Publications
Non-equilibrium gas-liquid interface dynamics in high-pressure liquid injection systems
Dahms, Rainer N.; Oefelein, Joseph C.
The transition of classical spray atomization processes to single-phase continuous dense-fluid mixing dynamics with diminished surface tension forces is poorly understood. Recently, a theory has been presented that established, based on a Knudsen-number criterion, that the development of such mixing layers is initiated because the multicomponent two-phase interface becomes much wider than the mean free molecular path. This shows that the transition to mixing layers occurs due to interfacial dynamics and not, as conventional wisdom had suggested, because the liquid phase has heated up to supercritical temperatures where surface tension forces diminish. In this paper we focus on the dynamics of this transition process, which still poses many fundamental questions. We show that such dynamics are dictated by substantial statistical fluctuations about the average interface molecule number and the presence of significant interfacial free energy forces. The comprehensive analysis is performed based on a combination of non-equilibrium mean-field thermodynamics and a detailed modified 32-term Benedict-Webb-Rubin mixture state equation. Statistical fluctuations are quantified using the generally accepted model of Poisson-distributions for variances in systems with a small number of molecules. Such fluctuations quantify the range of pressure and temperature conditions under which the gradual transition to dense-fluid mixing dynamics occurs. The interface begins to deteriorate as it broadens substantially. The related interfacial free energy forces do not instantly diminish only because vapor-liquid equilibrium conditions do not apply anymore. Instead, such forces along with the present interfacial statistical fluctuations are shown to gradually decrease as the interface transitions through the molecular chaos regime and to diminish once the interface enters the continuum regime. Then, the interfacial region becomes a continuous gas-liquid mixing layer with diminished free energy forces that is significantly affected by single-phase real-fluid thermodynamics and transport properties.