Suppression Limits of Low Strain Rate Non-Premixed Flames

Published

January 5, 2011

Author(s)

Matthew F. Bundy, Anthony P. Hamins

Abstract

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.

Citation

Combustion

Pub Type

Journals

Keywords

combustion, extinction, halon, micro-gravity, non-premixed flames, suppression

Citation

Bundy, M. and Hamins, A. (2011), Suppression Limits of Low Strain Rate Non-Premixed Flames, Combustion (Accessed December 14, 2024)