Numerical Simulation And Experimental Study On Pulp Flow In Hydraulic Headbox

Headbox is the heart of paper machine, its function is flowing the uniform and stable pulp onto wire along the full width of paper machine, creating good premise for the sheet forming. Hydraulic headbox rectify pulp rely on reasonable structure, therefore, each component structural design should be very consistent with the principles of fluid mechanics, in which design of rectifying device is the key, directly determines the work performance of hydraulic headbox.Based on the hydraulic headbox mechanism analysis and structural design of the lessons learned, this study starts from pulp flow characteristics and mechanism of fiber dispersion, numerically simulates the flow inside rectifying device and nozzle using computational fluid dynamics software, predicts the flow characteristics inside hydraulic headbox, optimize the structure and size of rectifying device and nozzle according to the simulation results, designs and manufactures the experimental setup, tests the flow field of step diffusers for verifying the optimization program, provides a reliable basis to guide the hydraulic headbox design and optimization.First, the pulp suspension is a three-phase coexistence of complex distributed system, its flow characteristics change with the nature of the pulp, concentration and flow rate. At higher concentrations and lower flow conditions, the fibers flocculate easily. In turbulent flow pattern, fiber flocculations disperse by shear force of turbulence. By analyzing mechanical properties of fiber network and flocculations, critical shear stress calculation formulas of decentralizing fiber networks and flocculations are concluded, the necessary condition of pulp fibers dispersion is sumed up.Second, this study contraposes the structural characteristics of hydraulic headbox, probes into the structure and action mechanism of each component, sums up the hydraulic headbox design requirements, discusses the tapered header design principle and calculation formula used in the production, gives design points of pulp rectifier such as orifice plate, tube and step diffuser, analyses the impact of nozzle structure on the jet angle and distance between jet nit point and lip mouth, introduces control mode on wire speed setting and lip aperture opening.Third, evaluation criterion is raised by analyzing the turbulent flow pattern inside headbox, computational fluid dynamics software FLUENT 6.2 was used to simulate internal flows of hydraulic headbox, for different structure of rectifying device and nozzle, flow characteristics in the hydraulic headbox was forecasted changing in inlet velocity and geometry, adopted turbulence intensity and velocity distribution as evaluation criterion to evaluate verifying effect of various factors, further provided guidence on hydraulic headbox structure optimization.Fourth, as an efficient rectifying component, step diffuser plays a crucial role on the complex flow field inside headbox. Step diffuser performance test device was set up, the particle image velocimetry system was used for the flow field tests, velocity distributions of axial cross section, resistance loss and cavitation were obtained, impact of inlet velocity and step diffuser shape, size parameters on the paper quality were analyzed in order to guide the structural design of step diffuser.Fifth, low pressure areas appear in the rectifier device, cavitation bubbles flow with pulp onto forming net, directly affect the quality of paper. Therefore, analysis of area distribution of low pressure is essential, occurrence of cavitation could avoid by improving the structure design and reasonable speed setting.The test results indicated that step diffuser with square tube is more suitable for high-speed paper machine, and appropriate high speed, geometric shape and size are guarantee of quantitatively uniform and evenness good paper. At high speed and unreasonable structure design cases, prone to occur, high air concentration will lead to loose paper structure, cavitation should be avoided.By fluid dynamics analysis, combined with computer numerical simulation and particle image velocimetry, this paper systematically studies flow characteristics inside the the rectifier device and nozzle, but still need simulate other types of the rectifier device, test overall headbox, and validate for numerical simulation conclusion.