Experimental and Numerical Studies of Two- and Three-Phase Jets in Crossflow
Creators: Zhang, Huan
Publisher: name: University of Alberta Library
Year: 2024

<p>Multi-phase jet in crossflow (MJC) garnered significant attention across a range of applications, including artificial aeration in oceans, lakes, and rivers. As of yet, the vast majority of the MJC literature focuses on pure gas or solid injection into crossflow, and limited studies pay attention to the much more complex two-phase (e.g., injecting air-water mixture) or three-phase (e.g., injecting air-water-sand mixture) jets in crossflow. This thesis addresses existing knowledge gaps in the field of MJC by conducting fundamental studies on two-phase and three-phase jets in crossflow through a combination of laboratory experiments and numerical simulations.<br>
Bubbly (i.e., air-water mixture) jets in crossflow have been mainly investigated based on physical experiments. To unveil more hydrodynamics of bubbly jets in crossflow, a 3-dimensional model was developed, calibrated and validated by coupling the Euler-Euler two-fluid model with unsteady Reynolds-averaged Navier Stokes (URANS) approach in OpenFOAM. The results showed that the modeled gas void fraction, bubble velocity, water jet centerline trajectory, and jet expansion agree well with the experimental data. Compared to pure water jets, bubbly jets are stretched wider in the vertical direction due to the lift of bubbles and thus dilution is larger. Interestingly, the vorticity at water jet cross-sections of bubbly jets evolves from two vertical “kidney-shapes” to two axisymmetric “thumb-up-shapes”. <br>
Although previous studies of bubbly jets in crossflow have been conducted in free-surface crossflow, relevant studies in crossflow with the top solid boundary effects (e.g., in a pipe/conduit or under ice-cover) are much scarce. A series of physical experiments were conducted to investigate the bubble characteristics for bubbly jets in pipe crossflow. The centerline gas void fraction and bubble size increase with distances after bubbles touch the top wall, mainly because bubbles are prone to gathering to the centerline to form larger bubbles. Good agreement has been achieved between horizontal bubble velocity and the 1/7th power law for water velocity in the pipe, and a correlation was proposed for predicting bubble rise velocity. Furthermore, turbulence characteristics of bubbles (e.g., root-mean-square of bubble fluctuating velocity, bubble turbulence intensity) were investigated for the bubbly jets.<br>
Compared to two-phase flows, gas-liquid-solid three-phase flows (GLSTPF) are inherently complex. The utilization of the Eulerian-Eulerian-Lagrangian (E-E-L) approach for modeling GLSTPF, especially incorporating the population balance model (PBM), has been scarcely reported. A new solver coupling E-E-L approach with PBM was developed in OpenFOAM to simulate GLSTPF. The new solver was successfully compared with the experimental results of bubble size distribution and phase velocities in a three-phase bubble column. The introduction of PBM significantly improved the predictions of bubble rise and solid velocities (by up to 20%) and phase holdups (by up to 30%). <br>
Air-water-sand three-phase jets in crossflow, as an extension of two-phase bubbly jets and a specific case of GLSTPF, have not been previously investigated. A series of physical experiments were conducted to explore air-water-sand three-phase jets in crossflowing water. The results reveal that sand particles that tend to separate from the bubbly region enhance downstream bubble dispersion. The sand concentration at the same level as the nozzle exit typically follows a parabolic distribution in the streamwise direction, and the peak position shifts closer to the source with an increased gas flow rate or reduced slurry flow rate. Finally, dimensionless prediction equations are proposed for the gas void fraction and sand concentration, which agree well with the experimental data (R2 = 0.91 - 0.94).<br>
To comprehensively explore the flow field and particle movement in air-water-sand three-phase jets in crossflow, a large eddy simulation (LES) was conducted based on the developed Eulerian-Eulerian-Lagrangian solver. The results showed that the predicted concentration and velocity of both gas and sand agree well with the experimental data. A pronounced jet expansion of the scalar concentration for the liquid-phase was observed at the location where the leading and the trailing edges of resolved turbulent kinetic energy collapse into one single peak. Sand particles escape from the jet region with a low rising velocity, and then turn downwards and accelerate rapidly to ultimately achieve a relatively uniform settling velocity as they descend to the bed. </p>