Large Eddy Simulation Of Spray-flame Interactions

Turbulent two-phase flow plays a critical role in the combustion process of aeroengines and internal combustion engines.It is related to very complex multiscale multi-physicochemical processes,including fuel injection,evaporation,atomization,breakup,collision,autoignition,flame stabilization and so on.The strong interactions among these processes determine the economical efficiency and pollutant emissions.In this thesis,large-eddy simulation（LES）was employed for spray flames under engine-like conditions based on the experiments from the Engine Combustion Network（ECN）.Focus was given on the turbulent spray flame interactions and flame stabilization process,which can be used to organize advanced combustion strategies.Firstly,a high-accuracy numerical model（LES-LEM）was developed based on the KIVALES code by combing a third-order Monotone Upstream-centered Schemes for Conservation Laws（MUSCL）differencing scheme and the linear eddy model（LEM）.Compared to the Quasi-second-order upwind（QSOU）scheme,the MUSCL method shows better performance in the prediction of turbulent vertex structures and the mixing process between fuel and air.The combination of LEM model with the LES method improves the prediction of the ignition delay（ID）and flame lift-off length（LOL）.The prediction of LOL improves by more than 20%at a low temperature of 850 K.The further study shows that the lowest differences in the ID and flame LOL between the predicted and measured results are as low as 7.4%and 4%at different temperatuers,oxygen concentrations and densities for different fuels without changing the parameters for spray combustion models.Secondly,the mult-stage ignition processes and the stabilization process under different conditions were studied by the LES-LEM method.Results show that the initial gas temperature determine the mixture activity and the initial ignition location.At 850K,the activity for fuel/air mixture is very low due to the limited heat absorbed from the ambient gas,thus restricting the chemical reaction rate during the early stage.The longer mixing time leads to fuel-lean ignition for the low-temperature combustion.However,the mixture activity is much higher because more heat can be absorbed from the ambient gas and the first-stage ignition is initiated at fuel-rich regions.A further discussion on the local flow time scale and chemical time scale was conducted based on the Chemical Explosive Mode Analysis（CEMA）method.Results show that a balance is observed between flow time scale and chemical time scale,indicating that‘cool flame’propagates through the flow field during the eary oxidization stage.When high-temperature reactions occur,the chemical time scale is much shorter than the local flow time scale,indicaint that autoignition controls the stabilization mechanism during the quasi-steady state.Thirdly,the turbulent spray flame interactions are investigated by using the doule-injection strategies at different temperatures,and different injection strategies（duration and dwell time（DT））.Results show that the mixture activity is strongly restricted at750 K and high-temperature reactions only occur after the end of the second injection,leading to fuel-lean premixed combustion.However,high-temperature ignition will occur separately for two injections.Compared to the first injection,the ignition delay is much shorter,leading to fuel-rich nonpremixed combustion due to the limitation in fuel/air mixing process.A further discussion is conducted to interpret the acceleration mechanisms.The‘slipstream’effect after the first injection is the main reason under non-reacting conditions.For reacing spray flames,gas expansion due to combustion plays the dominant role.Besides,the current work also focuses on the effect of dwell time on the spray flame interaction and the ignition mechanism for the second injection.For short DT,the combustion of the first spray plays a very important role and the second one is ignited by the hot flame formed in the first spray.By increasing the DT,the influence of the interaction becomes less important.The local chemical time scale is smaller than the flow time scale.Autoignition controls the ignition process for the second spray.Finally,the interactions between spray flame and premixed fuel/air are further analyzed based on the above discussion.LES study is conducted for the ignition and flame development processes at methane/air conditions.At low temperatures,methane prolongs the first-stage ignition,thus prolonging the ignition delay.And then high-temperature reactions accelerate the autoignition for premixed mixture.By increasing the initial gas temperature,the early oxidization in methane leads to temperature increase,the formation of intermediate species,and pressure increase,which shorten the ignition for spray flames.The temperature increase plays the dominant role,followed by the formed intermediate species,for example,CH₂O.And the pressure increase plays a less important role.