Abstract
Non-intrusive measurement techniques are required to gain a comprehensive understanding about the processes of soot formation, growth and oxidation. Time-resolved laser-induced incandescence (TiRe-LII), commonly performed 0D or 2D within a flame, has proven to be a very suitable tool for the in situ sizing of soot primary particles. In this work, the technique is expanded to the third dimension by employing volumetric illumination and coupling it with a tomographic approach, which allows to computationally gain 3D information from 2D images taken at various angles. To minimize experimental cost, an approach using nine fiber bundles arranged in a semicircle around the flame and imaging the light onto a single camera is used. The technique is demonstrated on an ethene diffusion flame on a standard burner, providing spatially resolved 3D particle sizes. One focus of this work is to reveal the influence of input parameters such as the local bath gas temperature, which we measured by two-color pyrometry, and local laser fluence, which are both required for an accurate evaluation of the local particle size. It is shown that the assumption of an average temperature may result in a wrong picture even of qualitative soot size evaluation. In the end, a concept is proposed for a simultaneous determination of the 3D distribution of particle sizes through TiRe-LII and the required bath gas temperature via two-color pyrometry using a tomographic approach with only three cameras.
Highlights
In fuel rich combustion zones soot nanoparticles form from polycyclic aromatic hydrocarbon (PAH) clusters [1]
As local bath gas temperature is a key parameter for accurate particle sizing, in the end we suggest an approach how the present work can be extended by a simultaneous tomographic measurement by two-color pyrometry, which is performed sequentially in this work
There are some artifacts from the reconstruction procedure and empty voxels below the signal threshold of 4% of the signal maximum [33] especially at the boundaries and in the lower center region of the flame
Summary
In fuel rich combustion zones soot nanoparticles form from polycyclic aromatic hydrocarbon (PAH) clusters [1]. LII is based on heating an ensemble of soot particles with a short laser pulse up to approximately 3000–4000 K and detecting and analyzing the thermal radiation signal during the subsequent cooling to bath gas temperature Because of their larger surface-to-volume ratio small particles show a faster signal decay as compared to larger ones. We used an approach employing nine customized fiber bundles arranged in a semicircle around the flame and imaging the light onto a single camera, which avoids the need for many cameras, yet at the cost of the image resolution To our knowledge, this is the first demonstration of 3D-TiRe-LII for primary particle sizing employing the evaluations for 3D temperatures and laser profiles. As local bath gas temperature is a key parameter for accurate particle sizing, in the end we suggest an approach how the present work can be extended by a simultaneous tomographic measurement by two-color pyrometry, which is performed sequentially in this work
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