Fluid flow and heat transport near the critical point of H₂O

Abstract

Near‐critical extrema in the properties of water may influence flow patterns in hydrothermal systems, but singularities in equations of state for H2O at its critical point have inhibited quantitative modeling. Posing governing equations in terms of pressure (P) and enthalpy (H) avoids these singularities and facilitates computation. Numerical simulations with a P‐H based model show little near‐critical enhancement in heat transfer for systems in which flow is driven by fixed pressure drops. However, in density‐driven systems, near‐critical variations in fluid properties can enhance convective heat transfer by a factor of 10² or more (“superconvection”) if permeability is sufficiently high. Near‐critical two‐phase processes (“heat pipes”) are at least equally effective at dissipating thermal energy. The restriction to high‐permeability environments within a fairly narrow P‐H window suggests that superconvection may be quite rare in natural systems

On the dynamics of NaCl‐H2O fluid convection in the Earth's crust

“…Higher pressures shift the temperature to higher values and the peak in Ra L is wider (Figure 16). Using numerical simulations, Ingebritsen and Hayba [1994] have made a similar observation. They could show that at high permeabilities a hydrothermal system is cooled most efficiently at pressures and temperatures in the vicinity of the critical point, leading to superconvection.…”

“…Hence heat can be removed most efficiently by convection from a hydrothermal system at these conditions. This was also shown by Ingebritsen and Hayba [1994], using numerical simulations of pure H 2 O convection around the critical point of pure H 2 O. For pressures typical at mid‐ocean ridge hydrothermal systems (200 to 300 bars), the ∣∂ F /∂ T ∣ is maximized at ∼400°C and is likely the reason for the large difference between the temperature of a black smoker at 350°C to 400°C and its magmatic heat source at ∼1200°C [ Jupp and Schultz , 2000].…”

Application of the computer code TOUGH2 to the simulation of supercritical conditions in geothermal systems

“…HYDROTHERM Ingebritsen and Hayba, 1994) and the HOTH2O extension to the STAR simulator (Pritchett, 1994(Pritchett, , 1995. Both of these simulators are limited to regular rectangular or radial computational grids.…”

Physical balances in subseafloor hydrothermal convection cells

“…At hydrothermal pressures, the density and viscosity of water are highly nonlinear functions of temperature near ∼400°C, and it has been suggested for some years that this behavior might be responsible for the upper limit on vent temperatures [e.g., Bischoff and Rosenbauer , 1985; Johnson and Norton , 1991]. It is known that the thermodynamic properties of water affect the onset of convection and the overall rate of heat transfer in convection cells operating across a small temperature difference (say Δ T = 10°C) [ Straus and Schubert , 1977; Dunn and Hardee , 1981; Ingebritsen and Hayba , 1994]. If Δ T is sufficiently small, the variation of fluid properties with temperature can be linearized or ignored (the “Boussinesq approximation” [ Phillips , 1991]), and it can be shown that cells operating near the critical point of water (∼22 MPa, ∼374°C) transfer heat much more rapidly than cells driven by the same Δ T at other points in p − T space.…”