Ultrafast X-ray tomographic imaging of multiphase flow in bubble columns - Part 2: Characterisation of bubbles in the dense regime

Opaqueness and visual accessibility in turbulent bubbly flows are the main cause of difficulty for conventional experimental techniques to extract properties from the bulk flow. Ultrafast X-ray tomography presents an unique possibility to visualise the dense bubbly flows and to provide the ability to characterise the bubbles. With this technique, temporally resolved measurements can be obtained from a scanning plane. The post-processed images are stacked in time, resulting in a three-dimensional matrix with two spatial and one temporal resolutions. The gas flow structures are given straightforward by the image stack, which provides unique insights of the bubbly flow dynamics. However, sizing the bubbles requires the velocity to be known, which can be achieved by means of measurement or assumption. Typically, a second measurement plane is needed for the velocity estimation. However, the employment of dual plane measurement data is limited to dilute bubbly flows. To characterise bubbles in the dense regime, a method is presented using only single plane data for size extraction without the need of velocity data. The procedure comprises determinating the Sauter diameter (d32), which depicts the ratio between volume and surface area of the bubble. Contrary to the use of dual plane measurements, here, the bubble diameter is firstly determined and from the bubble size with the assumption of the bubble’s shape, the absolute bubble velocity can be roughly estimated. This method is verified with synthetic data of simple ideal-shaped bubbles. Subsequently, real bubbly flow measurements in bubble columns (with and without internals) are applied for the assessment of the resulting bubble sizes and velocity trend.