An explicit method for the analysis of transient compressible flow in pipe networks

Abstract

This thesis deals with the development of an explicit pipe network analysis code for the benchmarking of Flownet. Flownet is a general-purpose thermal-fluid network analysis code used as a tool in the design of the Pebble Bed Modular Reactor. The National Nuclear Regulator (NNR) requires the use of benchmarks to verify and validate such a design code. RELAPS was selected as an additional commercial code to benchmark Flownet. Various benchmark cases were identified to determine the validity of specific models and components. Some of the models or components validated include pipes, boundary conditions, heat transfer, non-inertial elements and junctions. Comparisons were also done to validate the integrated use of these components in an arbitrary structured pipe network. For virtually all steady-state flow cases, Flownet compared very well with analytical results. Only for isothermal flow at high Mach numbers were some differences noted. Some anomalies were identified with the use of RELAPS. Comparisons between the three codes for transient flow were very favourable, except for cases were the anomalies iniRELAP5 came into play. For these cases Flownet and Lax-Wendroff still compared very well. The only noteworthy anomaly for transient flow was found when solving total temperature at pipe boundaries. This anomaly was only found at boundaries with a combination of little or no flow and sharp pressure transients. A difference in boundary modelling between Flownet and Lax-Wendroff is most likely the cause of this difference. This localized effect had virtually no impact on the solution elsewhere in the network. Upon successful completion of the selected benchmark cases, one can conclude that Flownet uses a valid implementation of the basic components tested. Further, these models are also integrated in a way that enables valid modelling of complex pipe networks.