On the intricacies of soot volume fraction measurements in counterflow diffusion flames with light extinction: Effects of curtain flow

Counterflow diffusion flame is an attractive platform for fundamental research on kinetics of soot formation. Accurate determination of soot volume fraction in the flame is a prerequisite for in-depth analysis of the sooting characteristics and assessment of predictive soot models. Light extinction has been proven to be an efficient technique for measuring soot volume fraction thanks to its non-intrusiveness and its simple optical setup. Nevertheless, tomographic inversion needs to be performed if spatially resolved soot volume fraction is to be obtained from the measured light extinction data which is essentially a projection along the line-of-sight. In this regard, radial distribution of soot volume fraction would affect the accuracy of the measurement through its influences on the inversion processes. In this work, we show that the curtain flow, which is necessary to avoid the formation of the undesired secondary diffusion flame and to keep the core counterflow from ambient disturbances, has notable effects on spatially resolved soot volume fraction measurements with line-of-sight measurements. In particular, different flow rate settings of the curtain flow can result in different soot distributions at the edges of soot fields: upwards curved, outwards extended, and downwards curved, which may influence the measurement of centerline soot volume fraction distribution. The necessity of tomographic inversion, the minimal region of the projection image necessary for tomographic inversion (when necessary), the quasi-one-dimensional feature of soot distribution, and the sensitivity of measurement to slight flame asymmetry were investigated where possible to determine the most suitable curtain flow configuration for soot volume fraction measurements by light extinction. Recommendations on curtain flow setting are finally made.