Use of laser-induced ionization to detect soot inception in premixed flames.

Experimental measurements of laser-induced ionization were performed for ethene-air premixed flames operated near the soot inception point. Soot was ionized with a pulsed laser operated at 532 nm. The ionization signal was collected with a tungsten electrode located in the postflame region. Ionization signals were collected by use of both single-electrode and dual-electrode configurations. Earlier laser-induced-ionization studies focused on the use of a single biased electrode to generate the electric field, with the burner head serving as the path to ground. In many practical combustion systems, a path to ground is not readily available. To apply the laser-induced-ionization diagnostic to these geometries, a dual-electrode geometry must be employed. The influence of electrode configuration, flame equivalence ratio, and flame height on ionization signal detection was determined. The efficacy of the laser-induced-ionization diagnostic in detecting soot inception in the postflame region of a premixed flame by use of a dual-electrode configuration was investigated. Of the dual-electrode configurations tested, the dual-electrode geometry oriented parallel to the laser beam was observed to be most sensitive for detecting the soot inception point in a premixed flame.