Suppression Limits of Low Strain Rate Non-Premixed Flames
Matthew F. Bundy, Anthony P. Hamins
The suppression of low strain rate non-premixed flames was investigated experimentally in a counterflow configuration through the development of a methodology that allows establishment of laminar flames free from conductive heat losses. The method allows the range of global strain rates investigated in normal gravity to be extended to lower values than previously achieved and facilitates studies of flame suppression at low strain rates. This was accomplished by isolating the burner to reduce disturbances by ambient currents and by varying the velocity ratio of fuel to oxidizer to adjust the flame position such that conductive losses to the burner were reduced. This was confirmed by temperature measurements using thermocouples (0.05 mm wire diameter) near the reactant ducts.The critical concentration of N2, CO2, and CF3Br added to the fuel and the oxidizer streams required to obtain extinction of methane-air non-premixed flames was measured as a function of the global strain rate. In agreement with previous measurements performed under micro-gravity conditions, limiting non-premixed flame extinction behavior in which the agent concentration obtained a value that insures suppression under all conditions was observed. A series of extinction measurements varying the air:fuel velocity ratio showed that the critical N2 concentration was invariant with this ratio, unless conductive losses were present. In terms of fire safety, the measurements demonstrate the existence of a fundamental limit in the suppressant requirements in normal gravity flames, analogous to agent flammability limits in premixed flames. The critical mole fraction of agent in methane was found to be 0.841 0.01 for N2, 0.773 0.009 for CO2, and 0.437 0.005 for CF3Br. The critical mole fraction of agent in air was found to be 0.299 0.004 for N2, 0.187 0.002 for CO2, and 0.043 0.001 for CF3Br.