Complex interplay of power-law rheology and non-Oberbeck-Boussinesq effects on natural convection heat transfer in a confined domain

Abstract

As the first endeavor, the classical differentially heated square cavity problem is revisited to study the non-Oberbeck-Boussinesq (NOB) effects on laminar natural convection heat transfer to non-Newtonian power-law fluids. The augmentation and diminution in both momentum and heat transfer characteristics due to the interplay of power-law rheology and NOB effects are investigated numerically using a finite-volume based open-source solver OpenFOAM®. Moreover, significant non-intuitive changes in thermal and flow fields, due to the temperature-dependent fluid properties, are studied for practical temperature difference ranges. In the present study, both shear-thinning and shear-thickening power-law fluids are taken into account. The NOB results are compared with the corresponding Oberbeck-Boussinesq (OB) results to ascertain qualitative and quantitative inevitable changes in the intricate phenomena. The accelerating NOB effect on heat transfer trend augmented due to shear-thinning fluid behavior, whereas it diminished by the shear-thickening fluids, with reference to a Newtonian case. In particular, for the range of power-law index and temperature difference considered in this study, the average Nusselt number enhances in excess of 40% due to the consequent NOB effects at the extreme case. Results without ascertaining NOB effects would be quite misleading and thus, it should not be neglected in practical scenarios and for their implications. The primary objective of this work is to investigate the natural convection in a square cavity accounting the NOB effects and ascertain the regimes of validity of the Boussinesq approximation. This work finds application in thermal processing of polymeric melts and solutions in several process industries.