Experimental And Numerical Study On The Propagation And Geometry Characteristics Of The Methane Triple Flame

As a typical partially premixed flame, triple flame, which is consisted of lean premixed flame, rich premixed flame and diffusion flame, widely exists in many types of combustion devices. Since the ignition, propagation and structure characteristics of triple flame vary greatly with that of premixed or diffusion flame, it is of theoretical and practical improtance to study the fundamental characteristics and combustion mechanism of triple flame under different conditions. In this paper, taking methane as the fuel, the propagation and geometry characteristics of triple were studied experimentally and numerically. Through this study, the understanding on the propagation and the evolution of the flame geometry and their impact influence could be deepen, and theoretical basis on the control and utilization of partially premixed combustion could be provided.In the establishment of the triple flame experiment system, through the approach of theoretical calculation and numerical simulation, with the objective of uniform and stable laminar flow field, the design of the characteristic dimensions of the coflow burner (diameter and length of the center jet nozzle, length and inflow arrangement of the concentric coflow burner, et al) was studied and the rational design was determined according to the calculation results. Independently and precisely control of the outflow conditions of the center jet and concentric coflow was achieved based on the design of the control system of the gas line and burner outflow condition. The post-processing method on the flame image was improved via MATLAB programming, making the resolution of the data reduced to about 0.5 mm, which met the need of the measurement of the geometric features of the triple flame.The propagation of the methane triple flame in coflow flow field was studied using high-speed photography. Through the analysis of the propagation state, speed and other characteristic parameters under different conditions, the propagation characteristic of methane triple flame was discussed, the relationship between the state of propagation and combustion was identified. The influence of the inflow condition on the relation between propagation speed and local flow speed was obtained, thus the influence mechanism of that on the evolution in the propagation state was further clarified. The motion feature of the triple flame in the transition between different propagation states was discovered, "three stage attaching" or "three stage blowout" would appear when the propagation state was at the transition between "attaching and lifted off" or "lifted off and blowout", and the critical inflow condition causes the transition in propagation state was obtained. By studying the movement of the tip of the triple flame, the influence of the inflow condition on the flammable range of the flow field was identified, thus the effect mechanism of that on the flame stabilization and propagation state was further clarified.The evolution of the geometry of the propagating methane triple flame was studied using high-speed photography. Through the analysis of the global and local geometric features, the geometry characteristic of methane triple flame was discussed. The relationship between the width and height of the flame base was identified, it was found that the width of the flame base tended to increase with its height, and the influence of the jet velocity on the flame length was identified, which was that the increase in the flow velocity and fuel would restrain and promote the lengthening of the flame respectively. The variation regulation with time of the width of the flame base under different conditions of propagation state was obtained. The influence of the inflow on the flame length was revealed, it was found that the Kelvin-Helmholtz instability induced by the buoyancy in the flame tip was the main causing of the fluctuation of the flame length. The effect of the disturbance upstream on the flame length under different conditions of propagation state was clarified, when the local flow velocity roughly equaled the flame front propagation velocity, the disturbance in the flow field had a greater impact on the flame.The combustion characteristics of triple flame was studied using spectroscopy method. Through the analysis of the spectral information and the key intermediate content of the triple point under different inflow conditions, the variation regulation of the combustion characteristics of methane triple flame was discussed. The influence of the inlet velocity and stoichiometric ratio of the center jet and coflow on the local combustion characteristics and production rate of NOx of the triple point was identified. The variation regulation of the heat release intensities of rich premixed combustion, lean premixed combustion and diffusion combustion when the inflow condition changed was obtained, and the variety in the proportion of the combustion process of each combustion mode was clarified. It was found that comparing to the coflow, the increase of the velocity and stoichiometric ratio of the center jet prompted more the combustion of the rich premixed flame branch and the production rate of prompt NOx in the triple point.In order to further demonstrate the evolution of the triple flame geometry and its mechanism from the microscopic point of view based on the experimental results, this research adopted the laminar finite-rate model and detailed methane combustion mechanism (30 species and 184 reversible reactions) to numerically studied the propagation and geometry evolution of triple flame in confined flow under different inflow conditions. The influence of the boundary layer of the solid wall on the distribution of the velocity and concentration of the cold colflow flow field was identified. By the utilizing of coupled heat transfer method which could taking into account the impact of heat transfer among the reacting flow-nozzle and boundary wall-static air outside, the influence of the confined wall heated by flame to the flame itself and the unburned mixture upstream was revealed, it was found that the thermal expansion induced by the preheating of the unburned mixture by the hot wall was the main incentive of the flow redirection upstream the flame. Through the analysis of the elementary reactions in the triple point under different conditions of coflow diameter, it was found that the increase of the concentration of H and OH was the contributing factor of the combustion of triple flame.