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Title: Investigation on the interaction of a shock and a liquid droplet with and without a vapor cavity inside
Authors: Liu, Yao
Degree: Ph.D.
Issue Date: 2021
Abstract: The stripping breakup process of a single liquid droplet under the impact of a planar shock wave is the phenomenon that has been widely observed over the last 70 years. This droplet aerobreakup phenomenon is a fundamental and challenging two-phase flow problem that occurs in multi-application, such as secondary atomization and raindrop damage during an atmosphere reentry. A challenging problem which arises in this domain is how the internal structure and flow field inside the droplet affects the droplet's initial deformation. In this thesis, the droplet breakup experiment is conducted in a horizontal shock tube and the evolution of the droplet interface is recorded by direct high-speed photography. Compressible Euler equations are solved using an in-house inviscid upwind characteristic space-time conservation element and solution element (CE/SE) method coupled with the HLLC approximate Riemann solver. A reduced five-equation model is employed to demonstrate the air/liquid interface. First, we present the numerical and theoretical investigation targeting the establishment of an internal flow field inside a pure water droplet exposed to shock-wave impact. The main focus is on the description of the droplet internal flow pattern, which is believed to be one of the dominant factors in initial droplet deformation. The droplet internal flow pattern holds steady for quite a long time after the incident shock passage, and a saddle point is observed for the first time. Accordingly, the saddle point inside the droplet flow is used as a characteristic point to describe the internal flow. Cases of different incident shock strengths and liquid are tested, and a theoretical prediction is proposed to delineate the correlation between the saddle point steady position and the strength of the incident shock wave. The numerical cases are found to be in good agreement with the prediction. The present study helps to complete the understanding of the overall droplet aerobreakup phenomenon. Moreover, to bridging the knowledge gap that few studies have yielded the deformation and breakup regimes of a water droplet embedded with a vapor cavity in the high-speed airstream, the interaction of a shock wave and a water droplet embedded with a vapor cavity is experimentally investigated in a shock tube for the first time. The vapor cavity inside the droplet is generated by decreasing the surrounding pressure to the saturation pressure, and an equilibrium between the liquid phase and the gas phase is obtained inside the droplet. The formation of a transverse jet inside the droplet during the cavity-collapse stage is clearly observed. Soon afterwards, at the downstream pole of the droplet, a water jet penetrating into the surrounding air is observed during the cavity-expansion stage. The evolution of the droplet is strongly influenced by the evolution of the vapor cavity. The phase change process plays an important role in vapor cavity evolution. The effects of the relative size and eccentricity of the cavity on the movement and deformation of the droplet are presented quantitatively. A modified Rayleigh-Plesset equation is derived that reasonably predicts the bubble collapse process.
Subjects: Drops
Hong Kong Polytechnic University -- Dissertations
Pages: xx, 157 pages : color illustrations
Appears in Collections:Thesis

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