Nanoparticle migration in nanofluid film boiling: A numerical analysis using the continuous-species-transfer method

Investigating the boiling behavior of nanofluids remains a topic of considerable research interest. The existing theoretical framework often underscores Brownian motion and thermophoresis among the predominant mechanisms that influence nanoparticles' movement within nanofluids. However, most current numerical models addressing nanofluid boiling either neglect these mechanisms or incorporate them inadequately. A comprehensive review of the literature reveals two primary gaps. The larger set of numerical studies on nanofluids boiling does not account for any governing equation for nanoparticle concentration, neglecting the primary mechanisms of nanoparticle movement. The smaller set includes such an equation but limits its application to the vapor domain, thereby overlooking the liquid nanofluid phase. Moreover, in the context of film boiling involving nanofluids, an insulating vapor film forms between the nanofluid and the heated surface. This vapor film is dynamic, experiencing evaporation at its outer boundary, leading to an increase in nanoparticle concentration at the vapor-liquid interface—another aspect always neglected in existing numerical models. To address these research gaps, this study introduces a specialized governing equation employing the Continuous-Species-Transfer method within the framework of Computational Multi-Fluid Dynamics. Importantly, it also incorporates governing equations for thermophoresis and Brownian motion to account for the motion of nanoparticles. The method's efficacy is further corroborated by solving a two-dimensional axisymmetric film boiling problem on a vertical cylinder, with results aligning well with analytical studies that consider Brownian motion and thermophoretic behavior. Overall, the study fills a critical gap in the literature by providing a comprehensive tool for examining complex interfacial behaviors in nanofluid film boiling.