Projet de recherche PX/7/LP/39 (Action de recherche PX)
The dynamics of evaporating liquids is still poorly understood nowadays, despite widespread applications in various industrial processes and devices. In order to avoid costly case-by-case analyses and trial-and-error approaches, modeling tools must be developed, which need to be validated in idealized cases. For this purpose, the experiment ITEL (Interfacial Turbulence in Evaporating Liquids) has been proposed by ULB, and concerns the nonlinear dynamics of a layer of ethyl alcohol submitted to a flow of inert gas.
The experiment has already been flown onboard MASER-9 in March-2002. Actually, ITEL-Maser 9 is a precursor flight for the preparation of the CIMEX-1 experiment, originally foreseen for the International Space Station (ISS), in the frame of the CIMEX (Convection and Interfacial Mass Exchange) research programme of the European Space Agency. The present project concerns the upgrade of the ITEL module to the FOTON-M3 satellite, which is presently considered as an alternative to flying CIMEX-1 on ISS. This would indeed allow a much higher scientific return, as detailed hereafter.
The ITEL flight module has been built by the Swedish Space Corporation (SSC), while the optical diagnostics (Schlieren imaging of the evaporating surface and 3D re-construction of the liquid temperature field by optical tomography) have been designed by Lambda-X. The latter diagnostics have been extensively tested on ground by ULB-MRC and Lambda-X, and extensive bread-boarding of the flight module functionalities and software has been achieved in SSC in Stockholm, by both Lambda-X and ULB-MRC.
In the ITEL set-up, an inert gas (nitrogen) is forced to flow over the free surface of an evaporating liquid layer (ethyl alcohol), and refractive index gradients within the liquid are visualized using Schlieren imaging perpendicular to the free surface, and interferometry in six directions parallel to the interface. These six views are then grabbed at 25Hz and used for post-flight reconstruction of the 3D temperature field.
This combination of diagnostics has indeed allowed to follow the evolution of thermal ripples in the liquid due to Marangoni convection (module descriptions and results have been published in several papers, as seen in the reference list hereafter). During all ground tests of ITEL and even during the flight, it also appeared that the phenomenon studied is actually much more complex and interesting than it was thought at the beginning of this Prodex contract. In addition to interfacial turbulence, indeed also observed in microgravity, there are a number of other fluid behaviours : large-scale patterns composed of one thermal ripple within the experimental cell, oscillating patterns of various structures, and mixing of the latter with interfacial turbulence. Moreover, one of the important results obtained is that the gas phase (inert gas + vapour) above the liquid has a higher importance than expected on the flow patterns generated by Marangoni convection. In particular, the concentrational boundary layer driving the evaporation within the liquid is deeply influenced by the gas flow, and hence by the gas channel geometry.
However, even though the ITEL module behaved perfectly during all mission simulation tests, and even during countdown, a software parameter used to control the pressure within the experimental cell changed just before launch, likely due to an error in software development by SSC. This off-nominal pressure control was responsible for an overpressure in the cell during most of the flight, which greatly reduced the evaporation rate, and hence the interfacial turbulence. Despite of this failure, the post-flight analysis of the results shows that all other functionalities of the module worked quite satisfactorily (such as optical tomography, Schlieren, automatic surface flatness control, thermal regulation, …), and allowed to clearly demonstrate the occurrence of interfacial turbulence in microgravity (see publication list).
Due to the partial failure of the pressure control in ITEL, and the similarities between the ITEL set-up and the CIMEX-1 experiment foreseen for the International Space Station, the European Space Agency is presently considering to upgrade the ITEL module for flight onboard the FOTON-M3 satellite (foreseen for the first half of 2006). There are several advantages to proceed in this way :
• no need for re-condensation of the vapor, as foreseen for CIMEX-1 on ISS
• better microgravity level on FOTON than on ISS
• possibility of testing different liquids (actually, four)
However, upgrading ITEL to FOTON will also require a number of modifications :
• implementation of a polygonal glass cell for working with different liquids
• implementation of an active thermal control
• implementation of a fluid management system allowing to test 4 liquids
• implementation of a movable bottom to investigate the role of liquid depth
The present Prodex implementation contract, planned for three years from January 2004, should allow to conduct the ground preparatory work necessary to upgrade ITEL to the FOTON-M3 satellite, together with the analysis of flight results. This will require one experienced post-doctoral researcher during the full three-year duration of the project, together with the development and testing of a new experimental cell allowing to provide reference ground experiments and define scientific requirements related to the flight cell, to be developed by the industrial contractor selected by ESA.
