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

Sundaresan, Sankaran

Princeton University

关键词: Pneumatic Transport;    Sensitivity;    Mixtures;    Electrostatics;    Stresses;   

DOI  :  10.2172/836624
RP-ID  :  NONE
RP-ID  :  FC26-00NT40971
RP-ID  :  836624

美国|英语

来源: UNT Digital Library

【 摘 要 】

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.