Research On Stirring Flow Field And Dynamic Concentration Measurement Of Low Concentration Pulp Fiber Suspension Based On PIV

The rapid development of China’s paper industry keeps the higher demand for the papermaking industry equipment and production process.The rheological properties of the pulp fiber suspension was studied to provide the theoretical basis for the optimization design of the headbox in the papermaking equipment.Pulp suspension has been widely used in the textile,chemical,composite materials and other fields,it plays an important role in improving the rheological properties of materials and accelerating reactions between the.pulp and chemicals.In this paper,the rheological properties of pulp fiber suspension were studied by particle image velocimetry(PIV)and MATLAB.As its tracer particles,the flow field characteristics of pulp fiber suspension in magnetic stirring were studied by PIV measurement technique.The experimental results showed that: with the increase of the rotational speed of the magnetic stirrer,the maximum value of the flow field vorticity increased first and decreased later.With agitator rotation speed up to a critical speed N0,it was found that the vorticity peak was increased first and then decreased;From 150 r/min to 450 r/min,the relationships between the vorticity value and the radial distance at each point on the marker line passing through the vortex conformed to the Lorentz function distribution;The area of the (yc-y0)/2(yc is the maximum value of the vorticity,y0 is the minimum value of vorticity)vorticity was reduced with the increase of the rotational speed;With the increase of the distance from the vortex,the turbulence intensity continued to decrease until to the lowest point,then increased,and the turbulence intensity was the largest at the center of the main vortex.A concept of area concentration was proposed,and the knowledge of pixel and grayscale value was introduced in the image processing,which provided a new way to dynamically measure the concentration of pulp suspension.According to the difference of the gray-scale value between the fiber and the non-fiber in the PIV experiment image,the area concentration of the experiment image was calculated by PIV and MATLAB.The result showed that the pulp fiber concentration increased with the rotational speed;At the same rotating speed,the concentration was always fluctuated over time;the rotating speed of magnetic stirrer was the main factor in determining the area concentration.Based on the proven method of calculating the vortex core radius,the experiment was carried out by PIV measurement technique and MATLAB technology.The whirl water line images of pulp suspension under different rotating speed was snapped by CCD after analyzing those images by fitting curve and calculating area concentration by Matlab.The results showed that: At the different rotational speed(600,650,700,750,and 800 r/min),the whirl water line of the pulp suspension conformed to the Lorentz function distribution.The characteristic radius rc and the vortex core radius rm increased with the rotational speed,which met non-linear change rule.