Experimental investigation of acoustic forcing on temperature, soot volume fraction and primary particle diameter in non-premixed laminar flames

The influence of the acoustic-driven oscillation frequency on the structure and the soot formation of a laminar non-premixed ethylene flame is presented. The flames were forced at either 20 and 40 Hz, corresponding to Strouhal numbers of 0.23 and 0.46. Two-dimensional phase-resolved measurements of gas temperature, soot volume fraction and the diameter of primary particles were measured simultaneously for one steady and two forced flames with non-linear excitation regime Two-Line Atomic Fluorescence, Laser-Induced Incandescence and Time-Resolved LII, respectively. Simultaneous measurements of gas temperature and soot volume fraction provide details of the difference between the flame structure and the soot distribution in the forced and unforced flames. The distinctive features of the forced flames are the occurrence of necking near to the fuel tube and the formation of a hollow soot “shell”. Despite the distinctive structure, the soot region is confined to a restricted range of temperatures, approximately 1700-1800 K. The strong spatial correlation between the presence of soot and a narrow range of temperature is well established for steady laminar flames. The present measurements also reveal the complex relationship between diameter of primary particles and soot volume fraction for these flames. Planar gas temperature images and OH* chemiluminescence profiles show that the reaction zone extends periodically upstream from the exit plane of the fuel tube for the flame with St=0.23. This phenomenon, which only occurs for some conditions, is an important consideration for the modelling of these flames because the boundaries of the computational domain typically starts at the fuel flow exit plane.