Simultaneous 3D Lagrangian Particle Tracking of bubbles and fluid tracer particles in a bubbly water jet

Abstract

The experimental investigation of two-phase flows by means of particle-based optical field measurement techniques like PIV or Lagrangian Particle Tracking (LPT) pose significant challenges. Typically, the tracer particles within the fluid phase (e.g. water, air) are much smaller (Stokes numbers << 1), compared to the objects in the other phase (e.g. bubbles, solid particles, else). The illumination required to visualize the tracer particles therefore often significantly overexposes the larger objects, inhibiting a precise localization of both particle regimes. In such flows the grade of complexity for the particle position reconstruction and -tracking process even increases when applying 3D PIV or -LPT techniques using several camera projections. Two-phase turbulent flows are widely used in engineering and environmental applications (Balachandar & Eaton 1993; Mathai, Lohse & Sun 2020). This study describes a method for simultaneous measurements of time-resolved three-dimensional velocity and acceleration fields of the dispersed and continuous phases of a turbulent bubbly jet flow at a low void fraction using 3D LPT with the Shake-The-Box algorithm (Schanz et al. 2016, Schröder & Schanz 2023). Four high-speed cameras are used to acquire time series of images that include both bubbles and fluid tracer particles illuminated by a combination of two pulsed illumination sources with different intensities. A pulsed UV- LED array excites fluorescent tracer particles, which are imaged by four high-speed cameras using optical low-pass filters and a dimmer white-light LED array illuminates the bubbles (which are invisible for the cameras in the fluorescent light), leading to well exposed bubble and tracer images in the same image series. Bubbles are firstly tracked using intensity differences between tracer particles and bubbles, then the bubble images are removed from the camera images and all tracer particles are tracked using the residual images. Subsequently, FlowFit data assimilation (Gesemann et al. 2016) is applied to the obtained LPT results by phase separation to investigate the flow characteristics of the bubbly jet. This contribution is aiming at describing mainly the experimental set-up using two different LED arrays and fluorescent tracer particles and the image processing methods.