Dual solutions for mixed convective stagnation-point flow of an aqueous silica-alumina hybrid nanofluid

Hybrid nanofluid as an extension of nanofluid is obtained by dispersing composite nano-powder or several different nanoparticles in the base fluid. Hybrid nanofluids are potential fluids that offer better heat transfer performance and thermophysical properties than convectional heat transfer fluids (oil, water and ethylene glycol) and nanofluids with single nanoparticles. Here, a kind of hybrid nanofluid including silicon dioxide (SiO2) and aluminum oxide (Al2O3) nano-size particles with water as base fluid is analytically modeled to develop the problem of the steady laminar MHD mixed convection boundary layer flow of a SiO2-Al2O3/water hybrid nanofluid near the stagnation-point on a vertical permeable flat plate. The flow of nanofluids near the stagnation point has recently attracted the attention of many investigators because of its wide applications in the local cooling/heating processes, especially in industries of electronic devices and nuclear reactors. In first, analytic modeling of hybrid nanofluid is presented and using appropriate similarity variables, the governing PDEs are transformed into nonlinear ODEs in the dimensionless stream function, which is solved numerically applying the function bvp4c from MATLAB. Our results demonstrate that the developed model can be used with great confidence to study the flow and heat transfer of hybrid nanofluids. Moreover, dual solutions of hybrid nanofluid flow for both assisting and opposing regimes are observed, where the range of the mixed convection parameter for which the solution exists, increases with the volume fraction of second nanoparticle and magnetic field. Finally, the heat transfer rate of nanofluids and hybrid nanofluids with different values of nanoparticles volume fraction have been compared that HNF3 (phi(SiO2) = phi(Al2O3) = 0.1) has the largest heat transfer rate between all cases.