TED-AJ03-235 A Study of Dynamic Behavior of Liquid Droplet Impacting on Heated Surfaces

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The study of a single droplet impact and spread on a heated surface is motivated by its strong relevance to spray cooling technology. The generic view on the study of the spray cooling process exhibits the synthesis of fluid mechanics, heat transfer and surface thermodynamics. Due to the complex phenomena involved, no comprehensive theoretical models are available. The few works that appeared in the literature so far have been largely empirical; the applicability of several correlations proposed is limited. This paper reports an experimental study of fundamental aspects of the dynamic characteristics of a single droplet impacting on a heated surface. It aims to provide some insight into the underlying mechanisms of the spray cooling process. The objectives of this study are (1) to carry out thorough experiments to observe in situ the behavior of liquid droplets impact and spread on heated surfaces, and (2) to generate experimental data that would enable validation and improvement of the spray cooling models. The entire dynamic process of a droplet from the moment of collision with the substrate surface including the rebound was visualized and analyzed using a high-speed CCD camera. The experimental study focused mainly on the spread of a liquid droplet under the influences of substrate temperature varying from 26℃ to 240℃, the inclination angle of substrates at 0°, 30°, and 60°, the wettability of substrates with contact angles from 30°to 90°, the viscosity of liquids ranging from 0.00089 up to 0.9161kg/m・s, and surfactants of different concentrations. Experimental results show that the aforementioned parameters affect the spread and height of the droplets on the substrates in the following ways : 1. At room temperature, the droplet, upon impacting on a substrate, exhibits spreading, attaining the maximum of deformation, recoiling, then experiences damped oscillations, and finally reaches equilibrium. 2. When the substrate temperature is above the boiling point of water but below its Leidenfrost temperature, bubbles grow very fast with miniaturization occurring at a later time. The water droplet has a slightly higher fluctuating spread and higher peak compared with that which impinges the substrate at room temperature. 3. Miniaturization disappears and no bubbles are present if the substrate temperature is above the water's Leidenfrost temperature due to the occurrence of film boiling. After rising to a peak spread upon impact, the droplet rebounds from the surface. 4. Prior to the droplet spread reaching its maximum size, the effect of the substrate temperature is insignificant. 5. Compared to a water droplet, the glycerol droplet undergoes lighter damped oscillation after impacting on the surface and it stabilizes quickly due to its high viscosity. No miniaturization is present at 150℃ and there is no rebounding of the droplet even at 240℃. 6. The spreading diameter for the inclined surfaces is lower than that for the horizontal surface during initial period but it quickly surpasses that of the horizontal surface and low angles of inclination due to the effect of gravity force along the tangential direction. 7. Droplet spread increases with higher wettability or lower contact angles. 8. Droplet spread also increases with water of higher surfactant concentrations due to the reduction in surface tension. Adding SDS surfactant causes a decrease in the intensity of oscillation. Decay rates of such oscillations increase while their frequencies decrease as surfactant concentration increases.

TED-AJ03-235 A Study of Dynamic Behavior of Liquid Droplet Impacting on Heated Surfaces

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