Study Of Flames Propagation In Spray Combustion Using Direct Numerical Simulation

Spray.combustion is a common energy conversion mode in power system,in which the ignition and flame propagation affect the operation stability,combustion efficiency and pollutant emission to a great extent.In this paper,direct-numerical simulation(DNS)is used to study the flame propagation characteristics in spray combustion,aiming to analyze and explore the flame evolution and its main influencing factors during the forced ignition of spray.Firstly,the most common phenomenon of fuel stratification in spray combustion is studied with stratified gaseous flame.Secondly,the flame propagation in planar spray combustion is studied.Combining stratified gaseous flame with planar spray flame,the flame propagation in planar stratified spray combuston is studied.Finally,the flame propagation in spherical spray combustion is studied.The specific works are as follows:(1)One-dimensional direct numerical simulations are performed to study the flame propagation of laminar stratified syngas/air flames.Effects of fuel stratification,CO/H2 mole ratio and temperature stratification on flame propagation are investigated through comparing the distribution of flame temperature,heat release rate and radical concentration of stratified flame with corresponding homogeneous flame.For stratified flames with fuel rich-to-lean and temperature high-to-low,the flame speeds are faster than homogeneous flames due to more light H radical in stratified flames burned gas.The flame speed is higher for case with larger stratification gradient.Contrary to positive gradient cases,the flame speeds of stratified flames with fuel lean-to-rich as well as with temperature low-to-high are slower than homogeneous flames.The flame propagation accelerates with increasing hydrogen mole ratio due to higher H radical concentration,which indicates that chemical effect introduced by differential diffusion is more significant than thermal effect.(2)Two-dimensional direct numerical simulations with complex chemistry are performed to study the flame propagation of planar n-heptane spray with forced ignition.The effects of polydispersity,droplet number density,droplet diameter,initial air temperature and turbulence on flame propagation and structure are investigated.The flame front with high droplet number density and high air temperature are in the state of rich,the former possesses a thicker flame than base case,while the latter possesses a significantly thinner flame due to the shortening of preheat zone.The flame front with polydispersity,large droplets and turbulence are in the state of lean,which possess thinner flames than base case.In addition,the diffusion combustion mode is dominant in the early stage of flame propagation,while it transforms into premixed combustion mode by time.The heat release rate is mainly contributed by premixed combustion mode in small droplets and high droplet number density cases,while the contribution of diffusion combustion mode to the global heat release rate increases in large droplets,high air temperature and turbulent cases.Two typical ignition modes in spray with different droplet size are found:interdroplet spacing combustion and droplet sheath combustion.The decrease of spatial distribution uniformity of mixtures,obstruction from incompletely evaporated droplets and flow stretching and bending effects caused by polydispersity,large droplets and turbulence,respectively,make the flame propagation slow down;while high droplet number density and high air temperature,respectively,increase the continuity of mixing layer and enhances the chemical reaction,which accelerate the flame propagation consequentially.(3)Two-dimensional direct numerical simulations are performed to study the effect of droplet number density stratification on flame propagation and structure of n-heptane spray.The effects of stratification gradient,droplet diameter,initial air temperature and turbulence on stratified combustion are investigated.For stratified spray flame with droplet number density low-to-high,the flame speed is slower than homogeneous spray flame.And the flame propagation is relatively stagnant with a significantly thicker flame when the stratification gradient reaches a certain level.Different from negtive gradient cases,the flames are thinner and propagate stably in positive gradient cases.The reason for difference between this two types of stratified combustion is mainly related to the spatial distribution of mixing layer at flame front.Similar to stratified gaseous flame,the smaller stratification gradient is,the weaker stratification effect is.However,the "back-supported" effect only exists in the positive gradient case with large stratification gradient.Large droplet,high air temperature and turbulence are beneficial to weaken the adverse influence of spray flame propagating in negtive gradient cases,because the state of fuel supply(fuel vapor-air mixing layer)at flame front has been changed.(4)Three-dimensional direct numerical simulations with complex.chemistry are performed to study the flame propagation of spherical n-heptane spray with forced ignition.The effects of droplet number density,droplet diameter,turbulence intensity and spark location on flame.propagation and structure are investigated.The flame propagation in low droplet number density and small droplet cases is faster than that in high droplet number density and large droplet cases.Under the turbulence intensity adapted in this chapter,the flame propagation slows down under weak turbulent case,while the chemical reaction and flame propagation are improved under intense turbulent case due to the thickened fuel vapor-air mixing layer.Moving the spark location not only changes the flame structure,but also changes the heat release mechani sm of spray combustion.Premixed combustion mode is dominant in the interdroplet spacing,while diffusion combustion mode mainly surrounds droplets.In addition,the former occurs in lean mixtures,while the latter occurs in rich mixtures.The displacement speed Sd is mainly contributed by reaction component Sr(related to the flame temperature)when ζ<0.1 and by tangential diffusion component St(related to the concentration gradient of O2)when ζ>0.2,while the contribution of normal diffusion component Sn(related to the flame curvature)to Sd is always small.There is a negative correlation between displacement speed Sd and flame curvature km*.Sr and Sn are positively correlated with km*,while St is negatively correlated with km*,which means that the negative correlation between Sd and km*comes from St.