~ Pergamon www.elsevier.com/locate/actaastro Acta ,4 ~tronautica Vol.48. No. 5-12, pp 629-638,2001 © 2001 ElsevierSoence Ltd All rightsreserved Printed m Great Britain Plh S0094-5765(01)00032-7 0094-5765101$- see front matter NEAR-CRITICAL FLUII~ UNDER MICROGRAVITY : STATUS OF THE ESEME PROGRAM AND PERSPECTIVES FOR THE ISS D. Beysens Commissariat/t I'Energie Atomique Equipe du Supemritique pour i'Environnement, les Mat~iaux et I'Espace, Service des Basses Tempc~atares, lnstitut de Chimie de la Mati~re Condens~ de Bordeaux, F-33608 Pessac Cedex (France) Y. Garrabos Cenffe National de la Recherche Scientiflque Equipe du Supereritique pour I'Environnement, les Mat~iaux et I'Espace, lnstitut de Chimie de la Mati~re Condenfde de Bord~u-~, Universit~ de Bordeaux i, F-33608 Pessac Cedex (France) Abstract. Started 16 years ago, the ESEME program has led to a number of important findings. We note a simple and unified view of phase transitions, which has been applied to the development of biological patterns, and a very fast thennalization mode that we coined the "piston effect". This effect has been applied to control the cryogenic reservoirs of the Ariane 5 rocket. All these findings have been obtained thanks to the good coordination of the ESA and CNES space facilities and the constngtion of high technology eXlXa-kaental modules. The fut~e of the wogram is linked to the CNES DECLIC facility and the ESA Fluid Science Laboratory (FSL). DECLIC has been designed to increase the temperature regulation above the critical point of water (550 K) so as to investigate chemical reactions under conditions of supercritical water, and in relation to the promising applications of waste treatment by supercritical oxidation. Thanks to the construction of a special vibrational Experiment Container for FSL, the thermal and mechanical behavior of fluids under forced vibration can be investigated. The results of such studies will help to estimate the effect of g- jitter on fluids, and control gases and liquids in space. © 2001 Elsevier Science Ltd. All rights reserved. I. INTRODUCTION Our program is mostly devoted to the study of fluids, in the vicinity, inmaediate or not, of their critical point. To give some examples, the critical point of carbon dioxide (CO2) is observed at 3 l°C and 72 bar, that of water (I-120) at 375°C and 225 bar and that of hydrogen at 33 K and 13 bar. Above the critical temperature and pressure, fluids are called "supercr/t/cal" (Fig.l). They exh~it there a nomber of specific properties (large density, low viscosity, large diffusivity) which make them intermediate between fiquids and gases t. In addition, their isothermal compressibility and thermal expansion can become very large, especially when they approach the critical point. The highly variable properties of near-critical fluids make them very attractive for studying many interesting phenomena that hold for all fluids because of the critical universality. Fluids in their supereritical state are increasingly used by the food and waste management industry 2 for thek solubilization properties (e.g. supercriticai CO2), as host of "cold" combustion (e.g. supercritical water), in energeties (supererifical thermal or nuclear plants), and in astronautics (e.g. storage of cryogenic fluids). However, their behavior under terrestrial (I-g) or space (zero-g) conditions is not well studied. Their use raises fundamental questions concerning fluid dynamics, heat transfer, interfacial phenomena and chemical processes. Experiments in the absence of effects due to gravity, and especially those in the International Space Station, is a tremendous opportunity to answer these questions and enhance knowledge in this field, which is of both fundamental and industrial interest. 2. NEAR - CRITICAL FLUIDS AND GRAVITY Fluids in their near-critical or supercritical state are strongly affected by gravity. As they are highly compressible, gravity compresses them under their own weight, thus ~preventing a close approach to their critical I~'in! . Super- crilical Liquid Fluid ~ Solid O _ O ~- ~riti~"Point oas ~nt Temperature Fig.I. Phase diagram of a pure substance in the temperature - pressure plane. The supercritical -state" corresponds to a compressed gas that shows the density of a liquid. 629