| Fiber suspension flow is widely distributed in the process of pulp and paper-making,textile and composite material production,etc.Investigation of the flow characteristics plays an important role in product quality and efficient operation of the related processes.Supported by the National Natural Science Foundation of China(51309118),an open impeller centrifugal pulp pump is used as a carrier in this paper,and the flexible fiber suspension with different mass concentrations is used as the research medium.The effect of fiber concentration on pump performance and internal flow field under different operating conditions is investigated by different means,i.e.,the external characteristic test system,the Particle Image Velocimetry(PIV),and the high-speed photography system.For fiber particles,the distribution and motion in the pump internal flow field are also analyzed.The coupled CFD-DEM method is used to simulate the flow process of flexible fiber suspension in the pump and to study the flow field and the motion characteristics of the fiber in the pump.The main research work and achievements are listed as follows:(1)The absolute velocity in the middle section of the impeller is obtained by the PIV device,and the velocity conversion program of the PIV device is developed in Python language to convert the absolute velocity into relative velocity.The program is then used to study the relative velocity distribution in the impeller passage when the test pump conveys different mass concentrations of fiber suspension at rated working conditions(1.0Qd).Then the effects of different concentrations of fiber suspension and different flow rates on the relative velocity distribution in L-2 channel are analyzed.The results show that under the working condition,the relative velocity of each passage from the impeller inlet to the outlet tends to increase.Especially when transporting clear water and 0.1% fiber suspension,the relative velocity in the middle part of the impeller passage decreases.However,this phenomenon gradually disappears with the increase of fiber concentration,and the relative velocity in each passage continues to increase along the impeller inlet to outlet.At the same time,the relative velocity gradient in the L-2 passage from the suction surface to the pressure surface direction stabilizes with increasing flow and fiber concentration.(2)The external characteristic curves of the test pump during conveying clear water and different mass concentrations of fiber suspensions are obtained through the external characteristics experiment.Through the analysis of the external characteristics of the test pump results,it is found that when the medium is transported under larger working conditions((above 0.9Q_d),the head and efficiency of the test pump increase with the increase of the mass concentration of fiber suspension.When the mass concentration of the fiber suspension is 0.7%,its efficiency increases by up to 3.40percentage.To conclude,it is largely due to the presence of the fiber particles,the test pump shows an increase in its efficiency,and then it achieves an effect of reduced resistance.(3)During conveying different mass concentrations(0.3%,0.5%,0.7%)of fiber suspension,the high-speed photography system is adopted to measure and analyze the movement and orientation distribution of fiber particles within the test pump.In the impeller passage,the results show that the fiber particles under each working condition are more distributed on the side close to the blade working surface,and which maintain the direction of movement from the inlet to the outlet of the impeller,and the fiber orientation is close to the profile of the blade working surface.The particle group appears around the trailing edge is similar to the weak flow.With increasing flow rate and fiber concentration,it becomes more and more obvious.In the volute,the fiber particles totally maintain the direction of movement along the volute streamline,and the fiber orientation is orientated toward the direction of the streamline,the phenomenon of fiber particle reflux,flip and twist appears in some areas.In addition,bubbles and flocculation occur at the impeller inlet and vary somewhat with the change in flow rate.The increase of fiber concentration and flow rate can reduce the collision between fiber particles and the wall,reduce the phenomenon of fiber particles turning and twisting,and to a certain extent make the fiber distribution more uniform,so that the presence of fiber particles to improve the flow of fluid in the pump state.(4)Based on the CFD-DEM method,a suspended flow model considering fiber flexibility is established,and simulations are performed for pulp pumps conveying clear water and 0.1%mass concentration of fiber suspension at different flow rates.At a higher flow rate,the results show that the existence of fiber particles can effectively improve the flow line distribution and pressure distribution in the pump,improve the vortex in the pump and reduce the wall shear force of the blade and volute wall,to achieve the pump head and efficiency improvement.The motion and orientation distribution of fiber particles in the pump are consistent with the results of high-speed photographic tests,which verifies to a certain extent the feasibility of using CFD-DEM method to analyze the flow field of fiber suspension in the pump. |
