In biochemical engineering, bubble columns and stirred tanks are key equipment where a wide range of gas–liquid reactions and processes takes place. At our facility, we evaluate various designs and scales of bubble columns and stirred tanks to better understand the physical phenomena occurring in different multiphase systems. Our research focuses on characterizing bubble hydrodynamics and interfacial mass transfer in complex liquids to determine optimal designs and operational conditions for industrial biochemical processes. Additionally, we aim to bring our research further to various liquids that resemble biosystems, such as non-Newtonian fluids containing salts or solid particles, to better replicate industrial and biological processes.

Experimental setup for a bubble column where the dynamics of rising multiple bubbles in complex liquids are recorded using high-speed cameras and analyzed with image processing algorithms.

Experimental investigation of bubble size distribution in a stirred tank using an optic probe.

Research objective: characterization of turbulence, fluid dynamics, and multiphase interactions.

Flow turbulence, characterized by the chaotic movement of fluids and velocity fluctuations, is frequently encountered in real systems due to its vital contribution in enhancing process efficiency, such as improving mixing homogeneity and mass transfer. The influence of turbulence on fluid dynamics is governed by various factors, including flow velocity, liquid rheology, reactor design, and operating conditions. In stirred-tank multiphase reactors, for instance, turbulence generated by impellers can deform and break bubbles, increasing their surface area and promoting efficient gas–liquid interactions. By investigating the effects of these key parameters on fluid dynamics, we aim to develop predictive models applicable to a wide range of fluid conditions.

Turbulent flows induced by stirring, resulting in chaotic motions and bubble deformation.

Research objective: investigation of energy and mass transport phenomena in multiphase flows.

Our pilot-scale laboratory serves as a center of experiments for advancing the understanding of mass transfer in both single and multiple bubble systems. Imaging techniques are employed to predict the liquid-side mass transfer coefficient, a critical parameter for the design and operation of multiphase reactors. Studies with single bubbles reveal gas–liquid contact time as an affecting parameter, which remains unmeasurable in multiple bubble setups. One of our objectives is to bridge the gap between fundamental insights from single bubble experiments and their application in complex bubbly flows. We are currently expanding our investigation to various Newtonian and non-Newtonian liquids to better characterize complicated rheological behavior existing in biochemical processes.

Experimental studies on bubble hydrodynamics and interfacial mass transfer using photographic techniques and dissolved oxygen probes.

Further validation is performed with high-fidelity direct numerical simulations of species transfer in multiphase flows, utilizing various in-house Computational multi-Fluid Dynamics solvers. The sub-domain and whole-domain conservation law-based formulations enable us to model complex heat and mass transfer-induced phase change phenomena in high-density-ratio flows with significant interface deformations. We aim to further extend these approaches to multiphysics and multidisciplinary applications.

Mass transfer simulation: Species distribution in a quiescent liquid from (a) a single rising bubble and (b) multiple rising bubbles, simulated using sharp and diffuse interface solvers, respectively.

Heat transfer simulation setup: Sketch of the computational domain where the flow moves from left to right. Two immiscible phases (carrier and dispersed) are enclosed between two differentially heated walls. In this simulation, the temperature is treated as a passive scalar.

Research objective: investigation on the influence of flow structures on drop dynamics.

The production and control of immiscible liquid–liquid dispersions and emulsions require a deeper understanding of drop evolution, coalescence, and breakup phenomena under different flow conditions. While these phenomena have been extensively studied individually, a fundamental understanding of how flow structures interact with drops is not well established. To address this gap, we use high-performance direct numerical simulations to examine drop and bubble dynamics in both turbulent and laminar flows, focusing on how coherent structures influence drop behavior.

In addition to traditional approaches for quantifying coherent structures, such as the Q criterion, we also incorporate Lagrangian coherent structures (LCS) and data-driven techniques for structure identification. By leveraging LCS, we can address many of the limitations inherent in traditional methods. Our primary research objective is to establish a link between coherent flow structures and drop dynamics.

Lagrangian coherent structures (LCS) to identify rotating flow structures.

Research objective: characterization of interface stability and its impact in multiphase systems.

Interfacial phenomena, including bubble deformation, interface contamination, and fluid particle interactions, play a key role in determining the efficiency of processes that involve multiple phases. Understanding these phenomena is essential for industrial applications, which has driven us to explore the intricate mechanisms governing the interaction between gas bubbles or liquid drops and the surrounding fluids. Through experimental studies and computational models, we aim to investigate how these phenomena are influenced by physical parameters like fluid properties, bubble size, turbulence, and interfacial properties, to further analyze their effects on process efficiency.

Fluid particle breakage caused by interface instability in the dynamics of a single rising bubble.

Research objective: characterization of fluid rheology and its impact on fluid dynamics.

Newtonian and non-Newtonian fluids exhibit distinct rheological characteristics that significantly influence gas–liquid interactions. While Newtonian fluids maintain a constant viscosity regardless of applied shear stress, non-Newtonian fluids are characterized by shear-dependent viscosity. In multiphase systems involving non-Newtonian liquids, dispersed bubbles may generate shear forces which can alter the viscosity around the bubble and eventually influence the bubble evolution. This complexity arises when the bubbles deform or encounter instability. To better characterize these phenomena, we aim to further examine the effect of liquid rheology, including shear-thinning inelastic and viscoelastic rheology, on bubble dynamics and mass transfer.

A single bubble rising in a stagnant liquid that may exhibit either Newtonian or non-Newtonian behavior. The shear forces acting on the bubble can influence the fluid rheology in non-Newtonian liquids, resulting in variations in the viscosity field.

Experimental observation on various shapes of bubble deformation in Newtonian and non-Newtonian liquids.

Research objective: detection of multiple bubble edges.

The detection of bubble edges is crucial for tracking the time evolution of bubble sizes, positions, and rising speeds. The edge-detection algorithms have been well developed for single deformable bubbles. However, this technique becomes more complex in the case of multiple bubbles, where bubble edges may overlap. The development of AI-based algorithms is required to identify individual bubbles and reconstruct overlapping bubble projections from the large data sets of recorded images.

Illustration of bubble edge detection using AI-based algorithms.