An experimental investigation of Li and SF6 wick combustion

Abstract

An experimental investigation of liquid-metal wick combustion at subatmospheric pressures is reported. Liquid lithium and gaseous sulfur hexafluoride are used as fuel and oxidizer. The combustion is a multiphase process with white-color, submicron flaky powder (particulates) formed on the fuel-lean surface and black coatings of granular or crystal-like structures formed on the fuel-rich surface. The element analysis suggests that LiF is the major species in powder combustion products and Li2S in black-coating products. The experiments also quantified the flame emission spectrum over the spectral regime 400–700 nm. The measured emission spectrum is found to coincide with the six spontaneous emission lines of lithium atoms in this spectral regime. The resonance doublet centered at 670.78 nm is the strongest component of the emission spectrum. Simultaneous measurements of the emission lines as a function of spatial locations are accomplished using an intensified CCD camera for the spectral regime 400–500 nm, and the results are used to establish an emission thermometry. The thermometry measurements show that the temperature decreases as combustion proceeds. A mean temperature of 3500-2500 K is found for the region bounded by the stoichiometry and the wick surface and the corresponding “apparent” flame temperatures are 4000 and 2600 K. An error of 400-200 K is estimated for the temperature measurements. The temperature data also suggest that the lithium-sulfur hexafluoride wick combustion is not a steady-state process for the burner employed and future experiments are needed to quantify the interfacial conditions to advance our understanding of the liquid-metal wick combustion.