High-resolution numerical simulations give clear insights into the three-dimensional structure of thermal convection associated with black-smoker hydrothermal systems. We present a series of simulations that show that, at heat fluxes expected at mid-ocean ridge spreading axes, upflow is focused in circular, pipe-like regions, with the bulk of the recharge taking place in the near-axial region. Recharging fluids have relatively warm temperatures. In this configuration, the system maximizes its heat output, which can be shown to be linked to nonlinearity in the fluid properties. Furthermore, we present a series of simulations with different permeability scenarios. These show that when permeability contrasts are moderate, convection maintains this pipe-like fluid flow structure. The permeability contrast has a dominant effect on flow patters only at early, immature, stages of convection, focussing upflow in high-permeability regions and downflow in low-permeability regions. In such early stages of convection, diffusive vent styles can emerge, which look remarkably similar to diffuse vent fields in natural systems. Finally, simulations in which permeability is defined as a function of temperature indicate that the brittle-ductile condition is likely to occur at temperatures not lower than 650°C. At lower brittle-ductile transition temperatures, the system cannot remove the heat delivered from the magma chamber and vent temperatures are substantially lower than 4000C. This result is in agreement with estimates of the brittle-ductile transition temperature from rock-mechanical studies and the occurrence of earthquakes in the oceanic lithosphère. Copyright 2009 by the American Geophysical Union.