High spatial resolution and sensitivity laser cavity extinction measurements of soot volume fraction in nitrogen diluted methane flames

A highly sensitive extinction-based laser cavity soot measurement system with increased spatial resolution is demonstrated, using two partially reflective concave mirrors on either side of the flame under investigation. This configuration makes the beam convergent inside the cavity, allowing for a spatial resolution within 200 µm, whilst increasing the absorption by two orders of magnitude via a multi-pass technique. A series of nitrogen diluted laminar diffusion methane-air flames are tested. Measurements of the soot distribution across the flame show good agreement with results acquired using laser-induced incandescence (LII). The effect of fuel dilution is measured, and the mechanism for the reduction of the net soot production is discussed.N/