Inlet conditions effect on bubble to slug flow transition in mini-channels
Gregorc, Jurij, strojnik ; Žun, Iztok
The paper deals with the impact of inlet conditions on bubble to slug flow transition in mini systems. A new experimental test loop with a glass mini-tube (D=1.2 mm ID) has been constructed to assess ...the effects of inlet conditions on the two-phase flow pattern development in the spatial and temporal domains. The interchangeable inlet part of the test section allowed different geometrical combinations for the mixing of gas and liquid prior to it entering the mini-tube. Porous media mixer and cross-junction mixer were considered. High speed video recordings were taken of 70 combinations of flow rates, corresponding to superficial velocities ranging from 0.2 to 11 m/s and 0.25 to 3 m/s for air and water, respectively. The following discernible flow patterns are considered: bubbly, slug and semi-annular flow. No significant differences were found when comparing flow pattern maps for each mixer. Digital image post processing of high speed video recordings was used to estimate the equivalent diameter for every gas structure. A comparison of the distribution of bubbles with equivalent diameter revealed the inlet mixer's strong impact on bubble size and bubble distribution along the mini-tube and thus, the bubble to slug flow transition. The VOF method, implemented in ANSYS Fluent was used as the numerical tool to predict the flow patterns in a mini-tube of 1.2 mm ID. A novel approach to mimic continuous mixing is presented. By changing only the prescribed flow rates, different flow patterns can be simulated. Similar interfacial structures were obtained by numerical simulation and experiment for both mixers. Reasonable quantitative agreement was also achieved when analyzing bubble to slug flow transition.
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