| Author: | Chen, Xinmeng |
| Title: | Experimental investigation on the collision of binary droplets of water in atmospheric air |
| Advisors: | Zhang, Peng (ME) |
| Degree: | M.Sc. |
| Year: | 2017 |
| Subject: | Hong Kong Polytechnic University -- Dissertations Drops -- Mathematical models Fluid dynamics Collisions (Physics) |
| Department: | Faculty of Engineering |
| Pages: | xi , 101 pages : color illustrations |
| Language: | English |
| Abstract: | There has been a significant meaning to understand the results that occurring during a gravitational falling droplet head-on colliding with a hemispherical droplet attaching on hydrophobic material, with emphasis on the transitions from permanent coalescence with deformation to separates after temporary coalescence and the transitions from permanent coalescence with deformation to recover pressed shape before temporary coalescence by adjusting two important parameters, namely collision Weber number and size ratio. This experiment explored only one liquid which is water, so the viscosity and surface tension remained in a certain value in every droplet collision experiments. Weber number is different because of different collision velocity, which is dominated by falling distance of freefall droplet and droplets’ diameter. Weber number here is ranged from 0 to 1.2 and from 7 to 8. Size ratio is increased from 0.6 to 8. To ensure the accuracy of outcomes and eliminate the effects of environmental vibration, the roundness of falling droplet is used and has to be fluctuated around 1 with small variation. Binary droplets collisions produced by compressed air were visualized in a particular vertical plane by shadow-graphic method, where a high speed camera together with a LED backlighting is used which can provide photographic of collision motion. Values of the collision Weber number, which based on the small droplet size and droplets’ relative velocity, as well as the values of the size ratio, calculated by two droplets diameter, can be both decided by using a series of time-resolved photographic images. What’s more, collision outcomes can also be identified by micro-photographic images. Plotting these results into We-size ratio graph which make collision regimes with different outcomes can be distinguished. It is found that the critical size ratio is decreased with the value of Weber number increased. For Weber number changing from 0 to 1.2, the average transition boundaries are size ratio at 3.65 and 5.5 for the transitions from permanent coalescence with deformation to separates after temporary coalescence and the transitions from permanent coalescence with deformation to recover pressed shape before temporary coalescence. For Weber number ranging from 7 to 8, the average transition boundaries are size ratio at 1.8 and 2.3 respectively. So the collision reasults is more easily influenced by size ratio with Weber number increasing. Furthermore, it was found that thin layer of gas mass plays an important role in droplets collision process, and even lead to bounce in off-center collision case and help droplets recover their shape. |
| Rights: | All rights reserved |
| Access: | restricted access |
Files in This Item:
| File | Description | Size | Format | |
|---|---|---|---|---|
| 991021962224603411.pdf | For All Users (off-campus access for PolyU Staff & Students only) | 4.12 MB | Adobe PDF | View/Open |
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