Large Eddy Simulation On The Mechanisms Of Prechamber Jet Ignition At Engine Low-Reactivity Condition

Low-carbon fuel,e.g.,methane and ammonia,has the advantages of low carbon emission.Therefore,the utilization of low-carbon fuel in internal combustion engines is an important way to accomplish the strategy of"3060"carbon emission.However,those low-carbon fuels often have low reactivity,high ignition temperature and slow flame speed,thus making they are difficult to ignite and completely combust.That becomes a bottleneck when they are utilized as the fuel for internal combustion engines.Alternatively,prechamber jet ignition（PJI）technology could provide high ignition energy and multiple ignition kernels for triggering the ignition.In addition,PJI generates turbulence when hot jet issued from prechamber,which is beneficial for accelerating flame propagation in the main-chamber.Hence,PJI has the potential to effectively address the challenges of igniting low reactivity low-carbon fuel.The current work focuses on the ignition mechanism of PJI in engine at low reactivity condition,such as methane addition and ammonia addition.To account for the intensive turbulence-chemistry interaction（TCI）in jet ignition,a new adaptive linear eddy model（LEM）was proposed based on the conventional linear eddy model,called adaptive LEM.The adaptive LEM was incorporated into the framework of large eddy simulation（LES）,resulting in an LES-LEM platform for simulation.Based on the LES-LEM simulation platform,both the auto-ignition in prechamber and turbulent jet ignition in main-chamber were investigated in low temperature,dilute oxygen and methane/ammonia low reactivity atmosphere condition.In this study,the effect of low reactivity atmosphere on auto-ignition in prechamber was comprehensively investigated.In addition,this work explored the ignition pattern of turbulent jet ignition（TJI）at the ultra-lean condition and found a physical boundary between successful ignition and ignition failure.Moreover,the ignition mechanism of micro pilot jet ignition in prechamber was also revealed in this work.The current study provides theoretical guidance for stable and rapid ignition of low reactive fuels,e.g.,methane/ammonia.The main conclusions are as follows:First,to account for the intensive TCI effect in the jet ignition problem,a new adaptive LEM was proposed based on the conventional LEM.The adaptive LEM could refine its spatial resolution to get adaptive to the minimum turbulent eddy in multi-scale turbulent jet flow.In comparison with the convention LEM,it has a more adequate capture on TCI,and thereby accurately predict the extinction/reignition phenomenon in high hear jet flow.In addition,the adaptive LEM significantly improves the computational efficiency by achieving a speedup ratio up to 3 times in both turbulent reacting flow.The adaptive LEM was incorporated into the framework of LES,and the LES-LEM method was validated in the high-shear reacting jet and n-heptane spray combustion,which supports its further applications.Focusing on the auto-ignition mechanism of prechamber pilot fuel at low reactivity condition,the effect of ambient temperature,oxygen mole fraction and methane addition.were investigated by using LES-LEM method.Results show that:the two-stage ignition characteristic becomes more pronounced when diluting oxygen concentration or decreasing ambient temperature.In specific,the auto-ignition is significantly delayed,especially for the second-stage auto-ignition.It also noticed that dilution of oxygen or decreasing temperature alters the most reactive mixture in the mixture fraction space.Besides,the addition of methane into the atmosphere significantly inhibits the auto-ignition of pilot fuel,especially delaying the second-stage auto-ignition.Specifically,the second-stage ignition delay increases by 4.3～6.7 times with theλ_CH4 in the range of 1.25～2.0.In addition,in the micro pilot ignition scenario,a new auto-ignition mode was found at a short fuel injection duration,in which two independent auto-ignition kernels start from spray tip and spray root respectively and then propagate to each other,named‘two ends auto-ignition’mode here.The‘two ends auto-ignition’tends to appear when the fuel injection duration t_injis shorter than ignition delayτ_ig,i.e.,τ_ig/t_inj<1.Focusing on the jet ignition mechanism of methane/ammonia fuel jet at ultra-lean condition（λ=2.0）,the present work studied the effect of jet parameters and fuel reactivity on jet ignition performance,including jet temperature,jet velocity,nozzle size.Results found three ignition patterns exist in turbulent jet ignition（TJI）of methane/ammonia fuel,including flame propagation pattern,jet re-ignition pattern,and complete failure pattern.By performing dimensionless analysis in Re-Da space,it is revealed that ignition patterns are determined by Damk（?）hler number Da in both methane and ammonia fuel jets.Thereby,a criterion for distinguishing ignition failure/success was established using Da≈0.1.The utilization of low-reactivity low-carbon fuel would reduce jet Da,and thus jet ignition gets harder to succeed.As for ignition rate,results show that the combustion rate of TJI is mainly controlled by the growth of the flame front,which depends on the stretch effect.The larger the Reynolds number Re,the stronger the stretch effect.To pursue rapid combustion,the jet energy flux should be enlarged,and the Da and Re of the injected jet should be increased simultaneously.For methane/ammonia low reactive fuel,TJI can significantly improve the combustion rate.In particular,the flame propagation speed of ammonia fuel can be increased to 20Sl⁰,making up for its shortcoming of low laminar flame speed Sl⁰.At last,focusing on the jet ignition mechanism of prechamber micro pilot ignition（PMPI）,the fully coupled ignition progress was investigated using LES-LEM,including auto-ignition in prechamber and jet ignition in main-chamber.The results show that,from the view of heat release rate,the PMPI combustion can be broadly split into three stages,including auto-ignition of pilot fuel in prechamber,jet formation and jet ignition in main-chamber and the self-sustained combustion in main-chamber.The prechamber orifice size has a significant influence on the jet formation stage.A small orifice has a lower mass flow rate,and thus the jet duration lasts longer,and also the peak heat release at ignition time is higher.Both oxygen dilution and methane enriching inhibit auto-ignition in prechamber,whereas the latter could increase jet Da in favour of successful ignition.The jet Re and Da gradually increase during ignition progress,and a successful ignition would be triggered as jet Da increase larger than the critical value（Da≈0.1）of the success/failure boundary of steady-state TJI.The CEMA was performed to analyse the jet reactivity,and results indicate that the jet Da depends on the jet reaction reactivity.The jet reaction reactivity is not only affected by the above factors but is also related to the auto-ignition mode in the prechamber.The current study provides theoretical guidance for stable and rapid ignition of methane/ammonia low reactive fuels.