Analysis of Basic Phenomena in Boiling Channel Instabilities with Different Flow Models and Numerical Schemes

The results of a computational study on boiling channel stability are here discussed. The study compares the information obtained by two programs, developed in previous work for the linear and the nonlinear stability analysis of boiling channels basing on a simplified flow model, with the predictions of a well known system code. The phenomena highlighted by the results of the system code, adopted with different flow models and numerical methods, are discussed in a systematic way with the aid of the description obtained by the simplified model, in order to obtain a clearer picture than could be achieved by the mere application of the system code to conditions of interest for boiling water reactor stability. The effects on the obtained results of non-equilibrium, numerical discretization and number of nodes are considered, evaluating their influence on the predicted stability boundaries. Some physical aspects of the density-wave mode of instability are finally discussed basing on the predictions obtained by the different models

Sensitivity Analyses on Natural Circulation in a 8:1 Tall Enclosure using Finite-Volume Methods

The results herein presented are an extension of those obtained in previous work by the Authors in a benchmark problem dealing with flow driven by buoyancy in an 8:11 tall enclosure. A simple finite-volume model purposely set up for this application has provided the preliminary results reported. The adopted modeling technique was a direct extension of the one previously adopted by the Authors to deal with single-phase natural convection and boiling channel instabilities. This extension to two-dimensional flow is based on a finite-volume scheme using first order approximation in time and space. Despite its simplicity, results were reasonably good and detected the flow instabilities due to proper selection of cell Courant number and a semi-implicit solution algorithm. In this paper, results using the same code with different discretisations are presented in a more detailed way and are further discussed. They show proper capture of all the main characteristics of the flow, also reported by other authors and considered as "converged" solutions. Results show that, as expected, first order explicit or semi-implicit methods can be considered reliable tools when dealing with stability problems, if properly used. Some initial results obtained using a second order upwind method are also presented for the purpose of comparison. Additionally, results obtained using a commercial code (FLUENT) are also reported

On the convergence of RELAP5 calculations in a single-phase natural circulation problem

It is shown that, using a single volume to lump the heat source and sink, it was not possible to reach convergence to steady state flow rate when the heated (cooled) length was diminished and the heat transfer coefficient increased to keep constant the total heat transferred to (and removed from) the fluid. An algebraic justification of these results is presented. Concerning the effect of nodalization on the damping of instabilities, it was shown that 'reasonably fine' discretization lead to damping. However, the search for convergence of numerical and theoretical results was successful, showing the expected nearly chaotic behaviour. This search lead to very refined nodalization. The results obtained have also been verified by the use of simple, ad hoc codes

Linear and Nonlinear Analysis of Density-Wave Instability Phenomena

In this paper the mechanism of density-wave oscillations in a boiling channel with uniform and constant heat flux is analyzed by linear and nonlinear analytical tools. A model developed on the basis of a semi-implicit numerical discretization of governing partial differential equations is used to provide information on the transient distribution of relevant variables along the channel during instabilities. Furthermore, a lumped parameter model and a distributed parameter model developed in previous activities are adopted for independent confirmation of the observed trends. The obtained results are finally put in relation with the picture of the phenomenon proposed in classical descriptions