Flow regimes of air-water two-phase flow were experimentally studied in a horizontal upward U-bend unit with an inner diameter of 16 mm and a radius of curvature as 100 mm. Visualization technique on flow observation and analysis of corresponding pressure drop fluctuations were used to objectively identify flow regimes. The fluctuation characteristics of pressure drops at various flow regimes were summarized and a new method for recognizing flow regimes quantitatively was proposed. A total of five flow regimes

i.e.

stratified-churn

plug-bubble

slug-wavy

annular-wavy and annular-dispersed flows were identified within the scope of experiment. They are different from flow regimes in a horizontal or vertical straight tube. Compared to standard deviation of pressure drop signals

power spectral density (PSD) distributions can better reflect evolution characteristics and dynamic properties of different flow regimes in the U-bend unit. Combination of skewness or kurtosis of PSD distributions and ratio of gas-liquid superficial velocities could objectively and quantitatively identify flow regimes in the U-bend unit. Flow regime transition occurs at ratios of gas-liquid superficial velocities as 1 and 13.