Vapor bubble dynamics in microgravity.

The problems of spherical vapor bubble growth in initially uniformly and non-uniformly superheated liquids are considered. A review of earlier analytical and numerical models is made, followed by the development of a model whose growth is initiated from a critical size vapor nucleus by providing a small temperature disturbance at the liquid-vapor interface. Momentum, energy, and surface tension are incorporated simultaneously. The numerical models are considered in two parts: a spherical bubble model and a hemispherical model. The former is applicable to bubble dynamic processes such as in cavitation or ultrasonic cleaning devices, while the latter is particularly applicable to pool boiling on a flat heater surface. The hemispherical bubble model cannot give a precise prediction of the bubble growth for pool boiling, but rather provides upper and lower bounds. The model was tested by comparison of the computed results with pool boiling experiments conducted as part of the present research in microgravity, which provides a known initial temperature distribution at nucleation in the absence of any liquid disturbances. It was demonstrated that the concept of upper and lower bounds for bubble growth has merit. Rapid vapor bubble growths at superheats far below the homogeneous nucleation superheat limit were observed in the pool boiling experiments in microgravity. Photographic evidence revealed a distortion of the bubble liquid-vapor interface as growth took place, which is believed to be a consequence of an interface instability. The growth of the instability is explained as the result of heat conduction at the distorted surface, where the heat transfer to the liquid-vapor interface increases at the trough of the interface perturbation while decreasing at the crest. The rapid bubble growth is modeled by introducing a simple expression in terms of a circular function to account for the increase in surface evaporating area. The onset of the interface roughening can be considered to originate from either a Rayleigh-Taylor or Landau instability; however, neither of these models successfully provides the necessary quantitative physical description.PhDApplied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/129205/2/9409744.pd