Observations and analysis of near-critical fuel injection

Abstract

Results from a study of n-heptane jets injected into a nitrogen environment under supercritical pressures with respect to n-heptane are presented. The temperature of the injected jet varies from supercritical to subcritical values (with respect to the n-heptane critical temperature) while the ambient environment is maintained at temperatures above the critical temperature of n-heptane. Three types of jets are observed and described: (1) subcritical, where the liquid/gas interface persists until complete evaporation of the liquid phase (2) supercritical, where there is no sharp interface between the injected jet and the ambient gas, and (3) transcritical, where distinct liquid/gas phase interface exists at injection but disappears before complete evaporation of the liquid. The subcritical and transcritical jets exhibit Rayleigh-Plateau instability where the jet breaks up into droplets. For subcritical jets, the droplets maintain their sharp interface whereas for the transcritical case the droplets transition from evaporative mixing to diffusive mixing with the surrounding fluid. The experiments are conducted under microgravity conditions at the Zero-Gravity Facility at NASA Glenn Research Center to minimize effects of buoyancy. To interpret these observations, the evolution of a transitioning droplet is modeled with a Regularized Interface Method that can describe both subcritical and supercritical processes as well as transcritical dynamics. Model predictions of critical transition are compared with experimental results and show good agreement.