Analyse thermodynamique des absorbeurs à film tombant pour l’amélioration de l’efficacité des machines à absorption

In the context of greenhouse gases reduction, an increasing attention is dedicated to carbon–free power plants solutions. To answer to this growing demand, tiny nuclear reactors or Small Modular Reactors (SMR), are being developed such as the 170Mwe Pressurized Water Reactor within the NUWARD project. This technology is downscaled, modular, with a very compact Steam Generators (SG) design in comparison to current recirculating SG. Moreover, the secondary fluid is vaporized through one unique passage in millimetric channels. However, such devices potentially include static (Ledinegg) and dynamic (density wave oscillations, …) two-phase flow instabilities. These instabilities can alter the SG’s efficiency, lifetime, and even integrity from modifying the temperature, mass flow and pressure levels. Consequently, it justifies a more precise analysis and understanding of the instability’s mechanism. In this PhD, a thorough study of the Ledinegg instability and the flow maldistribution phenomenon is carried out in the compact plates SG’s operating conditions. In a capillary dominated regime we consider a localized, infinitesimally thin interfacial front plunged into a forced longitudinal temperature gradient whereby vaporization arises leading to successive liquid-gas phases distribution within the channel. Whereas the liquid and vapor velocity profiles are provided by the Poiseuille’s law, the temperature fields in the solid and the fluid are obtained using the generalized Graetz modes method, specifically adapted to the considered vaporization model. The generalized Graetz modes decomposition permits a semi-analytical solution of the 3D convection-diffusion problems provided that the velocity field, domain’s section and Peclet’s number are longitudinally invariant along the flow direction. In the first chapter, this methodology is used to analyse heat transfers in single-phase natural convection circulation loop. A new universal scaling law for the relation between the Grashof and the Reynolds numbers is obtained, this is confirmed by an asymptotic analysis and direct numerical simulations and is successfully compared with experimental data sets. This analysis has highlighted the influence of boundary conditions, boundary layers, and fluid to solid thermal conductivity ratio in the heat transfer control. In the second chapter, the generalized Graetz modes method is extended to solve the temperature fields as well as the two-phase interface position within the vaporization model. This methodology is applied to two configurations: a uniformly heated single channel and a co-current heat exchanger. The vaporization’s numerical computation with imposed heat flux in a microchannel depicts the proportionality between the front’s position and the liquid Peclet’s number. The results are consistent with the theoretical energy balance analysis as well as with experimental data obtained in the literature for moderated mass flows and heating powers. Using the resulting interface’s position law into a pressure drops model, the boundaries of the stability areas in a single heated microchannel and many parallel channels have been computed and analysed. In the case of the co-current heat exchanger, the state-of-the-art remains spotty because most of stability studies deals with imposed heat flux and thermally insulated channels, not relevant for conjugated heat transfers in a heat exchanger which deviate from such simplified assumptions. Our confined vaporization model predicts a logarithmic dependence between the two-phase interface’s position and the secondary inlet Peclet’s number. The influence of the fluid properties, primary mass flow and the wall thermal conductivity on this law has been studied and allowed to specify the stability criteria for a single heat exchanger and a network composed of parallel heat exchangers, closer to the compact plates’ SG.