Mie-scattering imaging and μPIV in porous media burners with TPMS-based topologies

In this work, Mie-scattering imaging, ${\mathrm{CH}}^{\star }$ chemiluminescence and $\mu$PIV are applied in various optically accessible Porous Media Burners (PMBs). The present PMBs are generated using Triply Periodic Minimal Surfaces (TPMS) and produced via Additive Manufacturing (AM). These topologies feature optical pathways that are both orthogonal and coincident, so that one can be used for illumination and the other for imaging. This enables the application of laser diagnostics in fully 3D structures whilst avoiding altering the geometry of the porous medium. These techniques are applied in homogeneous porous burners where the position of the flame is determined by the operating conditions. First, Mie-scattering imaging is combined with ${\mathrm{CH}}^{\star }$ chemiluminescence to analyze the influence of the flame position on the preheating distance of the reactants. For that, the flow is seeded with micrometric oil droplets that evaporate at approximately 500 K. The light scattered by these particles delineates an evaporation front in the Mie-scattering images and this can be compared to the actual location of the reaction region, which is deduced from chemiluminescence images. Then, Mie-scattering imaging and $\mu$PIV are used to obtain the flame shape and the velocity field in the unburned gas region for inlet-anchored flames. The influence of the hydrogen content, the pore-size and the burner topology is analyzed. The topology is found to have a major impact on the interstitial flow and flame stabilization. A new topological parameter, namely the linear porosity, is proposed to quantify the influence of local hydrodynamic effects on flame stabilization.

Novelty and Significance Statement

The novelty of this work is the accomplishment of Mie-scattering imaging and $\mu$PIV measurements in a Porous Media Burner (PMB). The application of these laser techniques in an homogeneous PMB requires two orthogonal and coincident optical accesses. In this work, this is achieved via TPMS-based topologies and Additive Manufacturing (AM) techniques. It is significant because it opens the door for the application of laser diagnostics in 3D porous structures without altering the topology. Mie-scattering imaging on small oil droplets seeded in the flow offers a new way to study heat recirculation in PMBs via characterization of the solid-diffusion distance. $\mu$PIV measurements reveal the importance of linear porosity and its influence on the interstitial flow and on the hydrodynamic stabilization of flames within the pores.