Multiple-bubble-size models

A non-isothermal dynamic model has been developed for a shallow fulidized bed combustor, which can be used to predict, at least qualitatively, the transient and steady-state characteristics of such systems. Parametric studies have been conducted to examine the effects of excess air flow rate, bubble size and carbon feed rate. It has been shown that an appreciable carbon concentration gradient does exist in the bed. This explains why it is necessary to use multiple feed points in large fluidized bed combustors. A surprising result obtained is that the temperature iii the bed is essentially uniform under all conditions studied even though the carbon concentration is not uniform laterally. 

Lo [91, 92, 93[ developed the extended two-fluid model named the MUSIG (MUltiple-SIze-Group) model in which the dispersed phase is divided into multiple groups. In the original two-fluid MUSIG model only one set of continuity and momentum equations are considered for the dispersed phase. The model describes the dynamic evolution of a number of bubble size groups in... 

The bubble size distribution resulting from purely random binary coalescence is well-represented by the geometric and exponential distributions. But these distributions completely miss the behavior of measured bubble size distributions that show relatively fewer of the smallest bubbles and more of the very largest ones. But a simple cluster coalescence model follows the Pareto distrihution, which matches these characteristic trends quite well. We conclude that multiple bubble interaction... 

In the furnace model we can trace multiple bubbles with varying sources, location and sizes to see if they end up in the product. If they end up in the product, we can compare their size and composition with measured compositions fhtm bubbles in the real product. Based on this... 

Let us consider a shallow fluidized bed combustor with multiple coal feeders which are used to reduce the lateral concentration gradient of coal (11). For simplicity, let us assume that the bed can be divided into N similar cylinders of radius R, each with a single feed point in the center. The assumption allows us to use the symmetrical properties of a cylindrical coordinate system and thus greatly reduce the difficulty of computation. The model proposed is based on the two phase theory of fluidization. Both diffusion and reaction resistances in combustion are considered, and the particle size distribution of coal is taken into account also. The assumptions of the model are (a) The bed consists of two phases, namely, the bubble and emulsion phases. The voidage of emulsion phase remains constant and is equal to that at incipient fluidization, and the flow of gas through the bed in excess of minimum fluidization passes through the bed in the form of bubbles (12). (b) The emulsion phase is well mixed in the axial... 

Shah YT, Kelkar BG, Godbole SP, Deckwer W-D (1982) Design parameter estimations for bubble column reactors. AlChE J 28(3) 353-379 Shi J, Zwart P, Frank T, Rohde U, Prasser H (2004). Development of a multiple velocity multiple size group model for poly-dispersed multiphase flows. Aimual Report 2004. Institute of Safety Research, Forschungszentrum Rossendorf, Germany... 

In monolith reactors, the distribution of fluid into the channels is typically at least somewhat uneven [9] this is why it is very important to predict the flow distribution and include it in the quantitative modeling. Experimental techniques can also be used to study the flow distribution in monolith channels this method is introduced in Figure 9.9 [10]. CFD calculations make it possible to obtain the flow characteristics of the experimental system. In this case, the calculations were performed using the software CFX.4.4 [7]. The flow profiles in the gas and liquid phases were described by the turbulent k-e method (320,000 calculation elements), and to evaluate the distribution of gas bubbles, the multiple size group method was applied. The results from the CFD calculations gave the flow velocities for gas... 

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