Time-Domain Nonlinear Investigation of Two-Phase Flow Instability in HTGR OTSG Using SIMULINK

Abstract Two-phase flow instability may occur in the once-through steam generator (OTSG) of High-Temperature Gas-cooled Reactors (HTGR). A time-domain theoretical model of HTGR OTSG was established to investigate this phenomenon. The heat transfer tube of OTSG was divided into three regions, namely the subcooled water region, the saturated two-phase region and the superheated steam region. The influence of inlet resistance coefficient could not be accurately predicted by a single-node model of the saturated two-phase region. Consequently, the saturated two-phase region was subdivided into several regions, each experiencing the same enthalpy increase. The dimensionless one-dimensional partial differential governing equations were transformed into nonlinear ordinary differential equations (NODE) using integration and the Leibniz formula under constant pressure drop boundary conditions. The physical parameters characterizing flow instability were dimensionless inlet flow velocity and the dimensionless length between the entrance and different nodes. These NODE were solved using SIMULINK (a toolbox of MATLAB). The typical characteristic of DWO was captured by NODE and SIMULINK, where the phase of dimensionless inlet and outlet mass flow rates are opposite. The multi-nodes (in the saturated two-phase region) time-domain theoretical model can accurately predict the stability boundary of High-Temperature Gas-cooled Reactor-Pebble bed Module (HTR-PM) OTSG. This multi-nodes model was used to predict stability boundary at start-up conditions further, and there is enough safety margin for HTR-PM OTSG.