Title: Buoyant convection resulting from dissolution and permeability growth in vertical limestone fractures 

Abstract: Upward flow through vertical fractures in limestone formations under a geothermal gradient favors dissolution of rock. Thus the fracture aperture and overall permeability of the fractured rock system grow with time. In some circumstances the permeability growth largely affect the coupled processes such as water flow, heat transfer etc. We have investigated the transition from conductive and forced convective regimes to instability and buoyant convection as a result of permeability growth. The onset time for instability and roll height at onset depend on the initial aperture and driving pressure (or initial flow rate). In general the thermal instability is quantitatively described by dimensionless Rayleigh number. As the system evolves with time aperture and density gradient do not remain uniform across the fracture plane. So, a modified Rayleigh number criterion has been proposed, which provides a unified interpretation of the instability across a wide range of initial aperture and driving pressure. Interaction between buoyant convection and aperture alteration leads to narrow upward flow paths supporting dissolution and precipitation in surrounding downward flow regions.