Entropy generation of radiative Oldroyd 8-constant fluid over a Riga plate with temperature-dependent properties and heat gain

This study investigates the coupled effects of Riga-plate electromagnetic actuation, Oldroyd 8-constant viscoelasticity, temperature-dependent viscosity and thermal conductivity, thermal radiation, internal heat generation, and bimolecular reaction kinetics on the flow, heat transfer, and entropy generation of a reactive non-Newtonian fluid. The governing momentum and energy equations are nondimensionalised and solved as a coupled boundary-value problem using a partitioned weighted-residual method with polynomial trial functions on subdivided domains. Residual integrals are evaluated using Simpson’s 1/3 rule and implemented in Maple 18. Results reveal opposing effects of the two Oldroyd 8-constant parameters: the elastic-dominated parameter suppresses velocity and increases temperature, whereas the retardation-dominated parameter enhances velocity and decreases temperature. Variable viscosity enhances velocity while reducing temperature. The Riga-plate parameter accelerates the flow, confirming its role as a wall-parallel Lorentz-force actuator. Increasing the modified Hartmann number from 1.5 to 4.5 increases the Nusselt number by approximately 5.2% but raises skin friction by 24.8%, demonstrating a trade-off between thermal enhancement and drag. Thermal expansivity and radiation reduce temperature, whereas heat-source strength and Brinkman number increase it. Entropy generation increases with all parameters, with heat-transfer irreversibility dominating (Be ≈ 0.99). The novelty lies in demonstrating the coupled interaction of these mechanisms and quantifying the associated thermal enhancement–drag trade-off. The findings provide useful guidance for polymer processing, lubrication, thermal management, and MHD energy systems, where enhanced flow control must be balanced against increased wall shear stress.