Study On Flow Behavior Of Fresh Concrete During Pumping Using CFD Simulation

Concrete pumping technology has become one of the most important and most commonly used techniques in modern engineers.However,the theoretical study of concrete pumping and the prediction of pumping loss mainly depend on empirical formulas or full-scale pumping tests.While the empirical formulas are not accurate in some cases,and experiments cost a lot of manpower and material resources,it is difficult to get accurate internal information of pump.Therefore,based on the numerical simulation method of Computational Fluid Dynamics(CFD),this paper simulated fresh concrete pumping process by computer,and provides a solution for effectively predicting the pumping pressure loss,and analyzed the flow behavior of concrete during pumping.Firstly,the accuracy of the simulation results under different simulation methods was verified.The selection of the turbulence model has a significant impact on the CFD simulation of concrete pumping.The results showed that the simulation results based on the SST k-? turbulence model were better,and the error of pressure loss simulation results against the experimental results in the literature was within ±20%.Based on the simulation results of SST k-? model,the effects of rheological properties of concrete on flow pattern and pressure loss were studied.The results showed that there were three regions in the pipeline: plug layer,shear layer and lubrication layer.The thickness of the lubrication layer was not related to the rheological properties and flow rate.The thickness of the shear layer depended mainly on the yield stress and decreased with the increase of the yield stress.The higher flow rate could lead to the increase of the thickness of the shear layer and the decrease of the effect of yield stress.And the velocity profile of concrete with higher yield stress and lower viscosity was more uniform and smoother.In addition,the simulation results showed that the pumping pressure loss was greatly affected by viscosity,and decreased with the increase of yield stress,increased with viscosity increased.Secondly,based on the SST k-? model,the 116 m long full-scale pumping test was simulated.The results show that the error of simulated pressure distribution and pressure gradient results compared with experimental results were less than 10%.The influence of the elbow and the rheological properties of the concrete on velocity profile and pressure loss was studied.The results showed that the velocity profile in the upstream straight pipe change in advance and recover after a long distance in the downstream straight pipe.And the disturbance distances were affected by the rheological properties of concrete.The velocity profile in elbow section was divided into three regions: the lubricating layer,the shear layer and the plug-like flow layer,wherein the thickness of the plug-like layer decreases with the increase of the viscosity or flow rate,and increases with the increase of the yield stress,decreasing as the curvature radius of the elbow section increases.The pumping pressure loss decreases with the increase of the yield stress,and increases with the increase of the viscosity.The concrete in upstream straight pipe section was affected by the elbow structure,thus the influence of the yield stress on pressure loss increased.Finally,based on the above CFD simulation method,the Eulerian multiphase flow model was introduced to simulate the distribution and migration of particles in the pump tube.The results showed that the prediction error of pumping pressure loss was within ±20%.The particles migrate from the tube wall to the center in the horizontal straight pipe and maintain a constant mass fraction in the plug layer region.The migration of particles in the curved section and the downstream straight pipe section was a slow progress.The larger the particle diameter,the greater the mass force of one single particle,and the greater the degree of outward deflection due to the influence of the curved pipe.