Transition from mixed to forced convection for opposing vertical flows in liquid-saturated porous media

Mixed-convection phenomena can occur within liquid-dominated geothermal reservoirs due to interactions of injected flows, or ground water flows, with the buoyancy-induced fluid motion. This problem was studied experimentally and numerically for the case of opposing flows about a vertical heat source in a liquid-saturated porous medium. The ratio of the Rayleigh number (Ra) to the Peclet number (Pe) was identified as the nondimensional parameter which characterizes the relative influence of buoyancy-driven to pressure-gradient-driven fluid motion. The transition from mixed to forced convection was numerically determined to be (Ra/Pe) approx. = 0.5, where the minus sign denotes superimposed downflow. Agreement between measured and predicted thermal-field results showed that the finite-element code of Gartling and Hickox can be used to model low-temperature (single-phase) geothermal reservoirs throughout the natural, mixed, and forced convection regimes.