The Numerical Investigation of Boiling Flow Instability in Heated Channels

Abstract

To ensure the stability of boiling working fluid flow is an important problem in such applications as Boiling Water Reactors¹ and Two-Phase Thermal Control Systems for Spasecrafts². The given problem was the objective of numerious investigations since 1960’s. The monographs^3–5 contain the methods of linear stability analysis of boiling flow in a heated channels. The flow is assumed to be transient one-dimensional homogeneous with local losses. The system of non-linear equations is linearized. Laplace transforming is used for getting an analytical solution. The stability analysis represents the search for the boiling flow system parameters dividing the regions of stable and unstable operation. The reviews of two phase-flows stability investigations perfomed in the West by the end of 1970’s were given^6–7. Bundles of rods and tubes of nuclear reactor core or steam generator, evaporative heat-exchangers networks of two-phase thermal control system represent the systems of parallel heated channels. In such systems the oscillatory instability manifests itself as the periodic flow rate oscillations under conditions of fixed pressure drop. Parallel channels instability was studied both analytically (linear and quasilinear approaches)⁸ and numerically with the use of simplified nonlinear models^9–10. When analysing the parallel channels operation it is important to know not only the threshold of instability but also to predict the limits of parameter variations in each channel under unstable operation. Because of that and due to advances in high-speed computers and finite-difference schemes in the last years computer programs solving non-linear equations are developed even suitable for microcomputers¹¹.