Flow field of non-Newtonian fluids in impinging jets confined by slopping plane walls

Abstract

An experimental investigation was carried out to characterize the flow field in a liquid impinging jet confined by slopping plane walls and emanating from a rectangular duct for various non-Newtonian fluids. These jets are frequently found in processes within the food and pharmaceutical industries, and they are formed when a high velocity fluid impinges a solid surface leading to intense levels of heat and mass transfer. The experimental work is complemented by results from a numerical investigation for purely viscous fluids. This work continues previous research, Cavadas et al (2006), on the same flow geometry for Newtonian fluids in laminar and turbulent flow regimes. Here detailed measurements of mean flow fields were carried out by laser-Doppler anemometry at inlet duct Reynolds numbers of Kozicki (1966) (Re*) of 200 pertaining to the laminar flow regime. The two non-Newtonian fluids were aqueous solutions of xanthan gum (XG) and polyacrylamide (PAA) at weight concentrations of 0.2% and 0.125%, respectively. For Newtonian fluids, Cavadas et al (2006) found a characteristic three-dimensional helical flow inside the recirculation, starting at the symmetry plane and evolving towards the flat side walls. This helical flow eliminates the separated flow region near the side walls and was also visualized in the non-Newtonian cases. Before reaching the flat side walls, the fluid in helical motion exits the recirculation and joins the main flow stream creating a near-wall jet which can be seen as velocity peaks near the walls in the spanwise profiles of streamwise velocity. The numerical simulations investigated the roles of shear-thinning and inertia on the main flow characteristics for purely viscous fluids at Reynolds numbers between 10 and 800. The length of the recirculation (XR) is constant in the central portion of the channel and decays to zero before reaching the flat side walls. At high Reynolds numbers a slight increase in XR at the edge of the core of the flow is apparent. As expected, inertia increases the length of the recirculation as for Newtonian fluids, but somewhat surprisingly it also increases the three-dimensional nature of the flow by reducing the size of the central core. Shear-thinning enhances the role of inertia especially at high Reynolds numbers, whereas at low Reynolds numbers the behavior is quite similar for all fluids. All flow fields were found to be symmetric relative to x-z and x-y centre planes