High-Speed Imaging of Unstable and Stable Homogeneous Nucleation of Diethyl Ether Droplets at the Superheat Limit

Abstract

Immersed diethyl ether droplets, sized 0.5 to 3.5 mm in diameter, undergoing homogeneous boiling at the limit of superheat, 147˚C, are examined experimentally. The boiling process and resulting vapor bubble behavior filmed at 100,000 frames per second is studied in conjunction with far-field pressure data. It is shown that at low pressures, 1 – 2 bar absolute, homogeneous nucleation manifests as unstable explosive boiling, where the evaporating bubble interface becomes unstable in a process believed to be similar to Darrieus-Landau instabilities, leading to an evaporative velocity and mass flux an order of magnitude greater than traditional boiling. In comparison, at elevated pressures, 3 – 5 bar absolute, homogeneous nucleation is stable, resulting in a smooth evaporative interface devoid of instabilities and a vaporization velocity and mass flux rate comparable to traditional nucleate boiling. Vapor bubble growth during unstable nucleation is found to be linear with constant interface velocity and mass flux rate. In addition, vapor bubble growth during stable nucleation vaporization shows a roughly logarithmic shape with time, with an interface velocity and mass flux that decays rapidly at first and slowing overtime. The normalized droplet diameter combined with the resulting vapor bubble diameter is examined from the beginning of nucleation until the vapor bubble stops expanding. For unstable nucleation, the normalized diameter growth generally forms an S-shaped curve. For stable nucleation, the normalized diameter growth appears mostly linear. Increasing ambient pressure is shown to decrease growth rate, interface velocity, and mass flux rate. Increasing initial droplet diameter is shown to not have an impact on vapor bubble growth rate, interface velocity, or mass flux rate. However, when normalized by initial droplet diameter, smaller initial diameter droplets are found to grow to larger vapor bubble diameters than larger initial diameter droplets. After unstable explosive boiling is complete, the behavior of the resulting vapor bubble is studied as oscillations occur, a result of initial growth kinetic energy, and Rayleigh-Taylor interface instabilities form. Rayleigh-Taylor instabilities on the surface of the vapor bubble are quantified in a wavelength parameter, which is shown to first decrease in length with subsequent oscillations before increasing in length as oscillations are dampened by presumably some combination of thermal, viscous, and acoustic radiation effects. Pressure data is shown during vaporization and during subsequent vapor bubble oscillation. An increased ambient pressure is found to correlate to a decreased Rayleigh-Taylor instability wavelength, a decreased overpressure amplitude, and an increased oscillation frequency. An increase in initial droplet diameter has been shown to increase Rayleigh-Taylor instability wavelength, increase overpressure amplitude, and decrease vapor bubble oscillation frequency.