Discrete Particle Simulaiton of Gas-Solid Flow-Effect of Inter-Particle Collision

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Y Tsuji, T Tanaka S Yonemura and Y Yamamoto
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Powder Flow
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This final report describes the method and results of discrete particle simulation which we have been developing under the support of IFPRI from 1994 to 1997. Calculations have been performed for dispersed gas-solid flows where particle concentrations are so high that particle-to -particle collision is essential. The method is based on calculation of trajectories of individual particles. In this report the method is described first, and then some results of a preliminary calculation are shown. Main calculations made in three years are classified into three cases. Calculations of the first case were made for comparison of our method with those by conventional numerical analysis based on a continuum model. The second case was made to investigate structure of gas-solid flows in detail. The third case concerns turbulence modification due to particles. The most important aspect of our work is that the method uses very simple equations of motion and yet it succeeds in producing complicated phenomena in gas-solid flows. It is shown that particle clusters in a riser can be explained by repeated in-elastic collision. Since cluster generation was predicted by another numerical method by anothe,r group. Their method is based on the model which regards the mixture of gas-solid as being consisted of two fluids. To compare both our method and their method, we performed the discrete particle simulation under the same conditions as their work. In these calculations we clarified similarity and dissimilarity of results between bothmethods. It is shown in the second case that the existence of such clusters causes large scale turbulent flow. In the third case, we extended OUT numerical analysis to analysis based on large eddy simulation. In the firstand second cases, the equations of gas motion based on inviscid gas, and thus fluid phase does not have any turbulence in the absence of particles. Thus, as long as we use the method based on inviscid gas, we are not able to predict turbulence phenomena near the wall. We used the large eddy simulation technique to consider the turbulence of gas phase. The empirical constant used in the present LES is the same as has been established in single phase flows. Like the first and second cases, we took into account the particle-particle collision. We found that the effects ofparticle-particle collision on velocity fluctuation and concentration of particles are significant even in dilute phase flows (volume fraction is the order of O(lO-'). Most researchers consider that the particle-particle collision can be neglected at such a low concentration. Turbulence suppression due to particles was also observed in the calculation as in experiments.

Finally in this report the future work which should be made as an extension of the present work is briefly described. First, the importance of the work combining the discrete particle model and the continuum model is described. Second, the effects of inter-particle collision on flow structure should be made in more detail.