Thermal Performance and Entropy Generation of MHD Casson Fluid with Temperature-Dependent Viscosity

Abstract

The thermal management and thermodynamic assessment of non-Newtonian fluid flow over stretching surfaces remain important in manufacturing and engineering processes involving electrically conducting fluids. This study examines the thermal performance and entropy generation of a steady, two-dimensional magnetohydrodynamic Casson fluid over a linearly stretching sheet, considering temperature-dependent viscosity and viscous dissipation. The flow is formulated under boundary-layer assumptions, with a transverse magnetic field applied normal to the stretching surface and the induced magnetic field neglected because of the small magnetic Reynolds number. The governing mass, momentum and energy equations are transformed into coupled nonlinear ordinary differential equations through similarity transformations. The resulting boundary-value problem is solved numerically using a shooting technique combined with the Runge–Kutta–Fehlberg method with adaptive step-size control, and the procedure is validated against limiting cases reported in the literature. The results show that the magnetic parameter suppresses the velocity field through the Lorentz force and increases thermal resistance within the boundary layer. The Casson parameter, viscosity variation parameter, Prandtl number and Eckert number produce distinct changes in the wall shear stress and local Nusselt number. Viscous dissipation acts as an internal heat source, increasing the fluid temperature and reducing the surface heat transfer rate. Entropy generation increases with stronger magnetic effects, higher viscous dissipation and larger Brinkman number, while the Bejan number indicates the relative contributions of heat transfer and fluid friction irreversibilities. These findings provide a numerical basis for assessing heat transfer and thermodynamic losses in MHD Casson fluid systems with variable viscosity under the stated assumptions.