Scrutinization of Mixed Convection in Variable Viscosity Casson Nanofluid Flow Over a Stretching Sheet Subject to the Cattaneo–Christov Flux Model

This study explores mixed convection in a variable viscosity Casson nanofluid flowing over a stretching sheet, incorporating the effects of a magnetic field, viscous dissipation, and Joule heating within the framework of the Cattaneo–Christov flux model. The Buongiorno model is employed to capture the influence of Brownian motion and thermophoresis. The governing partial differential equations are transformed into first-order ordinary differential equations and solved numerically using the Keller-box method in MATLAB. The findings reveal that the velocity profile increases with variable viscosity, buoyancy ratio, and concentration relaxation time, whereas it decreases with higher thermal relaxation time and the Forchheimer coefficient. The temperature profile rises with the buoyancy ratio and thermal radiation but declines with increasing concentration relaxation time and variable viscosity. Similarly, the concentration profile increases with the Forchheimer number and viscous dissipation while decreasing with thermal relaxation time, variable viscosity, and buoyancy ratio. A detailed analysis of the skin friction coefficient indicates a 0.33% increase when both the buoyancy ratio and thermal relaxation time rise from 0.1 to 0.2, while a 24% reduction occurs as the variable viscosity increases from 2 to 3. The local heat transfer rate improves by 0.46% with an increase in variable viscosity (from 2 to 3) and the Forchheimer number (from 0.1 to 0.2), yet decreases by 2.28% when the thermal relaxation time, solutal relaxation time, and buoyancy ratio simultaneously increase from 0.1 to 0.2. Furthermore, the local mass transfer rate increases by 7.97% when the thermal relaxation time, solutal relaxation time, and buoyancy ratio increase from 0.1 to 0.2, whereas it decreases by 0.68% when the variable viscosity rises from 2 to 3 and the Forchheimer number increases from 0.1 to 0.2. A comparative analysis with existing literature demonstrates strong agreement with the present findings, reinforcing the validity of the study.