Numerical modeling of two-phase flow in the NaCl-H2O system: 2. Examples

In an earlier paper, we presented a new multiphase numerical code for NaCl-H2O fluid called FISHES. Here we present some example simulations using FISHES that show its applicability to 1-D and 2-D geometries that are commonly used in describing two-phase hydrothermal flows. We simulate a 1-D heat pipe with an open top upper boundary in order to study mass and energy balance properties within a region of phase separating fluid. The results show the formation of a dense brine layer at the base of the heat pipe as a two-phase zone expands toward the top. We then simulate 2-D circulation in a single-pass geometry with a highly permeable discharge zone, a geometry that has often been used in the study of seafloor hydrothermal systems. We show that when active phase separation occurs at the base of the system, low-salinity fluid emerges from the top as a dense brine layer forms at the base and that increased vent fluid temperatures result from decreasing the width of the discharge pipe. Following a period of active phase separation, we show that lowering the basal temperature results in flushing the brine from the system on a time scale similar to the period of active phase separation. Lowering the permeability does not significantly affect vent fluid salinities during active subsurface phase separation but inhibits the flushing of brine upon lowering of basal temperatures; also, low permeabilities increase the time scale for the evolution of the system. Finally, we conduct simulations of two-phase flow in rectangular 1 km(2) homogeneous fluid-saturated open top boxes heated from below with permeabilities of 10(-12) and 10(-13) m(2). During these simulations a strong plume forms above the region of strong lateral temperature change, and brine collects at the base of the system. Initially, low-salinity fluids vent from the top of the box; eventually, higher-salinity fluids emerge. Vent fluid salinities and temperatures associated with plumes vary with time, resulting in larger temporal variation in fluid properties than those obtained from single-pass simulations.