Experimental investigation of flow and pressure drop characteristics of air-oil slug flow in a horizontal tube

•Two-phase flow and pressure drop characteristics associated with slug flow are examined.•Flow visualization images and temporal records of pressure drop oscillation are presented.•Relevant slug flow parameters are measured and compared with previous correlations.•A new correlation for elongated bubble length in air-oil slug flow is developed.•A theoretical slug flow model for predicting total pressure gradient of slug unit is presented. This study examines two-phase flow and pressure drop characteristics associated with slug flow in a tube with an inner diameter of 4 cm. Experiments are conducted using air and low viscosity mineral oil as working fluids at atmospheric pressure and two different oil temperatures, 25 and 45°C. The superficial gas and liquid velocities are in the range of 0.33–2.43 m/s and 0.20–1.20 m/s, respectively. Flow visualization images and temporal records of pressure drop oscillation are presented for different gas and liquid superficial velocities. The relevant flow parameters associated with slug flow, such as the elongated bubble velocity, elongated bubble length, liquid slug length, and liquid film thickness, are measured, and the relationship between the slug flow parameters and the superficial gas and liquid velocities is investigated. The measured pressure gradient data are also compared to predictions of previous homogeneous equilibrium models and semi-empirical correlations for two-phase frictional pressure gradient. A theoretical slug flow model for predicting the total pressure gradient of slug unit is presented using empirical correlations for the slug flow parameters. The new model accurately predicts the experimental pressure gradient data, evidenced by an overall mean absolute error of 19.3%.