Opaque multiphase flows

The multiphase fluid systems of interest are often opaque, and thus noninvasive techniques based on optical methods or using laser beams are not effective. Various experimental techniques are available and continue to be developed to characterize opaque multiphase flows. 

Dudukovic, M.P. (2001) Opaque multiphase flows experiments and modeling. 4th International Congress on Multiphase Flow, 2001, New Orleans. 

Dudukovic, M.P. (2002), Opaque multiphase flows Experiments and modeling, Experimental Thermal and Fluid Science, 26(6-7) 747-761. 

Research and flow visualisation techniques allow detailed interrogation of mixing patterns and phase distributions in opaque multiphase systems. Recently developed... 

During the last years tomography has obtained intensive research to characterise multiphase flows (Fransolet et al. 2001). EIT is a non-invasive technique that applies to opaque dispersions. In EIT experiments resistivities are measured between the electrodes that cover the part of the walls of the vessel. The continuous phase must be conductive and the difference in conductivity between the continuous phase and the dispersed phase must be distinct. The resistivity distributions are reconstructed to produce three-dimensional images of the resistivity field. Tomography techniques are relatively slow compared to the time scale of flow in a mixed tank so it is not suitable for the determination of BSDs. 

Detailed reviews of the CARPT technique have been given by Devanathan et al. [51, 52], Moslemian et al. [142] and Sannaes [175] including the principles of operation, calibration and discussions of problems/difficulties related to this technique. A brief description of the set-up can also be found in the thesis by Sannaes [175]. Duducovic [55] presented a survey on the use of nuclear techniques to characterize opaque multiphase reactor flows, and Chaouki et al. [36] reviewed the non-invasive Tomographic and velocimetric techniques for monitoring multiphase flows. 

Fluidized beds, slurries and many other multiphase flows are opaque and the powerful, laser based techniques of fluid mechanics can not be used to characterize these systems. However, solids are to some extent transparent to electrical fields and radiation such as 7- and X-rays that may penetrate these multiphase systems [147]. 

The above discussion and examples illustrate that reactor scale models have not advanced much during the past decades and this hinders our ability to reduce risk in implementation of new more efficient catalytic technologies. To make progress, it is necessary to develop improved and more accurate descriptions of flow and mixing in typical multiphase reactors. Multiphase reactors frequendy encountered in practice (Fig. 1.10) such as risers, bubble columns, fluidized beds, packed beds, and stirred tanks are opaque so that not all flow visualization tools (Table 1.6) are suitable. 

Principles and Characteristics The use of acoustic monitoring techniques for process analysis and control is becoming more relevant in industry. Ultrasonic signals have attributes that are well suited for characterisation of multiphase fluids and flows. The signals have the ability to interrogate fluids and dense opaque suspensions, penetrate vessel and process walls, and are not degraded by noisy process conditions because the signal frequencies differ from that of machinery. 

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