In this study, pressure losses in three horizontal pipes of 25 mm, 52.7 mm and 80.1 mm in internal diameter in which two immiscible fluids (oil and water) were made to form a core-annular flow (CAF) were measured. Two types of viscous liquids (oil M680, viscosity of 2.82 Pa s at 20 °C ; oil M1800, viscosity of 8.92 Pa s at 20 °C) were used in the experiment. The oil-water flow rate ratios (ratio of oil flow rate to water flow rate) were set at 2.6, 3.0, 3.4, 4.0 and 4.6 for the pipe of 25 mm in

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... diameter and at 8.0 for the pipes of 52.7 mm and 80.1 mm in diameter. Based on the results of the experiment, an empirical equation was derived to estimate CAF pressure loss at various oil-water flow rate ratios and pipe diameters. The pressure losses obtained from the equation showed good agreement with the results of the experiment.