Origins of Pulsing Regime in Cocurrent Packed-Bed Flows

The mechanism of the formation for cocurrent downflow pulse flow was studied experimentally in a packed-bed of inert spheres of 3, 6, and 8 mm using an air-water flow. By measuring the flow distance until pulses are observed, the spatial growth rate of convective disturbances within the pulsing-flow regime were determined. Observations indicate that pulses form from trickling-flow as the result of a global convective instability. Further, experiments indicate that an analogous transition exists for the formation of pulses from the dispersed bubble flow regime, except that pulses form as the flowrates are adjusted to become less severe. Existing global instability models based on averaged (dispersed flow) momentum equations were modified to explain experimental results. A key uncertainty in modeling pulse formation from trickle flow is the regularization (i.e., stabilization) force. Reexamination of this issue suggests some mechanistic inconsistencies with surface tension which had been used in previous studies. Consistent with the present experiments, it is proposed that gravity may be the primary restoring force. Incorporating gravity stabilization into the dispersed flow equations provides predictions that are at least as good as the previous models. A similar dispersed flow model is used to explain the bubbly flow to pulse transition. While predictions agree with experimental data for part of the range, model accuracy is limited by the accuracy of constitutive expressions for interaction forces between phases.