Study On The Rheological Properties Of Pulp Fiber Suspensions And CFD Simulation Of Pulp Flow In Special Shaped Channel

The development of papermaking industry put forward the higher request of pulp & paper equipment and technology in China. According to the characteristics of the pulp fiber suspension, the structure optimization of paper machine headbox is of great significance, this article mainly aims at the rheological properties of the pulp fiber suspension and its flow behavior in special shaped channel through computer simulation study.This study independently design and processing a set of complete experimental visualization headbox system, the experiment result with visual headbox system in the application of PIV measurement is good, it can be used in the CFD simulation verification.This study presents a novel device based on an oscillating torsion resonator with continuously varying frequency capability to characterize the rheological properties of pulp fiber suspensions. Fiber suspensions made from unbleached wheat straw pulp at concentrations ranging from 0.1 wt% to 1 wt% were used. Diluted pulp fiber suspensions exhibit water like Newtonian behavior. PIV measurements were performed in a magnetic stirring flow, tracking a PSt tracer suspension and a dilute cellulose suspension in two sets. The measurements show that the streamwise velocity and swirling strength of cellulose fibers is lower than the liquid flow velocity.The special shaped channels of step diffser and turbulent generator in headbox play a key role in this study, for the reason that these structures can generate micro-turbulence.Realizable κ-ε turbulence model, the SIMPLEC algorithm and second-order windward format were used for CFD simulation of flow in step diffuser and turbulent generator, the comparison with PIV measurement results shows that the model can well reflect the actual situation of flow in the channels. Simulation results show that the sudden expasion at the step diffuser can enhance the turbulent kinetic energy and turbulent kinetic energy dissipation rate of flow; the size of sudden expansion in step diffuser must be big enough to cause micro-turbulence effectively,but not too large to cause dead section. Effects of the taper of turbulent generator on the turbulent kinetic energy and turbulent kinetic energy dissipation rate in the axial line are not significant. Flow in turbulent generator with no taper or large taper is uneven.CFD simulation was performed using mass transfer model based on CFD models in previous sections. Simulation results show that, structure of step diffuser can shorten the flow length that dilution water completely mixed needed, a “step” feature appeared in the pulp concentration curve. In the flow after the sudden expasion, the pulp concentrations in cross section horizontal center line of step diffuser curve in "V" shape, its distribution accords with the characteristics of gaussian model. The pulp concentration in the center of step diffuser outlet decreases linearly as the dilution water injection rate increases, the pulp concentration in the center of step diffuser outlet increases linearly with the increase of concentration of diluted water injection. When dilution water is mixed in turbulence generator, the pulp concentrations in cross section horizontal center line of turbulent generator curve in "W" shape, its distribution accords with the model proposed by D.G orinevsky.