UC Irvine

The injection of liquid fuel at supercritical pressures is a necessary but overlooked topic in combustion. Under the premise that the liquid will swiftly transition to a supercritical state, the role of two-phase dynamics is typically ignored. Nevertheless, a transcritical domain exists for which a sharp interface prevails before substantial heating occurs. This transcrit- ical scenario may persist, for example, during the early deformation of liquid hydrocarbon fuels injected into high-pressure combustion chambers. Local thermodynamic phase equi- librium (LTE) enhances the dissolution of the oxidizing species into the liquid phase, and vaporization or condensation may occur simultaneously in several regions along with the in- terface. Following species and thermal mixing, fluid properties vary greatly, with liquid- and gas-mixture properties being more comparable near the interface. Small surface instabilities emerge early due to the combination of low, variable surface-tension force and gas-like liquid viscosities.

The mixing process, interface thermodynamics, and early deformation of a cool liquid n- decane jet submerged into a hotter moving gas initially composed of pure oxygen are studied under different ambient pressures and gas velocities. For this purpose, a two-phase, low- Mach-number flow solver for variable-density fluids is proposed. Since the jet flow evolution corresponds to transitional turbulence, no turbulence models are considered. The interface is captured using a split Volume-of-Fluid method, generalized for a non-divergence-free liq- uid velocity. Both phases exchange mass across the interface. Numerical challenges emerge compared to incompressible two-phase algorithms, which are successfully addressed for the first time in this type of flow. The importance of transcritical mixing effects for increasing pressures is demonstrated. At first, local deformation features emerge, which differ signifi- cantly from earlier incompressible works. Then, instead of the traditional atomization into droplets, the minimal surface-tension force is responsible for the folding and layering of liquid sheets. Thus, rather than spray formation, surface-area increase at transcritical conditions is primarily the result of gas-like deformations under shear. Moreover, the interface may be easily perturbed in hotter regions submerged in the faster oxidizer stream under specific triggers. The net mass exchange limits the liquid-phase vaporization to the small liquid structures at high pressures. Then, the dynamical vortex identification method is used to identify the interaction between vortical structures and the liquid surface, explaining various deformation mechanisms. Details on the evolution of vortex structures are provided, showing that layering traps and weakens the vortices.