Numerical And Experimental Studies On Ignition And Propagation Of Laminar Premixed Gas Mixtures

Premixed flames are widely utilized in advanced engine systems.Because it is easy for premixed flames to control the ratio of fuel and oxidizer,adjust the flame temperature,and reduce pollution emissions.In the present study,the fundamental properties of premixed flames such as initiation and propagation are systematically investigated using detailed numerical simulations,and experimental measurements.First,we study the ignition of premixed flames.The lean burn is the effective way for reducing pollution emission by dilunt addition,but this will lead to the ignition failure.Water vapor dilution has great impact on fundamental combustion processes such as ignition,flame propagation and extinction.In the literature,there are many studies on how water vapor addition affects flame propagation and extinction limit.However,the influence of water vapor addition on ignition receives little attention.In this study,numerical simulations considering detailed chemical mechanisms are conducted for the ignition of methane,n-butane and n-decane/air/water vapor mixtures.The emphasis is spent on examining the effects of water vapor dilution on the ignition of these fuels at normal and reduced pressures.The minimum ignition energies(MIE)at different dilution ratios and initial pressures are obtained.It is found that at normal and reduced pressures,the MIE is proportional to the inverse of pressure and it increases exponentially with water vapor dilution ratio.A general correlation among the MIE,pressure and dilution ratio is proposed for each fuel.Furthermore,for stoichiometric methane/air/water vapor mixtures,the chemical and radiation effects of water vapor dilution are isolated and quantified.It is found that the three-body recombination reaction greatly increases the MIE and reduces the dilution limit.Second,we investigate the propogation of premixed flames.With the Laminar flame speed is one of the most important combustion properties of a combustible mixture.It determines the combustion rate of the gas and reflects the stability of the premixed flame.It is an important target for chemical mechanism validation and development,especially at low and elevated pressure conditions.In this study,the laminar flame speeds of lean hydrogen/oxygen/helium mixtures were measured at low and elevated pressures up to 7 atm using a dual-chambered high pressure combustion facility.Similar to experiments,numerical simulations of outwardly spherical flame propagation were conducted.Four chemical mechanisms for hydrogen available in the literature were considered in simulation and their performance in terms of predicting the stretched flame speeds,laminar flame speeds were examined through comparison between experimental and numerical results.It was found that at low pressures,especially at P<0.7 atm,the present experimental results of laminar flame speed are much higher than those predicted by these four chemical mechanisms.The laminar flame speed measured in experiments decreases as the pressure increases.However,the predicted laminar flame speed first increases and then decreases as the pressure increases.At high pressures,the experimental laminar flame speeds agree well with those predicted by chemical mechanisms.It was also found that the overall reaction order n at low pressure is above 2,while it is negative at high pressure.We also study the propogation of lean high-hydrogen syngas/oxygen/helium mixtures at normal and evaluated pressures up to 10 atm using a dual-chambered high pressure combustion facility and numerical simulation.The research is focus on the comparison of the laminar flame velocity measurements and numerical predictions of the premixed gasification under high pressure with spherical flame method and heat flow method is focused on.It was found that at both normal and elevated pressures,the present experimental results agree well with those predicted by simulations using these three chemical mechanisms.Therefore,these chemical mechanisms for syngas can well predict the laminar flame properties of lean high-hydrogen syngas.Besides,the laminar flame speeds measured in the present work were compared with those measured from the heat flux method and large difference was observed.Cool flame due to low-temperature chemistry(LTC)has received great attention recently.However,previous studies mainly focused on cool flames in homogenous systems without transport or non-premixed cool flames in droplet combustion or counterflow configuration.There are only a few studies on premixed cool flames,and the transient initiation and propagation of premixed cool flames are still not well understood.In this study,the initiation,propagation and disappearance of one-dimensional premixed cool flames in dimethyl ether(DME)/air mixture is investigated through transient simulation considering detailed chemistry and transport.The premixed cool flame governed by LTC can be initiated by a hot spot.When the hot spot temperature is not high enough to directly trigger the high-temperature chemistry(HTC),only the LTC reactions take place initially and thereby a cool flame is first initiated.During the cool flame propagation,HTC autoignition occurs at the hot spot and it induces a hot flame propagating behind the cool flame.Therefore,double-flame structure for the coexistance of premixed cool and hot flames is observed.Since the hot flame propagates much faster than the cool flame,it eventually catches up and merges with the leading cool flame.A welldefined cool flame speed is found in this study.We inverstigate different factors affecting the cool flame speed and the appearance of hot flame.It is found that at higher equivalence ratio,higher initial temperature or higher oxygen concentration,the premixed cool flame propagates faster and the hot flame appears earlier.Three chemical mechanisms for DME oxidation are considered.Though these three mechanisms have nearly the same prediction of hot flame propagation speed,there are very large discrepancy in the prediction of cool flame propagation speed.Therefore,experimental data of premixed cool flame speed are useful for developing LTC.