Study Of Turbulent Dilute Spray Autoignition Using Direct Numerical Simulation

Spray autoignition is a basic issue in many combustors,e.g.,diesel engine and aeroengine,involving atomization,evaporation,fuel/gas mixing and chemical reaction.Due to the complexity of temperature and pressure condition in diesel engine and gas turbine engines,the spray ignition and subsequent combustion process are significantly influenced by the low temperature chemistry.Hence there exsist complicated interactions among turbulence,droplet and different chemistry.To improve combustion efficiency/stability and reduce emissions,detailed investigations are needed to understand the physical-chemical processes involved in spray autoignition.Based on the method of direct numerical simulation,the major work and conclusions are as follows.(1)Three dimensional direct numerical simulations are performed to investigate the autoignition behavior of n-heptane droplets in high-temperature products of premixed H2/air combustion.Five cases of diverse initial droplet size and global equivalence ratio are carried out to study their effects on the ignition process.For larger droplet diameter,the evaporation rate is slower,and the ignition occurs later.For larger global equivalence ratio,the spray evaporates faster and the ignition occurs earlier.In addition,it is also found that the peak heat release rate shows a decreasing trend when initial droplet size increases.The joint PDF of the mixture fraction and scalar dissipation rate is studied,which is found a key parameter relating to the happening of ignition.(2)Three dimensional direct numerical simulations coupled with complex chemistry are firstly adopted to investigate fundamental issues of n-dodecane spray autoignition at high temperature.Droplets are initially distributed in a slab layer of a cubic box.The ignition processes are analyzed both in physical and mixture fraction space.The effects of initial droplet temperature are studied for the first time.The performances of typical heat release indicators are also quantitatively evaluated,and the relative contributions to heat release of elementary reactions are examined to explain the complex correlations between the indicator and heat release rate,which is helpful for the analysis of heat release imaging obtained with experimental technique.(3)N-dodecane spray autoignition at a typical low temperature,NTC-affected condition,is investigated through OD,1D,and 2D direct numerical simulations in this work.The ignition location and delay time,the structure and combustion modes of cool and hot flames,and subsequent combustion mode evolution during spray autoignition are discussed in detail for the first time.The results indicate that the most reactive mixture fraction of main ignition no longer exists due to the coupled influences of droplet evaporation and low temperature chemistry,and the main ignition is greatly affected by cool flame propagation.The propagation of cool flame helps to decrease the temperature stratification in the flow field,resulting in better uniformity and relatively larger kernels for the second-stage ignition.In addition,influenced by the cool flames,there is a higher proportion of spontaneous ignition for high temperature reaction fronts.(4)Zero dimensional and two dimensional direct numerical simulations are conducted to identify the role of cool flames in the process of spray autoignition.Two types of ignition processes are observed:1)When initial air temperature is relatively low,the entire vaporization and mixing region could be influenced by the low temperature reactions.The first-stage ignition first occurs near stoichiometry and then cool flames propagate to richer regions.With elevated temperature and chemical reactivity in the domain with cool flame occurrence,the second-stage ignition is triggered and eventually leads to hot flames.2)When initial air temperature is relatively high,two-stage ignition only occurs in the rich region with cool flames surrounding individual droplet,while a hot flame is triggered directly in the lean region and further propagates into the rich core region.The very rich mixture with low temperature in this scenario prevents the combustion from being fast and complete,and local extinction of the hot flame is observed during the interaction with cool flame and droplet.Budget analysis is further conducted to analyze the local flame structure and identify all the combustion modes of reaction fronts during spray autoignition,suggesting fundamental requirements for the development of comprehensive spray combustion model.