An experimental investigation of the leading edge of diffusion flames

This research is devoted to clarifying the nature and behavior of the Leading Edge portion of diffusion flames by experimental observations and the development of a phenomenological model. The motivation of the research originates from the need to better understand the combustion of composite solid propellants in rocket motors, especially the role played by the leading edge part of the oxidizer-fuel diffusion flame. Since experimental investigation of actual solid propellant flames is presently not possible in detail because of the microscopic scale of the combustion zone, this research has used an atmospheric pressure gas burner to investigate the characteristics and controlling processes in the leading edge flame zone. Specifically, tests using combustion photography and CH flame radiation measurement were conducted to examine the heat release rate of the LEF region and the subsequent diffusion flame. The effect of systematic variations of controllable parameters such as gas velocities and dilution levels were studied with the help of a Schlieren system, an M-Z interferometer and thermocouple measurement, to understand the characteristics of the LEF. The results indicate that the leading edge flame zone has a heat release rate significantly higher than the rest of the diffusion flame. The value of this maximum varies with the change of control variables and the flame adjusts its position accordingly. The heat release rate results suggest that the LEF will contribute the most to the upstream heat transfer into the oxidizer-fuel flows, and the presence, stability, and velocity of the flame as a whole will be heavily dependent on the LEF behavior. Other results from the parametric studies indicate the increase of mixing time necessary for a stable flame to sustain itself against higher gas velocity or higher dilution level. Current studies also suggest that the stabilization of the LEF could be argued through the balance between the flow divergence effect (lowering velocity) and the heat loss effect (measured flame temperature at the stoichiometric point reflecting the level of lateral heat loss) in a 2-D mixing region. The results reveal that flame accommodation to increased flow velocity involves both reduced heat losses and increased divergence factor. However, the main factor determining establishment of a new stable location resulting from dilution is not the establishment of a flame with less heat loss (i.e., the adjustment in flame position to reactant dilution does not involve much change in heat loss), but one with a larger flow divergence factor associated with a larger LEF further from the burner surface.