https://doi.org/10.31871/IJNTR.6.3.8 International Journal of New Technology and Research (IJNTR) ISSN: 2454-4116, Volume-6, Issue-3, March 2020 Pages 07-13 7 www.ijntr.org Abstract—Two-phase flow is present in many and of different kind of industrial applications, especially in the form of gas-liquid two-phase flow. Despite of its common occurrence, its understanding is quite limited in comparison to single-phase flow due to its complexity. One critical parameter to characterize a gas-liquid two-phase flow and thereby to appropriate size the required industrial equipment is void fraction. The latter is affected by flow rates, fluid properties and flow gradient, whereas in turn it influences other important flow parameters such as pressure drop and heat transfer among others. Though there is available a plethora of void fraction correlations, most of them are limited to be accurate for similar fluid flow problems to that developed for, and thus they cannot be considered suitable to be utilised irrespective of the applied inlet conditions. However, a slip ratio correlation proposed by Premoli et al. has been proven quite accurate in void fraction predictions, and thus it is recommended to be used in case of horizontal and upward inclined pipes regardless the established flow regime, whereas its implementation does not require complex or iterative procedure to be performed. This hypothesis was evaluated by conducting numerical analysis and comparing the obtained results for void fraction, average velocity, and two-phase mixture density with the corresponding predictions of Premoli et al. correlation. It was found that in general Premoli et al. correlation over predicts the void fraction and average water velocity values, while under predicts the average air velocity and two-phase mixture density. However, the observed error does not indicate that the adoption of Premoli et al. correlation is a mistaken option a priori with the final choice of its usage being determined by the desired accuracy required from the application. Index Terms—CFD model, two-phase flow, void fraction, Premoli et al. correlation. I. INTRODUCTION Multiphase flow can be defined as the concurrent flow of more than one of any of the three discrete physical phases of matter or a combination thereof, whereas its occurrence range from simple natural processes to complex industrial applications, especially in the form of two-phase flow. Two-phase flow can be considered as the most common class of multiphase flows and includes gas-solid, gas-liquid, and liquid-liquid flows. In spite of its common presence, the understanding is quite limited in comparison to single-phase Georgios K. Makrygiannis, Department of Mechanical Engineering and Aeronautics, University of Patras, Patras, Greece Dionissios P. Margaris, Department of Mechanical Engineering and Aeronautics, University of Patras, Patras, Greece flow due to its nature [1]. Among two-phase flows the gas-liquid flow is probably the most important due to its existence in many industrial processes and to the comprised characteristics of a deformable interface as well as to the compressibility of one of the co-existing phases. Moreover, the interfacial distribution that occurs during two-phase flow leads to the formation of flow structures called flow regimes, which in case of horizontal flow can be bubbly, stratified (or stratified-wavy), intermittent (slug and/or plug), or annular flow regime, whereas the developed flow regime is depended on parameters such as flow rates, fluid properties, and pipe geometry [2]. Once the phases' redistribution is known, the two-phase flow phenomenon may be simplified and dealt in a similar way to single-phase flow [3]. One crucial parameter in any gas-liquid system that is affected by flow rates, fluid properties and flow gradient is the void fraction [4], [5], which in turn is a key parameter for sizing the entire equipment of industrial applications by influencing important flow parameters such as pressure drop and heat transfer among others [3]. There is a plethora of available void fraction correlations but most of them are limited to the fluid flow problems developed for (i.e. flow patterns, gas and liquid flow rates, and pipe geometries), and thereby they cannot be considered robust and suitable to be utilised irrespective of the applied inlet conditions. However, according to extensive comparison works of available void fraction correlations such as the work presented by Woldesemayat and Ghajar [3], who performed a comparative analysis of sixty-eight void fraction correlations against a wide range of experimental data, Premoli et al. correlation is found to be as accurate in void fraction predictions as the drift-flux correlations, and thus it is recommended to be utilised in case of horizontal and upward inclined pipes regardless the established flow regime. In addition, Premoli et al. correlation does not require a complex or iterative procedure to be performed facilitating thereby its adoption. In this perspective, the present study aimed to assess via direct comparison with numerical data the predictive capability and thus the applicability of Premoli et al. correlation to estimate important two-phase flow parameters apart from void fraction in a stratified-wavy horizontal air-water two-phase flow. The computational model generated to conduct the numerical analysis was first validated in order to ensure the robust and accurate predictions of numerical modelling, whereas its scale was dictated by the observed lack of published data regarding the Assessment of Premoli et al. Correlation Predictions in Horizontal Stratified-Wavy Two-Phase Flow Encountered at Small Diameter T-Junction Georgios K. Makrygiannis, Dionissios P. Margaris