Suppression of a baffle-stabilied spray flame by halogenated agents

Abstract

A series of experiments was conducted on baffle-stabilized spray flames in an effort to provide an improved understanding of the influence of various parameters on the processes controlling flame suppression. Measurements were made of the agent mass required to suppress the spray flames as a function of the agent injection duration (which was designed to deliver a constant mass flow of agent to the burner for a controlled duration), the air velocity, the oxidizer temperature, the ambient pressure, and the fuel flow. The agent mass fraction required to extinguish the flame was estimated from agent mass divided by the agent injection duration. Extinction measurements were performed with the gaseous agents CF3Br, CF3I, C2HF5 (CFC-125), and C3HF7 (HFC-227ea). The results showed that, in general, CF3Br was the most effective agent on a mass basis, followed by CF3I and then C2HF5 and C3HF7, which had similar effectiveness. For elevated air temperatures (T> 100°C), the three candidate replacement agents had similar effectiveness on a mass basis. As the air velocity increased, the agent mass fraction required for suppression (β) decreased. The fuel flow had little effect on β. An expression, based on treating the recirculation zone as a well-stirred reactor, was developed to describe flame suppression as the agent delivery duration varied. Two parameters were determined as crucial in that description. They were a characteristic mixing time that described the rate of agent entrainment into the recirculation zone downstream of a flame holder (or baffle) and the agent concentration at extinction for long agent injection durations. The model facilitated a comparison of the effectiveness of agents in suppressing baffle-stabilized spray and pool fires, two very different combustion configurations.