Shape-dependent motion estimation of solid fuel particles in two-phase flows using wavelet-based optical flow velocimetry (wOFV)

The accurate measurement of the motion of rigid solid fuel particles is important for the study of many applications in energy science. These dynamics influence the particle-turbulence interaction as well as the heat and mass transfer processes governing reactor performance and chemical conversion. While many methods for the tracking of the translational velocity of such particles are available, methods for the robust measurement of angular velocity are scarce, especially for nearly circular particles and particles with complex shapes. To address the issue, this study presents a novel methodology for computing the angular velocity of dispersed particles in two-phase flows using a wavelet-based optical flow velocimetry (wOFV) algorithm. First, the applicability of wOFV for particle velocimetry was validated against a synthetic ground-truth dataset of walnut shell fragments as a representative biomass and the influence of the regularization parameter 𝜆 was investigated. It was found that for 0 . 0 1 ≤ 𝜆 ≤ 5 , particle rotation can be accurately determined using the presented wOFV method. Additionally, findings reveal that particle shape significantly influences the performance of both wOFV and a commonly used ellipse fit tracking method, which is used as a benchmark. While the accuracy of ellipse fit tracking correlates strongly with the particle aspect ratio, the accuracy of wOFV is more strongly determined by the ellipticity of fuel particles. The implementation of a hybrid approach that integrates wOFV and ellipse fit tracking has been demonstrated to leverage the strengths of both algorithms. This is achieved through a shape-dependent selection of the most suitable algorithm for each specific particle based on a support vector machine classification. Overall, this hybrid approach yields a reduction in the median relative error of angular velocity predictions from 4.2 % (benchmark method ellipse fit tracking) to 3.7 % (hybrid method) for the employed test set. The proposed framework demonstrates considerable potential for enhancing the precision of rotational measurements in the field of particle dynamics.