Calculated Interaction of Sprays With Large-Scale Buoyant Flows

Turbulent, recirculating gas flows resulting from interactions of water droplet sprays with large-scale buoyancy sources are difficult to predict without the use of numerical techniques, especially when spray-induced gas motion is considered. One such flow occurs when a negatively buoyant methane cloud, generated during LNG spills in a wind, is dispersed by a line water spray. Numerical predictions of the ratio of average methane vapor concentration downwind of the line spray to the upwind value correlate as a function of the ratio of methane momentum in the vapor cloud to water momentum in the spray. Warming of the cloud, which occurs when small drops in the spray freeze, leads to the production of positive cloud buoyancy and the possibility of cloud lift off from the ground. Numerical calculations have also been used to predict how a near-ceiling, downward-directed spray interacts with an opposed, buoyant jet issuing from floor level. Recirculating gas motion induced by droplet trajectories is again an important part of the problem. This opposed spray-plume arrangement, which is important in the process of fire suppression by automatic sprinklers, allows the effectiveness of spray cooling of the near-ceiling environment to be determined as a function of droplet injection characteristics. Because of the excessive amounts of computer time required for the solution of both turbulent, buoyant flow problems, it is concluded that much more efficient numerical techniques are needed.