Non-Newtonian Casson fluid double diffusion magneto-hydrodynamics in a concentric ring with different cylinders containing radiative ternary nanofluids

Purpose This study aims to numerically investigate the double-diffusive convection of a Casson ternary hybrid nanofluid (Al2O3–CuO–Ag/fluid) within a concentric annulus featuring an inner polygonal cylinder. The purpose is to quantify the impact of fluid rheology (Casson parameter) and geometry (circular, triangular, square inner cylinder) on heat and mass transfer rates, providing insights for the design and optimization of advanced thermal management systems. Design/methodology/approach The governing nonlinear partial differential equations are solved using a Galerkin finite element method. The implementation uses a Newton–Raphson iterative scheme to handle the system’s nonlinearity. The analysis is conducted for a range of key parameters, and the results are validated against established benchmark studies to ensure numerical accuracy and reliability. Findings Increasing the Casson parameter (γ) from 0.1 to 10 significantly enhances transport rates, boosting the average Nusselt number by 20.1% (circular), 32.4% (triangular) and 30.0% (square). The average Sherwood number sees even greater enhancement, rising by 137.0%, 154.0% and 158.0%, respectively. The circular inner cylinder geometry consistently outperforms the polygonal shapes, achieving superior heat and mass transfer due to its streamlined geometry which minimizes flow obstruction. Originality/value This work provides a novel analysis of double-diffusive convection for a Casson ternary hybrid nanofluid in a complex annular geometry with inner polygonal cylinders. The combined investigation of non-Newtonian rheology, multi-component nanoparticles and geometric effects on both thermal and solutal transport represents a significant extension of existing literature, offering new quantitative insights for system optimization.