In this first phase of the project, we have laid the groundwork for the experimental investigation of bubble-particle collisions in turbulence. Besides the development of a microfluidic approach to fabricate particles and capsules of varying properties at a high rate, this relates to the construction of a new turbulence generator for the Twente Water Tunnel facility. It consists of a rectangular array of 112 individually computer-controlled water jets that are aligned streamwise to the measurement section of our 8 meter tall vertically recirculating water tunnel. The maximum exit velocity of each jet is designed to be 8 m/s. This so-called ‘jetting grid’ allows for high turbulence intensities and Taylor-Reynolds numbers inside of the measurement section, while preserving lateral homogeneity and near isotropy in the core of the measurement section. Novel to our jetting grid is the full stainless-steel design and the protocol that we use to drive the individual jets. The protocol is based on 4-dimensional OpenSimplex noise, a type of gradient noise that features temporal and spatial coherence.
For the simulations, we have developed a framework to study the geometric collision rate by means of direct numerical simulations in homogeneous isotropic turbulence using the point-particle approach over a range of the relevant parameters, including the Stokes and Reynolds numbers. We employed this approach to stud the spatial distribution of bubble and particles, and quantify to what extent their segregation reduces the collision rate. Our results showed that this effect is countered by increased approach velocities for bubble–particle compared to monodisperse pairs, which we related to the difference in how bubbles and particles respond to fluid accelerations. Furthermore, we used our data to critically examine existing models. These results are published in Chan,Ng,Krug, JFM 959 (2023) for the case without buoyancy and a manuscript discussing gravitational effects is currently in preparation.
Additionally, we have adopted a Lattice-Boltzmann method using the Immersed Boundary method to enable interface resolved simulations for the bubbles. Based on this approach, we investigated the impact of turbulence on the collision efficiency between a fully contaminated bubble and small inertial particles in a moderately turbulent flow. We observed that collisions in the turbulent flow occurred for particles coming from a significantly wider region ahead of the bubble compared to non-turbulent flow. This led to a remarkable enhancement in collision efficiency, reaching approximately 100% compared to the results in the quiescent flow. We developed a statistical model based on turbulent sweeping which involves the results obtained in a quiescent flow. A manuscript on these results will be submitted shortly.
