Research On Numerical Simulation For The Mixing Mechanism Of Dilution Water And Pulp Fluid In Dilution Control Headbox

Dilution control headbox is a new part of technology development in paper-making equipment. It is mainly used to adjust the cross direction basic weight profile through feeding dilution water. The cross direction basic weight profile should be ensured uniform during the stock flowing through the headbox, which should avoid fiber flocculation effectively. During the stock flows through the headbox, the fluid should keep uniform distribution in the CD (cross direction) and MD (machine direction).Therefore, the structure and size of all components of the headbox should be in accordance with the principle of hydraulic mechanism in the process of model design. The structures and parameters would influence the flow characteristics in the process of adjusting the CD basic weight profile, and then the dilution water dilution effect will change also. Thereinto, the structures of turbulence generator and nozzle is crucial to the performance and dilution effect of dilution control headbox.In order to study the influence of flow characteristics and structure parameters on the dilution effect in dilution control headbox, in this paper computational fluid dynamics (CFD) was adopted to simulate the mixing characteristics of pulp and dilution water and the status of the basic weight CD profile, then obtain the calculation and comparison of dilution effect according to optimize the structure of dilution system in dilution control headbox. Simulating the adding unit of dilution water and optimizing the structure of turbulence generator and nozzle can provide us with designing basis and control means.First of all, on the basis of the regulation mechanism of dilution water and according to the situation of the technology development of domestic headbox, we analyzed the superiority of dilution water regulation in papermaking process and the impact on the flow characteristics. Moreover, we discussed the process of mixed hybrid mechanism of dilution water and pulp. We introduced the adding method and control system of dilution water. According to the dilution unit, we established mathematical model of two kinds of fluid flow in the dilution control headbox through reasonable assumptions and simplifications.Secondly, we chose the turbulent two-phase fluid computational model in FLUENT that is the closest to actual theoretical model for numerical simulation, in order to study flow characteristics of mixing fluid under different structural parameters. At the same time, we apply flow velocity, pressure, turbulence intensity, turbulent scale and fiber concentration to measure and compare the capability and dilution effect of headbox. On the basis of overall simulation, we improved the structures of the turbulence generator and nozzle, then the size of the improved model was optimized, which made the dilution effect of headbox the best.At last, the test results indicated that the effect on quantitative regulating of dilution water is best at the adding angle of 30°,the adding location away from the entrance of stilling chamber 50mm.The performance of headbox with turbulence generator of widening structure and the nozzle jet with stable passage is better. The influence of traditional structure at fiber flocculence and directional position is poor, which should be improved.By analyzing the mixing mechanism of dilution water and stock flow, combining with the computer numerical simulation technology, this paper systematically studies flow characteristics and the effect on dilution result of stock in adding unit of dilution water, rectifier device and nozzle of dilution control headbox, but still need simulate other types of devices in dilution control headbox, and validate for numerical simulation conclusion.