The Effect of Axial Boundary Motion on Pressure Surge Generation

Abstract

The basic relationships for transient flow in a straight tube are investigated for the case of axial boundary motion. Viscous effects and other small terms are neglected allowing the problem to be described by the wave equation subjected to a moving boundary. Using this equation the case of pressure surge generation due to instantaneous load rejection in a finite straight tube is investigated. The axial boundary motion is due to the internal pressure stretching the pipe wall along the pipe axis. A closed form analytical result is obtained using a differential difference equation and shows that the pressure surge magnitude can be significantly greater than that predicted neglecting the boundary motion. This result is used to obtain a simple expression for estimating the maximum value of the pressure surge following the load rejection. An experiment is conducted which shows the significance of the axial line motion and compares well with the prediction of this effect.