# Slug Regime Transitions in a Two-Phase Flow in Horizontal Round Pipe. CFD Simulations
## 1 Introduction
### Experimental Investigation of Slug Regime of Gas-Liquid Flow in Horizontal Round Pipe Section  
The slug flow mode can cause dangerous vibrations of the pipe and associated process equipment, large pressure surges, and, as a consequence, rapid mechanical wear of pipeline elements [13]. That is why it is necessary to clearly understand which flow regime of the two-phase mixture will be implemented with specific parameters in the inlet section, and what measures must be taken to avoid the occurrence of undesirable flow regimes. There are two variants of the slug flow regime, called circulation and bypass [1]. These regimes differ in streamlines drawn around the slug (Figure 1). For technical applications of macro-tube flows, it is essential to know the pressure drop [14], the presence or absence of a slug flow pattern, and the frequency of the slugs. This problem has not been adequately solved for straight pipes [10; 15; 16], although there is research on the more complex case of air-water two-phase flows in horizontal U-bend pipes [17; 18; 19; 20]. The slug regime in a two-phase gas-liquid flow in horizontal round pipes was experimentally investigated with different methods [10; 21; 22; 23; 24]. The transition from plug to slug flow in a horizontal pipe was experimentally studied with electrical capacitance tomography [21]. The researchers used air and silicone oil for two-phases flow simulation. The horizontal gas-oil flow was studied with non-intrusive electrical capacitance tomography measurements [10]. The frequency and time span of slugs was assessed. The detailed flow visualization studies of air-water two-phase flow in a round pipe was performed with a high-speed video camera in a wide range of two-phase flow conditions [22; 23; 24]. Ultrasonic detection was applied for moving slug interfaces detection in gas-liquid two-phase flow [25]. Figure 1: Two variants of the slug flow regime, called circulation and bypass: a—circulating flow; b—bypass flow; streamlines of the liquid: 1—in circulation; 2—in bypass modes. Two-phase flow regime transition mechanisms were exploited with high-resolution virtual experiments [26]. It was shown that the transition out of the slug flow regime could take at least two pathways:  
* Interfacial wave-induced instability development in the Taylor bubble, leading to its disintegration. * Intense bubble shearing at the tail of the bubble. The estimation of pressure drop in two-phase gas-non-Newtonian liquid flow in long horizontal pipes was made with the multiple-layer perceptron artificial neural network model [27]. The model is based on a large number of experimental data. The input parameters of the model are a wide range of operating conditions, pipe diameters, and fluid characteristics. The authors claim that the estimations are in good agreement with the experimental results; but, such a model is a way of interpreting previously obtained experimental data and does not have predictive properties. In the absence of experimental data on two-phase flow in a pipe with local resistance, this kind of model could not be used for prediction of flow regime's transitions on local hydraulic resistance element. The statistical approach for the slug flow often aims to predict the slug frequency [14]. Statistical features of the flow in horizontal liquid-gas slug flow was experimentally investigated [28]. The probability density functions of the slug frequency and the translational velocity was determined. Based on a detailed review of slug frequency for a gas-liquid plug flow, a new empirical correlation has been proposed [16]. Despite the large amount of experimental data obtained by researchers, there is no theory and method for calculating such flows. The methods for calculating the pressure drop and flow regime transition of a multiphase flow are mainly empirical and semi-empirical. The application of these methods is mainly limited. Two-phase flow is simulated using the computational fluid dynamics (CFD) method [29; 30; 31; 32] and others.