| Summary: | 博士 === 國立成功大學 === 航空太空工程學系碩博士班 === 92 === More than seventy years has passed since Weber’s theory was proposed to modify Rayleigh’s instability theory in 1878. However, the influences of viscosity and surface tension on the frequency band of mono-sized droplet generation have not been clearly explained. Also, it is not assured whether the frequency band of mono-sized droplet generation of “molten metal (Sn63 Pb37)” can be described by both theories. Thus, this dissertation tries to clarify above questions through the experiments. The working fluids include the water solutions, glycerine solutions and the molten metal (Sn63 Pb37). The major apparatus is a mono-sized droplet generator, which breaks a laminar stream of fluid to produce mono-sized droplets through the forced excitation by a piezoelectric disk. The mono-sized droplets are formed if the excitation is applied in the proper frequency range. The results show the fluids can be divided into the low viscosity and viscous fluids to describe the effect of viscosity and surface tension on the working frequency band. The separation margin is around 25 cp to 60 cp. For low viscosity fluids, the influence of viscosity variation on the working frequency band is not significant because the fluids with different viscosities have the same disturbance growth rate for each disturbance wavelength. The influence of surface tension variation on the working frequency band is not obvious because the disturbance growth rates for two different fluids have the same ratio for each disturbance wavelength. For viscous fluids, as the viscosity increases or the surface tension decreases, the wave number at the center of the working frequency band decreases because optimum wave length increases. The maximum working frequency decreases because Relative growth rate ratio is smaller than 1. The minimum working frequency decreases because Relative growth rate ratio is bigger than 1. Additionally, the fluids with the same Ohnesorge number have the same working frequency band because both optimum wavelength and growth rate are the function of Ohnesorge number.
In the investigation of mono-sized molten metal droplet generation, the results show that the conditions under which molten metal jet is affected by the increase of oxygen concentration can be approximately divided into three regimes by two “critical oxygen concentrations, [O2]*.” They are the “breakup regime,” the “incomplete breakup regime,” and the “breakup failure regime,” respectively. [O2]*1st is equal to around 500 ppm with a diameter of 152 �慆 and is also the limit of the breakup regime at the natural breakup of a molten jet in this research. As long as the excitation is applied to the jet, the molten metal jet can completely break up again in a certain frequency band. However, if [O2]*2nd is reached, the molten metal will never be broken up, even when the excitation is applied. [O2]*2nd is around 900 ppm with a diameter of 152 �慆. The behaviors of molten metal jets conform to the Rayleigh’s theory in “breakup regime” and “incomplete breakup regime” under excitation. Rayleigh’s theory is not applicable in breakup failure regime because it does not predict any breakup failure due to the change of surface tension. It can be explained by integrating the research of Haj-Hariri and Poulikakos (2000), and Artem’ev and Kochetov (1991)--- i.e., the oxide film islands grow and join until reaching high surface flexural rigidity at the critical oxygen concentration, and resulting in a sudden failure of the breakup process
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