Computational Investigation of the Flow Through Elbows in the Approach Piping System to a Headbox, 1992 Engineering Conference Proceedings

Successful operation of the wet-end in a papermachine and paper quality partially depend on the approach piping system. In particular, insufficient pipe length between the headbox and the last elbow would result in cross direction profile variation and paper curl. Optimum approach piping should eliminate hydrodynamic phenomena, like cross-flow eddies and pressure gradient, which disturb the uniform and stable delivery of the stock to the headbox. Although elbows are commonly used in practice, many questions regarding their optimum arrangement still remain unanswered. For this study we modeled the three-dimensional turbulent pipe flow through single- anddouble-elbows configurations under conditions of practical interest. Within an accuracy of 6-12%, simulations compared well with previous experimental results for flow velocities and pressure. Single-elbow flow at a tubular Reynolds number (Re) of 10 to the 5th requires a distance of 22 pipe diameters to eliminate swirling flows, 34 diameters to attain a uniform pressure profile, and 45 diameters to obtain a fully-developed turbulent velocity profile. An increase of Re to 2x10 to the sixth does not change these requirements. Double-elbows require longer distances to eliminate the elbow effects, only if they are separated closer than 15 diameters. In that case, out-of-plane elbows demand longer pipe lengths than two in-line elbows.