CFD Simulation Of Flow And Mass Transfer In Nanofluids

It was proved in some experimental studies that mass transfer process can beintensified by adding nanoparticles into traditional fluid （such as water, ethanol andetc.）, which formed a new working fluid called nanofluids. Numerical investigationson nanofluids were presented in this paper to obtain a comprehensive understandingon the mass transfer enhancement mechanism of nanofluid.Due to the size effect of nanoparticles, the continuum hypothesis was not anymore valid in nanofluid, and macroscopic continuity equation was not capable todescribe the motion in nanofluid. In view of this reason, lattice Boltzmann method（LBM）, an ideal approach for meso-scale and scale-bridging simulations, wasintroduced to simulate the flow and mass transfer process in nanofluid in this paper.One-phase and two-phase models were used separately to simulate the diffusionprocess of Rhodamine B in water-based Cu-nanofluid with LBM incorporatingpractical external force. And the calculation results of two-phase model were found tobe in good agreement with the experimental values in the literature. The diffusioncoefficient of Rhodamine B in water-based Cu-nanofluid was calculated exactly bythis model. Especially, the calculated diffusion coefficient fit well with theexperimental values in the literature when Brown force adjustment coefficientC0was107.4845.Sedimentation of quiescent water-based Cu-nanofluid in square cavity wasnumerically studied. It was observed that sedimentation was not obvious in initialstage and the distribution of nanoparticles was relatively uniform. However, theconcentration of nanoparticles on the bottom increased with the time. Almost all thenanoparticles would settle at the bottom over a sufficiently long time.Flows of pure water and water-based Cu-nanofluid were compared under thesame external conditions, respectively. Pronounced micro-convection initiated by theBrownian movement of nanoparticles was observed in water-based Cu-nanofluid.Absorptions of CO₂in pure ethanol and ethanol-based Cu-nanofluid were studiedunder the same external conditions, respectively. The occurring time of Rayleighconvection, average velocity, average CO₂concentration and average vorticity in pure ethanol and ethanol-based Cu-nanofluid were analyzed. The comparison resultsdemonstrated that absorption of CO₂in ethanol could be enhanced by addingnanoparticles.The traditional flow field was changed because of the non-stop Brown motionarisen by nanoparticles in base fluid, pronounced micro-convection was observed,which made turbulence extent of local flow field intensified and speeded up thediffusion of gas molecular in nanofluid, enhancing the mass transfer process.