RELAP5/MOD3.3 study on density wave instabilities in single channel and two parallel channels

摘要

The subject of instability is of great interest in two-phase flow studies and a constantly up to date research field concerns the determination of stable and unstable operating regions of different industrial systems. Density Wave Oscillations (DWOs) are probably the most common type of instability affecting vapor generation in boiling systems. They result from multiple dynamic feedback effects between the flow rate, the vapor generation rate and the pressure drops in the boiling channel. DWOs may constitute a critical issue in all the industrial applications operating with water-steam mixture, as steam generators and boiling water nuclear reactor cores. In this paper, DWOs are investigated by means of the RELAP5/MOD3.3 code. Available literature works address single channel configuration by working with an imposed Ap, kept constant throughout the simulation. In this work the attempt is to reproduce more realistic experimental apparatus for DWO investigation. In experiments, a bypass tube is usually connected to the heated channel to maintain the constant pressure drop boundary condition. The influence on stability of the bypass area is hereby examined. The analysis is completed by studying twin parallel channels. In particular, different configurations are simulated to underline the influence of some geometric parameters on RELAP5 predictions. Channel geometry has been modified to reproduce an experimental facility including two helically coiled tubes. Simulation data are clustered in dimensionless stability maps, generally adopted in such stability investigations. The results aim to be a contribution to the assessment of the code capability to detect the onset of twophase flow instabilities in boiling channels. Within this respect, RELAP5 can provide useful results for comparison with analytical models and in preparation of experimental campaigns on the matter. Therefore, purpose of the paper is to highlight strengths and weaknesses of the code to simulate DWOs as well as to evaluate carefully the suitable numerical settings necessary to assure a correct prediction of the phenomenon.