Detailed Numerical Simulation On The Role Of Low Temperature Chemistry In Turbulent Combustion

Combustion will continue to play an important role in energy supply systems for the decades to come as it is one of the most efficient and mature energy conversion approaches.In the transportation sector,it is essential to improve the engine combustion efficiency simultaneously with low engine-out emissions for the decarbonization and environment protection.Market diesel fuel naturally owns the Low Temperature Chemistry（LTC）,e.g.,the two-stage ignition characteristic,which has been proved to be significant for advanced engines combustion technologies which may be operating at low temperature combustion mode.Understanding the underlying physics of LTC,its coupling with heat and mass transfer and chemical kinetics,and the impact of LTC on flame structures,flame propagation speed,mixture auto-ignition and spray flame liftoff can greatly benefit the improvement of engine performance.With the rapid development of laser combustion diagnosis,e.g.,plane laser-induced fluorescence,relevant experimenters have provided valuable and detailed information of key species,such as CH₂O,OH and CH,in the turbulent flame.But owing to the difficulties and limitations of measurement technology in experiments,the turbulent flow velocity,other key thermodynamic quantities are not available.To shed light on LTC physics in turbulent combustion relating to advanced Internal Combustion Engines（ICEs）combustions conditions,high-fidelity numerical simulations are needed.In this thesis,Large Eddy Simulation（LES）and Direct Numerical Simulation（DNS）are adopted to investigate LTC involved flames of diesel surrogates,such as n-heptane and n-dodecane.Due to the complexities of spray combustion,two simple flames are firstly selected to assess the LTC effects on premixed flame propagation and mixture auto-ignition.Then,the spray combustion in a constant volume vessel is simulated and the LTC and other physics is discussed.Finally,this thesis ends with a newly developed novel hybrid Adaptive Mesh Refinement（AMR）method for spray combustion simulation.In the present research,n-heptane/air premixed flame in the Low Temperature Ignition（LTI）regime relevant to preheated premixed combustion is firstly studied based on the slot burner flame experiment of Won et al.（CNF,2014）.The LTI regime is defined as that LTI takes place before the mixture reaching the hot flame front.Otherwise,it is Chemically Frozen（CF）regime.Current numerical results show that the flame in LTI regime owns a broadened preheat zone compared to that in CF regime due to more diffusive and reactive species from LTI.However,radicals OH and H yet show similar distributions in the reaction zone.By analyzing the budget term of flame displacement speed,it is confirmed that the increased flame speed in LTI regime is a net effect of enhanced reaction and diffusion towards the preheat zone and the lower gradient of the reactant mass fraction（e.g.,O₂）.Therefore,differential diffusion is shown to be more significant in LTI regime on the ground of the enhanced diffusion of small species,such as H and H₂,between the high temperature reaction zones and the preheat zone.Moreover,the role of the composition change and temperature rise from LTI is clarified.The temperature rise from LTI is the dominated factor to increase the flame speed,while the laminar flame speed of LTI products is actually smaller than that of the fresh mixture at the same initial temperature.Based on results of the above investigation,effects of LTC products on the auto-ignition characteristics relevant to“single-fuel”reactivity controlled compression ignition engines are studied based on the experiment of Geng et al.（CNF,2019）.N-heptane is partially reformed to fruitful intermediate species（reforming products）via the LTC under an extremely rich condition（equivalence ratio>8）before it is rerouted into the cylinder.Compared to n-heptane/air mixture,n-heptane/reforming products/air owns a weaker Negative Temperature Coefficient（NTC）phenomenon because of the lack of the complete LTC pathways of reforming products.Chemical reaction pathway analysis shows that LTC reactions are intensified at the beginning in the presence of reforming products resulting from the decomposition of KET（ketohydroperoxide）to OH,but it may delay the onset of High Temperature Chemistry（HTC）.The basic reason of reforming products’effect on HTC timing attributes to the heat accumulation in the early phase from the oxidization of large as well as small hydrocarbons,such as C₇H₁₆ and CH₂O.The heat accumulation depends on not only the production of active radicals before the critical temperature,but also the competition of active radicals between small hydrocarbons in reforming products and fresh n-heptane.Furthermore,spray combustion relevant to modern dual-fuel reactivity controlled ignition engines are numerically studied by using the Eulerian-Lagrangian two-phase method,LES and Eulerian stochastic fields combustion model.This part of research is based on well-documented experiments of n-heptane sprays series（Spray H）of Engine Combustion Networks（ECN,https://ecn.sandia.gov）.A very good agreement between present LES results and experiments of Spray H has been obtained for spray liquid penetration length,vapor fuel penetration length,mixture fraction profile,pressure rise profile,Ignition Delay Time（IDT）,and flame Liftoff Length（LOL）.The spray LTI is found to be started at fuel lean region where the temperature is high.It is observed that a successful spray HTI is determined by the cool flame propagation.Specifically,cool flame propagates into the rich mixture in the mixing layer of n-heptane jet to reach the critical temperature from the heat release of LTC.The effects of the ambient mixture on spray ignition of dual-fuel combustion are threefold:i)introducing the extra fuel to the ambient mixture may dilute the oxygen concentration and change the mixture thermo physics properties;ii)owing to the high temperature of ambient mixture,it may spontaneously react to generate radicals and heat,which is beneficial to the spray ignition;iii)ambient mixture,such as H₂ and CO,can consume OH radicals in LTC stage to suppress the cool flame propagation thus retarding the spray ignition.This conclusion is drawn from a series of dual-fuel spray combustion with varying ambient mixture compositions.For the LOL of spray flames,current results show the flame is mainly stabilized by a LTI assisted hot flame propagation front.The soot formation is consequently affected by the spray LTC.In addition,the flame structure of dual-fuel spray combustion and its difference from the single-fuel spray combustion are discussed.It is highlighted that the premixed flame front of the ambient fuel/oxidizer mixture is back-supported by the diffusion flame of the injected n-heptane.Finally,a novel hybrid Adaptive Mesh Refinement（AMR）method for spray combustion relevant to the advanced low temperature combustion engines is developed in current thesis.This is motivated by the fact that the motion,breakup,and evaporation of the Lagrangian particles are sensitive to the mesh resolution employed.Therefore,treating the spray liquid region and vapor region with different mesh resolution is an efficient way to achieve mesh-convergence results.LES tests of hybrid AMR method under ECN Spray A（n-dodecane）combustion are conducted by using the Eulerian-Lagrangian two-phase solver,combined with partially stirred reactor combustion model.It is found that a coarse mesh in the vapor region and a fine mesh in the liquid region can provide comparative results with those of the overall fine mesh in terms of liquid and vapor penetration lengths,low temperature ignition time,mean pressure rise profile and overall flame structure.However,high temperature ignition time,flame liftoff length and soot emission yet show nonnegligible sensitivity to the mesh resolution in the vapor region.