Rotordynamical Concept Evaluation of Fibre Refiners

There are many industrial examples of rotating machineries, e.g. turbines, pumps, compressors, hydro power plants, etc. This thesis deals with fibre refiners whose purpose is to break cellulose raw material into slender fibres. Such fibres are then used to manufacture paper, panel boards, etc. Insufficient tools currently exist regarding dynamics to supply the development of refiners. This means that the research problem is to provide knowledge about the dynamics of refiner concepts so that tools can be developed. The thesis is limited to rotordynamics at the early stage of refiner development (concept phase). Hence, the aim is to support this phase with relevant information regarding rotordynamics. The research was carried out by analysing concepts with and without clearance in the bearing assembly when the machines were running both at idle and in production. For the studies of bearing assembly clearance, a two degrees of freedom model is developed. The other concept studies are conducted using a four degrees of freedom model, including the gyroscopic effect. Most analysis is done numerically, since most equations of motion cannot be solved analytically. One essential tool for evaluating different refiner concepts is the visualisation of a design space that is spanned by design parameters (usually nondimensional) and one or many requirement variables. The idea of the requirement variables is to be able to distinguish dynamically good concepts from bad ones. By dealing with simple, rigid body models and scaled equations of motion it is possible to visualise a significant part of the design space. Poincare maps, bifurcation diagrams, etc., were used to study nonlinear dynamics due to clearance in the bearing assembly. At the initiation of this project no validated load model existed for the refining process. By coupling axial force measurements to the gap between the rotor and stator, such a load model was developed in this thesis, providing an opportunity to evaluate refiner concepts when the process is applied. It was found that certain combinations of design parameters lead to low vibration amplitudes both at idle and in production. Further, concepts with clearance in the bearing assembly can give rise to complicated motions and high bearing forces. However, by reducing the clearance, introducing more damping or increasing the friction, impacts can be avoided. Another solution to get rid of this problem is to introduce stabilising rods. In this thesis, analytical expressions for the required preload on these rods are derived to avoid impacts. Little is known about the pressure distribution in the refining zone when the machine is in production. However, it is concluded that non-axisymmetric pressure distribution can excite both forward and backward whirl, which must therefore be considered in the design. Axisymmetric pressure distribution (about the rotation axis) on the other hand leads to no external load; hence, concepts with such distribution are preferable. Regardless of the pressure distribution, the process can give raise to fluctuations of the stiffness, i.e. the dynamics depend on the process frequencies. In this thesis, certain process frequencies are found to lead to large vibration levels and should hence be avoided.