Computational characterisation of intermittent hydrodynamic behaviours in a riser with Geldart A particles

In the riser of a gas-solid circulating fluidised bed (CFB) with Geldart A particles, the multiscale interactions and diverse coherent structures give rise to an important hydrodynamic phenomenon called flow intermittency. In this work, the two-fluid model incorporating the energy minimisation multi-scale (EMMS) drag model is employed to simulate the gas-solid flow in the riser. The predicted fluctuating signals are processed to acquire the intermittency indices, wavelet flatness factors, power spectra of solids volume fraction fluctuation, probability density function (PDF) of wavelet coefficients for solids fluctuating velocity, and PDF of solids volume fraction, based on which the flow intermittency and effects of coherent structures are characterised. The results presented in this paper reveal that the EMMS-based computational fluid dynamics (CFD) simulation in combination with the fluctuating signal analysis provide an in-depth understanding of the intermittent flow behaviours in the riser with Geldart A particles. Particle clusters and particle vortices are identified as typical coherent structures in the riser, and the flow intermittency, caused by the flow field heterogeneity and the presence of coherent structures, is found to be significantly dependent on the radial locations and operation conditions.