Large Eddy Simulation Of Partially Premixed Flame Based On DTF Model

In practical combustion devices,complete premixing is difficult to achieve in the mixture of fuel and air.Therefore,the research on partially premixed flame has an important engineering value and useful purpose.This thesis employed DTF(Dynamic Thickened Flame)combustion model coupled with LES method to study two important and different types of the partially premixed flame with inhomogeneous inlets cases: FA and FJ,available on University of Sydney database.In this work,we not only improved the DTF combustion model,but also systematically analyzed the flame structure and local flameout phenomenon and predicted the extinction limits,so that a better understanding obout partially premixed flame characteristics can be obtained.In this thesis,we calculated series of freely propagating premixed flames by GRI3.0 detailed mechanism,so the changes in the laminar flame thickness and the flame propagation speed with different mixed fraction can be obtained.Based on these,two fitting functions were obtained to modify the DTF model,and the improved model could better capture the partially premixed flame.Next,the accuracy and efficiency of JL1,JLR,19 steps and 28 steps of methane/air reaction mechanism comprehensively evaluated in one-dimensional flame simulation.The results show that the combination of 19 steps reaction mechanism and DTF model is a good coupling scheme,which can meet the requirements of the subsequent partially premixed flame calculation.The Sydney flame results show: in the FJ flame case with the blowout ratio equals to 0.9,the original DTF model overestimated the combustion state,which is far away from the central place and,about 200 K higher than the experimental values.While the modified DTF model has improved the ability of predicting the temperature field at same place.The analysis of flame blowout ratio equals to 0.7 shows: in the FA flame case,the flameout phenomenon firstly appeared at x/D=7;while in FJ flame case,the flameout firstly appeared at x/D=13.In addition,the "holes" on the flame surface caused by the local flameout of two cases are obviously different.The former has large and sparse holes,while the latter has small and dense holes.This make great differences of the flame stability between FA / FJ layout,while FJ case has about twice blowoff value than FA case.From the scatter plots of temperature versus mixed fraction,we found that in the FJ flame case,stratified premixed combustion existed in upstream,and in downstream locations,it gradually transformed into diffusion dominated combustion;while the FA flame exhibit stratified combustion at upstream.Both flame cases have more and more points appeared between premixed branche and diffusion branche with developing of the flow field,which indicates that local flameout has become serious.Also with the increase of jet velocity,the two flames became more and more dangerous until all extinguished.The statistical data of velocity,temperature,species and mixed fraction calculated by Large Eddy Simulation(LES)are in good agreement with the experimental results.Meanwhile,LES also well predicted the local flameout and flame extinction under FA/ FJ layout.The calculated extinction velocity of FA/ FJ flame case are 74m/s and 128m/s respectively,which are close to the experimental results of 64.5m/s and 114.3m/s.Obviously the FJ flame layout has better flame stability than the FA flame case.In order to check the model and method used in thesis quantitatively,newly developed Wasserstein metric analysis method was used to conduct multi-dimensional joint statistical analysis on simulation results,it also shows that the accuracy of present work is satisfactory.