| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 105 === This study presents a theoretical model for a spherical Leidenfrost droplet under different states of motion: stationary and combined rotational and translational motion. The velocity field inside the rotating droplet is measured by microscale image velocimetry method. Using this as the boundary condition and coupling the lubrication theory and the continuity equation, we are able to determine the velocity field and pressure field in the vapor film underneath the Leidenfrost droplet. In addition, scaling analysis is carried out to evaluate the temperature field in the vapor film and the heat transfer rate. The unique feature of our model is that it is able to iteratively solve the minimal thickness of vapor film, the rotational speed, the translational speed, the heat transfer rate, and the variation of droplet radius with time for a given wall temperature and an initial radius of the droplet. Herein, results of water and mineral oil, both are commonly used in spray quenching, are presented. The results show that the velocity field inside the Leidenfrost droplet resembles forced vortex, and heat transfer of the Leidenfrost droplet is dominated by thermal conduction through the vapor film, but contribution of thermal radiation from the heated surface to the entire droplet grows as the surface temperature increases. In addition, using mineral oil leads to a heat transfer rate lower than a water droplet. This is mainly abscribed to the low thermal conductivity of oil vapor. Moreover, less thermal energy can be transported when the motion of the Leidenfrost droplet is incorporated into the model. This is because droplet motion break the symmetry of the pressure profile and leads to a much thicker vapor layer . During the evaporation process, the Leidenfrost droplet both spins and moves faster and faster, and droplet shrinkages accelerate and there exists a critical radius below which both vapor film.
Keywords : Leidenfrost droplet、spray quenching、droplet motion、evaporation、Vapor film、heat transfer.
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