The Impact of Swirl in Turbulent Pipe Flow

Abstract

The impact of swirl (i.e., flow with axial and azimuthal velocity components) on the turbulent flow in a pipe is studied using two-component laser-Doppler velocimetry (LDV). There are practical motivations for the flow geometry. For example, previous studies demonstrate that introducing swirl in the tube bank of a paper machine headbox can significantly increase mixing, and hence increase fiber dispersion and orientation isotropy in the finished paper product. The flow characteristics in a pipe downstream of a single straight tapered fin, a single fin with 180??ist but otherwise identical geometry, and four twisted fins were therefore studied at a pipe-based Reynolds number of 80,000. Radial profiles of the mean and rms fluctuations of the streamwise and azimuthal velocity components are measured; results for the straight and twisted single fin are compared to determine the effects of fin geometry and swirl on the turbulent wake downstream of the fin. From a practical viewpoint, it is also desirable to have adjustable swirl, where swirl can either be turned on or off depending upon the type of paper product being produced. The next generation swirler concept consists of fins fabricated from two-way shape memory alloys. Using the two-way memory effect, the fins will be in their straight configuration when cold and twisted configuration (hence acting as a swirler) when hot. This study is the initial phase in developing new active control mechanisms, known as the Vortigen concept, for increasing productivity, and hence reducing wasted raw material and energy, in the pulp and paper industry.