An experimental and theoretical study of thermal-hydrodynamic instability at low mass flow rates in two-phase upflow systems

An experimental and analytical investigation of thermal-hydrodynamic instabilities that are associated with Boiling Water Reactors is conducted. The cooling channels of a BWR core is simulated by a single stainless-steel pipe which is heated with an electric current. The flow system consists of this heater, a fluid supply section and a recovery section. These sections together form an open loop that operates under constant inlet and exit pressures. The experiments indicate that the system is subject to thermal-hydrodynamic instabilities which are confined to a certain range of mass flow rates. There exists a narrow stable region at high mass flow rates. Contrary to previous expectations, there exists another stable region at very low mass flow rates. Parallel to the experimental study, an analytical study is conducted in which a mathematical model based on the assumptions of homogeneous fluid and phase equilibrium is used to generate the flow characteristics of the system. Using numerical experimentation techniques, oscillations are simulated and the stability-instability boundaries of the oscillations are determined. It is concluded that the mathematical model also predicts a thermal-hydrodynamically stable region at low mass flow rates