An Experimental Study of Evaporation Waves in a Superheated Liquid

Abstract

The dynamical behavior governing the propagation of evaporation waves in chlorinated fluorocarbons is studied in a constant-diameter vertical glass test cell which exhausts into a large, low-pressure reservoir. Care is taken to suppress heterogeneous nucleation within the liquid column. The test liquid is initially in equilibrium with its own vapor, sealed by a foil diaphragm. Upon diaphragm rupture, a series of expansion waves depressurizes the liquid to approximately the reservoir pressure, during which nucleation and subsequent rapid vaporization begin at the free surface. After an approximately 10 ms long start-up transient, a quasi-steady process develops during which the wavefront propagates into the stagnant liquid column at constant average velocity, generating a nonuniform high-speed two-phase flow. The leading edge of the wavefront consists of smooth and rough bubbles with maximum diameters of order 1 mm and characteristic lifetimes of order 1 ms. High speed movies show that the nucleation rate is both spatially nonuniform and temporally nonsteady, which leads to significant unsteadiness in the propagation of the wave. Fragmentation of the liquid into fine droplets occurs primarily as the result of the violent break-up of the leading-edge bubbles coincident with explosive bursts of aerosol, which occur in the region extending about 1 cm downstream of the leading edge bubble layer. These two processes appear to be mutually interactive. Three distinct modes of flow initiation are observed depending on the liquid superheat. Moreover, a self-initiation threshold is observed, below which waves do not occur. We observe that waves can propagate at slightly lower superheats if they are started artificially. However, an absolute threshold for wave propagation exists which is related to the nonsteady processes alluded to above.

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