Assessment of Halon-Free Agents for Fire Safety Application in Aircraft Propulsion System

Abstract

Halon 1301 is an effective total flooding fire suppression agent which is widely employed in aircraft cargo and propulsion systems for emergency fire scenarios. The production of Halon 1301 was banned by the Montreal Protocol in 1994 due to its environmental impact on ozone depletion and long atmospheric lifetime. Thus, a credible alternative is required for fire safety application on-board aircrafts. This study forms a follow-up to the work published by A. Dinesh et.al and M. Diakostefanis et.al which studied halon-free fire suppression in aircraft cargo bay through CFD and experimental validation respectively. Due to constraints in cost and logistics of testing fire suppression in a gas turbine engine, a computer modelled fire zone of a hypothetical high-bypass turbofan engine was used to simulate and evaluate the fire suppression performance of three Halon-free agents - HFC-125, HFC-227ea and FIC-13I1. Halon 1301 was also simulated to provide baseline comparison to the alternative agents. The capacity to inert was based on the distribution of agent concentrations subjected to a heptane-based fuel fire. Simulations were conducted at ISASLS and 40,000 feet to represent the engine’s operating environment. Overall, it was found that HFC-125 and HFC-227ea satisfactorily suppressed fire at both ambient scenarios. It was found that agent density and suppression concentration significantly influenced the agent’s ability to mix with ambient air and flood the fire zone. Density of the agent combined with higher inertion concentrations also influenced formation of localised recirculation zones which influenced mixture distribution in the fire zone. The simulations enabled identification of individual changes to agent flow characteristics at different operating conditions and demonstrated the performance criteria of Halon 1301 along with the proposed alternative agents for successful fire suppression. Through this study, it was found that all the agents except FIC-13I1 passed the minimum suppression criteria making HFC-125 and HFC-227ea suitable agents for replacement of Halon 1301. The minimum concentration and simulation design in the engine fire zone were validated against the experimental tests conducted by NIST in Boeing 777’s dual-spool high bypass turbofan engine for Halon 1301 and HFC-125. The failure of FIC-13I1 was attributed to the complex design in fire zone and could be overcome by considering a higher factor of safety for this design and injecting higher mass flow rate of the agent into the fire zone. The outcome of this study proposes further work to be carried out through experimental validation of the CFD simulations which will be key to progress the technology to higher TRL and meet aviation certification requirements. Further work may also explore blended agent configurations to tailor the agent characteristics to meet suppression requirements for maintaining safety and reliability of the system.