Abstract
Pouring one liquid to another is a frequent occurrence in everyday life and industrial production, extensively utilized in numerous mixing processes. Introducing high-viscous fluid to a lighter and low-viscous liquid can lead to unexpected outcomes. Understanding the principles behind these phenomenons could enable us to apply them to improve efficiency in other production processes. In this dissertation, I will discuss the distinct phenomena, their mechanisms, and practical uses, when the viscous flow impacts and passes through another liquid in forms of streams and droplets.I will first report a peculiar phenomena of a viscous jet deflected on the free surface which we name as fluid ladder. Unlike the common phenomenon, the jet despite being denser and miscible with the surrounding liquid free surface, it still flows stably on the surface. By virtue of the laser experiment and interferometry, this is primarily found because an air film fully envelops the jet so as to avoid its direct contact with the surround liquid, greatly enhancing the effect of surface tension. Via the analysis of force balance and time span upon jet impacting on free surface, the presence of fluid ladder is predicted to be related with Weber number and Froude number. Besides, the relations between the characteristic length versus the Capillary number and ratio of surface tension and density is found by the collapse of data. More excitingly, the jet is discovered to deform along the flow, whose shape factor is modeled to have relation with liquid’s surface tension, jet radius, jet flow rate and air viscosity. Taking every cross-section for analysis under force equilibrium, the local pressure distribution, air flow direction and rate, and force conditions when the three phase line is moving are solved in MATLAB. It ultimately reveals that once the fluid deforms to a certain extent, the triple line will reach the top, and the water surface no longer has sufficient surface tension generated by deformation to support the jet, resulting in the jet’s eventually falling. We extended this phenomena by injecting stream on moving free surface and producing super long ropes and trying two reactive liquids to realize the production of gels, and also making trials to manipulate the stream to create unique patterns.
I will then present the droplet breakup phenomena by releasing the viscous drop from air to another miscible free surface. Three breakup modes including Jellyfish, Bag and Folding according to their shape changes are found to fragment at different depths and related with Reynolds number. The topological, width and thickness changes, and velocity changes with time evolution are captured and analyzed for each modes. Meanwhile, we visualized the trajectory, velocity, and vorticity field as droplets descent via the PIV experiments. The single vortex in Jellyfish breakup mode indicates that the shear flow on the other side results in the inward contraction of droplets’ skirt, while the symmetrical vortex in Bag breakup mode leads to the increase of droplet’s diameter, thickening of the rims and thinning of the central region. Finally, we compared the mixing effects by injecting the viscous flow in the form of stream and droplets at three breakup modes, all of which are at same flow rates. It is surprising to find that droplets breakup especially in Bag and Folding modes terrifically enhanced the blending comparing with directly pouring fluid columns. In the current situation where liquids of a high viscosity difference (1000) exhibit poor mixing efficiency, the leverage of this droplets fragmentation phenomena could have potential for becoming one of the new low-cost and high-efficient mixing approaches.
| Date of Award | 9 May 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Steven WANG (Supervisor) |