Solid flow pattern visualization

APPLICATION OF A MONTE CARLO METHOD TO THE SOLID FLOW PATTERN VISUALIZATION IN CFB... 

Application of a Monte Carlo Method to the Solid Flow Pattern Visualization in CFB... 

Especially for multiphase systems flow visualization (Wen-Jei Yang, 1989 Merzkirch, 1987) can provide valuable initial information on the prevailing flow patterns and should at least always be considered as a first step. Of course, in applications that involve extreme conditions such as high temperature and/or pressure it is very difficult if not impossible to apply flow visualization and other techniques should be considered. Here the use of cold flow models which permit visual observation might be considered as an alternative as an important first step to obtain (qualitative) information on the flow regime and associated flow pattern. Of course, multiphase flows exist such as dense gas-solid flows that do not permit visual observation and in such cases the application of idealized flow geometries should be considered. A well-known example in this respect is the application of so-called 2D gas fluidized beds to study gas bubble behavior (Rowe, 1971). 

A schematic diagram of the experimental apparatus is shown in Fig. 1. A glass apparatus was used so that the flow and solid formation could be visually observed. This precluded using high upstream pressures. It was felt, however, that preliminary information where visual observations could be made would aid in understanding the flow patterns and the formation of solids. 

Figure 17 Variation of instantaneous heat transfer rate with bubble passage synchronized with the visualization of flow patterns in the vicinity of the heat transfer probe in a liquid-solid fluidized bed of low-density gel beads. (From Kumar et al., 1993a.)...

At high solids concentrations, visual observations become difficult The solids screen most of the vessel from view. If a process with a high solids concentration is being investigated, observations at lower solids concentrations will aid in understanding the mixing mechanisms. However, observations should stiU be carried out at high solids concentration since the flow patterns in the vessel often vary considerably with solids concentration. 

The flow patterns of bulk solids while discharging from silos have been visualized by many authors, e.g. Johanson [5] and Schwedes [7]. Usually the investigations were carried out with optical access, using tranclucent silo walls and filled with material in different colors laid in layers into silos. Photographic techniques have been uses by Rotter et al. [6] which have been transferred to the use of CCD cameras with which measurements were carried out by Bosley et al. [1] and Carson et al. [2]. These techniques have in common that the displacement of particles is measured between consecutive images and thereby determining the velocity. 

The iirteraction of a fluid flow with the surface of a solid body is a subject of great interest. Matty technical measurements are aimed to determine the shear forces, pressure forces, or heating loads apphed by the flow to the body. A possible means of estimating the rates of momentum, mass, and heat transfer is to visualize the flow pattern very close to the body surface. For this purpose, the body surface can be coated with a thin layer of a substance that, upon the interaction with the fluid flow, develops a certain visible pattern. This pattern can be interpreted qualitatively, and in some cases, it is possible to measure certain properties of the flow close to the surface. Three different interaction processes can be used for generating different kinds of information. 

Dynamic mechanical analysis in polymeric multiphase systems in solid state, as part of rheology, is associated with oscillatory tests that are employed to investigate all kinds of viscoelastic material from the point of view of flow and deformation behavior. In particular, it evaluates the molecular mobility in polymers, the pattern of which may be an indication of phase-separated systems. Although there are certain preferred tools for visual examination of phenomena for these kinds of systems, dynamic mechanical analysis has the advantage of examination in dynamic conditions and of the prediction of properties. 

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