Numerical Simulation of Multiphase Reactive Flows

Abstract

This study deals with mathematical modeling and numerical simulations of reactive multiphase flows in dense fluidized beds. These flows involve complex physical mechanisms related to the coupling between the bed hydrodynamic and the reactions,which are still poorly understood. In this context, numerical simulations can provide explanatory access to the underlying physics taking place in the reactor, thus supplementing the experimental results. The present contribution focuses on the natural gas combustion in a dense fluidized bed reactor, for which experimental results are available from the literature (Dounit, 2001; Dounit et al., 2001, 2008). In their experiments, the authors pointed out the essential role played by the particle projection zone, above the bed surface, in the global thermal efficiency of the reactor operating at relatively low temperatures (600°C–800°C). In the present study, this point is further investigated by analyzing the results obtained by the numerical simulations. The unsteady 3D numerical simulations were performed using NEPTUNE_CFD code which is based on an Euler–Euler approach; the latter computes both the gas and the particulate phases in an Eulerian framework, accounting for specific closures modeling the interphase momentum and energy transfers. Time-averaged quantities were then computed and compared with the available experimental measurements. Numerical results (especially the gas temperature) were found to be very sensitive to the mesh refinement for the selected operating point. A further analysis at mesoscopic and macroscopic scales was carried out. This analysis pointed out the crucial role of the toroidal loop, which extends from the bed, close to the ejection zone, to the freeboard, on the fuel conversion. In numerical simulations, this loop must be accurately reproduced in order to provide reliable combustion predictions.