The US Federal Aviation Administration (FAA) observed during explosion tests that at low concentrations candidate halon replacement agents increased the explosion severity instead of mitigating the event. At UTC Aerospace Systems a test program was developed to assess the behaviour of alternative agents at values below inerting concentration. Two agents were selected, HFC-125 and Novec ™ 1230. Baseline tests were performed with unsuppressed propane/air mixtures and fuel/air mixtures with Halon 1301 and nitrogen (N 2 ). Using Halon 1301 or N 2 at below inerting concentrations mitigated the explosion. HFC-125 was tested against propane at stoichiometric (4 vol%) and lower explosion limit (LEL) (2 vol%). Against 4 vol% propane the combustion was mitigated, proportional to agent concentration; however, low concentrations of HFC-125 with 2 vol% propane enhanced the explosion. Tests with N 2 against a volatile mixture of propane with HFC-125 showed that N 2 mitigated the events.Final tests were performed with low concentrations of Novec™1230 against propane/air mixtures. This showed similar behaviour to that observed with the HFC-125 tests. Normally during qualification tests for new agents the stoichiometric concentration of a fuel is deemed to be the worst case scenario and the baseline against which agents are tested. The above described test results show that this assumption may need to be reconsidered. This work shows that contrary to common assumption the agents investigated did not act chemically at the flame front, but mainly cooled the flame and changed the stoichiometry, i.e. the ratio of components of the flammable mixture.
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