Technologies for clean energy generation from fossil or bio-based fuels and the capture of resulting CO2 emissions use fluidized bed reactors with particles of size and density corresponding to the Geldart A and Geldart B classification. These gas-solid systems exhibit non-uniform and heterogeneous flow structures arising from hydrodynamic instabilities at the particle scale. While theoretical estimates for the stability of a homogeneous gas-solid suspension exist for Stokes flow, the regime of finite fluid inertia is yet unexplored. Design and scale-up of industrial devices that implement clean energy technologies increasingly rely on computational fluid dynamics (CFD) simulations that solve yet untested averaged conservation equations. An important validation test for averaged multiphase flow models is their ability to accurately capture the stability limits of a gas-solid suspension. It is difficult to obtain data from experiments because of limited optical access, but particle-resolved direct numerical simulation (DNS) can provide fully characterized field data to reveal flow physics and to serve as a benchmark for comparative assessment of multiphase flow models.
This project will employ a first-principles approach to characterize the stability limits of gas-solid suspensions at finite Reynolds number using particle-resolved DNS. The effects of volume fraction fluctuations and granular temperature will be incorporated to develop a comprehensive stability theory for gas-solid suspensions. The objectives of the project are to: (i) quantify the stability limits of homogeneous gas-solid suspensions with finite fluid inertia for Geldart B particles; (ii) quantify the competing mechanisms of particle-fluid (hydrodynamic) and particle-particle interactions that result in generation of volume fraction fluctuations in gas-solid suspensions using a set-theoretic approach; and (iii) understand the stability and characterize the base state of gas-solid suspensions with finite fluid inertia for Geldart A particles.
The DNS data will fill an important gap in our knowledge regarding the stability of gas-solid suspensions with finite fluid inertia. The improved set-theoretic stability theory accounting for volume fraction fluctuations will give additional insight into understanding the dynamics of both Geldart B and Geldart A suspensions, and will remove classical assumptions of scale separation. The DNS data on suspension stability will be made available to the multiphase flow community and will serve as a useful benchmark for CFD codes. The DNS data and stability theory will also be useful for developers of multiphase CFD codes, such as the popular multi-fluid code (MFIX) at the National Energy Technology Laboratory, who may use it to improve existing models that are used to evaluate and optimize fluidized bed design for CO2 capture and other clean energy applications.