Influence of gravity on gas–liquid two-phase flow in horizontal pipes

Based on volume of fluid (VOF) method, a three-dimensional unsteady mathematical model for gas–liquid two-phase flow in horizontal pipe under various gravities is developed. The flow patterns, void fraction distribution and fluctuation in circular pipes with diameters of 7 mm and 10 mm under various gravities of 10−4g0, 0.17g0, 0.38g0, and g0 (g0 = 9.8 m/s2) are all presented. The quantitatively characterization for intermittent features of gas–liquid two-phase flow under various gravities is realized by using power spectral density (PSD) analysis to extract the dominant frequencies from massive void fraction fluctuation signals. The results indicate that the gravity plays a significant role in the flow pattern, void fraction distribution and void fraction fluctuation. The flow pattern transition appears as a result of the coalescence of bubbles/plugs induced by bubbles/plugs accumulation and gravity driven drainage of liquid film between coalescing bubbles/plugs with increasing gravity. Moreover, the thicker liquid film at the bottom of bubble/plug unit occurs with increasing gravity, resulting in a larger void fraction at the top layer of pipes and a longer region with zero void fraction at the bottom layer. And also with the increasing gravity, both the average peak value and dominant frequencies of void fraction fluctuation decrease. For the slip ratio, it not only increases with increasing gravity but also with increasing two-phase superficial velocity. However, the influence of gravity on the two-phase flow is weakened by the enhanced role of surface tension due to the reduction of diameter as well as the enhancement of inertial force due to the increase of two-phase velocity.