Experimental study of the dynamics of passive thermosiphons

Two-phase thermosyphons allow the efficient and passive transfer of thermal energy from a hot source to a cold source. Their physical principle is based on the natural circulation induced by gravity without the need for so-called active means (such as pumps). They can operate in a closed circuit according to the heat transfer mechanisms of evaporation and condensation, with the main advantage of benefiting from a very high value of latent heat of phase change compared to sensible heat. They are used in many industrial applications involving heat exchangers: cooling of electronic components, cooling of rotating machinery, etc.In the context of work on new nuclear reactors, particular attention is being paid to passive systems to ensure safety functions based on two-phase thermosyphons. They are considered as a credible alternative to the essentially active systems present in the current and future EDF fleet. Active systems require an external energy input (electrical or mechanical) to operate, such as hydraulic pumps, control valves, etc.Passive systems, on the other hand, are defined by their activation and operation based on a natural balance between the driving forces available in the reactor during accident situations, generally of moderate intensity (pressure, temperature, density differences, etc.) and the desired low resistance forces (pressure drop in pipes or tubes, thermal resistance through heat exchangers, etc.). These systems do not require any external energy input, except possibly in the initial phase to activate, for example, an opening valve.The physical phenomena associated with passive systems are generally induced by time-varying potentials of moderate intensity, governed by complex and multiple heat transfer mechanisms (condensation, boiling), which can lead to poorly controlled phenomena such as priming and dynamic instabilities of the thermosyphon.This thesis consisted of the design, construction, instrumentation and operation of a new test facility, THEDI (Two-phase THermosiphon). The test campaign allowed-the analysis of the many physical phenomena that occur in a passive two-phase system, both at local and system scale-the evaluation of the performance of the thermosyphon in terms of energy extraction;-the study of transient phenomena: the establishment of fluid circulation and various instability phenomena.These different aspects have been studied for a wide range of operating conditions: heating power, pressure, subcooling, section restriction or presence of non-condensable gases. The database thus created allowed comparison with several predictive models of thermosyphon or two-phase system operation: prediction of flow rate, frequency or amplitude of oscillations, or instability maps. Most of these models are derived from the literature, but the comparison was also made with a one-dimensional model developed during the thesis.