Multi-parameter measurements of laminar sooting flames using thermophoretic sampling technique

Abstract

The dual exposure-time thermophoretic sampling (DTTS) method had successfully realized multi-parameter measurements for flame synthesis TiO2 nanoparticles (Xu and Zhao, 2015), which was further researched and developed to apply to sooting flame diagnosis in this study. However, the traditional sampling probe (TEM grid) cannot satisfy the assumption of convection dominant for heat transfer between probe and flame when moving to a sooting flame that radiation would affect the temperature of the probe dramatically owing to very high emissivity of soot particles. After a comprehensive investigation, we found that the radiative heat transfer between the sampling probe and the sooting flame can be effectively eliminated when tailor-made ultrathin quartz glass slices (UQGSs) were used as novel sampling probes due to a very low emissivity of UQGSs (high radiative transparency). Thereby the interaction between the flame and the UQGS probe conformed to an experiential convective heat transfer - thermophoresis coupling mechanism. As the amount of particles deposited on the probe surface by thermophoresis was a function of soot volume fraction, probe exposure time, gas temperature and flow velocity, two thermophoresis samplings with different exposure time (one is far less than the time constant τ of the UQGS and another is close to τ) in a same position were conducted to simultaneously measure soot volume fraction and flow velocity in the location. The collections of particles with tunable exposure time of probes in the flame were observed and analyzed by a field-emission scanning electron microscope (FESEM) and FESEM-image processing for soot particle size. The novel sampling probe as well as the DTTS technique provided a practical way to perform multivariate measurements of soot volume fraction, flow velocity, and soot particle size distribution.