Translated Abstract
Droplet impact is of fundamental importance that has been investigated by extensive scientists since two centuries ago. These investigates have shaded some light on different fundamental scientific principles as well as practical applications in this rather simple looking but complex phenomena. Study of this phenomena is not only because it is fascinating but it also have a wide range of practical application in various fields that involve the spray scenario such as industrial heat transfer process by spray cooling, internal combustion engine mixture formation, spray coating and painting. The impact outcome between a high inertia droplet and different targeting surface such as solid surface, thin liquid film or deep liquid pool over a wide range of test conditions are actually affecting the designed performance of the spray. One governing parameter of the impact outcome is impact inertia which is typically non-dimensionalized as Weber number (We) by the surface energy of impacting droplet before impact. In addition to the surface tension effect, the impact outcome will be also affected by viscosity, which is another physical parameter of the fluid and typically non-dimensionalized as Ohnesorge number (Oh). On the bases of parameter space of the Weber number and Ohnesorge number, appearance of different impact outcomes are expected to be observed such as bouncing, partial coalescence, coalescence, jetting and splashing. Extensive literatures have attempted on finding the boundary between different outcomes for a fixed target surface. However, accurate determination of the transition boundaries is still an open issue, especially at extremely low impact inertia condition where the capillary effect dominates. In addition physical understanding on the dynamics of this fascinating process is still insufficient, though several theoretical models have targeted to describe the energy transformation during the impact process. In this work, we have investigated on the dynamics of droplet and deep liquid pool interactions with emphasize on the effect of impact inertia (We) and the viscosity (Oh).
We have done a comprehensive literature review of the topic together with background of present research in Chapter 1 and it showed that in past there were some researches on droplet impact on deep liquid pool focusing on finding the effect of different parameters on impact outcomes for example how diameter of droplet can affect the result of droplet impact or how physical properties of immiscible liquids effects this phenomena etc. However there is not much information in literature about very low impact Weber number case, the first objective of this thesis is to extend the experimental conditions to a wider range and to provide more experimental data related to this field. Our second objective is to investigate the effect of Weber number and Ohnesorge number for each kind of impact outcome to reveal underlying mechanism that governs the impact phenomenon. Finally, the literature models for transition boundaries between the coalescence and jetting will be compared with the present experimental data and for the boundaries between partial coalescence and coalescence where no previous models have been developed, we will propose an empirical model by fitting our experimental data. To conduct these experimental investigation, we have adopted the high speed technique to capture the impact process with a long focus microscope, the experimental setup and procedures, test conditions and liquid properties will be specified in Chapter 2.
High speed photography of the droplet liquid pool impact process as presented and discussed in Chapter 3 show that low impact Weber number impact (We~1.0) water shows complete coalescence. However, for decane and tetra-decane at this magnitude of Weber number, the drop sits on the pool for a while and then coalesce partially to the pool fluid. This is caused by the unbalanced surface tension force which initiate a capillary wave that greatly deforms the drop that coalesce partially and ultimately pinches off a daughter droplet. The daughter droplet bounces and come to rest on surface, repeating the partial coalescence for several rounds. In addition, if Weber number decreases partial coalescence is also observed for water. We have determined the critical Weber number that triggers partial coalescence for three fluids. Furthermore, other phenomena like coalescence, jetting and splashing are also observed and relative dimensionless numbers are calculated and finally graphs were plotted between these dimensionless numbers to compare the transition boundaries among all three liquids under observation. A model for each transition boundary is proposed based on the experimental data fitting.
Present work extends the previous literature research on the dynamics of droplet liquid pool interaction to a wider experimental range and the proposed models for different transition boundaries are of merits for controlling the dynamics of droplet pool impact scenarios, as concluded in Chapter 4.
Translated Keyword
[deep liquid interaction, Droplet impact, extremely low Weber number, model for transition boundaries of droplet impact]
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