Analyzing variable characteristics effects on an unsteady hybrid nanofluid flow over a rotating sphere near a stagnation point with thermal stratification — A comparative study

Efficient dissipation of heat is crucial in various heat transfer applications. Hybrid nanofluids outperform conventional coolants in both absorbing and transferring heat, rendering them ideal for cooling purposes in machinery, electronics, and various industrial processes. Given these intriguing properties of coolant-based hybrid nanofluid, this numerical study aims to analyze the unsteady flow of hybrid nanofluid around a rotating sphere with thermal stratification, and variable heat source and sink effects. This investigation incorporates aluminum alloys AA7072 and AA7075 as nanoparticles in the Fluorinert liquid named FC-72 base liquid. The AA7072 alloy is a composition of silicon, ferrous, and copper added to aluminum and zinc in the ratios of 98:1. Similarly, AA7075 comprises a combination of aluminum ∼90, zinc ∼6, magnesium ∼3, and copper ∼1, together with magnesium and silicon ferrous. What sets this model apart is its incorporation of variable porosity and permeability, along with temperature-dependent viscosity and thermal conductivity. The governing equations are converted into systems of differential equations, and a boundary value problem solver for ordinary differential equations of fourth order using the collocation (bvp4c) technique is employed to obtain the numerical solution. Two alternative scenarios for porous media — (i) constant porosity and permeability, and (ii) variable porosity and permeability — are determined for relevant parameters versus the velocity and temperature profiles. Additionally, the heat transfer rate is computed versus numerous parameters. It is perceived that secondary velocity profiles decrease more significantly with higher estimates of the variable viscosity parameter when a constant porous medium is assumed. Furthermore, the heat transfer rate of the FC-72-based hybrid nanofluid increases with higher estimates of the rotation parameter and the particle volume fraction of aluminum alloys.