EXPERIMENTS ON THE PERIODIC INSTABILITY OF BUOYANT PLUMES AND POOL FIRES

An experimental study of buoyant propane diffusion flames was undertaken to identify the mechanism responsible for the periodic oscillations near the source of these flames. This phenomenon, often referred to as ''puffing'' in the literature, exhibits itself as quasi-periodic oscillations of the diffusion flame front near the axisymmetric source of a fire with formation of large scale flaming vortical structures. Experimental diagnostics primarily involved flow visualization, velocity, and pressure measurements under a variety of experimental conditions. First, the behavior of buoyant, noncombusting plumes originating from 0.10- and 0.30-m-diameter sources was investigated with either isothermal helium or high temperature combustion products as the buoyant fluid. It was found that the helium plumes exhibited puffing at both scales with puffing frequencies similar to the flames scaling with D-1/2. The plumes of hot combustion products on the 0.30-m-diameter burner were highly turbulent and puffing was very weak compared with the flames that were generated upon ignition of the latter hot gas plume of vitiated combustion products. Second, effects of heat release were studied by dilution of fuel with a noncombustible gas. It was found that puffing persisted as long as a diffusion flame was sustained by the fuel stream, although its intensity diminished with increasing dilution. Third, effects of disturbances both internal and external to the flame were studied. These experiments strongly suggest that there exists a coupling of the flame front motion near the burner surface with the downstream development of large-scale flaming structures. Based on the reported experiments, a puffing mechanism is suggested. The puffing mechanism involves (1) acceleration of buoyant plume gas in stagnant surroundings resulting in formation of a toroidal vortical structure within one burner diameter above its surface, (2) the decaying influence of the toroidal vortex on the flame surface near the burner lip as it convects upward, and (3) the accumulation of buoyant gas inside the flame envelope and its buoyant acceleration to form the next vortical structure. The scaling of puffing frequency with the burner diameter is connected to the convection speed of toroidal vortices within one diameter height above the burner surface as it was shown with the use of a simple kinematic model.