Flushing of Dense Gas from Bottomlands by Wind

Abstract

Mappings of the areas of occurrence of fatalities in the aftermath of disasters such as those in Bhopal, India and at Lake Nyos, Cameroon, reveal strong influences of topography on channeling and inhibiting diffusion of dense gases. Wind tunnel experiments were carried out at the USEPA Fluid Modeling Facility to determine how fast a crosswind removes dense gas that is otherwise trapped in a topographic depression. V-shaped valleys set into the tunnel floor were used. A series of 92 steady-state experiments, with inflow of CO₂ at the valley bottom balanced by outflow due to entrainment, established relationships between outflow rate, height of the dense gas pool in the valley, reference wind speed upwind of the valley, and relative gas density difference times gravity, g. Measurements of wind speed just above pool height, U_s, confirmed that the outflow is proportional to U_s ³/g’ for Froude number <1 and large Reynolds number, Re; it does not depend on valley width, W. At low Re a weak dependence on W was found; an algebraic expression fitting both large and small Re asymptotes revealed that molecular diffusion can dominate entrainment even at real-world scales if U_s < 2 m/s. “Flushing” experiments using CO₂ and SF₆ with no inflow provided further support for relationships developed from the steady-state experiments.