Thermal gradient stabilized supercritical droplets and bubbles

In the pursuit of higher efficiencies, combustion pressures in rockets, gas turbines, and piston engines are being increased, often exceeding the thermodynamic critical pressures of the respective fuels. At these nominally supercritical conditions, a pure fluid neither exhibits a phase equilibrium with vapor and liquid in coexistence nor a surface tension at the interface. Instead, experimentally observed droplets are attributed to mixture vapor liquid equilibria. In this paper, we identify an alternative mechanism that causes the formation of stabilized droplets at supercritical conditions, even in pure fluids: thermal gradient stabilized interfaces. We show in computational fluid dynamic simulations and analytical solutions that, surprisingly, initially diffuse and blurry interfaces of (cool) dense and (warm) light regions self-stabilize to a sharp density gradient as time progresses, instead of diffusing further, even in absence of surface tension. A dimensional analysis of the transient vaporization process reveals a D^2 law of supercritical vaporization, akin to Spalding's classical result for subcritical evaporation. Thus, the experimental observation of droplet shapes at supercritical conditions does not necessarily imply the existence of phase equilibrium and surface tension.