Radiative Extinction of Gaseous Spherical Diffusion Flames in Microgravity

Abstract

Radiative extinction of spherical diffusion flames was investigated experimentally and numerically. The experiments involved microgravity spherical diffusion flames burning ethylene and propane at 0.98 bar. Both normal (fuel flowing into oxidizer) and inverse (oxidizer flowing into fuel) flames were studied, with nitrogen supplied to either the fuel or the oxygen. Flame conditions were chosen to ensure that the flames extinguished within the 2.2 s of available test time; thus extinction occurred during unsteady flame conditions. Diagnostics included color video and thin-filament pyrometry. The computations, which simulated flow from a porous sphere into a quiescent environment, included detailed chemistry, transport and radiation, and yielded transient results. Radiative extinction was observed experimentally and simulated numerically. Extinction time, peak temperature, and radiative loss fraction were found to be independent of flow rate except at very low flow rates. Radiative heat loss was dominated by the combustion products downstream of the flame and was found to scale with flame surface area, not volume. For large transient flames the heat release rate also scaled with surface area and thus the radiative loss fraction was largely independent of flow rate. Peak temperatures at extinction onset were about 1100 K, which is significantly lower than for kinetic extinction. One observation of this work is that while radiative heat losses can drive transient extinction, this is not because radiative losses are increasing with time (flame size) but rather because the heat release rate is falling off as the temperature drops.

Effects of C/O Ratio and Temperature on Sooting Limits of Spherical Diffusion Flames

“…For the flames of Table 1, peak flame temperatures at 2 s span a broad range of 1479 -2262 K. These temperatures are too high to cause radiative extinction at 2 s. 19 The calculation of mixture fraction, Z, for these flames is complicated by the presence of N 2 in both the fuel and oxidizer in most cases. Furthermore, even for cases without fuel dilution, the conventional definition of Z CH = Y C + Y H does not yield peak temperatures at Z st .…”

“…Flame expansion causes temperatures to drop owing to increased radiative losses, which are proportional to flame surface area 19 . At 2 s, the radiative heat loss fractions are around 0.42 for all flames except Flames 10, 11 and 12, whose heat loss fractions are about 0.28.…”

“…The code was adapted for spherical diffusion flames allowing for optically thick radiative heat losses. 19 Both steady state and transient flames can be modeled.…”

“…Variable Y k,0 is the mass fraction of the k th species in the burner gas supply. Because drop tower experiments did not show a significant increase in burner surface temperature, 19 T b was taken to be constant. The outer boundary temperature (295 K), species mass fractions, and pressure are held constant and specified as input data.…”

Effects of finite-rate droplet evaporation on the extinction of spherical burner-stabilized diffusion flames

“…1. The spherical burner-stabilized diffusion flame has been popularly adopted to study the extinction and soot formation in diffusion flames (e.g., [17][18][19][20][40][41][42]). Similar to that of Liu et al [42] and Wang and Chao [21], the burner consists of a void core region (0 < e r < e r i ) in which a stream of fuel flow (containing sufficiently small fuel droplet and fuel vapor at the temperature of e T 0 ) is supplied and a porous region (e r i < e r < e r b ) in which the fuel flow is regulated to be uniform at its exist.…”