Numerical simulation of spray combustion considering soot formation and thermal radiation

Abstract

Abstract The main aim of this work is to simulate the combustion process through a model gas turbine combustor. The effects of mass flow distribution and inlet air swirl on soot formation, temperature distribution, species concentration and the efficiency of combustor have been discussed thoroughly in this work. Moreover the selection of the turbulent model among different two equation turbulent models for the simulation of swirl flow has been studied. It is validated with the experimental results obtained by Rahim et al.[126]. It has been found that Realizable k-ɛ model is the best suited turbulent model for mild swirled flow. This turbulent model is used throughout the simulation of combustion processes. LISA model is used to simulate the atomization process of a pressure swirl atomizer. Eulerian-Lagrangian frame has been considered to simulate the interaction in between droplet phase and continuous phase. Modelling of soot is one of the major contributions of this work. A semi-empirical two equation soot model is used [98] to simulate the soot in flame. The source term has been modified for kerosene fuel (C12H23). It is compared with the experimental results of Young et al. [142] and the computational results of acetylene and PAH model of Wen et al. [104]. In this work, benzene and phenyl are not considered as surface growth species. Here acetylene is considered as a precursor and surface growth species as well. The predictions through our proposed acetylene model becomes very good with experiments. However the constants proposed by Brookes and Moss for methane fuels are modified to a suitable value. Discrete ordinate (DO) radiation model is used to simulate radiation effect. The gas is considered as participating media. Effect of soot in radiation also has been incorporated. Three different combustion models such as: Eddy dissipation model, Laminar flamelet model and Constrained equilibrium model are compared with the experimental results which has been obtained from the experiments conducted in our laboratory. It has been found that Eddy dissipation model predicts very poor as compare to other two models. However a major conclusion from this part is that, the Constrained equilibrium model can predicts good if a proper value of RFL (rich flammability limit) will be chosen. On the other hand Laminar flamelet model does not need any such uncertain parameters and still it predicts nicely. But the computational economy of this model is little poor as compared to Constrained equilibrium model. However a detailed chemical kinetics is required to simulate in Laminar flamelet model. The effect of airflow distribution on flame formation and heat transfer through model gas turbine combustor has been studied. It has been found that more air supply through primary swirled inlet decreases the flame length. When the primary air increases to 50%, the flame is broken. Therefore with a higher supply of primary air, the centreline temperature decreases. Moreover in same case the soot formation also becomes lesser but the wall temperature increases. Due to the higher wall temperature heat loss increases through wall. Therefore the overall combustor efficiency decreases but the pattern factor at outlet becomes better. The effect of radiation on injector tip has also been studied. The effect of swirl level on soot and heat transfer also has been studied. It has been depicted that, at lower swirl, the flame length increases. As the swirl level increases the flame length become shorter. At a certain swirl level (60o vane angle), the flame is broken and the centreline temperature decreases. However the soot formation does not affect much with swirl level. But at higher swirl the soot laden zone become closer to injector tip. The injector plane area also becomes hotter at swirl level increases. The combustor efficiency is being better at lower swirl due to comparatively a small heat loss has been occurred in this case. The species mass fraction distributions along the axis also have been studied. The effect of radiation on injector tip has also been studied.