Research On The Method Of Measuring Equivalence Ratio Based On Femtosecond Laser-Induced Plasma

Equivalence ratio is a crucial physical parameter of combustion,and it has a significant impact on the engine’s combustion processes.In recent years,with the development of combustion laser diagnostic technology,femtosecond laser-based combustion diagnostic technology has attracted a lot of attention.The femtosecond laser’s pulse width is only a few tens of femtoseconds,so it has a very high peak power,allowing the femtosecond laser to generate a self-focusing effect when it propagates in the air,forming a plasma channel called"filament".The plasma will radiate fluorescence signals,from which the gas composition information can be extracted.This paper mainly studies the relationship between the fluorescence signal emitted by the plasma,formed by the interaction between the femtosecond laser and the flow field in the cold premixed methane/air flow field,and the equivalence ratio of the flow field.Two types of equivalence ratio calibration methods are proposed.Firstly,we analyze the one-dimensional spatially-resolved spectra of seven kinds of fluorescence signals emitted by the plasma,and it is found that there is a linear relationship between the relative spatial position of the peak signal intensity of different components and the equivalence ratio.We establish two sets of linear calibration curves about N₂（316 nm）-H（656 nm）and C₂（516 nm）-H（656 nm）’s relative spatial position with equivalence ratio.The phenomenon might be related to the second-order nonlinear refractive index of the mixture.The equivalence ratio changes the second-order nonlinear refractive index of the flow field,which leads to the shift in the component’s peak signal position.Secondly,we study the relationship between the intensity of the spectral signal emitted by the plasma and the equivalence ratio.The experimental results show that the lifetime of the plasma fluorescence is about 10 ns.Affected by the fluctuation of the femtosecond laser energy,the relationship between the peak signal intensity and the equivalence ratio cannot be accurately obtained.To reduce the influence of the energy fluctuation,the ratio of the peak signal intensity of different components is used to characterize the equivalence ratio of the mixture.We establish six sets of linear calibration curves including CH（431 nm）/N₂（337 nm）,CH（431 nm）/N₂（357 nm）,CH（431 nm）/CN（388 nm）,O（777 nm）/N₂（337 nm）,O（777 nm）/N₂（357 nm）and O（777nm）/CN（388 nm）with the equivalence ratio.To improve the quality of the linear fitting,the peak area of the spectral signal of different components is used to characterize the emitted fluorescence signal.The results show that the linear fitting has been greatly improved,and the degree of linear fitting of CH（431 nm）/N₂（357 nm）is as high as0.99.This means that the ratio of the signal intensity of the plasma spectrum can be used to measure the equivalence ratio of the flow field.In this paper we prove the feasibility of using a femtosecond laser to measure the equivalence ratio of methane/air mixtures experimentally,and we propose a new method for measuring equivalence ratio and establish the relationship between the spatially resolved position of the spectral signal and the fluorescence signal intensity with the equivalence ratio.It is also useful to improve the understanding of the physicochemical properties of the plasma formed by femtosecond laser in gas mixtures.