Research On Effects Of Buffer Gases And Fuel Properties On Spray Combustion By Means Of Optical Diagnostics

The diesel spray combustion process is directly affected by the oxygen concentrations and the composition of the buffer gases.Meanwhile the effects of fuel properties on the development of spray combustion still remain to be revealed under oxygen enriched conditions.In this study,based on the experimental study of different buffer gases and wide range of oxygenconcentrations,mutiple optical diagnostic methods were applied to investigate the effects of fuel properties on spray flames in a constant volume vessel with high-temperature and high-pressure.In the oxygen concentration ranging from 10%-21%,the effets of different buffer gases（Ar and N₂）on the ignition and development process of diesel engine flame were studied by high-speed imaging.In the low oxygen level conditions,two color method were adopted to study effects of buffer gas components（Ar and N₂）on soot flame temperature and soot distribution.In addition,OH*chemiluminescence was introduced to study the effects of different buffer gases（Ar,N₂and CO₂）on diesel spray flame in the oxygen concentration ranging from 10%-70%.The results can be summerized as follows:As oxygen concentration increases,the natural flame luminosity of diesel spray flame becomes stronger.Besides the width of flame reduces in size,and the initial formation of soot moves upstream of the diesel spray.When oxygen level rises to 60%,flame lift-off length at three buffer gases tend to be the same and remain unchanged.Under low oxygen concentrations,the natural flame luminosities of O₂-N₂ are much weaker than that of O₂-Ar.the distribution of a rich soot region is broader in the midstream of flame.Besides,A faint blue chemiluminescence is observed at initial stage of combustion at 10%O₂+90%N₂case.As nitrogen is replaced by argon,the flame lift-off length shortens significantly,and overall temperature of the flame increases under the same oxygen concentration.Under ultra-low oxygen concentration,using argon as buffer gas rather than N₂ can effectively improve the ignition stability of diesel spray flame.Based on this,the main fuel properties were separated by contrasting different fuels,including diesel（D100）,70%diesel+30%n-heptane（D70H30）,70%diesel+30%iso-octane（D70O30）,70%diesel+30%n-butanol（D70B30）,all of which were mixed by volume.The effects of fuel properties on diesel spray were studied under high oxygen concentrations（21%-70%,buffered with N₂）.Experimental results show that Fuel properties have more effects on flame ignition and soot formation under 30%oxygen concentration.With the increase of oxygen concentration,the effects of fuel properties on the ignition and development of the spray flame are weakened,and the time differences to reach the quasi-steady state of spray flame for the four fuels reduce continuously.The flame lift-off length（FLo L）experiences a fast decrease firstly and then goes down slightly until it remains basically unchanged.At high oxygen concentration,the FLo L of D100 is the shortest,while the FLo L of D70B30 is the longest.Fuel properties such as viscosity and density change the fuel injection velocity,affecting the convective flow downstream at the lift-off region.Therefore,they have a greater impact on the FLo L under high oxygen concentrations（more than 50%）.In this study,the use of Ar as a buffer gas can effectively improve the ignition stability of diesel spray flames at the extremely low oxygen concentration.Moreover,the effects of using buffer gases and alternative fuels on spray flame stabilization were studied.Results showed that the formation rate of vaporized fuel and injection velocity of spray become the deciding factor for FLo L at high oxygen concentrations（more than50%）,which establishes a theoretical foundation for the use of buffer gases and alternative fuels under oxygen-enriched conditions.