Onset of convection in a hybrid nanofluid porous layer

Thermal instability in nanofluid–porous systems is of significant interest due to its direct relevance in advanced thermal management, energy storage, and heat transfer enhancement technologies. Although extensive studies exist on convection in conventional nanofluids, the stability characteristics of hybrid nanofluids saturated in porous media, particularly under combined solutal and nanoparticle effects, remain insufficiently explored in the literature. The present study aims to investigate the onset of thermal convection in a hybrid nanofluid layer heated from below while accounting for the influence of a saturated porous medium. A linear stability framework is employed, and the governing equations are analysed using the normal mode technique to determine the conditions for stability and instability. The results reveal that the incorporation of two different nanoparticle types in the base fluid significantly delays stationary convection compared with single-nanoparticle suspensions, thereby enhancing thermal stability under top-heavy particle distributions. Furthermore, the presence of solute gradients is found to promote earlier convection onset in the heated layer. These findings provide new insights into controlling convection in hybrid nanofluid–porous systems for next-generation thermal applications.