Momentum and Heat Transfer Characteristics of Axisymmetric Particles in Non-Newtonian Fluids: Sphere and Spherical Segments

Abstract

This thesis contributes towards understanding the flow and heat transfer characteristics of generalized fluids past a particle of the shapes: sphere and spherical segments. In processes like fluidization, the interaction between the fluid and solid components plays a crucial role in enhancing heat and mass transfer, as well as promoting fluid mixing. The pneumatic transport of the particulate matter also involves the hydrodynamic interactions of the particle and the conveying medium. Various scenarios, such as solid-fluid interaction in sewage sludge, suspensions, drilling muds in oil recovery, sedimentation, thickening of slurries, and waste treatment in mineral industries, involve non-Newtonian fluids frequently interacting with non-spherical particle shapes. Furthermore, in biological applications of fluid-particle systems, instances like the motion of red blood cells in capillary flow and drug delivery systems showcase the relevance of understanding these interactions. Thus, the current interest in studying the momentum and heat transfer from spherical segments and a sphere as a limiting case of spherical segment in different types of fluids and/or under different conditions stems from both fundamental and pragmatic considerations. Generalized Newtonian fluids are encountered in a wide range of applications: toothpaste, butter, jam, cosmetic creams, mortars, foams, polymeric solutions melt, etc. A great many industrial processes involve Generalized Newtonian fluids, power-law, and Bigham plastic, ranging from the creation of chocolate to concrete used for the construction of buildings and the paper pulp suspensions, dairy products, polymers and polymeric solutions, etc. To achieve the underlying objective of this thesis, a finite element based numerical approach has been employed to study the crossflow of Bingham plastic or power-law fluids and Newtonian fluids as a limiting case past a spherical segment in creeping, forced-, and natural-convective regimes. The numerical experiments were conducted using the finite element-based simulation software COMSOL Multiphysics. The Bingham plastic fluid behavior was modeled using a regularized continuous constitutive relation Papanastasiou model. The evaluation of yielded and unyielded sub-domains in the flow field due to the Papanastasiou model has been further substantiated by reproducing similar results using the other regularization approaches available in the literature, i.e., the bi-viscous model and Bercovier and Engelman model. The range of nondimensional parameters considered in this study is such that the flow and temperature fields remain steady. Using the numerical technique outlined above, two noteworthy contributions were made toward understanding the flow and heat transfer characteristics of Bingham plastic or power-law fluid flow past spherical segments. First, the influence of the shape of the particle, i.e., sphericity of the spherical segment () on the flow and thermal characteristics has been examined for the unconfined/confined flow of Newtonian, Bingham plastic or power-law fluid in forced/free convection regime. In each case, extensive results for the topology of the yielded/unyielded regions, recirculation zones, isotherms, drag and heat transfer coefficients as a function of the pertinent parameters have been presented and analyzed in detail to elucidate the effect of the shape of the particle and of the confinement on momentum and heat transport. The second contribution was the numerical results for the creeping flow of power-law fluids past a sphere, which have been used to develop a scheme to construct the shear stress–shear rate curves using the Falling Ball Method. The work was concluded by presenting extensive comparisons with experimental results for Newtonian fluids and shear-thinning polymer solutions in the low-shear region including the zero-shear viscosity and the shear-thinning region.