Numerical Hydrothermal Investigation of Multiphase Flow of Aqueous Alumina Nanofluids in Millichannels

The increasing power density of modern electronics necessitates advanced thermal management solutions beyond the capabilities of conventional cooling methods. While multiphase flow boiling in millichannels is a promising approach, there is a scarcity of data on the performance of nanofluids within this regime, particularly concerning the establishment of stable and hydrodynamically efficient annular flows. This study numerically investigates the heat transfer and hydrodynamic characteristics of multiphase flows of aqueous alumina (Al₂O₃) nanofluids (0.25% and 0.5% volumetric concentrations) within a rectangular millichannel to address this gap. A key contribution of this work is the use of an integrated modeling approach, where a one-dimensional (1D) analytical model is first used to define the boundary conditions required to achieve a stable, thin-film annular flow across the entire channel length. These conditions are then implemented in a comprehensive three-dimensional (3D) multiphase computational fluid dynamics (CFD) model. Results demonstrate that increasing nanoparticle concentration enhances heat transfer, with the nanofluids providing an improvement in heat transfer up to 35.6% compared to pure water. This improvement, however, is accompanied by an increase in pressure drop of 10.9% and 18.2% for the 0.25% and 0.5% concentrations, respectively. By providing detailed performance data for nanofluids in a controlled annular flow regime, this study contributes to the design and optimization of next-generation, high-heat-flux cooling systems and directly contributes to the underexplored area of multiphase nanofluid dynamics in millichannels.