Extinguishment of methane diffusion flames by inert gases in coflow air and oxygen-enriched microgravity environments

Abstract

Extinguishment of laminar coflow diffusion flames in microgravity (μ g) have been studied experimentally and computationally. The μ g experiments were conducted using a methane cup-burner flame aboard the NASA Reduced-Gravity Aircraft. Transient computations with full methane chemistry and a gray-gas radiation model were performed to reveal the flame structure and extinguishment processes. In μ g, as an inert gas (N 2, He, or Ar) was added incrementally to the coflowing O 2–N 2 mixture with the initial oxygen volume fraction ( X O 2 ,ox ) of 0.21 at 101 kPa or 0.3 at 70.3 kPa, the flame tip opened, and the flame base gradually lifted off the burner parallel to the axis until blowout. The predicted minimum extinguishing concentration (MEC) of each agent in the oxidizing stream was in a reasonable agreement with the measurement. The measured MEC was nearly independent of the mean oxidizer velocity. In μ g, the MEC for each diluent was nearly at a critical air-inertization point of the Coward–Jones flammability-limit curve (the maximum diluent concentration that sustains premixed combustion); whereas in earth gravity, studied previously, the flame prematurely blew off after oscillations at ≈70% of the critical condition. The maximum oxygen volume fractions at extinguishment (converted from the MECs) were nearly the same for X O 2 ,ox = 0.21 and 0.3 despite the different atmospheric pressures. The computation of the lifted flame with a sufficiently long fuel-oxidizer mixing time (≈0.2 s) revealed that: (1) a peak reactivity spot (i.e., reaction kernel), formed in the flame base, broadened laterally, thereby supporting a super-lean reaction wing on the oxidizer side and a trailing diffusion flame on the fuel side; (2) the flammable mixture layer was, nevertheless, radially thin (≈0.4 mm); and (3) the unburned mixture velocity at the flame base was comparable to the stoichiometric laminar flame speed found in the literature.