Channel flow that reduces pressure drop, minimizes friction, improves heat transfer, and facilitates cleaning

Abstract

The two main themes presented in this thesis are the effects of temperature-dependent viscosity in fluid flows and the influence of complex fluids usage for cleaning purposes. Viscosity gradients occur in many industrial and manufacturing processes such as 3D printing, heat exchange cooling systems, injection molding, etc. A proper understanding of how these viscosity gradients affect the flow can lead to appropriate allocation for pumping power and improved energy management. Moreover, complex fluids can have constituents such as fibers, colloids, or polymers that cause their rheological and thermal properties to change. Therefore, if we can predict how these changes occur, we can optimize these changes to our advantage. Here, by coupling analyses of the momentum and energy equations, we derive a low-Reynolds number analytical expression for the pressure drop across a heated narrow pipe and then extend the results to a converging channel with an arbitrary cross-section. We highlight how changing the temperature boundary conditions at the wall impacts the pressure drop correction. Then we assess how changing the shape of a converging channel and contraction ratio affects the pressure drop. Additionally, we will outline the trade-off between improved heat transfer effectiveness and pressure drop reduction due to viscosity changes as these are important design considerations. Subsequently, we consider how increasing a fluid viscosity's sensitivity to temperature changes affects the wall slip velocity on a flat plate enhancing the reduction in friction and improving heat transfer effectiveness. Finally, we discuss a purely experimental work focused on the cleaning efficacy of a micro-fibrillated cellulose (MFC) suspension. We highlight the spatial variations in cleaning effectiveness in small channels and how the fibrillar network formed by the cellulose material causes an increase in the viscosity leading to higher shear stress at the wall which is favorable for effective cleaning. Additionally, we consider varying the concentration of fibers in the fluid and the flow rate. Finally, we examine how the corresponding changes in the rheology are linked to the cleaning efficiency of the respective suspensions.