| Author: | Wu, Zhuofeng |
| Title: | Experimental investigation on the collision of unequal size binary droplets of water in atmospheric air |
| Advisors: | Zhang, Peng (ME) |
| Degree: | M.Sc. |
| Year: | 2021 |
| Subject: | Drops Fluid dynamics Collisions (Physics) Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | x, 68 pages : color illustrations |
| Language: | English |
| Abstract: | Liquid droplets collision is a frequent phenomenon in many natural and technical processes. Especially, theoretical, and experimental analyses for droplets head-on collision are worth studying in a gaseous environment, with the bouncing and coalescence outcomes. Therefore, we designed an experiment for investigating the two physicals phenomenon by the collision between a large stationary droplet on a super-hydrophobic surface and a small falling droplet, with emphasis on the conversion from coalescence to bouncing. This experiment contains two important control parameters Weber number (We)and size ratio (Δ). Besides, Ohnesorge number (Oh) and impact parameter (B) are also needed to be considered. In the study, we used pure water in experiment. Therefore, the viscosity and the surface tension are the same in all experiments. Because of the different velocity of the free-fall droplet, the weber number is different in every experiment, with range from 0.3 to 10, and size ratio is from 1 up to 3. To make sure that the environment factor would not affect the accuracy of the experiments, the roundness of the free-fall droplets should be around 1 with little diversification. Moreover, to explore if the contact angle will influence the collision result or not, two kinds of hydrophobic materials are used in the experiment. Binary droplets collisions, which were produced by the electric droplet generator, were visualized in a special plane, where a high-speed camera is used with a LED lighting which can provide photographic of collision motion. Weber number based on the size and the relative velocity of the smaller droplet which generated by the electric droplet generator. The size ratios can be calculated by the sizes of two droplets and the weber numbers can be decided by a series of time-resolved pictures. What is more, the experiments outcomes can be plotted into We-Δ graph by identifying the images took by the high-speed camera, which make different experiment results can be distinguished. It is found that the size ratio hardly effects the results of experiments and the most important factor, which plays an important role in the experiments is the Weber numbers. To eliminate the reaction force of the board, this study discussed two experiments. One used super hydrophobic material and the other used ordinary hydrophobic material and the difference between the two materials are the contact angles. However, no matter what materials are used, the critical weber number is about 1.7 to 2. Furthermore, it was found that it is easier to find bounce phenomenon in off-center collision case because of the thin layer of gas. To draw a conclusion, the bounce boundary of water collision is at the Weber number about 2, and size ratio has little effect in this experiment. Big collision parameter can help to get bounce phenomenon. |
| Rights: | All rights reserved |
| Access: | restricted access |
Files in This Item:
| File | Description | Size | Format | |
|---|---|---|---|---|
| 5939.pdf | For All Users (off-campus access for PolyU Staff & Students only) | 2.07 MB | Adobe PDF | View/Open |
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