Microgravity burner-generated spherical diffusion flames - Experiment and computation

Abstract

Microgravity experiments were conducted in the 2.2-s drop-tower facility at the NASA Glenn Research Center to study the transient response of the burner-generated spherical diffusion flame caused by its initial displacement from the steady-state position. The experiment involved issuing H 2 /CH 4 /inert mixtures of constant fuel mass flow rates from a bronze, porous, 1.27-cm-diameter, spherical burner into atmospheric air. The experimental results on the flame trajectory were found to agree well with those obtained through fully transient computational simulation with detailed chemistry and transport, and appropriate initial conditions. Furthermore, although steady-state behavior should exist for such flames, the experimental and computational results indicated that it cannot be reached within the 2.2-s microgravity duration for the fuels and mass-flow rates tested.To assess the role of radiation on the flame dynamics and extinction, computations were performed without radiation, with radiation employing the optically thin approximation, and with radiation utilizing a detailed emission/absorption statistical narrow band (SNB) model. The computation showed that while the influence of radiative heat loss on the position of the flame is small, proper consideration of radiative effects is crucial in assessing the state of flame extinction. Specifically, while all simulations of the experimental cases studied incorporating radiative heat loss revealed that the flame extinguishes well before the attainment of steady state, simulations accounting for gaseous reabsorption of radiative emissions were required to adequately represent the experiments in terms of extinction time, with the optically thin simulations predicting premature extinction during the flame expansion process. Effects of heat loss to the porous burner were also examined, and the lack of correspondence between the visible flame luminosity and flame strength, as related to flame temperature and heat release rate, was noted.

Effects of Lewis Number on Temperatures of Spherical Diffusion Flames

“…A similar discrepancy was observed in a previous study 2 and was accounted for by noting that the peak temperature occurs on the oxidizer side of the visible flame. Although this explanation would improve the agreement for Flames (a) and (b), it would reduce the agreement for Flames (c) and (d).…”

“…Following the approach adopted in Tse et al, 2 the initial (ignition) conditions for the transient cases were prescribed as the steady-state solutions of flames without radiation and with the same outer boundary values, but with the outer boundary brought to 1.2 cm from the burner exit. The temperature boundary condition at the burner exit was adiabatic.…”

“…[1][2][3][4][5] These flames are one-dimensional and are straightforward to model. The flames can be generated in microgravity using droplets or by issuing a gaseous or liquid reactant from a porous sphere.…”

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

“…Following the approach adopted by Tse et al, 21 the initial (ignition) conditions were prescribed as the steady-state solution of the same flame in a compressed domain. These ignition conditions had no radiation and assumed adiabatic conditions at the burner exit and a constant temperature at the outer boundary.…”

Experimental study on diffusive solid combustion behavior during transition from normal- to reduced-gravity

“…Combustion behavior and flame characteristics under microgravity conditions have attracted extensive research interest [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Under micro-gravity conditions, the buoyancy force owing to gravitational acceleration disappears.…”