Description
The valorization of microalgae for energy or pharmaceutical applications strongly relies on the harvesting stage, which consists of the separation of microalgae from the culture media to obtain a more concentrated biomass.
The aim of this study is to investigate the potentiality of microfluidics as an alternative separation process, with reduced resource consumption compared to standard processes. We consider the particular case of microalgae with rigid membranes such as Chlorella Vulgaris. Previous studies have revealed that using spiral microchannel, with small height-to-width ratio ($\lambda$), allows efficient separation of inert and biological microparticles, like algae or pathogens, when the flow inertia is finite at the particle scale. While the spiral device seemed to allow good separation performance, the particle transport properties were not well characterized. To fill in this gap, we first study the dynamics of the carrier fluid and second the forces experienced by the microparticles in curved microchannel flows.
The study is mainly based on numerical simulations of the fluid flow using the in-house code JADIM. Source terms based on the Force Coupling Method (FCM) allows capturing the hydrodynamic interactions between the freely moving particles and the flow. The forcing term, written in the form of a multipole expansion based on Stokes flow, is adapted to capture finite flow inertia at the particle scale in curved flow geometry. Moreover, the numerical results are compared with experiments carried out (in our group) using microalgae with similar size.
The flow features are investigated as a function of the Dean number (product of the Reynolds number $\mathrm{Re}$ and the square root of $\delta$, the channel curvature) for $\lambda=0.17$.
Secondary flows, in the form of a vortex pair, take place in the curved channel flow. While their intensity increases with $\mathrm{Re}$ and $\delta$, two scaling laws are proposed for flow regimes dominated by viscous and inertial forces, respectively. Freely moving neutrally buoyant single particles, of diameter equal to the fifth and eighth of the channel height, are studied. While the particles are transported by the main flow and by the secondary vortices, they also experience inertial lift along the local strongest gradient of the streamwise velocity. The balance between the drag along the secondary vortices and the inertial lift across the streamwise flow, leads the finite size particles to focus near the inner channel wall for $0.006\leq\delta\leq0.04$ and $30\leq\mathrm{Re}\leq160$. Nevertheless, we show that the maximum of the streamwise velocity shifts towards the outer wall as the Reynolds number is further increased, suggesting the decrease of the lift force along the radial direction near the inner wall, and subsequently the potential shift of particle equilibrium away from the inner wall.
Furthermore, we carried out simulations of suspension flow with volumetric concentration up to $10\%$. These simulations allow to study the suspension dynamics as a function of flow inertia and to evaluate the efficiency of microalgae concentration in microfluidic devices.
Keywords: particle transport, inertial focusing, curved channel flow, microalgae suspensions