Absorbent falling film with free-surface instabilities: exploration – FRAISE

FRAISE project intends to optimize energy conversion through falling-film absorption processes. Its main technological outcome is the development of innovative concepts for the design of efficient desorbers, which represents the bottleneck for the conception of new compact absorption machines adapted to automotive air-conditioning, and more generally to the design of efficient heat pumps, chillers and recovery systems to limit energy waste. The project focuses on the automotive application for which compactness is crucial. Yet, the investigated design solutions will benefit to the development of compact absorption machines adapted to abundant low-grade temperature sources (industrial waste, marine transports) and renewable energies (solar cooling, domestic heating). Desorbers are key elements of the absorption machines where coupled heat and mass transfer occur. The correct sizing and the compactness of these components represent the principal challenges to the aimed technical application. We propose to develop new concepts of desorbers using plate exchangers with falling films, which have the advantage to be easily operated in vacuum conditions as required whenever low-temperature heat sources are considered. In this project, we propose to optimize and control the wavy motion of a falling film in order to intensify heat and mass transfers across the film. Indeed, it is known that the mixing and surface renewal mechanisms generated by surface waves may enhance heat and mass transfer rates several folds. This project is thus devoted to the wavy regime that mostly develops at moderate Reynolds numbers. Passive control by means of wall corrugations will be considered and tested under external vibrations. The design of new strategies of transfer intensification requires (i) to understand how the wavy dynamics is affected by the coupling with the transfer due to the induced variations of physical properties at the free surface, (ii) to identify the most efficient wavy structures and their optimum dynamics (rates of creation and merging, spatial and temporal distribution etc.) to promote transfers and (iii) to propose efficient strategies to control the hydrodynamics of the flow, generate these wavy structures and their distribution in time and space and to test these strategies under external vibrations. To meet such requirements, we propose a strategy combining an advanced fundamental research effort and a latter-stage more applied study with the adaptation of a dedicated prototype of absorption machine and a test campaign on an experimental bench developed by the industrial partner. This exploratory project, oriented to fundamental research combine theoretical and numerical approaches based on direct numerical simulations (DNS), advanced shallow-water mathematical modelling and thermodynamic modelling at the component and system levels, to experimental studies using avant-garde non-intrusive optical techniques, and in particular, a two-colour Laser-Induced Fluorescence technique that will be adapted to to measure in-depth temperature gradient across the wavy film. The project will take advantage of the skills of three complementary laboratories in the field of Heat/Mass Transfer using specific non-intrusive optical techniques (LEMTA), Applied Mathematics and shallow-water approaches (LAMA/LOCIE) and Engineering with the design of absorption machines (LOCIE). The projects benefits from the involvement of the industrial partner (PSA) (two already financed test benches, one of which at PSA). The synergy between the theoretical and experimental investigations will be fostered by the proximity between two partners (LOCIE and LAMA on the same campus) and the regular meetings of the informal CNRS group GDR FILMS.