An automated image analysing routine for estimation of equivalent diameter in high-speed image sequences with high accuracy and its validation

Bubbly flow plays an important role in industrial processes. Obtaining detailed information on the flow is, however, a challenging task. One of the key process parameters is the diameter of the bubbles, which is often determined as equivalent diameter. In this study, an automatic image analysis routine for time-resolved analysis and estimation of equivalent diameter of rising single bubbles in high-speed image sequences with high accuracy is developed using the open source software KNIME. Common estimation techniques for the equivalent diameter, e.g. based on the major/minor axes, are compared to a novel rotation algorithm, where the detected bubble segments are rotated by 180° in 1° steps, for the determination of the Feret diameter. Additionally, the elongation is measured as a morphological parameter and the centroid positions are determined. In combination with the frame rate of the image sequences the ascent rates can be calculated in form of a Bubble Tracking Velocimetry (BTV). The routine was validated using static computer generated geometrical shapes as well as precision grade solid glass spheres with diameters of 0.7-2.5 mm under dynamic (settling) conditions. High-speed image sequences were recorded and analysed with a critical statistical evaluation. It could be shown that the deviation between measured diameter and real diameter of the glass spheres is less than 1.5% when using the rotating Feret diameter algorithm. Settling velocities were determined with a maximum error of 3%. A first test of the analysing routine in a bubbly flow showed that it is unaffected by dirt, small tracer particles or internals in the viewing area, which makes a combined Particle Image Velocimetry (PIV) and Shadowgraphy analysis feasible.