Numerical modelling and Ultrasound Velocity Profiling of Wood Chips Flow in an Industrial Application

Licentiatavhandling, 2025

Large amounts of energy and raw material are required to produce pulp and understanding the process and equipment is the key to optimization of energy consumption as well as chemicals and overall yield of the process. Conducting experiments on lab-scale for impregnation vessels remains a challenge and a numerical model enables a cost-effective test environment where to gain insight of the complex process. In this work a \textit{computational fluid dynamics, CFD model} is examined. The solid and liquid phases were both treated as continua, and it was found that the continuum model for the solid wood chips phase could capture the previously observed oscillating formation of arches in the contracting part of the vessel. This has been observed in more refined models, for smaller particle systems, which are not feasible for industrial-scale pulping equipment. The present work also highlights the importance of correct material data when using the model as a design tool. As an initial step to validate the models for wood chips flow, \textit{Ultrasound Velocity profiling}, UVP measurements were performed on an industrial impregnation vessel. The measurements could successfully capture the velocity of the wood chips and the normal production variation at the mill. The velocity profiles of the wood chips show a non-zero velocity at the wall, a shear zone and a plug flow, which indicates that the wood chips flow can be regarded as a dense granular flow and the rheology can be described with the Bingham fluid model and that the partial slip condition could be used to model the interaction with the wall.