Influence of gravity and pressure on pool fire-type diffusion flames

Abstract

Experiments are conducted to study the effects of gravity and pressure on the characteristics of diffusion flames of the pool fire type, that is, a diffusion flame stabilized on a burning horizontal fuel surface. In the experiments, the pool fire is simulated by injecting at very low velocity, a gaseous fuel (ethane) through a small-scale, porous, flat, horizontal surface burner and generating a diffusion flame over the burner by the reaction of the gaseous fuel with air. The resulting diffusion flame is characterized by a low Froude number. The diffusion flame characteristics (visual appearance, height, radiant output and temperature, and velocity distribution) are investigated at gravity levels ranging from microgravity (parabolic trajectory of an aircraft) to 12 times normal gravity (centrifuge facility—atmospheric pressure), and at ambient pressures ranging from 0.03 to 0.3 MPa (normal gravity). The results provide information about the effects of these variables on the flame characteristics and data for validation of numerical models of diffusion flames. Furthermore, they also help in understanding some of the limitations of Froude, or pressure, modeling of fires. The experiments indicate that the effect of gravity and pressure on the flame characteristics appears primarily through their effect on the buoyantly induced entrainment of air by the flame plume. Although at elevated pressures the effects are similar on the flame size and shape, important differences are observed on their effect on soot formation. It is found that for pressures above atmospheric, pressure has a major influence in soot formation and, consequently, on the radiant characteristics of the flames, increasing as pressure is increased. It is also found that at pressures below atmospheric pressure and gravity have opposite effects on flame size and soot formation and that consequently their effects on flame radiation also differ.