Deformation behavior of two droplets successively impinging obliquely on hot solid surface

Significance Statement

The impingement of a jet of liquid droplets on a hot solid surface is a common phenomenon in spray cooling. The collision of individual droplets of water with a dry solid generates a crucial topic in jet impingement that involves extremely small flow rates. The flow characteristics of the liquid jet significantly rely on the volume flow rate. A low flow rate produces distinct droplet impingement on a hot and dry solid surface where as an increased flow rate amplifies the collision of incoming droplets with the previous ones.  This leads to the formation of a stable liquid film on the hot surface, upon which most of the incoming droplets impinge.

In a recent paper published in Experimental Thermal and Fluid Science Dr. Hitoshi Fujimoto and colleagues from Kyoto University in Japan proposed a study with the main objective being to understand the flow interaction of two droplets that successively obliquely impinge a hot substrate. They hoped to achieve their objective by empirically investigating the successive oblique collisions of two droplets with a hot sapphire surface.

First, a pair of water droplets, of diameter 0.6 mm at room temperature, was vertically dropped to impact a tilted smooth sapphire substrate successively. The impact velocity varied between 1.6 and 2.1 meters per second. The researchers then varied the spacing between the successive water droplets between a range of 0.8 and 1.5 mm. The substrate was then tilted to an angle less than 45° and its temperature varied between 170 °C and 500°C.

The researchers observed that the leading droplet impacted the substrate and slid downwards over the surface. This was then followed by the off centered collision of the trailing droplet relative to the deformed leading droplet. The research team was then able to observe that the resultant subsequent motion of the combined liquid droplets was essentially three dimensional. Concurrently, they noticed that at the substrate temperatures 200-300°C, the combined liquid was significantly distorted by the bursting of boiling vapor bubbles at the free surface. At a substrate temperature of 500°C, the liquid motion was approximately linearly symmetric. Rebounding phenomena were also observed in the liquid. Eventually, the research team measured the residence time of the droplets and used the results obtained to derive an expression for predicting the parameter.

From this empirical analysis, it can be concluded that the sapphire substrate temperature significantly influence the effect and resulting motion of the impacting water droplets. The spacing between the droplets affects the liquid motion but not the residence time of the droplets. The residence time of the droplets can therefore be roughly correlated with its equivalent Weber number.

Reference

Hitoshi Fujimoto, Soushi Yoshimoto, Ken Takahashi, Takayuki Hama, Hirohiko Takuda. Deformation behavior of two droplets successively impinging obliquely on hot solid surface. Experimental Thermal and Fluid Science volume 81 (2017) pages 136–146.

Graduate School of Energy Science, Kyoto University, Kyoto 606-8501, Japan.

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