Discrete Particle Simulation of Riser Flow

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Toshitsugu Tanaka, Takashi Noda and Yutaka Tsuji
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Powder Flow
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It is desired to understand the flow with particle clusters, because particle clustering has a definite effect on transport, phenomena in risers of circulating fluidized beds (CFBs). For example it is known that particle clustering largely increases the particle slip velocity against gas, consequently, the pcarticle residence time in the riser is largely changed from that of an isolated particle. In addition, properties of gas turbulence must be significantly modified due to the clusters. Frorn the view point of application to industrial facilities, macroscopic models for predicting flows in the risers should be developed finally. On the other hand, to develop such models, it is important to study the physics or the dynamics of the clusters from both of the experimental and numerical stand points.

According to this context, Tanaka et al., (1998) have carried out the first, year project. They applied their numerical model, in which inviscid gas and a stochastic particle-particle collision model were assumed (Tanaka et al., 1995), in the three-dimensional flows corresponding to the experiments by Louge et al. (1999). They found that their numerical model is capable of predicting the fully developped cluster flows, and examined the effects of pressurized gas condition on the flow structure. Furthermore, they evaluated the quantities for characterizing the cluster structure, such as number density distributions of cluster diameter, probability density functions of solid volume fraction, etc.

In the second year project, quantitative comparison between the sirnulation and the corresponding experiment at Cornell University was intcndcd. The collaboration between Osaka and Cornell Universities began with discussions at the Brighton Annual Meeting in July 1998. There, Tanaka asd Louge proposed to compare the predictions of the numerical simulations at Osaka with solid volume fraction mcasurcments carried out at Cornell. The Cornell group obtained solid volume fraction with an optical fiber bundle in the fully-developed region of the Cornell riser. The Osaka group then carried out the numerical simulation at the conditions of the Cornell experiments. They then compared the probability density functions and power spectra of the data sets and the corresponding simulations.

In usual industrial applications, particles do not have a uniform diameter but a wide distribution. Tarmka et al. (1995) performed Lagrangian/Eulcrian numerical simulations of two-dimensional cluster flows, and found that the spatial scale of cluster structure largely depends on the particle diameter. Therefore, it is expected that the particle size distribution may affect the flows. The Osaka group has ttaken particular care to match the particle size distribution of the experimental powders.

Tanaka then visited Cornell in March 1999 to prcscnt these comparisons and to discuss details of the simulations with Profs. Louge, Jenkins, Koch and their respcctive collaborators. The status of their collaboration was reported at the Spring TC meeting in Newark, NJ, and at, the Annual Meeting in Somerset, NJ.