Dafne Molin

Enhanced heat transport during phase separation of liquid binary mixtures

We show that heat transfer in regular binary fluids is enhanced by induced convection during phase separation. The motion of binary mixtures is simulated using the diffuse interface model, where convection and diffusion are coupled via a nonequilibrium, reversible Korteweg body force. Assuming that the mixture is regular, i.e., its components are van der Waals fluids, we show that the two parameters that describe the mixture, namely the Margules constant and the interfacial thickness, depend on temperature as T−1 and T−1∕2, respectively. Two quantities are used to measure heat transfer, namely the heat flux at the walls and the characteristic cooling time. Comparing these quantities with those of very viscous mixtures, where diffusion prevails over convection, we saw that the ratio between heat fluxes, which defines the Nusselt number, NNu, equals that between cooling times and remains almost constant in time. The Nusselt number depends on the following: the Peclet number, NPe, expressing the ratio betwee...

Unified framework for a side-by-side comparison of different multicomponent algorithms: Lattice Boltzmann vs. phase field model

Lattice Boltzmann models (LBM) and phase field models (PFM) are two of the most widespread approaches for the numerical study of multicomponent fluid systems. Both methods have been successfully employed by several authors but, despite their popularity, still remains unclear how to properly compare them and how they perform on the same problem. Here we present a unified framework for the direct (one-to-one) comparison of the multicomponent LBM against the PFM. We provide analytical guidelines on how to compare the Shan-Chen (SC) lattice Boltzmann model for non-ideal multicomponent fluids with a corresponding free energy (FE) lattice Boltzmann model. Then, in order to properly compare the LBM vs. the PFM, we propose a new formulation for the free energy of the Cahn-Hilliard/Navier-Stokes equations. Finally, the LBM model is numerically compared with the corresponding phase field model solved by means of a pseudo-spectral algorithm. This work constitute a first attempt to set the basis for a quantitative comparison between different algorithms for multicomponent fluids. We limit our scope to the few of the most common variants of the two most widespread methodologies, namely the lattice Boltzmann model (SC and FE variants) and the phase field model.

Protocols to compare infusion distribution of wound catheters

Multi-holed wound catheters are increasingly used in clinical practice to administer analgesic/anaesthetic locally to the painful region. The distribution of flow infused during controlled (continuous or intermittent) administration of medication is believed to be an important issue for successful pain relief. Nevertheless, this information is not available from the literature. In this paper, we propose protocols to screen the performance of wound infusion catheters in the laboratory environment. Four wound infusion systems (PAINfusor by Baxter, OnQ Pump with Soaker catheter by I-Flow, PolyFuser Polymedic by Temena and Infiltralong by Pajunk) have been tested. Test results demonstrate that the distribution of the infused flow is different for the four catheters and closely connected to the catheter design (i.e. hole size and position, lumen diameter). Catheters characterized by small size holes (e.g. Baxter, Pajunk) distribute the flow more homogeneously than catheters characterized by large size holes (e.g. I-Flow, Temena). The distribution of infused flow does not change significantly during continuous or intermittent infusion.

Diffuse Interface (D.I.) Model for Multiphase Flows

We review the diffuse interface model for fluid flows, where all quantities, such as density and composition, are assumed to vary continuously in space. This approach is the natural extension of van der Waals’ theory of critical phenomena both for one-component, two-phase fluids and for liquid binary mixtures. The equations of motion are derived, showing that the problem is well posed, as the rate of change of the total energy equals the energy dissipation. In particular, we see that a non-equilibrium, reversible body force appears in the Navier-Stokes equation, that is proportional to the gradient of the generalized chemical potential. This, so called Korteweg, force is responsible for the convective motion observed in otherwise quiescent systems during phase change. Finally, the results of several numerical simulations are described, modeling, in particular, a) mixing, b) spinodal decomposition; c) nucleation; d) heat transfer; e) liquid-vapor phase separation.

Direct Numerical Simulation of Microbubble Dispersion in Vertical Turbulent Channel Flow

In this work, direct numerical simulation of turbulence is coupled to lagrangian tracking to study the behavior of 220 μm bubbles in a vertical turbulent channel flow. Both one-way and two-way coupling approaches and both upward and downward flows are considered. For each simulation, the same external imposed pressure gradient is considered. In one-way simulations, this leads to a shear Reynolds number of Re = 150. In the coupled cases, the presence of bubbles increase/decrease the driving pressure gradient, respectively in upward/downward flow, thus yielding to an increase/decrease of the wall shear stress and of the shear Reynold number. For the considered bubble average volume fraction (α = 104 ), the corresponding shear Reynolds number are about Re τ,2U= 174 for the upflow case and Re τ,2D= 121 for the downflow case. Statistics of the fluid and of the bubble phase are presented. The interactions between bubble and the near-wall turbulence structures is also investigated and a preferential bubble segregation in high-speed/low-speed zones is observed for the upflow/downflow cases respectively. An attempt to describe the transfer rate between the gas and the liquid will be included with some preliminary results.