Gas-liquid-solid three-phase slug flow in a vertical pipe is observed. The motions of small bubbles, small particles, large bubbles and large particles were explained qualitatively by comparing their spatial distributions and velocities with each of other flows.It become clear in the case of three-phase slug flow that the rising velocity of cluster composed of small bubbles or small particles in liquid slug is larger than it in three-phase bubbly flow.When large particle collides with large bubble, it enters into it and falls there, then large bubble is destroyed by large particle.When large particle leaves large bubble and enters into liquid slug, first it is accelerated at a big acceleration then rises with approximately constant velocity. But this rising velocity in three-phase slug flow is smaller than it in liquid-solid two-phase flow or gas-liquid-solid three-phase bubbly flow.Also small particle falls in the liquid film around large bubble with a declining acceleration to be bigger than large particle.In gas-liquid-solid three-phase slug flow with small particles, the velocity of large bubble is faster than it in gas-liquid two-phase slug flow, but when becoming gas-liquid-solid three-phase slug flow with large particles, the large bubbles show a situation contrary to the above.
