Experimental study of analogue vent erosion towards nozzle shapes

Abstract

Volcanic vent shapes evolve over the course of an explosive eruption, as the high-pressure ejecta erode the solid conduit walls. The changing shape affects the fluid behavior of the expelled gases, potentially the plume rise, column collapse, and the type and amount of accidental debris ejected. Field observations of evolving vent shape are difficult or impossible. While the mechanics of this process have been studied numerically, almost no laboratory studies have examined these behaviors. Thus, a series of test samples was manufactured using 3D printing techniques, which were eroded using high-pressure compressed air. Four material combinations were considered, using stainless steel and alumina powders consolidated with binder concentrations of 16% and 38%. Material properties were evaluated using compression testing, while the erosion behavior was studied through high-speed visualization and pressure measurements with identical samples at constant inflow conditions. In all cases, the erosion showed an initial rapid erosion of the vent exit, followed by quasi-steady state conditions. All samples produced converging/diverging nozzle shapes, with exit areas increasing by 1.5 to 6 times, depending on the sample properties. Behavior correlated strongly with binder concentration, as low-binder materials had significant increases in nozzle area. When using high-density particles, these weaker samples disintegrated due to particle abrasion from higher kinetic energy impacts. The nozzle shapes produced supersonic exit conditions, with Mach numbers from 1.48 to 2.51 for the various samples. However, exit pressures were not balanced, suggesting that the optimum is not the isentropic, pressure-balanced shape previously postulated.