Research On The Complex Inner Flow Of Fiber Suspension And Its Applications To The Development Of Stock Pump

Fiber suspensions are involved in many application backgrounds, which play an important role in the chemistry, textile, composite material and paper-making industries. The movement of the fibers immersed in the suspension, and the rheological properties due to the fiber orientation distribution states are two major aspects in the research of the fiber suspensions. Observation from its microstructure, the stock flow is a typical flow of the fiber suspension. To obtain the macroscopic properties via the microstructure of the fiber suspension, many fields such as multiphase fluid dynamics, non-Newtonian fluid mechanics, statistical mechanics, multi-body dynamics, etc. are always involved.To avoid the complexity of the microstructure of the fiber suspension, the previous researchers always considered the fiber suspension as the solid-liquid two-phase flow. It is obviously that this model could not reflect the physical characteristics of the fiber suspension, i.e., it neglects the fiber orientation and the orientation distribution. The enlarged flow rate method is widely used in the design of the stock pump. But this method needs the experimental results and the design experience. Therefore, the criterions with clear physical meanings are still not available in the design of the stock pump.The low consistency refining technology had been used for a long time in the domestic paper-making industry. But the dedicated stock pumps are less efficient. To promote the design innovation, the internal flow investigation is employed in this paper. Taking the fiber suspension as the physical model, a series of theoretical and numerical researches are performed. The main contents are:(1) Numerical prediction of the fiber motion and the fiber orientation distribution in a 2D rotating expansion channel. The results show that, first, with decreasing inlet velocity, the fiber needs more time to be oriented to the flow direction; second, the fiber initial location will influence the subsequent orientation evolution, and along the central line, the fiber rotates more slowly to the flow direction; third, there is a critical fiber aspect ratio, i.e.,5. When fiber aspect is greater than 5, the fiber flips more frequently with decreasing aspect ratio, otherwise the fiber evolution is less sensitive to the aspect ratio; fourth, in the semi-dilute suspension with fiber interactions, the steady fiber orientation is totally along the flow direction, and this trend is more obvious in the wall regions.(2) Numerical prediction of the fiber orientation distribution in a 2D curved expansion duct. The results are similar to the above case, but the critical aspect ratio is 3;(3) The 3D fiber orientation prediction in the fiber suspensions through a 2D rotating curved expansion duct. The results show that, first, the fiber turns more quickly in the inlet region; second, the influence of inlet velocity and rotating speed is not obvious; third, the aspect ratio influences the fiber orientation evolution most significantly, for instance, in the wall regions, the in-plane orientation is more close to the flow direction, while along the central streamline, the in-plane orientation deviates from the streamline obviously; and the fiber flips more frequently with decreasing aspect ratio; fourth, the initial orientation influences the subsequent orientation evolution in the inlet and central regions, but it has little effect on the orientation evolution in the downstream region.(4) Numerical research on the direct solution of the Fokker-Planck equation for the fiber orientation distribution;(5) The research on the fiber-flow coupling, and its application in the fiber suspensions through the curved expansion duct to investigate the rheological properties. The results show that, first, the extra shear stress decreases gradually from inlet to outlet; second, along the lateral direction, the shear stress changes dramatically, and the lateral distribution becomes more uniform with the increase distance from the inlet; third, the normal stress difference also decreases from inlet to outlet, and with the increase distance from the inlet, the lateral distribution of normal stress difference becomes more uniform.(6) Based on the proceeding simulations, the fiber suspension flowing through the stock pump impeller is numerically investigated. The main results are:first, in the inlet region, the fibers have not been fully oriented and quickly move to the mid-region; second, in the mid-region, the flow has sufficient time to impose the effects on the fiber movement and the fiber orientation distribution; third, in the downstream region, the fiber orientation distribution are stabilized; fourth, the non-equilibrium distribution of the fiber orientation is most significant in the inlet and the concave wall regions, and it is obvious in the mid-region, while in the downstream region, the fiber orientation distribution tends to quilibrium. Based on the above flow characteristics in the stock pump impeller, four design criterions are obtained, that is, the large radius of curvature on the first third part of the blade, the channel expansion angle criterion, the large wrap angle criterion, and the blade outlet angle criterion.Subsequently, a new stock pump impeller was designed based on the criterions. And the impeller was assembled with the original volute. Through testing, the efficiency reached 72.49%, which is higher than the original machine by 6%, and is 3% higher than the Chinese stock pump standard. Thus the model and the criterions has been successfully validated.