Pressure Spike after Column Separation and Rejoining in a Horizontal Pipe Line

In experiments of water hammer phenomenon including column separation, pressure spike that is much bigger than the supposed water hammer pressure is often observed. That pressure arises not only in the experimental model but also in full scale system. This paper describes the mechanism of such pressure spike and the effect of valve closure time on water hammer with column separation. Computational model based on the Discrete Free Gas Model with 3-element Voigt model is used in this study. From the comparison of the numerical solution with experimental results, it is shown that the pressure spike occurs because of the difference of acoustic impedance of two connected pipes. Observed length of vapor slug at upstream end are slightly longer than that of numerical solution. The volume of free gas in the computational model has different meanings for various states of unsteady cavity flows.