FLUID-FLOW AND HEAT-TRANSPORT NEAR THE CRITICAL-POINT OF H2O

Near-critical extrema in the properties of water may influence flow pa tterns in hydrothermal systems, but singularities in equations of stat e for H2O at its critical point have inhibited quantitative modeling. Posing governing equations in terms of pressure (P) and enthalpy (H) a voids these singularities and facilitates computation. Numerical simul ations with a P-H based model show little near-critical enhancement in heat transfer for systems in which flow is driven by fixed pressure d rops. However, in density-driven systems, near-critical variations in fluid properties can enhance convective heat transfer by a factor of 1 0(2) or more (''superconvection'') if permeability is sufficiently hig h. Near-critical two-phase processes (''heat pipes'') are at feast equ ally effective at dissipating thermal energy. The restriction to high- permeability environments within a fairly narrow P-H window suggests t hat superconvection may be quite rare in natural systems