Research On The Flow Behavior And Control Of Fiber Orientation Distributioin In The Turbulent Fiber Suspension Flowing A Contraction Channel

Fiber suspension, which is the mixture of carrier flow and fiber particle, belongs to the liquid-solid two-phase flow typically. With the development of fiber-reinforced technology in modern industry, its significant value is becoming more and more obvious. The contraction is a common kind of internal flow field, mainly for the flow acceleration, such as jet nozzle. Different from the channel flow, acceleration effect and the shrinking streamline lead to a higher alignment of fiber orientation inside the contraction than the one in the channel under the same entrance conditions. The anisotropic orientation of fibers can significantly affect the performance of the corresponding products. So kinetics and orientation mechanism of fibers, constitutive relation of suspension, relevant prediction model and controllable orientation are of the importance not only for the theoretical research, but also for the industrial application.At present, the dynamic simulation of fiber suspensions is mainly using one-way coupling method, namely the fiber does not affect the turbulence. Such simplification generally applies only to dilute phase. The influence of fiber over turbulent flow varies from its concentration and scale especially in the semi-dilute phase and dense phase. When the extra-stress of the additives is roughly quivalent to or much greater than the viscous stress of fluid, suspensions may exhibit non Newtonian characteristics relying on the orientation of fiber. Thus the one-way coupling model does no longer work. Furthermore, the engineering application of fiber suspensions more and more concerns non-dilute phase structure in recent years, it is necessary to establish the appropriate two-way coupling turbulence model of suspensions. Therefore, combined with the existing achievements of the experimental and theoretical research, the more in-depth discussion and research is made below about the fiber orientation distribution calculation, the fiber coupled turbulence mode theory, controllable orientation approach and other related issues in the contraction.Fiber orientation probability distribution model is constructed based on the continuum theory and the Reynolds averaged model, in which rotational diffusion coefficient is allowed to change with the evolution of turbulence structure. Then fiber orientation distribution is obtained by the model above. The simulation results agree well with the Parsheh’s experimental data.Due to the anisotropic characteristics of the contraction turbulence, the Reynolds stress mode (RSM) is used. According to Batchelor’s constitutive theory, suspension RSM with coupling fibers’extra stress produces more unknown items than Newton fluid. Considering that the final derived results would have the same form as Newton stress tensor, these items can be expressed as the additional diffusion term and the additional viscous dissipation term. Then according to the modeling ideas for Reynolds stress mode, closed algorithm is provided for them. At the same time, in view of the solution complexity of the coupling dynamics modeling, successive iteration method is used to realize the numerical simulation.of Fokker-Planck equation and the coupled RMS model under a low computational cost. Based on this model, the influences of the fibers concentration and aspect ratio on the flow velocity profile, turbulence characteristics and the fiber orientation distribution are discussed by numerical calculation.A suitable contraction shape can realize the control of its anisotropic orientation as fibers is fully subject to the velocity gradient in the large contraction ratio flow. In order to meet the accuracy requirements of turbulent kinetic solution, parameterized shape curve, dynamic grid and quasi-static assumption are adopted to model the contraction with the Variable boundary and search the solution. Similarly to overcome the turbulence suppression under the tensile effect in the large contraction ratio flow, the longitudinal, transverse moving and small swing of turbulence rib are studied to understand their effects on the evolution of fibres orientation distribution, mean flow and turbulence with the dynamic grid when turbulence intensity generated by the stationary spoiler is not high and not easy to control.