The pressure drop has a significant importance in multiphase flow systems. In this paper, the effect of the volumetric quality and mixture velocity on pressure drop of gas-liquid flow in horizontal pipes of different diameters are investigated experimentally and numerically. The experimental facility was designed and built to measure the pressure drop in three pipes of 12.70, 19.05 and 25.40 mm. The water and air flow rates can be adjusted to control the mixture velocity and void fraction. The measurements are performed under constant water flow rate (CWF) by adding air to the water and constant total flow rate (CTF) in which the flow rates for both phases are changed to give same CTF. The drift-flux model is also used to predict the pressure drop for same cases. The present data is also compared with a number of empirical models from the literature. The results show that: i) the pressure drop increases with higher volumetric qualities for the cases of constant water flow rate but decreases for higher volumetric qualities of constant total flow rate due to the change in flow pattern. ii) The drift-flux model and homogenous model are the most suitable models for pressure drop prediction. KEYWORDS: Air-water flow, pressure drop, horizontal pipes, experimental measurement, drift-flux model Highlights: 1. The pressure drop in three horizontal pipes of 12.70, 19.05 and 25.40 mm is studied. 2. The pressure drop increases with higher volumetric qualities for the cases of constant water flow rate but decreases for higher volumetric qualities of constant total flow rate due to the change in flow pattern 3. The drift-flux model and homogenous model are the most suitable models for pressure drop prediction.

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Oil injected twin screw compressors are widely used in industry for gas compression because of their high volumetric efficiency and reliability. The injected oil has a significant influence on the performance of these compressors. Only an optimum quantity of oil supply would give best performance at a specified operating condition. Therefore, it is essential to determine the distribution of oil in the working chamber and thereby the compressor's performance. Using Computational Fluid Dynamics (CFD) to solve two-phase flow in deforming domains is numerically challenging due to the complex interactions between air and oil in rotating machines. In this paper, two Eulerian-Eulerian multiphase models have been investigated. Namely, inhomogeneous and homogeneous model with free surface modelling have been investigated for their stability, oil-air distribution and accuracy of compressor performance results. Deforming rotor mesh needed for the simulation is generated by SCORG TM grid generator while two-phase transient flow is solved with ANSYS CFX solver. Results obtained from the simulation have been compared with the measured data of mass flow rate of gas, indicated power and temperatures. Inhomogeneous model predicts data closer to the experimental results and an expected distribution of oil in the rotor tip clearances. For a high mesh deformation of a working domain in twin screw compressor, the inhomogeneous approach also provides better stability of the solver, but this is at an added cost of calculation time. In future, better initialisation and solver parameters with oil droplet characterisation will be required to further improve the model.

Growing demands for energy are motivating researchers to conduct in-depth analysis of positive displacement machines such as oil-injected screw compressors which are frequently used in industrial applications like refrigeration, oil and gas and air compression. The performance of these machines is strongly dependent on the oil injection. Optimisation of oil has a great energy saving potential by both increasing efficiency and reducing other impacts on the environment. Therefore, a three-dimensional, transient computational fluid dynamics study of oil injection in a twin-screw compressor is conducted in this research. This study explores pseudo single-fluid multiphase (SFM) models of VOF (Volume of Fluid) and a mixture for their capability to predict the performance of the oil-injected twin screw compressor and compare this with the experimental values. SCORGTM (Screw Compressor Rotor Grid Generator) is used to generate numerical grids for unstructured solver Fluent with the special interface developed to facilitate user defined nodal displacement (UDND). The performance predictions with both VOF and mixture models provide accurate values for power consumption and flow rates with low deviation between computational fluid dynamics (CFD) and the experiment at 6000 RPM and 7.0 bar discharge pressure. In addition, the study reflects on differences in predicting oil distribution with VOF, mixture and Eulerian-Eulerian two-fluid models. Overall, this study provides an insight into multiphase flow-modelling techniques available for oil-injected twin-screw compressors comprehensively accounting for the details of oil distribution in the compression chamber and integral compressor performance.