Experimental evaluation of solitary slugs in a horizontal pipe

In this work the existence of a bi-stable flow regime in a horizontal gas-liquid two-phase flow was experimentally confirmed. In this regime, both stratified and slug flows were found to be stable, and the occurrence of either regime depended on the inlet conditions imposed on the flow. When operating in the bi-stable flow condition, two types of slugs can occur. The first type was obtained by forcing a regular slug flow at the inlet of the horizontal pipe using an uphill section before the inlet. In this case, slugs in the uphill and horizontal sections have similar frequencies. The second type was obtained by forcing a stratified flow at the inlet, and then introducing a short pulse in the liquid flow rate. This triggered a solitary slug in the pipe, which grew linearly with the distance from the inlet. The behavior of the solitary slugs observed experimentally confirms the explanation and the model provided by Belt et al., 2024. The existence of such slugs can represent a threat in operations, since in long flowlines they can become extremely large and flood the downstream process installations. For instance, the experiments showed solitary slugs more than 200 pipe diameters long at a distance of 460 pipe diameters from the inlet. In order to evaluate the conditions under which the solitary slug phenomenon might occur, the extent of the bi-stable flow regime was experimentally determined. Its lower and upper boundaries match reasonably well with the stability conditions for slug flow and stratified flow, respectively, which were calculated within a 1D modeling framework.