Experimental Study On The Mechanism Of Turbulent Jet Ignition With Different Fuels And Dilution Methods

With the increasingly severe energy and environmental problems,lean combustion and exhaust gas recirculation（EGR）is attracting considerable attention due to their great potential in improving thermal efficiency and controlling emission.And turbulent jet ignition（TJI）has become an effective method to overcome the problem of slow combustion and ignition instability in lean combustion and EGR dilution combustion by providing multi-point ignition and turbulent disturbance.While TJI is a complex process coupled with turbulence and chemical reaction,and there are different re-ignition modes under different initial conditions.There is not enough research on fundamental mechanism of TJI,especially on the re-ignition modes,flame propagation and extinction phenomenon under lean fuel or EGR dilution cases.Therefore,ignition mechanisms of the TJI process,as well as the combustion characteristics and the optimization strategies for improving TJI flammability limit under EGR dilution cases,are investigated based on the constant volume combustion chamber（CVCC-TJU）and the self-designed single/dual injection pre-chamber system.First,the re-ignition characteristics of methane and hydrogen TJI with different orifices are investigated.The decrease of orifice has a greater impact on the methane TJI process,resulting in a narrower lean flammability limit and later re-ignition delay.The variation of re-ignition location is also analyzed.The reaction will be initiated near the jet tip in a short time with high mixture reactivity and jet energy.On the contrary,the location will move towards the orifice with a longer re-ignition delay.Furthermore,the shock waves and pressure oscillations with hydrogen TJI are discussed,and the special ignition mode transition of hydrogen jet in lean cases is analysed.Next,CO₂ is applied to simulate the introduction of EGR,to investigate the TJI combustion characteristics with EGR dilution.Results show that EGR dilution will cause buoyance instability and reduce combustion efficiency,which can be mitigated by TJI.In addition,the paper also explores methods for improving the combustion conditions of pre-chamber based on a single injection pre-chamber system.Unlike excess air dilution,the injection of auxiliary fuel could not improve pre-chamber combustion conditions efficiently under EGR dilution cases.However,the proper auxiliary oxidizer could reduce local EGR and improve combustion in pre-chamber,and even extend the EGR dilution limit.Finally,the influence of different injection strategies on TJI performance under EGR dilution is studied based on dual injection pre-chamber.Results show that dual injection can change the pre-chamber combustion conditions effectively,and appropriate injection strategies can improve TJI performance greatly.The reliability of the dual injection TJI system about improving pre-chamber combustion conditions and extending EGR dilution limit is further verified by experiments based on different reactivity mixtures.The variable TJI modes under varied initial conditions are found in this paper:jet ignition（initial ignition kernel in the middle of hot jet）,re-ignition at the jet tip,re-ignition failure,flame ignition,and re-ignition of the cold jet.