(583r) Stability-Constrained Multi-Fluid CFD Models for Multiphase Reactors

This presentation talks about the stability-constrained multi-fluid model (SCMF) for bubble column and fluidized bed reactors. It is recognized that mass and momentum conservative equations are still inadequate to acquire a complete description of the multi-scale structures in multi-phase complex systems. The relationship between meso-scale structure parameters and meso-scale energy consumption, and the stability condition reflecting the compromise between different dominant mechanisms might be indispensable for each specific complex system. The stability condition can be proposed as the minimization of micro-scale energy dissipation, and meanwhile the maximization of meso-scale energy dissipation. Because direct coupling of the stability condition into Eulerian-Eulerian multifluid CFD models is technically or computationally impractical, we further proposed several simplified approaches. For example, a Dual-Bubble-Size (DBS) model is established for bubble columns and then supplies the ratio of averaged drag coefficient to bubble diameter (SCMF-A), and/or other structure parameters (SCMF-B) to close the SCMF CFD model. A thorough comparison indicates that without any fitting parameters, the SCMF models are superior to the two-fluid model (TFM) with empirical drag correlations in the prediction of overall gas holdup, radial profile of gas holdup and liquid flow field over a wide range of operating gas velocities. By comparison, each SCMF model has its strength and weakness at different flow conditions. SCMF-B can better reproduce the plateau of overall gas holdup as a function of gas flow rate, and the relative error of radial profile of gas holdup is the smallest among all the models at relatively higher flow rate. SCMF-A is better for the lower and much higher gas flow rates. We also design a special and step-by-step strategy to ascertain the evolution from SCMF-A to SCMF-B. We find that the physical properties, flow rate and drag coefficient of different bubble classes have respective functions in the model evolution and only their synergistic effects could generate reasonable prediction. This suggests that the definition of two “fluids” in the SCMF-B in terms of the differences in dense and dilute phases, rather than the thermal physical properties, is reasonable only when all the above “external” and “internal” factors are taken into account.