旋流作用下的液膜初始破碎可视化实验研究

Visualization Experimental Investigation on Liquid Film Primary Breakup with Swirling Flow

利用背光照明和高速相机,对强剪切气动雾化喷嘴出口雾化进行了可视化实验研究,探究旋流中的液膜初始破碎特性及一二级旋流对液膜初始破碎的影响。唯象描述了液膜破碎过程和模式,并分析图像获得表征液膜初始破碎特性的物理量:液膜破碎长度和径向拍振频率。实验结果表明:旋流作用下液膜破碎主要为液袋破碎和液丝破碎模式,这与平面液膜相似,且受工况的影响规律也相同,但旋流作用使得破碎过程和模式叠加,更为复杂。液膜破碎长度主要由一级旋流决定,二级旋向的影响可忽略。径向拍振频率认为是由Kelvin-Helmholtz(KH)和Rayleigh-Taylor(RT)不稳定机理共同主导,且受一二级旋流共同影响;此外,在大气流流量时,同旋更有利液膜失稳破碎,即径向拍振频率更大,而小流量时反旋更利于破碎。进一步由实验数据得到拟合经验公式,两者吻合良好,且发现径向拍振频率可能与旋流数之间存在关联。最终认为旋流作用下液膜更易失稳破碎,且一级旋流决定了液膜初始破碎的基本形态,二级旋流起强化剪切和辅助作用。

The liquid film primary breakup characterizations under swirling flow and the influence of the primary and secondary swirling on the liquid film primary breakup were visualization experimental investigated by means of high-speed camera and backlighting illumination at the outlet of high shear air-blast injector.The pro⁃cesses and modes of liquid film breakup were phenomenological described.Meanwhile,two typical physical quan⁃tities of liquid film primary breakup were obtained by analyzing the high speed images:liquid film breakup length and radial flapping frequency.The experimental results indicate that liquid film breakup mainly presents bag-breakup and ligament-breakup modes under swirling flow,which is similar to the planar liquid film,and the influence patterns of operating conditions is same.While the swirling flow makes the breakup processes and modes superimposed and more complex.The liquid film breakup length is major determined by the primary swirl⁃ing flow,and the direction of secondary swirling can be ignored.The radial flapping frequency is thought to be jointly dictated by the Kelvin-Helmholtz(KH)and Rayleigh-Taylor(RT)instability mechanisms and affected by primary swirling flow as well as secondary swirling flow.In addition,at high air flow rates,co-rotation is more favorable for liquid film instability and breakup,namely higher radial flapping frequency.However,counter-ro⁃tation is more beneficial to breakup at low air flow rates.Furthermore,empirical equations are fitted from the ex⁃perimental data,which agree well,and a possible correlation between radial flapping frequency and swirl num⁃ber is also found.Finally,it is concluded that liquid film is prone to being instable and broken under the swirling flow,and the primary swirling flow determines the basic morphology of the liquid film primary breakup,the role of secondary swirling flow is to enhance shear and assist.