The Flow Dynamics of a Partially Filled Horizontal Helical Screw Reactor (PFHSR)

Abstract

Enclosed helical screw systems are used as screw feeders and conveyers for handling dry bulk solids. The design of such systems is based on the feed load, the properties of the bulk solid and the shape and parameters of the screw features. Similar designs are used as extruders in manufacturing deformable materials with help of pressure and a combination of pressure and temperature. Furthermore, identical arrangements also are used in mixing and transporting viscous fluid. The screw pitch, shape and size are some of the factors that determine the flow dynamics between the center screw (core shaft) and the outer cylinder (barrel), and also are found to determine dry bulk solids transport. The transport processes of helical screw systems for bulk solids, heat exchangers, passive mixers and high viscosity fluids are widely published. However, there are limited studies available that have investigated horizontally placed partially filled screw reactors. In the food processing industry, partially filled horizontal helical screw reactors (PFHSR) are used to transport and mix fluids and slurries and also to chill food products. Thus, understanding the flow dynamics of PFHSRs will lead to the design of effective PFHSR systems as well as open new areas of application. Our main objective is to understand the flow dynamics of a PFHSR system. The test system is a closed end horizontal cylinder with a screw agitator in the middle. The tests are designed to replicate the current industrial process and to investigate the flow dynamics change due to flow path and different screw rotational speeds. Water-based rheoscopic fluid is used to visualize the flow profile, and several replications of the same test were conducted. The PFHSR system length, outside cylinder diameter, screw (auger) pitch and clearance were fixed for all tests. The rotational speed and the flow return path were varied. The flow return path varied by opening and closing the auger’s core. The flow results indicate the annular flow between the auger core and outside cylinder is affected by the rotational speed and the flow return rate related to flow path. The open core region simulates the flow condition where the flow is pumped into and out of the PFHSR system. On the other hand, the closed core region simulates the effect of pressure (slip) flow as well as the implication of slip flow in mixing. The flow process has been studied by observing the flow pattern from different viewpoints. The experimental results are presented by relating the flow field with a Reynolds number (Re) that is defined using the rheoscopic fluid viscosity, the auger rotational speed and diameter. The bulk fluid flow is found to be the result of the moving surfaces and other boundary conditions in addition to the slip-flow through the flight and barrel clearance. Vortices appear at the trailing side of the screw flight and also show a periodic pattern. The flow fields observed from both open and closed core show the flow profile, as well as the flow type significantly affected by the flow path.