Compuitational Fluid Dynamics-based Polyurethane Low-pressure Numerical Simulation And Structure Optimation

A polyurethane low-pressure mixer is one of the key equipment in the pilot line of polyurethane thermal insulation board,and plays an important role in the production of non-halogen flame-retardant polyurethane thermal insulation board.Due to the actual production process,The mass transfer and chemical reaction could be influenced by the fluid flow which material A,material B and non-halogen flame-retardant powder will be mixed inside polyurethane low-pressure mixer.The excellent properties of polyurethane insulation board are influenced by the mixing of the material in the mixer.The small size of the reactor(diameter equal to 60mm),the exiting of the solid particles(non-halogen flame-retardant),wall-clearance is just 1mm and larger particle volume fraction make the flow field test method research difficulties.Based on this,researchers conducted design and optimation of polyurethane low-pressure mixer often according to experience formula and corresponding engineering experience,resulting in poor performance of mixer that can not reach the actual production requirements.In recent years,researchers adapted computational fluid dynamics(CFD)methods to explored fluid flow in the process of design and optimation of various industrial equipments.However,the reported CFD researches on the polyurethane low-pressure mixer’s multiphase fluid field are extremely limited.Taking polyurethane low-ressure mixer as research boject,adopted CFD methods to investigate single-and multi-phase flow field distribution in mixer.The results of this paper will provide theory guidance of design and optimization proposal used in polyurethane low-pressure mixer.Firstly,taking polyurethane low-ressure mixer as research object,using the commercial computational fluid dynamics software Fluent to conduct a 3D numerical simulation of single-phase turbulent flow field in polyurethane low-pressure mixer which aopted the Reynolds-averaged Navier-Stokes governing equations standard turbulence model.The calculation process was easier to convege for the finer-mesh cases in which 318,412 mesh cells which was the most resonable way of mesh generation were included in the calculation domain.The results illustrated that the flow field showed an obvious volatility change in the mixing period of low-pressure mixer,which was conducive to the mixing of the materials and there was no obvious dead zone.Below the position of the mixing head diameter of 1.5 times,material can be fully mixed and the design of the low-pressure mixer’s length was reasonable.Secondly,a unsteady species transport 3D numerical simulation was conducted by using Fluent,standard model,standard wall function and species transport equation(no reactions).The results indicated that fluid flow was in good condition,there was no obvious dead zone and the mixing time of unsteady species transport was 30s.The influence of impeller’s different structure to the species transport process in mixer had been also explored.The mixing effectiveness was best,when the impeller layers,impellers and impeller angel were 8,8 and 15° respectively.A species transport experiment was carried on to validate the reliability of simulation model.A good agreement was obtained between the experimental data of acetone volume fraction and the simulation results in the flow fields of ethanol-ace tone systems,and the maximum relative error was lower than 5%.This suggested that using CFD model to simulate and forecast the process of species transport was feasible and effective in the low-pressure mixer.Finally,it is the first to study fluid-solid two-phase flow in polyurethane low-pressure mixer by using CFD methods.Eulerian-Eulerian Multiphase model,RNG k-ε turbulent model,standard wall functions and Gidaspow model were employed to study the fluid-solid two-phase flow and to predict the pressure at the tip of impeller in mixer.The simulation results showed that the granular low velocity zone appears between impellers and it’s range was enlarged with the increasing of particle size,particle volume fraction,fluid viscosity.Also,improving rotation speed cannot effectively eliminate the low velocity zone.The mixing time of low-pressure with and without circle treatment were 60s,75s respectively.The impeller’s structure optimation results showed that the optimized impeller can effectively dispel the low velocity area in the case of higher fluid viscosity,larger particle diameters,higher particle volume fraction and achieve the goal of material’s completed mixing.Besides,a fitting anlysis of pressure at the tip of impeller was carried out by using orign8.5 in the case of different particle diameter,different particle volume fraction,different fluid viscosity and different rotation speed.The coefficient of determination of the pressure prediction expressions at the tip of impeller under different conditionswere both greater than 0.99,fitting results was good,it’s of some reference value for the mixer erosion wear study and new mixer design and research.