重力对火焰闪烁的影响及非线性分析

Effects of Gravity on Flame Flicker and Nonlinear Analysis

为研究不同重力环境下火焰闪烁行为的变化规律,采用ANSYS FLUENT软件模拟甲烷/空气火焰在同轴射流燃烧器中的燃烧过程,对比分析了不同重力(0~5g,g为重力加速度)下火焰结构、火焰闪烁频率的变化,并对燃烧系统进行了非线性分析。同轴射流燃烧器模型:内管直径为11.4 mm,长为33.3 mm,甲烷和空气按一定比例混合后从内管进入燃烧室;外管直径为95.2 mm,长为233.3 mm,伴流空气从外管进入燃烧室;内管燃料入口流速为Uf=0.37 m/s,外管伴流空气入口流速Uc=0.037 m/s。结果表明:引起射流火焰闪烁的原因是浮升力诱导的一种水力学不稳定性;随着重力加速度的增加,火焰面厚度逐渐变薄,火焰面外沿由平滑曲面变成褶皱曲面;火焰锋面周围的漩涡在浮升力驱使下运动加快,火焰闪烁频率增加;火焰的无量纲闪烁频率St和入口处无量纲数Fr的1/2次方成正比;随着重力加速度的增加,吸引子从不动点变为椭圆状,最后变为蝴蝶状,相应火焰燃烧状态从稳定燃烧变为周期振荡,最终进入混沌。

In order to explore the law of flame flicker in different gravity environments,the combustion process of methane/air flame in coaxial jet burner was simulated by ANSYS FLUENT software.The flame structure and flicker frequency under different gravity(0~5g,g is the acceleration of gravity)are compared and analyzed,and the combustion system is studied by non-linear analysis.The diameter of the inner tube is 11.4 mm,and the length is 33.3 mm.Methane and air are mixed in a certain proportion and then enter into combustion chamber from the inner tube.The diameter of the outer tube is 95.2 mm,and the length is 233.3 mm.Co-flow air enters into combustion chamber from the outer tube.The fuel inlet velocity of the inner tube is Uf=0.37 m/s,and the air inlet velocity of the outer tube is Uc=0.037 m/s.The results show that the cause of flame flicker is a hydrodynamic instability induced by buoyancy.With the increase of the acceleration of gravity,the thickness of the flame surface becomes thinner and the outer edge of the flame surface changes from a smooth surface to a folded surface.The vortex around the front of the flame is accelerated by buoyancy and flame flicker frequency increases.The dimensionless flicker frequency St of the flame is directly proportional to the 1/2 power of the dimensionless number Fr at the en trance.With the increase of the acceleration of gravity,the attractor changes from a fixed point to an ellipse shape and finally to a butterfly shape.The combustion state of the corresponding flame changes from stable combustion to periodic oscillation and finally chaos takes place.