This paper experimentally investigated the air-water intermittent flow in four helically coiled tubes with inner diameter of 0.016m, coil diameters of 0.24, 0.43 and 0.80m, and inclination angles of 5 degrees and 15 degrees A double conductivity probe was used to measure the hydrodynamic parameters at 48 points in the outlet cross section. The results showed that centrifugal force and Dean Vortices were the major mechanisms that influenced the void fraction distribution, elongated bubble length, slug length and slug frequency. Because of centrifugal force, the void fraction profile rotated an angle which approached to 90 degrees as liquid superficial velocity increased and coil diameter decreased. Due to the Dean Vortices, the elongated bubble length and slug length decreased as the coil diameter decreased, leading to an increase of slug frequency. A dimensionless parameter Z representing the ratio of the centrifugal force to the gravity was presented, which provides a quantitative criterion to evaluate the influence of centrifugal force. In terms of Z, correlations predicting the deviation angle, elongated bubble length and slug frequency were built, which met the experimental data well when the coil diameter is between 0.24 and 0.80m and the inclination angle between 5 and 15. Moreover, a drift flux model for HCTs was presented. (C) 2019 Elsevier Ltd. All rights reserved.