环形逆向射流流场结构特性的模拟研究

逆向射流是火焰稳定的重要技术手段,为探究环形逆向射流结构对煤粉逆喷式旋流燃烧器稳燃特性的影响,构建了不同尺寸圆形与环形射流管的逆向射流模型,以Fluent软件为计算平台,采用“可实现”的k-ε模型对流场结构进行模拟研究。通过计算不同流速比下各射流管的无量纲贯入深度变化,发现环形射流的贯入深度大于圆形射流,且随内径的增大而增大,其射流在环境流中的渗透能力增强,这有利于增加煤粉的逆向传播距离,增加停留时间,提高燃烧稳定性。与圆形射流不同,环形射流的贯入深度与流速比并不是单调的线性关系,在流速比7.5～12.5存在一个平缓的过渡段,表明在一定区间内,流速比的减少不会引起贯入深度的明显减小,对现有燃烧器的设计具有指导意义。通过计算不同流速比下各射流管的最大零流速半宽,发现环形射流在逆流中的最大零流速半宽大于圆形射流,并随射流管内径增大而增大,表明环形逆向射流在径向的扩展宽度更大,卷吸能力更强。环形逆向射流的最大零流速半宽随流速比呈线性增大,斜率随内径增大而增加,高流速比的环形逆向射流径向扩展能力强于低流速比的环形逆向射流。根据计算结果绘制分界流线,发现在较低速度比时,射流的分界流线与圆形射流相似;在较高速度比时,虽然射流的轴线速度在滞点减少为零,但轴线附近的一定区域,流体依然向下游传播渗透,经过一段距离后速度才降至零并转向,分界流线也变得不规则。环境逆流在轴向上对环形射流有一定程度的压缩和阻碍作用,使得射流在径向的扩展宽度增大,增强了射流的卷吸能力,有助于促进燃烧器内煤粉与助燃风的混合。通过研究不同射流管逆向射流轴线上轴向流速的沿程变化,发现环形射流轴线上的流速衰减较圆形射流更为平缓,表明其与环境流的掺混不如圆形射流强烈。随着内径不断减小,环形射流轴向速度峰值增大且位置提前,衰减速度增强,逐渐接近圆形射流中轴线上的衰减情况。 The counterflow jet is an important technical means for flame stability. In order to explore the effect of counterflow jet on steady burning of the reversed injection burner of coal particle combustor,the flow field structure of different annular and round jets into a counterflow were computationally investigated using the Realizable k-ε Model in Fluent platform. By computing the penetration length in variational velocity ratios of each jets,the results show that the penetration length of annular jet is larger than that of round jet,and it increases when the annular grows,The permeability of annular jet in ambient flow increases which is conductive to increase the backward propagation distance of pulverized coal and residence time and improve combustion stability. Unlike round jet,the penetration length of annular jet does not have a monotonic linear relationship with velocity ratio. There exists a smooth transition when velocity ratio between 7.5-12.5,which indicates that the decrease of velocity ratio does not cause significant decrease of penetration length within a certain interval and has guiding significance for the design of existing burners. By calculating the maximum zero velocity half-width of each jet tube at different velocity ratios,it is found that the half width of zero velocity of annular jet is wider than that of round jet,and increases with the increase of the inner diameter,which indicates larger radial expansion width and stronger suction capacity. The half width of zero velocity of annular jet increases linearly with the velocity ratio,and the slope increases with the increase of the inner diameter. The radial expansion capacity of annular jets in counterflow with high velocity ratio is stronger than that of annular reverse efflux with lo