Two-phase slug flows in helical pipes: Slug frequency alterations and helicity fluctuations

• Slug flow simulations are performed in helical pipes.
• Tangential velocity and helicity variations present high frequency fluctuations.
• Slug frequencies are altered depending on the helix lengths.
• Qualitative relations between helicity and tangential velocity are identified.
• Higher frequencies in helicity fluctuations are related to the change of regime.

Air-water numerical simulations in the slug flow regime have been performed in horizontal helical pipes and the effects of geometries on the flow regime have been investigated. Depending on the length of the helix, outlet slug frequencies have been reduced with various degrees of efficiency. Correlations between mean tangential velocity and helicity density fluctuations have been identified and investigated qualitatively. These flow fields show smaller time scales than those obtained in volume fractions fluctuations. Shifts observed in the tangential velocity and mean helicity fluctuations to smaller time scales (high frequencies) are associated with regime transitions. For a slug flow undergoing a continuous transition to the annular flow regime, it is shown that the presence of slower (low frequencies) helicity fluctuations is attributed to the variations in the axial velocity. Finally, the analysis of the helicity at gas-liquid interface confirms the presence of the mixing zone at the slug front.