LIQUID AND VAPOR FLOW IN SUPERHEATED ROCK

In this review, we describe the dynamics and thermodynamics of liquid and vapor flow through hot fractured rock. Such flows occur in geothermal reservoirs and have important implications for geothermal power generation; we describe both forced flows associated with liquid injection into such systems, and natural convective flows associated with the vertical heat transfer through such systems. First we focus on permeable media and describe the heat transfer of single-phase liquid or vapor flow through a medium of different temperature. Then we consider the dynamics and thermodynamics of a liquid front as it advances into a superheated region and boils. The morphological stability of such an interface is discussed, and we describe conditions under which the interface breaks down to form a two-phase zone between the liquid and vapor. We next examine the heat transfer and boiling in gravity-driven flows advancing through a superheated permeable rock, identifying that at large times such currents asymptote to a family of similarity solutions. In the second part of the review, we describe the analogous heat transfer and boiling processes associated with liquid flow along a fracture embedded in an impermeable rock. We describe some simple asymptotic solutions for the temperature distribution in the bounding rock, which reveal that in the fracture, a two-phase boiling region develops between the purely liquid and purely vapor zones. Model predictions are successfully tested with laboratory experiments. In the final section of the review, we briefly discuss natural convective flows, illustrating how single-phase and two-phase convective regions interact and in some cases produce instability.