Resolving biomass-turbulence interactions at the particle scale using ultra-high-speed wavelet-based optical flow velocimetry (wOFV)

The transition from coal to biomass as a sustainable solid fuel source is a critical step towards achieving climate goals. Biomass particle behavior significantly influences turbulent flow fields within combustion applications, making a detailed quantification of these effects crucial. Challenges arise due to small particle sizes and high flow velocities, requiring innovative methods to spatially resolve particle-turbulence interactions at the individual particle level. This study introduces wavelet-based optical flow velocimetry (wOFV) within a multi-phase flow framework, providing insights into resolving turbulent flow around individual biomass particles by generating a dense motion field with a vector spacing of one pixel. Leveraging ultra-high-speed velocimetry measurements from a biomass-laden turbulent jet by means of a novel fiber laser system, this innovative approach is demonstrated. The combination of ultra-high repetition rate diagnostics and wOFV enables the achievement of unprecedented simultaneous spatio-temporal resolution in multi-phase velocimetry. A regularization parameter selection approach grounded in the physical principles of turbulent single-phase flow enables a logical approach to resolve the flow structures around individual moving biomass particles. Global properties of particle dynamics, including dispersion in the jet and particle orientation within the flow, are systematically investigated through diffuse back-illumination. Comprehensive statistical analyses of turbulence modification by individual particles utilize the flow field data obtained through wOFV and is compared to a state-of-the-art particle image velocimetry algorithm. The method's versatility extends to other multi-phase flows relevant to energy conversion in combustion systems, enabling a detailed understanding of the turbulence modulation with respect to local particle properties.