High spatial resolution laser cavity extinction and laser-induced incandescence in low-soot-producing flames

Abstract

Accurate measurement techniques for in situ determination of soot are necessary to understand and monitor the process of soot particle production. 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 low signal-to-noise ratio, as the single-pass attenuation of the laser beam intensity is often insufficient. Multi-pass techniques can increase the sensitivity, but may suffer from low spatial resolution. To overcome this problem, we have developed a high spatial resolution laser cavity extinction 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 configuration makes the beam convergent inside the cavity, allowing a spatial resolution within 200 μm, 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 from around 20 ppb to 15 ppm.

Planar 2-color time-resolved laser-induced incandescence measurements of soot in a diffusion flame

“…based on the region of the valley compatible with the minimum error, as well as realistic distribution parameters s within 0.2-0.4 (Michelsen et al 2015;Oltmann et al 2012;Xu et al 1997). The values of volume fraction were obtained in a separate study at significantly higher fluences (0.26 J/cm 2 relatively to 0.17 J/cm 2 in the present study) to reach uniform soot temperatures (Tian et al 2015), as usual for LII concentration measurements. The issue is extensively discussed by Schulz et al (2006).…”

“…In the present study, 532 nm is used for excitation, often a preferred choice due to the visible beam, so the selection falls to the combination of 400/450 nm, which is close to the values used by Wendler (2006) in their previous study (390/450 nm). The top-hat profile of the laser sheet was carefully calibrated using the method described in a previous study (Tian et al 2015), with less than 2.5% fluctuation of the averaged intensity and 1% spatial deviation from the mean. A laser fluence of 0.17 J/cm 2 is used for LII signal excitation to optimize the signal-to-noise ratio and minimize sublimation (see Supplementary Material).…”

“…To obtain the ratio between the absolute signal intensities at the two detection wavelengths for the calculation of T p , a tungsten lamp (Thorlabs QTH10B) was used to quantitatively calibrate the two cameras, as described in the Supplementary Material. LII signals from this flame were used to determine f v after calibration using extinction measurements, as described in Tian et al (2015). The burner consists of an ethylene diffusion flame jet described in a previous study (Tian et al 2015), with a jet diameter of 10.5 mm, surrounded by a co-flow of dry air over a diameter of 96.8 mm, at a mean velocity of 4.43 cm/s.…”

“…LII signals from this flame were used to determine f v after calibration using extinction measurements, as described in Tian et al (2015). The burner consists of an ethylene diffusion flame jet described in a previous study (Tian et al 2015), with a jet diameter of 10.5 mm, surrounded by a co-flow of dry air over a diameter of 96.8 mm, at a mean velocity of 4.43 cm/s. The ethylene flame operating conditions correspond to Case B described in a previous study (Tian et al 2015), with a fuel flow of 0.22 slpm and air flow of 38.2 slpm.…”

“…Measured particle temperature T (red squares, left scale) using the two colour method; modelled T starting from the 2C peak measurement T p D 3181 K. Figure 8. Calculated T p with the 2C method (left); estimated D m with the 2-parameter fit (middle); measured f v in the tested flame (right) (Tian et al, 2015).…”