Experimental study of a two-phase flow thermosiphon

Thermosiphons are passive systems capable of transferring heat without the need for external mechanical energy. The difference in density from the bottom to the top of the system creates a buoyancy force that drives the circulation of the fluid. There is a compelling argument for using such a device in the event of an accident. However, thermosiphons tend to be unstable, especially in the case of two-phase flow. Furthermore, there is a lack of experimental data to validate numerical simulations. Therefore, there is a need for experimental results on two-phase flow thermosiphons. An experimental facility called THEDI has been developed. An analysis of the stability criterion and the modelization of mean mass flow rate, mass flow rate oscillation amplitude and frequency, are discussed in this work. Two different evolution trends of mass flow rate ; gravitational (increasing with heating power) and frictional (decreasing with heating power), as well as three flow regimes ; single flow, oscillating two phase flow and pseudo-stable flow are identified. The experimental data is plotted in a stability map and a criterion on quality (over 2.3%) is found to be the transition between the oscillating and pseudo-stable flow regime. The development of a simplified correlation on the oscillation frequency of the flow rate allows the prediction of the trend in evolution as a function of the heating power within an error interval of 40%.