Micro-PIV Measurement of Flow Upstream of Papermaking Forming Fabrics

Abstract

In the flow through porous media such as screens the structure of the screen affects the upstream flow field. This is relevant for filtration and deposition processes where the local filtrate deposition rate depends on the upstream flow field, while deposition also alters this flow field over time. Here, we investigate the flow through forming fabrics that retain pulp fibers during the papermaking process, while the liquid phase passes through. The associated drainage velocity through the fabric has a significant impact on the quality of the finished product. Microparticle image velocimetry (micro-PIV) was used to measure the drainage velocity distribution upstream of two different fabrics. To make measurements at the scale of the forming fabric filaments, an experimental micro-PIV procedure was developed to permit velocity measurements several millimeters into the fluid stream with a depth of correlation of $28\,\upmu \hbox {m}$ and an in-plane resolution of $40 \,\upmu \hbox {m} \times 40 \,\upmu \hbox {m}$. For the single phase flow of water, the experiments show the existence of a highly variable drainage velocity field upstream of both fabrics. The normalized standard deviation (NSD) of the velocity fields decays exponentially with distance upstream of the fabric. The decay constant is on the order of one filament diameter at moderate Reynolds numbers $(Re=52-140)$. At a constant freestream velocity, the deposition of a small number of wood fibers onto a fabric results in a higher NSD than that of the bare fabric. This is believed to be due to partial obstruction of some holes in the fabric, which leads to higher velocities through unobstructed holes. Once about 5 g per square meter of fiber has been deposited onto the fabric, the velocity field above the fabric with adhering fiber mat becomes uncorrelated with the velocity field through the bare fabric.