Investigation Of Turbulent Flames Based On Computed Tomography Of Chemiluminescence

In practical combustion systems,the combustion is generally operated in turbulent state.Investigations of turbulent flames are quite helpful to understand the interaction between turbulence and chemical reactions.The design and optimization of combustion systems also depend on the understanding of the flame shape and size.It is of great importance in both the scientific research and the engineering application.In this dissertation,the three-dimensional structures of turbulent flames were characterized by the computed tomography of chemiluminescence.Based on the theoretical analysis and experimental studies,systematic quantitative investigations have been carried out to obtain the correlations between the flame structures and the influencing factors.Coaxial jet combustion system and piloted jet combustion system were designed and manufactured.High quality and wide range jet flames with different premixed conditions were obtained to meet the needs of systematic investigation on turbulent flames.The computed tomography of Chemiluminescence system and two-component planar laserinduced fluorescence measurement system were designed and built.The multi-angle projection information were imaged simultaneously by introducing optical fibers for the reconstruction of the flame structures.The hydroxyl group and formaldehyde group in the combustion reaction were measured synchronously,and their pixel product images were used to characterize the heat release rate distributions.According to the structure characteristics of the flames and the imaging performance of the experimental systems,the method of the computed tomography of chemiluminescence was optimized.Simulations were carried out to study the effects of simplified weight coefficient calculation method,reconstruction algorithm,termination condition,prior information,number of projection angles and relaxation factor on reconstruction quality.The optimized configurations of the computed tomography of chemiluminescence method were obtained.Based on the simple structure of the twodimensional steady-state flame,a validation experiment was carried out to evaluate the feasibility of the computed tomography of chemiluminescence for combustion diagnostics.The structure of jet diffusion flames were systematically studied by means of theoretical analysis and experimental measurement.It is found that the shape and size of the jet diffusion flame are significantly affected by the jet Reynolds number.According to the different influencing laws,it can be divided into four stages: laminar stage,transition stage,turbulent stage and limit stage.The flame height is proportional to the jet Reynolds number in the laminar stage,but remains unchanged in the turbulent stage.The flame width significantly increases in the transition stage,whereas remanis unchanged in other stages.The flame volume is proportional to the jet Reynolds number in the laminar stage,and increases gradually in the turbulent stage,where the change gradually becomes gentle.The increase of the coaxial jet exit velocity is beneficial to the combustion,which results in the decrease of the flame overall structures with the coaxial jet exit velocity,but the trend is gradually weakened.The type of fuel does not affect the flame structure changing rules with key factors.However,the specific parameters and stages are changed because of the different physical parameters of fuels,The effects of Reynolds number,equivalence ratio,exit velocity and fuel type on flame shape and size were studied.It is found that the premixed flames can be divided into three stages according to the Mach number of the jet: incompressible stage,compressible stage and supersonic stage.The flame height varies linearly with the jet Reynolds number.The flame width increases gradually with the jet Reynolds number and reaches the extreme value at Mach number 1.The flame volume increases gradually with the jet Reynolds number,but the change amplitude decreases gradually.The flame structure parameters are directly proportional to the equivalence ratio.The exit velocity of the pilot flame affects the integrity of the flame structure of the central jet.When the exit velocity of the pilot flame reaches a certain degree,the flame structure parameters remain basically unchanged.The heat release rate distributions of premixed jet flames with high turbulence were studied using two-component planar laser-induced fluorescence technique.It is found that the preheating zone becomes wider and thicker,and the bilateral structure disappears gradually with the increase of the jet Reynolds number.The heat release rate distribution area becomes wider,while the average thickness remains unchanged at 0.5-0.7 mm,indicating that the minimum scale vortex can only enter the flame front preheating zone,but can not enter the reaction zone.