Experimental characterization of ozone-enhanced n-decane cool flames and numerical investigation of equivalence ratio dependence

Abstract

Low temperature combustion behavior is critical to understand for advanced combustor design and safety, but cool flames are challenging to study under chemistry-transport coupled conditions because of the generally transient nature of these flames. A series of studies have utilized a laminar flat flame Hencken burner platform and ozone enhancement at sub-atmospheric pressures to stabilize dimethyl ether, propane and heptane cool flames and measure flame characteristics such as propagation speed, flame temperature and product species. This work builds upon those prior studies to investigate n-decane, first through experimental characterization of ozone-enhanced n-decane cool flames, including measurement of propagation speeds and flame temperatures and imaging of formaldehyde planar laser induced fluorescence (CH2O PLIF) over a range of equivalence ratios and, second, through comparison of experimental results with one-dimensional cool flame simulations. Experimental findings showed moderate equivalence ratio dependence of cool flame propagation speeds and temperatures at lean equivalence ratios, and no significant equivalence ratio dependence for richer mixtures. Experimental CH2O profiles showed equivalence ratio dependence and no downstream consumption, as is characteristic of cool flames. Simulated n-decane cool flames showed significant dependence of propagation speeds and temperatures on equivalence ratio, as well as downstream consumption of CH2O, at the experimental conditions. Experimental and numerical discrepancies were first investigated by examining the influence of different ozone enhancement strategies on equivalence ratio dependence and the results indicate a minimal influence of ozone enhancement strategy for n-decane, while also providing insights to into equivalence ratio dependencies observed in previous ozone-enhanced and ozoneless cool flame studies. The effect of ozone concentration on simulated n-decane cool flames was then investigated, revealing a strong influence of ozone enhancement on equivalence ratio dependence. Simulated propagation speed and temperature dependence on equivalence ratio were shown to develop with increase in ozone enhancement levels. A kinetics analysis revealed enhancement of low temperature peroxy chemistry and other cool flame heat releasing reactions with ozone addition, increasing simulated flame temperatures and promoting the onset of early intermediate temperature chemistry (ITC), inducing equivalence ratio dependence which was not seen in experimental results. Finally, insights from this analysis were used to investigate reconciliation of differences in experimental and simulated cool flame temperatures and identify potential kinetics model discrepancies from experimental findings.