Experimental and Numerical Analysis of Water Hammer During the Filling Process of Pipelines

Abstract

During the start-up of the propulsion system of a spacecraft, the filling of an evacuated pipeline, a process known as priming, can generate severe pressure peaks due to the slam (water hammer) of the propellant against a closed thruster valve. The downstream conditions strongly affect the pressure surge; in the case of evacuated lines these pressure peaks can be as high as 250 bar and may lead to structure failure if not properly taken into account in the sub-system dimensioning. At DLR Lampoldshausen a new test-bench has been built in order to investigate priming process and other fluid transient phenomena. The experimental investigation presented in this paper includes priming test in both pre-pressurized and evacuated pipelines. Geometry of the test-section is a 2000 mm straight pipe with a relative large diameter (19mm) in order to examine high mass flow. From a physical point of view, the modeling of priming is a complex task, as it involves transient multiphase flow. As a first approach, an analytical model of the equations of motion is solved by applying the rigid liquid column theory, considering only the inertial and frictional effects of the liquid treated as a slug. The friction plays an important roles, in particular due to the unsteady conditions which increase its steady value. The cushion effect of an inert gas is also analyzed and the assumption of adiabatic vs isothermal compression compared. Results indicated that a polytropic compression can better model the realistic gas behavior. Finally, using a EcosimPro® numerical model of the test-bench (in conjunction with the ESPSS library, an ESAdeveloped tool capable of one dimensional, two-phase flow transient simulations), a comparison between numerical results against experimental data is presented and discussed.