The gravity-driven flashing of metastable water in a pool heated from below: An experimental study

The present doctoral research is focused on the gravity-driven flashing of metastable water, a phenomenon which is usually encountered in natural geysers. Specifically, the emphasis is put on the case of a water pool heated from below, motivated by the absence in the scientific literature of any known report of the phenomenon in this precise configuration. A wealth of data, close to the case of interest and related to genuine geysers or unheated depressurized pools can be identified. But the extrapolation of those available results to the studied case is not straightforward and some questions can be specifically formulated. Does the phenomenon exist in a pool-type geometry, heated from below? If yes, what are its main characteristics? Are we able to model as simple as possible the studied physics for further macroscopic transient simulations? Because of a lack of information about the studied case in the scientific literature, providing an answer to the above questions implies performing some new experiments. This has motivated the design of the test device called Aquarius, central to this doctoral research. For defining Aquarius, a novel downscaling methodology, presented in this thesis, has been developed by the present author. The method mainly consists in an operating pressure distortion, which allows conserving a large saturation temperature vertical difference even at a reduced scale, with an appropriate choice of system's pressure. Indeed, in the low-pressure case, because of existing steep variations in water saturation temperature against pressure, a little variation in hydrostatic pressure, provided by a reduced pool level, might be enough for achieving some required large vertical difference. By taking some height, this similarity approach, unmentioned yet in the scientific literature to our knowledge, may be beneficial to other types of applications where the gravity-driven flashing is expected to occur, providing away to reduce the size and hence the cost of the envisioned experiments and keeping water as the working fluid (e.g. for 1D-geysers or the pools constituting a passive cooling capacity for some light water nuclear reactors). Having developed this experimental tool, the phenomenon has been further studied and the obtained results are detailed in this thesis. First of all, it is shown that the test device allows obtaining an upper superheated area, located below the liquid free surface, from which the uprising metastable water may potentially turn into bubbles. In this area, the recorded water temperatures often exceed the local saturation and substantial superheats ranging from 1 to 5 deg. C are reachable. Next, under certain circumstances, the metastable water spontaneously flashes, as materialized by the emergence of numerous bubbles, thereby validating the proposed downscaling methodology. Most of all, the performed tests highlight the existence of the gravity-driven flashing of metastable water in the configuration of a pool heated from below, which constitutes the main result of the present research. Apart from the heating power, the operating pressure and the initial pool level, it is also shown that the phenomenon is particularly sensitive to the initial amount of dissolved gases in the configuration of the Aquarius experiments. Pointedly, the achieved tests have exhibited that the produced metastable water relaxes through the emergence of bubbles within the liquid bulk and almost never onto some immersed solid walls, contrarily to what can be envisioned on the basis of the scientific literature on flashing flows. A theoretical study of the observed nucleation processes is then conducted and provides an interpretation of this very fact. Nevertheless, the fundamental mechanisms standing behind the observed bulk nucleation are not fully unveiled yet. On the basis of some specifically conducted experiments, it appears that neither the so-called homogeneous nucleation nor the nucleation from gas nuclei entrapped into some suspended solid particles are serious candidates for explaining the bulk processes. Instead, the hypothesis of a main contribution of freely-floating gas nuclei to the observed bubble growths is very likely. But we are however left with the same questions that animate the cavitation and thermal-hydraulic communities for decades regarding the stabilizing mechanisms that may act on those freely-floating nuclei. At last, the characterization of the heat and mass transfers taking place within the liquid pool during a typical experiment is discussed. Precisely, some heat and mass transfer coefficients, derived from the available test data, are presented. Interestingly, the correlations that have been defined from the latter coefficients for the two-phase regime are rather original in their mathematical formulation. Those correlations link the noticed heat and mass transfers intensification in the presence of numerous bubbles to the so-called Gibbs number, which is a dimensionless form of the energetic cost required for a bubble nucleation. Moreover, the pre-factor of those correlations does appear related to the initial pool level, which is clearly surprising and could motivate further studies. At last, with those correlations defined and a lumped-parameter model of the liquid pool written, it has been possible to simulate some pre-selected tests, with moderate discrepancies. (author)