Research On Ammonia Measurements In Flow Fields And Combustion Flow Fields Using Femtosecond Laser

With the increasing energy demand and the ever-stringent restriction on conventional fossil fuel consumption,it is highly required to develop and apply renewable fuels with low or non-carbon.Ammonia（NH₃）,which has no carbon,is a promising alternative to fossil fuels in the future carbon-free economy.Ammonia has recently attracted much attention with the direct combustion as the most efficient way for energy utilization.In order to understand the characteristics of NH₃ combustion with the purpose of efficient combustion and low emissions for ammonia as a fuel,it is of significance to develop effective methods for in situ NH₃ measurements.Laser-based diagnostical techniques have advantages of non-intrusive,high temporal and spatial resolution,high sensitivity and visualization,which have been widely adopted in characterizing many aspects of flow fields and combustion flow fields.Various diagnostics based on nanosecond（ns）lasers,however,all have their own limitations.The advent of femtosecond（fs）lasers has enriched the laser-based diagnostic techniques.In view of the above background,this paper aims to develop fs laser-based diagnostic techniques for monitoring NH₃ in flow fields or combustion flow fields.Femtosecond laser-induced plasma spectroscopy（or called FLIPS）and femtosecond two-photon laser-induced fluorescence（or called fs-TPLIF）were introduced for NH₃measurements in this work,which are conducted through experiments.In the first part of this work,FLIPS was performed in NH₃/N₂ mixtures and premixed NH₃/air flames.An fs laser（800 nm）was adopted to photolyze NH₃,and then the NH fluorescence at 336 nm fromA³ï¼ˆ47）-X ³ï¼ˆ50）^-transition was detected.The experiment was initially conducted in non-reacting mixtures to investigate the feasibility of FLIPS.Besides,the main interference originated from N₂（C ³ï¼ˆ47）_u-B ³ï¼ˆ47）_g）at 337 nm was analyzed.Further,the detection limit of 120 ppm was obtained in NH₃/N₂mixtures.FLIPS was then performed in premixed NH₃/air flames,whose applicability was investigated.The results indicate that FLIPS has one-dimensional instantaneous measurement ability.In the second part of this work,fs-TPLIF was performed in NH₃/N₂mixtures and premixed NH₃/air flames.An fs laser（305 nm）was adopted to excite NH₃molecules from the ground state（X）to the excited state（C’）with two-photon excitation process,and then the NH₃ fluorescence at 565 nm from C’-A transition was detected.The experiment was initially conducted in non-reacting mixtures to investigate the feasibility of fs-TPLIF.Besides,the ability of one-dimensional instantaneous imaging for fs-TPLIF was analyzed.A detection limit of 720 ppm was obtained in NH₃/N₂mixtures.And then fs-TPLIF was performed in premixed NH₃/air flames,whose applicability was investigated.The results indicate that fs-TPLIF is a promising tool for visualizing NH₃ in a combustion environment,and the potential for simultaneous measurement of NH₃ and OH with only one laser was discussed.This work is the first attempt of fs-based techniques for NH₃ monitoring and flow field/combustion fields diagnostics.The results indicate that FLIPS and fs-TPLIF are promising tools for flows and combustion diagnostics,which will be beneficial for the fundamental research of NH₃ combustion.