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Title Splashing and Breakup of Droplets Impacting on a Solid Surface
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Publication Date
Date Available
Degree Level doctoral
University/Publisher University of Toronto
Abstract

Two new mechanisms of droplet splashing and breakup during impact have been identified and analyzed. One is the internal rupture of spreading droplet film through formation of holes, and the other is the splashing of droplet due to its freezing during spreading. The mechanism of film rupture was investigated by two different methods. In the first method, circular water films were produced by directing a 1 mm diameter water jet onto a flat, horizontal plate for 10 ms. In the second method, films were produced by making 0.6 mm water droplets impact a solid surface mounted on the rim of a rotating flywheel. Substrate wettability was varied over a wide range, including superhydrophobic. In both cases, the tendency to film rupture first increased and then decreased with contact angle. A thermodynamic stability analysis predicted this behavior by showing that films would be stable at very small or very large contact angle, but unstable in between. Film rupture was also found to be promoted by increasing surface roughness or decreasing film thickness. To study the effect of solidification, the impact of molten tin droplets (0.6 mm diameter) on solid surfaces was observed for a range of impact velocities (10 to 30 m/s), substrate temperatures (25 to 200°C) and substrate materials (stainless steel, aluminum and glass) using the rotating flywheel apparatus. Droplets splashed extensively on a cold surface but on a hot surface there was no splashing. Splashing could be completely suppressed by either increasing the substrate temperature or reducing its thermal diffusivity. An analytical model was developed to predict this splashing behavior. The above two theories of freezing-induced splashing and film rupture were combined to predict the morphology of splats typically observed in a thermal spray process. A dimensionless solidification parameter, which takes into account factors such as the droplet diameter and velocity, substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, was developed. Predictions from the model were compared with a wide range of experimental data and found to agree well.

PhD

Subjects/Keywords Fluid Dynamics; Heat Transfer; Droplet Impact; Droplet Splashing; Thermal Spray Coating; Water Jet Impact; 0548
Contributors Chandra, Sanjeev; Mechanical and Industrial Engineering
Language en
Country of Publication ca
Record ID handle:1807/17753
Repository toronto-diss
Date Retrieved
Date Indexed 2020-03-09

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…arrangement to study liquid jet impact on a pin………………….22 Figure 2.8: Jet tip displacement measurements with time…………………………………...23 Figure 2.9: A schematic diagram of molten tin droplet generator…………………………...26 Figure 2.10: Photograph of the water droplet…

…films are produced by the normal impact of a water jet on a solid surface. In the second method, films are produced by the normal impact of water and molten tin droplets on solid surfaces. A theory is developed to predict the internal rupture of films…

…Research Objectives The objectives of this research are: • To design and build an apparatus to photograph radially-spreading thin liquid films by normal impact of a water jet on solid surfaces of different material • To develop an analytical model to…

…films by using the normal impact of a water jet on a solid surface. It then depicts the rotating flywheel apparatus that was used to study the high-speed impact of droplets on solid surfaces. This is followed by a detailed description of the different…

…for the droplet impact study. Chapter 3 focuses on the water jet impact experiments done in order to investigate the stability of thin films against rupture through hole formation. It first describes the experimental CHAPTER 1. INTRODUCTION 12…

…designed and built to study the stability of thin liquid films (thickness ~ 100 μm), which were produced by impacting a 1 mm water jet on a solid surface for 10 ms. The process of impact, and the ensuing film spreading and retraction were…

…halogen lamp with a diffuser was used to provide illumination during the impact process. Shortly after the solenoid valve was opened, a water jet emerged from the needle and the CCD camera was triggered. These events were synchronized with the time delay…

…EXPERIMENTAL METHODOLOGY 18 Figure 2.4: A typical sequence of jet impact, film spreading and retraction (Pu = 97 kPa). CHAPTER 2. EXPERIMENTAL METHODOLOGY 19 A problem was encountered in producing water jets by the solenoid valve method as…

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