Research On Quantitative Concentration Measurements Of Hydroxyl Radical In Laminar Flames Using Bi-Directional Laser Induced Fluorescence

Diluted oxy-fuel combustion technology,which can effectively reduce the emission of NOx,is thought to be the hot research topic in the field of gas turbine syngas combustion.However,the difficulty of measuring the basic parameters,such as the OH concentration,which is closely ralated to the combustion characteristics and chemical reaction mechanisms of the diluted syngas flames,limits the further development of this combustion technology.Therefore,it has a crucial guiding significance for developing an effective quantitative laser spectroscopy technology for the mechanism research and practical engineering application of the gas turbine syngas flame.Planar laser-induced fluorescence?PLIF?is a kind of laser spectroscopy combustion diagnostics technology with the high sensitivity and the high spatial and temporal resolution.However,it is rather difficult to achieve the quantitative measurement for combustion intermediate species using the conventional PLIF because the laser-induced flourescence?LIF?signal is often subjected to the collisional quenching effect under the condition of atmosphere and high pressure.On the contrary,bi-directional LIF is a novel quantitative spectroscopy technology,for which the most prominent advantage is that it can not only retains the metris of high sensitivity and high spatial resolution,but also has ability to overcome the problem of calibration and collisional quenching effect encountered in the process of quantification using conventional LIF.In view of these reasons mentioned above,this dissertation made a quantitative research on the measurements of OH concentration profiles in the partially premixed methane/air flame and syngas flame by using a new laser spectroscopy method named bi-directional LIF.The dissertation firstly elaborates LIF and the developments of syngas flame using OH-PLIF,proposes the problems existing in the current research,presents the main contents of the work and the key problems to be solved.Then,the principle of bi-directional LIF is presented based on the theory of LIF,and the four factors influencing the measurement results are analyzed.Accordingly,the experimental scheme for measuring the OH concentration is established.The first domestic bi-directional PLIF experimental platform is designed and built.In addition,in view of the deficiency of effective peak absorption cross section,the experimental measurement formula of this important physical parameter has been given clearly.The OH effective peak absorption cross section for Q1?8?lines within the bands A2?+?X 2??0,0?and?1,0?,as well as the variations with flame heights are investigated in the partially premixed methane/air flame and the premixed syngas flame,respectively.The research result shows that the OH effective peak absorption cross sections in these two types of flame are basically unchanged at different flame heights.Compared to the OH absorption cross section at Q1?8?line of the band?1,0?,the expermental result for Q1?8?line within the band?0,0?is approximately six times higher.This result is found to be in agreement with the theoretical calculation,which proves the correctness of the experimental results.Secondly,the quantitative measurements of one-and two-dimensional OH absolute concentration profiles in a partially premixed methane/air flat fl ame under different equivalence ratios are performed for the first time,which provides the valuable experimental data for the chemical kinetics model and the related reaction mechanism.To our knowledge,this work has not been reported in the domestic and international research.The research result indicates that there is a large difference for the OH concentration profiles at lean-and rich-burn combustion zone in partially premixed methane/air flame.For the lean-burn condition,the OH radical is mainly distributed in a rather narrow zone above the burner surface,and the OH concentraion will be decreased rapidly with increasing the axial distance.For the rich-burn condition,the OH radical is mainly distributed in the outer side of flame,forming the two strong OH distribution bands,while the number of OH radical in the inner zone of flame seems rather scarce.It is found that the peak OH concentration has reached maximum at the equivalence ratio 0.9?with the corresponding peak concentration of?1.20±0.10?×1016 cm3?instead of 1.0.Thirdly,the quantitative measurements of temperature and OH concentration fields in the premixed and diffusion syngas flames with N₂,CO₂ and H₂ O diluents are investigated by using two-line PLIF thermometry and bi-directional PLIF,respectively,which has not been reported in the domestic and international research.The effects of different diluents on the temperature structure and the OH concentration profile in syngas flame are studied,providing a reliable experimental data for the chemical kinetics model and reaction mechanism research of the syngas flame.The research result shows that the spatial distributions of OH radical in the premixed and diffusion syngas flames are almost same,that is,all of which exhibit a symmertrical but non-uniform distributing shape.The OH radicals mainly distribute in the flame front near,and the number of OH radical at the edges of flame is found to be significantly less than that near the flame front.Further analysis indicates that both CO₂ and H₂ O diluents can promote the production of OH in the flame,but compared to H₂ O,the syngas flame diluted with CO₂ has a lower flame temperature.Finally,compared with the OH concentration measured before,the most classic GRI-Mech 3.0 mechanism for methane flame and the Davis mechanism for syngas flame are verified respectively,and then the problems existin g in current reaction mechanism and the need to be improved have been pointed out.The research result shows that the GRI-Mech 3.0 mechansim can accurately predict the variations of OH concentration with axial distance in each equivalence ratio.However,the predicted values are higher than the experimental values measured by bi-directional LIF,since the OH concentration and the flame temperature simulated by GRI-Mech 3.0 mechanism in this dissertation are adiabatic results,in which the radiation heat loss has not been considered.For the premixed syngas flame with N₂,CO₂ and H₂ O diluents?atmospheric pressure,preheating temperature 400K?,the predicted distribution regularity for OH concentration is found to be in good agreeme nt with the experimental result,but the predicted values of the peak OH concentration are lower than the experimental values.A more detailed analysis indicates that the Davis mechanism underestimates the chemical effect of diluent on the syngas flame,especially the chemical effect of CO₂ diluent.