Multi‐scale CFD simulation of operating diagram for gas–solid risers

Abstract

Our recently presented multi-scale computational fluid dynamics (CFD) approach has proven to be able to capture the choking phenomena in a circulating fluidized bed (CFB). However, how to transfer this capability to assist industrial operation remains to be explored. To this end, this paper presents further simulation results over the intrinsic flow regime diagram and the operating diagram for gas-solid risers, showing the variation of flow regimes with gas velocity and solids flux as well as riser height. It is confirmed that the choking in CFB risers, characterized by the saturation carrying capacity and the coexistence of both dense and dilute flows, holds clear-cut definition in hydrodynamics. In physics, both the choking, non-choking transitions, and the critical point in-between are intrinsic nature of gas-solid riser flows; they initiate as functions of gas velocity and solids flux. In engineering operation, however, their appearances vary with the riser height used. As a result, the intrinsic flow regime diagram can be defined by the combination of gas velocity and solids flux, although it is hard to obtain in practice owing to the limitation of riser height. The operating diagram of a CFB should be, accordingly, height-dependent in practice, demanding the riser height as a parameter besides commonly believed gas velocity and solids flux.La méthode CFD (dynamique des fluides par ordinateur) multi-échelle que nous avons présentée récemment a fait la preuve de sa capacitéà décrire les phénomènes d'engorgement dans un lit fluidisé circulant (CFB). Néanmoins, le transfert de cet outil pour améliorer les opérations industrielles demeure un problème ouvert.À cette fin, on présente dans cet article des résultats additionnels de simulation pour le diagramme de régime d'écoulement intrinsèque et le diagramme de fonctionnement pour des colonnes montantes gaz-solides, montrant la variation des régimes d'écoulement avec la vitesse de gaz et le flux de solides ainsi que la hauteur de colonne montante. Les résultats confirment que l'engorgement dans les colonnes montantes en CFB, caractérisées par la capacité de transportà saturation et la coexistence desécoulements dense et dilué, est un phénomène hydrodynamique bien défini. En physique, l'engorgement, les transitions de non engorgement et le point critique entre les deux sont intrinsèques auxécoulements gaz-solides en colonne montante. En génie des procédés, cependant, leur apparition varie en fonction de la hauteur de colonne utilisée. En conséquence, le diagramme de régime d'écoulement intrinsèque peutêtre défini par la combinaison de la vitesse de gaz et le flux de solides, bien qu'en pratique ce soit difficile de les obtenir du fait de la limitation de la hauteur de colonne. Ainsi, le diagramme de fonctionnement d'un CFB devrait dépendre de la hauteur, ce qui exige que la hauteur de colonne montante figure comme paramètreà côté de la vitesse de gaz et du flux de solides communément estimés.

Effects of riser height and total solids inventory on the gas–solids in an ultra-tall CFB riser

“…dense phase at bottom, dilute phase at the top section and a point of inflection dividing the two sections. This discovery was proved by several researchers [16][17][18][19][20][21]. However, a simple exponential shaped distribution in ε b rather than the S-shaped one was observed in some other studies [e.g., [22][23][24][25].…”

“…However, so far, the impact of I v on gas-solid flow characteristics in the riser is still in controversy [12,[16][17][18][19][20][21][22][23][24][25]. Table 2 summarizes the experimental studies and conditions on the I v effect found in the literature.…”

A review of multiscale CFD for gas–solid CFB modeling

“…EMMS/matrix model has basically different governing equations from those of TFM in that its conservation equations are doubled in terms of dense phase and dilute phase. As discussed in Wang et al ( , 2008b and summarized in the following section for the flow regime transition, EMMS-based models, in their own right, allow bifurcation or coexistence of two solutions for one given conditions. The intrinsically sub-grid contribution of structures can hence be captured.…”

Numerical simulation for the loop seal in the circulating fluidized bed and experimental validation

“…However, forces borne upon particles are usually not in balance in the local and transient sense, and hence inertial terms are required besides the original variables in the EMMS model for a CFD based transient simulation [36]. Gasesolid riser flow is characterized by two-phase structures consisting of particle-rich dense phase (marked by the subscript "c") in form of clusters and fluid-rich dilute phase (marked by the subscript "f") [36].…”