Round turbulent puffs and buoyant thermals in uniform Crossflows

The properties of round nonbuoyant turbulent puffs, and buoyant turbulent thermals in uniform crossflows were studied both experimentally and theoretically, motivated by applications to interrupted gas and liquid releases caused by process upsets, explosions and unwanted fires, among others. Emphasis was placed on self-preserving conditions far from the source where extraneous source disturbances have been lost, where flow properties are largely controlled by the conserved properties of the flow, and where properly scaled flow properties become independent of both distance from the source and time. The experiments involved injecting dye-containing turbulent round water puffs and thermals into a uniform crossflow produced by a 610 x 610 mm crosssection water tunnel facility and observing the flow with CCD cameras. Near-source properties varied significantly with source properties but the flows became turbulent and then self-preserving within 5 and 30-40 source diameters in the vertical direction (aligned with the source puff/thermal) from the source, respectively. Within the self-preserving region, the vertical penetration distance generally varied as a function of time in accord with self-preserving predictions. Flow in the self-preserving region, also satisfied the assumption of no slip crosstream convection at the mean crosstream velocity, yielding a simple and convenient way of predicting flow trajectories. Radial penetration distances of the flows also satisfied the general scaling relationships for puffs and thermals in still fluids but with the added complication that the flow was no longer axisymmetric about the axis of the trajectory. Finally, when amounts of source fluid injected became large for puffs and thermals, self-preserving behavior for starting jets and plumes was achieved before the source flow was terminated and the flow was best considered to be an interrupted jet or plume rather than a puff or a thermal.