On diffuse-interface modeling of high-pressure transcritical fuel sprays

At subcritical pressures, atomization devices in chemical-propulsion systems must ensure proper rupture of the liquid volume through aerodynamic shearing and homogeneous dispersion of the liquid droplets in the combustion chamber. The gas environment in the combustor is typically hot as a result of the heat released by chemical reactions, with part of its enthalpy being employed in vaporizing the liquid phase. The resulting vapor mixes and burns with the ambient gas in the combustor. Several studies have tackled the combustion dynamics of sprays and the resulting flame structures at low pressures (e.g. see Sánchez et al. (2015) and references therein). Conversely, not much is known about high-pressure sprays, partly due to the challenges associated with experimental diagnostics and analytical modeling for such extreme conditions. High-pressure combustion of liquid propellants is of relevance for a number of propulsion systems. However, injection configurations tend to be specific to the practical application under consideration. For instance, in liquid rocket engines, cryogenic liquid oxidizer is injected along with gaseous fuel coflows at extremely high pressures. In these conditions, the coflow is supercritical and the liquid oxidizer stream promptly reaches the same thermodynamic conditions upon completion of a short heating period that increases its temperature a few degrees up to the critical value. Downstream from the injector, in the supercritical mixture, the latent heat of vaporization vanishes and there is virtually no surface tension that prevents rupture of the liquid core and diffusive mixing with the gas environment (Mayer et al. 2013). Although supercritical phenomena have been studied extensively in rocket engines because of the high pressures involved (Sirignano & Delplanque 1999; Yang 2010), they have also been observed in recent experiments in high-pressure reciprocating engines (Dahms et al. 2013). Supercritical dynamics is also becoming increasingly relevant in gas-turbine engines given the current trends that gear design towards higher compression ratios to increase efficiency and performance (Mongia 2013), although much less is known of these systems in high-pressure operation regimes. In gas-turbine engines, the hydrocarbon liquid fuel is typically injected at ambient temperatures, while the oxidizer is provided by the air bled from the compressor. At pressures below the critical value, subcritical atomization dominates with primary and secondary stages taking place downstream from the injector (Lasheras & Hopfinger 2000). Conversely, supercritical dynamics occur in the combustor at pressures above the critical value for the mixture. Since the liquid fuel is seldom preheated to supercritical temperatures …