Analysis of fluid hammer occurrence with phase change and column separation due to fast valve opening by means of flow visualization

[Abstrac]: This paper presents an experimental investigation on the fluid hammer phenomenon generated when filling a pipe line under vacuum conditions with a closed end. This physical configuration, although it can be found in many piping configurations, it is of special interest in propulsion systems of satellites during priming operation. The fluid hammer taking place here not only leads to high pressure peaks in the fluid but also to low pressures, which can cause cavitation, gas desorption and liquid column separation. The study is carried out on a facility allowing flow visualization, which is achieved by replacing the pipe closed end by a quartz cylinder drilled with the same tube inner diameter. In this way, the flow can be recorded with high speed imaging at this location. The visualizations confirm that the pressure evolution is accompanied by a complex multiphase flow pattern. First of all, a foamy mixture of non-condensable gas, vapor and liquid droplets precedes the liquid front arrival at the bottom end. During the fluid hammer compression wave, the vapor condensates and the non-condensable gas gets compressed. Afterwards, the arrival of an expansion wave induces the movement of the liquid column backwards, with the corresponding pressure drop that generates a gaseous bubble referred to as column separation. Finally, the collapse of this bubble is at the origin of the next pressure rise.