Experimental and numerical study of the coupling between evaporation and thermocapillarity preparation of the Cimex-1 experiment

The present work has been organized in two main parts: in the first one, the focus will be on the scientific and theoretical aspects of the evaporation process in presence of an inert gas flow while in the second all the technical aspects and more practical tests related to the real implementation of the micro-gravity experiment CIMEX-1 will be detailed. In any cases, the discussion will always start from the phenomenology observed considering that ” Nature is far more reach of any speculations.”

Part I: Evaporation in presence of inert gas

In chapter 1, a detailed presentation of the experimental setups for the on-ground tests is given together with the presentation of the qualitative and quantitative results obtained. Actually, the main parameters that regulate such kind of experiments are the mass flow rate of inert gas, the total pressure of the cell and the geometrical shape and dimensions of the evaporating regions.

Consequently, the experiments aimed at covering the maximal possible combination of these three parameters with special attention to the variation of the inert gas flow and of the thickness of the evaporating liquid layer. More precisely, the liquid layer thickness was in the range 1.2 to 3.8 mm while the inert gas flow was set in the range 50 to 2500 ml/min. The pressure has been partially neglected as control parameter because its control was discovered not to be very reliable.

The visualization system used in all the experiments consisted in a opportunely calibrated infrared camera. It allowed for having a quantitative analysis of the temperature distribution at the interface of the evaporating liquid.

The infrared images also helped to follow the thermal history of the interface. In many cases, it has been possible to clearly observe the evolution of instability patterns at the interface that represent an original contribution to the understanding of such a kind of phenomena.

The physical and mathematical modeling of the observed phenomenology will be the subject of the chapter 2. One of the peculiar issue of the problem under consideration is that the thermal gradient normal to the interface is not directly imposed like in the usual Marangoni-Bénard experience, but is a result of the cooling of the interface due to the evaporation.

Moreover,the interface is subject to the shear stress of the inert gas flow and to the one due to the thermo-capillarity. Finally, the gas phase is to be considered as a mixture; this oblige to solve a diffusion problem in the gas phase. A physical model that takes into account the different aspects mentioned above is presented together with the governing equations and the appropriate boundary conditions.

Numerical issues involved in solving the model are also analyzed. Numerical results obtained are finally discussed and compared when possible with experimental results.

Part II: Preparation of the CIMEX-1 experiment on-board the International Space Station.

In chapter 3, we will describe the main platforms used to perform low-gravity experiments. They will be classified according to the low-gravity level and to the low-gravity interval duration that could be ensured for experiments. According to these criteria, the list of the low-gravity platforms will be as follows: Drop Towers with ≈ 4 sec. of micro-gravity, Parabolic Flights that can assure not more than ≈ 25 sec., Sounding Rockets with a low-gravity time of the order of several minutes depending on the rockets, Foton Capsules that assure for many days of high quality - i.e. without perturbations - low-gravity level and , last but not least, the International Space Station where the low-gravity duration could be even of several weeks which is a sufficient time duration for the most part of the experiments.

The chapter 4 will be entirely devoted to the ITEL experiment that is the precursor and the core of the CIMEX-1. After a brief overview of the experiment that has been performed twice on-board sounding rockets of the MASER class, the experimental setups used both on-ground and in micro-gravity will be detailed.

The focus will be on the experimental results obtained on-ground during the preparatory tests and during the two sounding rocket flights with special attention to the first one. The analysis will be supported by the presentation of many results obtained in numerical simulations.

The two parabolic flight campaigns performed to test one of the key sub-systems of the CIMEX-1 setup are the subject of the chapter 5. The separating-condensing unit is mandatory for performing the experiment on-board the International Space Station because the limitations on the crew intervention oblige to have a closed loop experiment.

The goal of the two parabolic flights will be detailed together with the setup and the experimental scenario. The main results will be also shown and some considerations on the efficiency of the system will be presented.

It is worthy to stress that the results obtained during these parabolic flights have been determinant at the European Space Agency level to fly the CIMEX-1 experiment on-board the International Space Station.

Finally, in the section conclusions and perspectives the main results obtained will be summarized together with the new scenarios opened by the present work and some guidelines for further development in the experimental, theoretical and technical analysis.