Experimental Study Of The Deformation And Breakup Of A Liquid Drop In Shock Induced Gas Flow

The deformation and breakup of an isolated liquid drop suddenly exposed to a high-speed flow is not only a classic problem of multiphase flow but also with rich engineering backgrounds. In this thesis, the deformation and breakup processes of a liquid drop in shock-induced high-speed gas flow are observed experimentally, and,with the help of numerical simulation and theoretical analysis, the drop deformation mechanisms as well as the influence mechanisms of the flow parameters to the drop development are investigated. The main works and conclusions include the following:First, a large amount of experimental tests (We>350) are systematically conducted based on a shock tube facility, and high-speed photography is performed to capture the images of drop deformation and breakup processes in shock-induced flows. The experimental images present plenty of details of drop deformation in the early stage of aero-breakup, such as the overall flattening, the fine wavy structures on the wind-ward surface, the circular lips, and so on. By testing different incoming flow conditions and different drop parameters, the features of drop deformation under varied flow den-sity, flow Mach number, Reynolds number (of outer flow) and viscosity of liquid are revealed. The results indicate that, although the drops generally follow the same aero-breakup mechanism under the similar Weber number condition, the early appearance of the deformed drop exhibits a variety of patterns which are mainly characterized by the leeward circular bulges and lips. To understand such drop deformation phenomena, the outer gas flow and the in-drop liquid flow are decoupled in the later numerical and the-oretical considerations. By this method, equations of the radial acceleration on the drop surface under the "shear induced mass accumulation” mechanism and the "normal pressure induced radial flowing"mechanism are derived, respectively. A comparison between the two mechanisms suggests that the uneven pressure distribution on the drop surface is the main cause of the liquid bulges and lips. When the outer flow is known,the theoretical model of “normal pressure induced radial flowing” may well predict the characteristics of early drop deformation. The predicted locations and relative am-plitudes of the bulges appear to agree well with those in the corresponding experimental results.Second, by summarizing the evolutional characteristics of the outer gas flow under different test conditions, the varied drop deformation patterns and their connection to the outer flow are explained. It is found that a low pressure area emerges quickly in the recirculation zone subsequent to the sweeping of the incident shock wave, and that the lasting time of the low pressure area is roughly proportional to the characteristic developing time of the outer flow. Thereby, the ratio between the characteristic devel-oping time of the outer flow and the characteristic time of drop deformation in a way represents the contribution degree of the low pressure area in the whole process of drop deformation, and thus largely influences the deformation pattern of the drop. With other test parameters remaining constant, the increase of flow Mach number tends to decrease the overall total pressure of the recirculation zone. The possible pressure gradients in this zone is then weakened, whereby the formation of bulges and lips is suppressed.The increase of the Reynolds number of the outer flow intensifies the unsteadiness of the flow structure. It tends to cause vortex fluctuations by which the number of bulges are increased whereas the amplitudes of them are suppressed. The viscosity of liquid plays a role of resisting deformation. It has little influence on the locations of the bulges and the characteristics of them, though.Third, multiple existing drop breakup models are comparatively examined accord-ing to the present experimental images as well as the data extracting from them. The examination of the overall flattening speed shows that the Burges' model agrees better to the present experimental results in the beginning of deformation whereas the TAB model turns to be better in the later stage along with effects of liquid viscosity and sur-face tension becoming gradually apparent. A modification to the Burgers' model is also proposed to take into account the compressibility of gas at relatively high Mach number in this study. The examination on the formation mechanisms of the mist shows that the mists formed on the equator of the drop under relatively higher Weber numbers and rel-atively lower Weber numbers follow respectively the 'sheet-thinning' mechanism and the 'shear stripping' mechanism. As the Weber number is low enough to approach the boundary between SIE and RTP regimes, there will be stripped liquid sheets developing from the drop equator, but the formation of mist occurs much later than the prediction of the ' sheet-thinning'.