The influence of the ambient pressure on the breakup process is investigated by means of PIV and shadowgraphy in the configuration of a planar prefilming airblast atomizer. Other investigated parameters are the gas velocity and the film loading. It is found that the gas velocity in the vicinity of the prefilmer partly matches the analytical profile from the near-wake theory. The shadowgraphy images of the liquid phase allow to extract characteristic quantities of the liquid accumulation at the atomizing edge. Two different characteristic lengths, the ligament velocity and a breakup frequency are determined. In addition, the spray Sauter Mean Diameter (SMD) and the mean droplet velocity are given for each operating point. A scaling law of these quantities with regard to ambient pressure is derived. A correlation is observed between the characteristic length of the accumulation and the SMD, thus promoting the idea that the liquid accumulation determines the primary spray characteristics. An threshold to distinguish the zones between primary and secondary breakup is proposed based on an objective criterion. It is also shown that taking non-spherical droplets into account significantly modifies the shape of the dropsize distribution, thus stressing the need to use shadowgraphy when investigating primary breakup. The ambient pressure and the velocity are varied accordingly to keep the aerodynamic stress $\rho_g U_g^2$ constant. This leads to almost identical liquid accumulation and spray characteristics, confirming that the aerodynamic stress is a key parameter to characterize prefilming airblast breakup. Finally, SMD correlations from the literature are compared to the present experiment. Most of the correlations calibrated with LDA/LDT measurement underestimate the SMD. This highlights the need to use shadrowgraphy for calibrating primary breakup models.

中文翻译：

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环境压力对预成膜鼓风雾化中积液和初级喷雾的影响

在平面预成膜鼓风雾化器的配置中，通过 PIV 和阴影成像研究了环境压力对分解过程的影响。其他研究的参数是气体速度和薄膜负载。发现预成膜器附近的气体速度部分符合近尾流理论的分析剖面。液相的阴影成像可以提取雾化边缘处积液的特征量。确定两个不同的特征长度，韧带速度和断裂频率。此外，还给出了每个操作点的喷雾索特平均直径 (SMD) 和平均液滴速度。推导出这些量相对于环境压力的标度定律。在积聚的特征长度和 SMD 之间观察到相关性，从而促进了液体积聚决定主要喷雾特性的想法。基于客观标准提出了区分初级和次级破裂区域的阈值。还表明，将非球形液滴考虑在内会显着改变液滴尺寸分布的形状，从而强调在研究初级破裂时需要使用阴影法。环境压力和速度相应地变化以保持空气动力应力 $\rho_g U_g^2$ 恒定。这导致几乎相同的液体积累和喷雾特性，证实空气动力应力是表征预成膜鼓风破裂的关键参数。最后，将文献中的 SMD 相关性与本实验进行比较。大多数使用 LDA/LDT 测量校准的相关性都低估了 SMD。这突出了使用阴影法校准初级破裂模型的必要性。