Nanoparticle-Dependent Viscosity and Viscous Dissipation Effects in Mixed Convection Flow of Ammonia-Water Nanofluid with Non-Linear Boussinesq Dynamics

This study integrates the single-phase nanofluid viscosity model into the generalized Buongiorno model to analyze the combined convection of an ammonia-water nanofluid over a vertical flat plate. A key contribution of this work in the field of mass and heat transfer is representing fluid viscosity as a composite function of nanoparticle concentration and temperature, while also achieving the nanofluid hybridization model. The model development assumes that fluid viscosity is directly influenced by nanoparticle concentration, temperature, and buoyancy. It indirectly controls the concentration gradient through the Dufour effect and dissipative heat. This physical phenomenon is addressed to achieve the quantitative improvements necessary for optimizing the design of engineering systems. The mixed convection problem, which incorporates the influences of the Dufour and Soret effects as well as the dynamics of the ammonia-water nanofluid (Sc = 445, Pr = 7.2, Le = Sc/Pr), is modeled over a semi-infinite plate subjected to double-diffusive and convective thermal boundary conditions. The model equations are simplified using an appropriate similarity transformation and are numerically solved via the Spectral Local Linearization Method (SLLM). The results highlight a reduction in fluid viscosity with increasing temperature, which consequently diminishes viscous drag; enhanced momentum and energy fields due to the involvement of the non-linear density–temperature relationship (Λ1);a pronounced effect of the non-linear density-solutal relationship (Λ2), particularly in the constant viscosity model (λ = 0.0), which exhibits significantly higher sensitivity compared to the variable viscosity model (λ = 0.2); increased contributions of dissipative heating with a rising Eckert number (Ec), acting as a thermokinetic effect on fluid flow. Further analysis reveals that, in the presence of Ec = 0.05, the skin friction in the constant viscosity model is reduced by 19.2%, while the Nusselt number, nanoparticle mass transfer, and regular mass transfer increase by 2.4%, 7.7%, and 7.4%, respectively, in the variable viscosity model.