Coarse-Grid Simulation of Reacting and Non-Reacting Gas-Particle Flows

Many processes involved in coal utilization involve handling of fine particles, their pneumatic transport, and their reactions in fluidized beds, spouted beds and circulating fluidized beds. One of the factors limiting our ability to simulate these processes is the hydrodynamics encountered in them. Two major issues that contribute to this limitation are lack of good and computationally expedient models for frictional interaction between particles, and models to capture the consequences of mesoscale structures that are ubiquitous in gas-solid flows. This project has focused on the development of these models through a combination of computer simulations and experiments. The principal goal of this project, funded under the ''DOE Vision 21 Virtual Demonstration Initiative'' is better simulation of circulating fluidized bed performance. The principal challenge funded through this cooperative agreement is to devise sound physical models for the rheological characteristics of the gas-particle mixtures and implement them in the open-domain CFD code MFIX. During the course of this project, we have made the following specific advances. (a) We have demonstrated unequivocally that sub-grid models are essential to capture, even qualitatively correctly, the macroscale flow structures in gas-particle flows in vertical risers. To this end, we developed sub-grid models of different levels of detail and exposed the sensitivity of the results obtained in coarse-grid simulations of gas-particle flow in a riser to the level of sophistication of the sub-grid models. (b) We have demonstrated that sub-grid model for the fluid-particle drag force is the most important additional feature and that the corrections for the granular phase viscosity and pressure are of secondary importance. We have also established that sub-grid models for dispersion of heat and mass are of secondary importance only. (c) We have brought forth the general character of the sub-grid model for the drag force. (d) We have performed for the first time in the literature a detailed analysis of the impact of unipolar electrostatic charges on gas-particle flow characteristics in a riser. (e) We have examined in detail the effect of wall friction and particle-particle contact (frictional) stresses on fluidization and defluidization behavior of particle assemblies, and brought forth their importance for stable operation of standpipes in a circulating fluidized bed circuit. (f) We have demonstrated that the general characteristics of contact stresses in particle assemblies and wall friction are similar for many different particles, establishing that a simple model framework can be widely applicable. (g) We have developed constitutive models for frictional regime, implemented them in MFIX and demonstrated the capability of simulating dense gas-solid flows in the frictional regime. (h) We have also performed detailed experiments to expose the nature of the stick-slip flows in silos, as a simple model system for under-aerated standpipes. All theoretical advances made in the study are implemented in MFIX and are available for public use

COARSE-GRID SIMULATION OF REACTING AND NON-REACTING GAS-PARTICLE FLOWS

The principal goal of this project, funded under the ''DOE Vision 21 Virtual Demonstration Initiative'' is virtual demonstration of circulating fluidized bed performance. We had proposed a ''virtual demonstration tool'', which is based on the open-domain CFD code MFIX. The principal challenge funded through this grant is to devise and implement in this CFD code sound physical models for the rheological characteristics of the gas-particle mixtures. Within the past year, which was the third year of the project, we have made the following specific advances. (a) We have completed a study of the impact of sub-grid models of different levels of detail on the results obtained in coarse-grid simulations of gas-particle flow. (b) We have also completed a study of a model problem to understand the effect of wall friction, which was proved in our earlier work to be very important for stable operation of standpipes in a circulating fluidized bed circuit. These are described in a greater detail in this report

Closures for Course-Grid Simulation of Fluidized Gas-Particle Flows

Gas-particle flows in fluidized beds and riser reactors are inherently unstable, and they manifest fluctuations over a wide range of length and time scales. Two-fluid models for such flows reveal unstable modes whose length scale is as small as ten particle diameters. Yet, because of limited computational resources, gas-particle flows in large fluidized beds are invariably simulated by solving discretized versions of the two-fluid model equations over a coarse spatial grid. Such coarse-grid simulations do not resolve the small-scale spatial structures which are known to affect the macroscale flow structures both qualitatively and quantitatively. Thus there is a need to develop filtered two-fluid models which are suitable for coarse-grid simulations and capturing the effect of the small-scale structures through closures in terms of the filtered variables. The overall objective of the project is to develop validated closures for filtered two-fluid models for gas-particle flows, with the transport gasifier as a primary, motivating example. In this project, highly resolved three-dimensional simulations of a kinetic theory based two-fluid model for gas-particle flows have been performed and the statistical information on structures in the 100-1000 particle diameters length scale has been extracted. Based on these results, closures for filtered two-fluid models have been constructed. The filtered model equations and closures have been validated against experimental data and the results obtained in highly resolved simulations of gas-particle flows. The proposed project enables more accurate simulations of not only the transport gasifier, but also many other non-reacting and reacting gas-particle flows in a variety of chemical reactors. The results of this study are in the form of closures which can readily be incorporated into existing multi-phase flow codes such as MFIX (www.mfix.org). Therefore, the benefits of this study can be realized quickly. The training provided by this project has prepared a PhD student to enter research and development careers in DOE laboratories or chemicals/energy-related industries

Non-random distribution of adsorbates on catalytic surfaces: the role of interactions between adsorbates

It has been repeatedly stressed in the literature that the commonly invoked assumption of a random distribution of adsorbates on the catalyst surface is suspect under certain operating conditions. Nonrandom distribution of the adsorbates can occur as a result of interaction between adsorbates and/or their inadequate mobility. We have studied the effect of adsorbate interactions on the rates and stability of catalytic reactions, and the salient features are outlined with several examples. An analysis of the thermodynamic data concerning the oxidation of SO2 on platinum is presented within the framework of the proposed model