi-component droplet heat and mass transfer in general ambient conditions

International audienceLiquid fuels applied to spray combustion processes are predominantly composed by a mixture of components. Following an injection process, multi-component liquid droplets are subject of heat and mass transfer processes in a multitude of atmosphere compositions, resulting in complex interaction processes. This example based on a spray combustion process summarizes the diversity of scenarios that a droplet may experience in a spray flow. Typically, available models in the literature have limitations to characterize such complex interactions. In view of this, the present work proposes a novel modeling strategy to account for such interactions in diverse scenarios grounded in a consistent computational approach. To accomplish this task, a new formulation is derived from general transport equations of the gas phase. The resulting model is validated by comparing numerical results with available experimental data. Herein, binary mixtures of liquids evaporating in diverse atmospheres are considered. In contrast to other reference approaches, the derived model demonstrates to overcome all tested scenarios, which includes severe atmosphere compositions and states. Additional effects could be observed regarding the different diffusivity among the participating species, which are not only perceived in mass transfer but also on heat transfer. A more comprehensive description of the different underlying phenomena relative to the spray combustion could be obtained with the proposed strategy

Numerical simulation of an ethanol turbulent spray flame\ud with RANS and diffusion combustion model

A detailed numerical simulation of ethanol\ud turbulent spray combustion on a rounded jet flame is pre-\ud sented in this article. The focus is to propose a robust\ud mathematical model with relatively low complexity sub-\ud models to reproduce the main characteristics of the cou-\ud pling between both phases, such as the turbulence\ud modulation, turbulent droplets dissipation, and evaporative\ud cooling effect. A RANS turbulent model is implemented.\ud Special features of the model include an Eulerian–\ud Lagrangian procedure under a fully two-way coupling and\ud a modified flame sheet model with a joint mixture fraction–\ud enthalpy\ud b\ud -PDF. Reasonable agreement between measured\ud and computed mean profiles of temperature of the gas\ud phase and droplet size distributions is achieved. Deviations\ud found between measured and predicted mean velocity\ud profiles are attributed to the turbulent combustion modeling adoptedThe authors would like to thank the Coorde- nac ̧a ̃ o de Aperfeic ̧oamento de Pessoal de Nı ́ vel Superior (CAPES) for supporting of this work under the grant ‘‘CAPES-PRO ́ -ENGENHA- RIAS/ESTUDO COMPUTACIONAL E EXPERIMENTAL DE CHAMAS TURBULENTAS DE ETANOL’’— PE004/2008 and, the Laboratory of Environmental and Thermal Engineering (LETE) of the Polytechnic School of the University of Sao Paul

A new robust modeling strategy for multi-component droplet heat and mass transfer in general ambient conditions

Liquid fuels applied to spray combustion processes are predominantly composed by a mixture of components. Following an injection process, multi-component liquid droplets are subject of heat and mass transfer processes in a multitude of atmosphere compositions, resulting in complex interaction processes. This example based on a spray combustion process summarizes the diversity of scenarios that a droplet may experience in a spray flow. Typically, available models in the literature have limitations to characterize such complex interactions. In view of this, the present work proposes a novel modeling strategy to account for such interactions in diverse scenarios grounded in a consistent computational approach. To accomplish this task, a new formulation is derived from general transport equations of the gas phase. The resulting model is validated by comparing numerical results with available experimental data. Herein, binary mixtures of liquids evaporating in diverse atmospheres are considered. In contrast to other reference approaches, the derived model demonstrates to overcome all tested scenarios, which includes severe atmosphere compositions and states. Additional effects could be observed regarding the different diffusivity among the participating species, which are not only perceived in mass transfer but also on heat transfer. A more comprehensive description of the different underlying phenomena relative to the spray combustion could be obtained with the proposed strategy