Air-blast atomization of liquid fuel jets is a crucial stage in the overall combustion process in various aero-propulsion systems. The coaxial configuration for air and fuel flow passages in the atomizers is relevant especially to liquid propelled rocket engines and also important for fundamental understanding of the jet breakup. The quality of atomization as a consequence of the liquid jet breakup close to the atomizer exit and the downstream droplet-air mixing is intimately related to the flame stability, overall performance and pollutant emission. The detailed understanding on the jet breakup process is important for control on the droplet size and the spatio-temporal distribution of the liquid mass near the reaction zone downstream of the atomizer exit. It is well known that typically the atomization of liquid jets occurs in two stages: the primary breakup of the liquid jet near the atomizer exit and the subsequent secondary breakup of the arbitrary shaped liquid ligaments as well as the large droplets generated from the primary breakup further downstream.¹ In the primary breakup region, the shear force at the liquid-gas interface is responsible for the disintegration of the liquid jet.^2–6 The extent of the primary atomization region is usually characterized by the jet breakup length, which is the length of the continuous liquid core corresponding to complete disintegration of the jet into droplets and ligaments. The liquid jet breakup length is typically few liquid jet diameters and in general, it depends on the liquid–gas relative velocity, the nozzle geometry and the physical properties of the two fluids^4,7,8 However, the primary breakup region can be additionally characterized by the unbroken length or liquid intact length referring to the axial location where breakup begins.⁹ The primary breakup length is very important for the performance of the fuel injectors and also for the development of computational models of the atomization process for numerical simulation of sprays as it defines the beginning of the fully developed multiphase flow region. In past, several studies have reported measurement of liquid jet breakup length in air-blast atomizer.^{1,4,8,10–12} Most of these studies are based on the shadowgraphic visualization of the jet. However, the presence of a dense cloud of droplets and ligaments around the liquid core often prevents accurate probing of the primary breakup region.¹³ In addition, the previous studies reported the mean jet breakup length based on identification of the breakup point either through visual inspection or image processing of an ensemble of images. However, the liquid jet disintegration process being highly unsteady, the instantaneous jet breakup length may vary substantially with time. According to Chigier and Farago¹⁴ and Chigier and Reitz,¹⁵ the jet atomization is often a pulsating process leading to a periodical temporal change in the spray characteristics (such as liquid volume flux and droplet number density) downstream of the atomizer even when the inlet liquid and atomizing gas flows are steady, oscillation-free and vibration-free. The unsteady flow of the liquid fuel mass into the combustion chamber can be of significance to practical issues such as the combustion instability. Hence, in addition to the mean breakup length, characterization of the jet breakup length fluctuations is also important since it indicates downstream unsteadiness within the spray. While only few studies have reported such measurements in the past (for instance, Charalampous et al.¹⁶), a comprehensive study on the fluctuations of jet breakup length for a wide range of operating conditions of the atomizer is still lacking.

中文翻译：

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同轴鼓风雾化器中的液体射流破裂不稳定

在各种航空推进系统中，液体燃料射流的鼓风雾化是整个燃烧过程中的关键阶段。雾化器中空气和燃料流动通道的同轴配置特别适用于液体推进火箭发动机，并且对于基本了解射流破裂也很重要。靠近雾化器出口的液体射流破裂和下游液滴与空气混合所导致的雾化质量与火焰稳定性，总体性能和污染物排放密切相关。对射流破碎过程的详细了解对于控制雾化器出口下游反应区附近的液滴尺寸和液团的时空分布非常重要。众所周知，液体射流的雾化通常分为两个阶段：¹在主要破碎区域，液-气界面处的剪切力负责液体射流的分解。^2–6初级雾化区域的范围通常以射流破裂长度为特征，该长度是连续液体芯的长度，对应于射流完全分解成液滴和韧带。液体射流的破裂长度通常只有很少的液体射流直径，并且通常取决于液体-气体的相对速度，喷嘴的几何形状和两种流体的物理性质^4,7,8但是，主要的破裂区域可以是附加的连续长度或液体完整长度是指破裂开始的轴向位置。⁹初始破裂长度对于喷油器的性能以及雾化过程的计算模型（对于喷雾的数值模拟）的开发非常重要，因为它定义了充分发展的多相流区域的起点。过去，一些研究报告了在鼓风雾化器中测量液体射流破裂长度的方法。^{1,4,8,10–12}这些研究大多数基于射流的阴影图像可视化。然而，在液芯周围存在密集的液滴和韧带云经常妨碍对主要破裂区域的精确探测。¹³另外，先前的研究报告了通过目视检查或图像整体的图像处理来确定破裂点的基础上的平均喷射破裂长度。然而，液体射流的分解过程非常不稳定，瞬时射流的破裂长度可能会随时间变化很大。Chigier and Farago ¹⁴和Chigier and Reitz ¹⁵喷射雾化通常是一种脉动过程，即使在入口液体和雾化气体流量稳定，无振荡和振动的情况下，雾化器下游的喷雾特性（例如液体体积流量和液滴数密度）的周期性变化也会引起周期性变化-自由。液体燃料团的不稳定流入燃烧室对于诸如燃烧不稳定性之类的实际问题可能具有重要意义。因此，除了平均破碎长度外，喷射破碎长度波动的特征也很重要，因为它表明了喷雾下游的不稳定。虽然过去只有很少的研究报告过此类测量（例如，Charalampous等人^16）），对于雾化器的各种工作条件，射流破碎长度的波动仍然缺乏全面的研究。