Research On Complex Interal Flow Of Fiber Suspension And Turbulence Generator For Medium Consistency Pulp Conveying

Fiber suspension flow is widely applied to paper industry, chemical industry, composite materials production and many other areas. Fiber suspensions internal flow is divided into simple and complex flow, simple internal flow including pipe flow, channel flow, wedge-shaped contraction flow; a typical of complex internal flow is the flow in a rotating impeller. As the properties of fiber suspensions mostly rely on the orientation distribution of fibers, the researching of the flow of fiber suspensions in coupling effect of suspending fibers and turbulence is the key point and difficult point. This thesis researches on the complex internal flow of fibre suspensions applying theoretical, numerical and experimental methods.Rheological properties of fiber suspensions flowing through turbulent pipe flows are studied numerically. The fluctuating equation for fiber orientation distribution function (FODF) in turbulent flows is derived and solved using the method of characteristics. By numerically solving the equation in a turbulent pipe flow with Reynolds number ranging from 2500 to 11000, the mean FODF is obtained which is in agreement with the experimental one qualitatively. Then the shear stress and first normal stress difference of suspensions are calculated based on the mean FODF.For dilute and concentrated suspensions the fiber orientation distributions are simulated numerically with Lattice Boltzmann method in fiber suspensions flow through a tube containing a sphere. In the simulations, the interactions between fibers, fiber and sphere, fiber and tube wall are taken into account. The numerical results of orientation distribution are in agreement with the experiment performed in a channel containing a cylinder qualitatively. The results show that the existence of sphere in the tube can change the fiber orientation in the downstream region of the sphere along the flow and transverse directions, that’s because of the stretching and shearing effect caused by the sphere. The effects of the sphere on the fiber orientation distribution are more significant for the concentrated suspensions than for the dilute one. The fiber orientation distribution in the upstream of the sphere is greatly influenced by the initial orientation at inlet, whereas no apparent difference in the fiber orientation in the downstream of the sphere is observed.The visualization measure of fiber suspension internal field in a curved expansion duct is experimentally studied using particle image velocimetry (PIV) technology.The results show that fiber orientation is mainly affected by velocity gradient, the fibers near the wall are aligned with the flow direction more quickly than the fibers in intermediate region, and the fibers near the concave wall are more quickly aligned with the flow direction than the convex wall. The larger inlet flow rate which will accordingly lead to increase inlet velocity enables the more quick adaptation and steady of fibers in flow direction.Finally, based on the research on the flow characteristics of the middle-consistency (MC) pulp, a new turbulence generator of MC pulp pumps are invented and optimum designed, the turbulent flow field of MC pulp suspension based on computational fluid dynamics (CFD) technique is simulated. The results showed that the MC pulp fluidized quickly by optimized turbulent generator before drawing-in the MC pulp Pump.