Effect of clustering on gas-solid drag in dilute two-phase flow

When coarse-grid calculations in two-phase flow are performed, mesoscale fluctuations, such as clusters, cannot be explicitly captured. Their impact on macroscale fluctuations, however, has to be taken into account by the introduction of an appropriate closure model. A new closure model to describe gas-solid drag is introduced. The effect of particle clustering on the interphase, momentum-transfer coefficient is taken into account by introducing the concept of effective drag. Clustering results in a decreasing value of the interphase momentum-transfer coefficient because the contribution of particles in the cluster is considered to be negligible in dilute phase flow. For solids fractions greater than 1%, clustering phenomena become increasingly important, resulting in an important decrease of the interphase momentum-transfer coefficient. Calculation results for a riser reactor, both neglecting and including the effects of clustering on the gas-solid drag, are presented and validated with experimental data. With the effective-drag model, the solids fraction profiles in a riser reactor are predicted more accurately.