Suppression Limits of Low Strain Rate Non-Premixed Methane Flames.
Suppression Limits of Low Strain Rate Non-Premixed
Methane Flames.
(275 K)
Bundy, M.; Hamins, A.; Lee, K. Y.
Combustion and Flame, Vol. 133, No. 3, 299-310, May
2003.
Keywords:
counterflow flames; extinction; diffusion flames; fire
suppression; temperature measurements; halons;
microgravity; combustion
Abstract:
The suppression of low strain rate non-premixed flames
was investigated experimentally in a counterflow
configuration for laminar flames with minimal conductive
heat losses. This was accomplished by varying the
velocity ratio of fuel to oxidizer to adjust the flame
position such that conductive losses to the burner were
reduced and was confirmed by temperature measurements
using thermocouples near the reactant ducts. Thin
filament pyrometry was used to measure the flame
temperature field for a curved diluted methane-air flame
near extinction at a global strain rate of 20 s-1. The
maximum flame temperature did not change as a function
of position along the curved flame surface, suggesting
that the local agent concentration required for
suppression will not differ significantly along the
flame sheet. The concentration of N2, CO2, and CF3Br
added to the fuel and the oxidizer streams required to
obtain extinction was measured as a function of the
global strain rate. In agreement with previous
measurements performed under microgravity conditions,
limiting non-premixed flame extinction behavior in which
the agent concentration obtained a value that insures
suppression for all global strain rates 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 for
suppressant requirements in normal gravity flames,
analogous to agent flammability limits in premixed
flames. The critical agent volume fraction in the
methane fuel stream assuring suppression for all global
strain rates was measured to be 0.841 + 0.01 for
N2,0.773 + 0.009 for CO2, and 0.437 + 0.005 for CF3Br.
The critical agent volume fraction in the oxidizer
stream assuring suppression for all global strain rates
was measured as 0.299 + 0.004 for N2, 0.187 + 0.002
for CO2, and 0.043 + 0.001 for CF3Br.
