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Spreading of granular pastes: from the particle to end use properties

Subject Area Mechanical Process Engineering
Term from 2019 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 431419392
 
Final Report Year 2024

Final Report Abstract

Pastes are highly concentrated particle suspensions which are important for many products in the chemical and food industries. The aim of this research project was to describe the complex rheological properties of pastes using physical based models to predict the spreading behaviour by the mechanical vibration. The spreading behaviour of pastes was investigated in the cooperation project using experimental and numerical methods. New measurement methods were developed to determine the particulate interactions in the fluid in order to obtain the parameters of the contact models for the Discrete Element Method (DEM). The influence of the fluid and the surface roughness of particles and apparatus walls on the restitution coefficient of microparticles was investigated experimentally and the Davis model for the "wet" restitution coefficient as a function of the Stokes number was extended with measurable parameters of the surface roughness. Furthermore, a new method was developed for parameter determination and validation of the DEM in order to apply marking points to microparticles for rotation determination. The calibration of friction and cohesion parameters was realised by measuring the angle of repose in the fluid. The rheological behaviour of particulate pastes was investigated experimentally with rheometer measurements and numerically with coupled CFD-DEM simulations. The flow functions of pastes made of glycerine and glass beads as well as pastes with Al2O3 and TiO2 particles were measured for different particle concentrations. The results of coupled CFD-DEM simulations explained the thixotropic flow behaviour of pastes by the particle movement and segregation. The spreading behaviour of glass beads on a vibrating plate was investigated for the dry case and for pastes. In the dry case, the final angle of repose depends on the applied vibration energy. By increasing the amplitude, the stationary angle of repose is reached more quickly. The DEM simulations showed that a shear band is created on the outer area of the pile during vibration, which leads to tangential particle movement. The investigations of pastes under vibration showed that the mechanical vibration energy that is needed for spreading of the paste decreases with increasing liquid content. By implementing a new dynamic liquid bridge model in the DEM, it was possible to reproduce the experimental spreading behaviour and investigate the contact behaviour of particles during vibration. Based on the simulations, the minimum vibration energy at which the spreading of the paste begins was determined.

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