Keywords: 
• unsteady flow /
• low concentration /
• coarse particles /
• followability

Abstract: Water hammer phenomena within solid-liquid two phases flow may cause serious damage to pipeline and transportation system, the motion characteristics of such unsteady flow are important values for calculating maximum pressure of water hammer. In this research paper, under water hummer condition, with various average flow rate and particle sizes in vertical pipe section, Particle Image Velocimetry (PIV) technique is utilized to study variation law of the velocity distribution of fluid medium and coarse particles as well as variation law of coarse particles followability. The following results are obtained: ① At the different moments of water hammer, the flow rate of coarse particles in the pipe flow is shown as irregular parabolic distribution in the cross section of the pipeline, mainly presented as the velocity of the particles near the bottom of the pipe wall is slightly less than the velocity near the top of the pipe, when the particle size is greater than 1.5 mm and the average flow rate is less than 2.5 m/s, coarse particles exhibit significant sedimentation characteristics; ② The followability of coarse particles is closely related to the force on particles, the change of particle velocity and fluid velocity are important parameters that affect the force on particles; ③ Based on the experimental data, the empirical formula of coarse particle followability coefficient k is obtained under water hammer, and the effects of different parameters such as particle size, pipe diameter, average flow velocity of two-phase flow and time of water hammer on the coherence coefficient of coarse particles were analyzed. The error between the calculated value and the measured value is less than 5%.