Solid Cryogen Formation in a Throttled Two-Phase Compressible Flow

When the pressure of a liquid drops below its triple point, a phase transformation occurs forming both solid and gas phases with no remaining liquid. Liquid cryogens vented from launch vehicles, such as the Centaur Upper Stage, will undergo such a transformation when exposed to pressures below their triple points. Solids thus formed may accumulate in the overboard vent, restricting flow and possibly blocking the vent. An investigation of vent blockage was performed using saturated liquid argon. Observations of solid adherence and accumulation were recorded for various argon delivery pressures. Wall adherence and vent blockage was strongly dependent on delivery pressure, with accumulation increasing for decreasing pressure. For low delivery pressures, relative to the triple point, vent blockage was immediate and complete. In addition, solid formation was observed to propagate into the liquid, against the direction of flow, revealing the porous nature of the solid as well as the impact on the flow.