Gas-solid flow

Total pressure drop for horizontal gas/solid flow includes acceleration effects at the entrance to the pipe and fric tional effects beyond the entrance region. A great number of correlations for pressure gradient are available, none of which is applicable to all flow regimes. Govier and Aziz review many of these and provide recommendations on when to use them. 

FIG. 12-33 Cocurrent gas-solids flow in a vertical-lift dilute-phase pneumatic conveyor. 

FIG. 12-860 Countercurrent gas-solids flow at the top disengaging section of a moving-bed catalytic reactor. 

For example, if the entering solids fraction tp is 0.4, the corresponding values of the local solids fraction relative slip velocities (Vr) of 0.01, 0.1, and 0.5 are 0.403, 0.424, and 0.525, respectively. There are many theoretical expressions for slip, but practical applications depend on experimental observations and correlations (which will be presented later). In gas-liquid or gas-solid flows, (pm will vary along the pipe, because the gas expands as the pressure drops and speeds up as it expands, which tends to increase the slip, which in turn increases the holdup of the denser phase. 

Fan L-S, C Zhu. Principles of Gas-Solid Flows. London Cambridge Univ Press, 1998. 

The scope of coverage includes internal flows of Newtonian and non-Newtonian incompressible fluids, adiabatic and isothermal compressible flows (up to sonic or choking conditions), two-phase (gas-liquid, solid-liquid, and gas-solid) flows, external flows (e.g., drag), and flow in porous media. Applications include dimensional analysis and scale-up, piping systems with fittings for Newtonian and non-Newtonian fluids (for unknown driving force, unknown flow rate, unknown diameter, or most economical diameter), compressible pipe flows up to choked flow, flow measurement and control, pumps, compressors, fluid-particle separation methods (e.g.,... 

Obviously, these two items are not strictly separated in contrast, the most fruitful approach is when they are simultaneously followed, so that they can mutually benefit from each other. In this chapter, we want to focus on the use of simulation methods as a design tool for gas-fluidized bed reactors, for which we consider gas-solid flows at four distinctive levels of modeling. However, before discussing the multilevel scheme, it is useful to first briefly consider the numerical modeling of the gas and solid phase separately. 

The prime difficulty of modeling two-phase gas-solid flow is the interphase coupling, which deals with the effects of gas flow on the motion of solids and vice versa. Elgobashi (1991) proposed a classification for gas-solid suspensions based on the solid volume fraction es, which is shown in Fig. 2. When the solid volume fraction is very low, say es< 10-6, the presence of particles has a negligible effect on the gas flow, but their motion is influenced by the gas flow for sufficiently small inertia. This is called one-way coupling. In this case, the gas flow is treated as a pure fluid and the motion of particle phase is mainly controlled by the hydrodynamical forces (e.g., drag force, buoyancy force, and so... 

C. The Multi Level Modeling Approach for Gas-Solid Flows... 

In Section I, we mentioned that the TFM can simulate fluidized beds at engineering scales (height 1-2 m), and that the large-scale industrial fluidized-bed reactors (diameter 1-5m, height 3-20m) are still far beyond its capabilities. Clearly, it would be highly desirable to predict the properties of gas-solid flows at the industrial scale however at present, there is no fully evolved model— based on fundamental principles—which is capable of this. In this section, we outline some new ideas in this direction that have been developed both at the... 

Princeton University and at the University of Twente. Before doing so however, it is first important to understand why the current class of TFMs is not suitable for describing large-scale gas-solid flows. 

In this chapter, we have discussed three levels of modeling for dense gas-solid flows, with the emphasis on the technical details of each of the models, which have not been published elsewhere. Up till now, the models have mainly been used to obtain qualitative information, that is, to acquire insight into the mechanisms underlying the gas-solid flow structures. However, the ultimate objective of the multiscale approach is to obtain quantitative... 

Thus, just as for incompressible single-phase flow, the pressure p constrains the velocity fields to ensure (in the case of multiphase flows) that the sum of the phase volume fractions equals unity. In the presence of mass transfer, the right-hand side of Eq. (148) is nonzero nevertheless, the role of the pressure is still the same. Finally, we should note that in gas-solid flows the maximum volume fraction of the solid phase is less than unity due to physical constraints (i.e., when particles are close packed there is still room for the gas phase so that 0solid-pressure term ps that becomes extremely large when ag approaches its minimum value (e.g., oc — 0.4). 

Mirzaie, H., and N. Syred. 1989. The use of multi inlet cyclone combustor to minimize NOj formations. 3rd Symposium (International) of Gas-Solid Flows— 1989 Proceedings. San Diego, CA (presented at the 3rd Joint ASCE/ASME Mechanics Conference). 

D. Mills, P. Marjanovic, J.S. Mason, An analysis of line pressure gradient for dense phase vertical gas-solids flow, Proceedings of the GAMM Congress, Dubrovnik, Yugoslavia, April 1985. 

P. Marjanovic, D.J. Mason, Gas—solid flows in an inclined pipeline, Proceedings of the 1st International Particle Technology Forum, vol. 3, Denver, USA, August 1994, pp. 466-471. 

R.J. Hitt, P. Marjanovic, A computer technique to develop a model of vertical dense phase gas-solids flow from experimental data. Proceedings of the 1st International Symposium on Two-Phase Flow Modelling and Experimentation, vol. 1, Rome Italy, October 1995, pp. 67-74. 

T. Mooney, A. Levy, P. Marjanovic, D.J. Mason, An investigation of gas-solids flow through inclined pipes, Proceedings of the 1997 Jubilee Research Event, vol. 1, Nottingham, UK, 1997, pp. 425 428. 

A. Levy, T. Mooney, P. Maijanovic, D.J. Mason, A comparison of analytical and numerical models with experimental data for gas—solid flow through a straight pipe at different inclinations, Powder Technol. 93 (1997) 253-260. 

J.S. Xiang, P. Maijanovic, Hydrodynamic model of gas-solid flow in circulating fluidised bed, Proceedings of the 7th International Conference on Bulk Materials Storage, Handling and Transportation, Newcastle, Australia, October 2001, pp. 825-832. 

D.J. Mason, J. Li, A novel experimental technique for the investigation of gas-solids flow in pipes, Powder Technol. 112 (2000) 203-212. 

Z. Mindziul, A. Kmiec, Modeling gas-solid flow in a pneumatic-flash dryer, Drying 96 Proc. 10th Int. Drying Symp. A (1996) 275-282. 

Optical techniques were developed - both in situ and on line - over a broad range of applications and various types of light sources. Main examples are solids concentration measurements in particle fluidisation [6,7], as well as gas-solid flow [8], or sample composition in dry blending operations, this latter beginning from the pioneering work of Harwood et al. [9] up to more recent advances in this scientific field [10-12], The main interest of these techniques lays in the fact that optical probes are easily available on the market, so that qualitative monitoring of mixers is possible at an industrial scale. However, these probes only provide local information of the mixture (typically 1/40 of a tablet), so that in essence, they still sample the powder flow in a way that may be intrusive and not always representative of the overall stream. 

T. Loser, M. Geweke, D. Mewes, Measuring the Phase Distribution in a Transient Gas—Solid Flow Using Capacitance Tomography. 3rd Israeli Conference for Conveying and Handling of Particulate Solids, Dead Sea, Israel, 2000, pp. 15.11-15.15. 

The EMMS model was proposed for the time-mean behavior of fluidized beds on the reactor scale. A more extensive application of the EMMS model to gas-solid flow is through its coupling with the two-fluid CFD approaches, which brings about an EMMS-based multi-scale CFD framework for gas—solid flow. For this purpose, Yang et al. (2003) introduced an acceleration, a, into the EMMS model to account for the... 

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