| Gas-liquid two-phase and multiphase slug flow is commonly encountered in the petroleum and chemical industries. Owing to the intermittence of flow rate and the complexity of phase distribution,the research of slug flow is still in the primary state. This paper studies the gas-liquid two-phase and oil-gas-water three-phase slug flow in the horizontal and slightly inclined pipes, with a diameter of 50mm and a length of 40m. It is very significant to improve the theory of multiphase flow and practical application. The main contents and conclusions of research are following.Going deep into the fluctuation characteristics of liquid holdup, pressure and differential pressure, and the mean liquid holdup within slug unit, the liquid holdup within slug and the liquid holdup within film are analyzed, and the regularity is obtained. By statistical and nonlinear analysis, it can be found that the probability density distribution of the liquid holdup is bimodal distribution. The high liquid holdup peak is in correspondence with liquid holdup within the slug and the low liquid holdup peak is in correspondence with liquid holdup within the film. Moreover, the distribution of pressure is unimodal distribution or bimodal distribution, and the differential pressure distribution is unimodal distribution. These characteristics provide dependable slug identities for flow pattern identification.The theoretical models of Kelvin-Helmholtz, the viscous long wavelength and slug stability are analyzed and compared through experiments. Kelvin-Helmholtz model is adopted to study the transform boundary of oil-gas-water three-phase slug flow, and it can be found that the critical liquid velocity and the critical liquid level will increase with the increasing of the water cut. The roll wave flow pattern is found in the process of transforming from oil-gas-water three-phase slug flow to annular flow. In this case, the liquid distribution of slug flow is W/O or O/W, the roll wave flow is three-layer flow without commixture in interfaces. The region of roll wave and its regularity are studied in depth.By studying the characteristics of the acquired signal, it is found that oil-water two-phase flow can be classified into six flow patterns: stratified flow, mixed interface stratified flow, O/W&W dispersed flow, O/W dispersed flow, W/O&O/W mixed flow, and W/O dispersed flow. Transform boundary correlations of different flow patterns are obtained through dimensionless variable analysis, and they coincided well with experimental data. The oil-gas-water three-phase slug flow is classified into eight flow patterns: stratifying-slug flow, mixed interface stratifying-slug flow, W/O&O/W slug flow, O/W&W slug flow, (slug)O/W-(film)O/W&W slug flow, O/W slug flow, W/O&O slug flow and W/O slug flow. The regularity of phase inversion and flow pattern transform is also studied.The characteristic parameters of gas-water two-phase and oil-gas-water three-phase slug flow in horizontal and slightly inclined pipes such as slug velocity, slug length, slug frequency, length of long bubble and slug unit length are studied systematically. The distribution of slug velocity accords with Gaussian distribution. The relational equation of slug velocity and the influence of oil-water phase distribution to slug velocity are obtained. The distribution of slug length accords with lognormal distribution and it increases at first and then decreases with the increasing of Frm. When Frm=3.5, the minimum slug length appears. The slug length is determined by gas and liquid superficial velocities, and oil-water distribution nearly has no effect on it. With the increasing of superficial gas velocity, the slug frequency decreases firstly and then increases, and there is a minimal value. The relational equation of slug frequency is also obtained. The length of long bubble and the length of slug unit are mainly determined by gas and liquid superficial velocity. Inclination angle and oil-water distribution nearly have no effect on them.
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