Driven by the demand for soot-free combustion, laser-based diagnostic techniques on soot have been developing since the late 1980s. One of these techniques is line-of- sight extinction, which is a fast, low-cost, and quantitative method to investigate the soot volume fraction in flames. However, the extinction-based technique suffers from relatively high measurement uncertainty due to a low signal-to-noise ratio, as the single-pass attenuation of the laser beam intensity is often insufficient. Multipass techniques can increase the sensitivity, but may suffer from a low spatial resolution. To overcome this problem, in the present study, we have developed a high spatial resolution continuous wave laser-cavity extinction (CW-LCE) technique to measure the soot volume fraction from low-soot producing flames. A laser beam cavity is realised by placing two partially reflective concave mirrors on either side of the laminar diffusion flame under investigation. This con guration makes the beam convergent inside the cavity, allowing a spatial resolution within 200 um, whilst increasing the absorption by an order of magnitude. Three different hydrocarbon fuels are tested: methane, propane and ethylene. The measurements of soot distribution across the flame show good agreement with results using laser-induced incandescence (LII) in the range of the soot volume fraction from 20 ppb to 15 ppm. The system is further applied to the measurement of soot volume fraction pro les of a series of nitrogen-diluted methane diffusion flames. The results of CW-LCE agree well with LII. The effect of fuel dilution on soot is interpreted by using a numerically calculated flow velocity and temperature eld. It is found that a previous one-step model for soot formation may underestimate the suppression effect by dilution. This is because diluent addition may shift the location of the sooting region away from the high temperature region, shortening the residence time for surface growth of soot particles. The effect of each factor (thermal, dilution, residence time) responsible for soot reduction is isolated and quantitatively compared.

The size of soot particles is another important aspect of comprehensive soot measurement, and to address this aspect, a planar two-dimensional two-colour time-resolved laser-induced incandescence (2D-2C-TiRe-LII) method is developed and employed to investigate the soot formation in a series of standard ethylene and methane laminar diffusion flame. The time resolution of the 2D-LII signal is realised by shifting the delay time of the ICCD cameras. A two-colour con- guration is applied to measure the peak temperature of soot particles after the laser pulse, rather than use the energy balance equations to compute the peak temperature, which may introduce signi cant uncertainty. By combining with the CW-LCE technique, both a soot particle volume fraction and a particle size distribution map of the flame are obtained, using a minimum error approach.