Research project PX/8/LP/07 (Research action PX)
This PEA is intended to cover part of the ULB and ULg activities related to the CIMEX experiments on evaporating liquid layers under inert gas flows (ITEL2, CIMEX1-3D and CIMEX1-2D), and on evaporating contact lines and porous media (CIMEX2).
1. ITEL2, CIMEX1-3D and CIMEX1-2D experiments: The presence of an inert (or non-condensable) gas, with or without flow, significantly complicates the classical problem of evaporative convection, since it strongly stimulates the apparition of surface-tension-driven instabilities of the evaporating liquid layer, even though the liquid-gas interface remains very close to thermodynamic equilibrium. For a two-component gas, it is indeed the partial pressure of the volatile component that is univocally linked to the interfacial temperature, and not the total pressure. Thus, although the total pressure in the gas remains quite constant, the fluctuations of the partial pressure of the vapor, which are coupled to the interfacial temperature fluctuations, can easily be enough to yield instability. This was clearly qualitatively demonstrated during the ITEL (Interfacial Turbulence in Evaporating Liquids) experiment, even though a re-flight of the latter in March 2005 is planned due to a deficient pressure control during the first flight in March 2002.
Given the increase of evaporation rate expected when convection occurs, and the opposite limiting effect of vapor diffusion through the inert gas, one of the main questions of ITEL and CIMEX1 experiments is to determine what is the net effect of an inert gas on the global heat/mass transfer rates. In the frame of this project, such target will be reached through systematic experimental investigation, supported by advanced theoretical and numerical methods based on the tools of Nonlinear Physics and recent developments in Non-Equilibrium Thermodynamics. In addition to understanding and quantifying the evaporation process, essential questions will also be tackled, such as the transitions between various types of flows, thermodynamics of interfaces, the nature of surface-tension-driven interfacial turbulence, its universal characteristics, …
As far as the state-of-the-art in this research field is concerned, it must be pointed out that despite the fact that evaporative convection is known for some decades (due to its applied interest, e.g. in chemical or thermal engineering), a systematic investigation and quantitative predictions of the influence of surface-tension-driven evaporative convection is still missing for several reasons. First, during ground experiments, buoyancy-induced currents and interfacial turbulence are always mixed and hardly distinguishable. In fact, in order to accurately measure the characteristics of interfacial turbulence (and to maximize its intensity), it is needed to conduct experiments with large liquid depths (of the order of 1-2 cm). On ground, such experiments are deeply influenced by the gravity-induced convection, which calls up for microgravity experiments allowing to isolate the effect of surface-tension gradients. The second reason for this lack of data on interfacial turbulence is its complexity, namely the presence of rapidly evolving small-scale structures, whose observation requires using fast and accurate optical diagnostics. Last but not least, on the modeling side, only the most recent progresses in Nonlinear Physics and Non-equilibrium Thermodynamics coupled with the parallel development of computers allow to expect the development of accurate theoretical and numerical prediction tools, essential to analyse experimental results and to extract the fundamental features of evaporative convection.
On the point of view of timeliness of the proposed research, it must be pointed out that several teams in Europe and in the United States are currently studying (theoretically and experimentally) evaporative convection in several contexts. Actually, given this international competition, it should be stressed that the microgravity experiments proposed in the framework of CIMEX1 are extremely challenging given their novelty, and should allow to answer most of the questions motivating the current interest in this field. Moreover, it will also be the first attempt to measure the three-dimensional characteristics of the thermal field during evaporation, thanks to a tomographic set-up based on interferometry, which was successfully validated during the MASER-9 sounding rocket campaign of March 2002. In addition to the Schlieren set-up also used during this mission, the CIMEX1-3D experiment will also make use of an infrared camera to measure the free surface temperature field, and of particle image velocimetry to measure the surface velocity field.
Moreover, one of the objectives of the CIMEX program is to study evaporative convection in a quasi-2D situation, namely by “sandwiching” the liquid between two rigid transparent plates separated by a very small gap (Hele-Shaw cell). This experiment, named CIMEX1-2D, is complementary to the CIMEX1-3D experiment : while the latter will allow to study the global three-dimensional convective pattern organization and the enhancement of heat transfer due to convection, the former will yield essential information about the specific mechanisms at the origin of secondary (boundary-layer) instabilities and transitions to time-dependent flows characteristics of interfacial turbulence. Note also that Hele-Shaw cells are often used to model real porous media, due to similarities (Darcy’s Law) in mathematical description. Hence, the CIMEX1-2D experiment is also strongly related to CIMEX2b tasks (see below).
As far as the overall objectives of the CIMEX research program are concerned, the research performed in the framework of CIMEX1 is determinant. Indeed, one of the main application field considered in CIMEX is the improvement of heat exchangers, and research is also conducted about two-phase systems allowing the more efficient transport of thermal energy. Actually, one of the promising candidate devices uses a thin liquid film in contact with the dissipating unit (e.g. electronic components), and submitted to a high-speed flow of inert gas (shear-driven liquid film evaporator), i.e. a situation quite similar to that studied in CIMEX1. This promising set-up should allow to remove very high heat fluxes, unreachable by conventional devices. Not only will the theoretical models be applicable to study this kind of evaporator, but it is also expected that several components of the ground hardware will be re-used for conducting new experiments aimed to measure performances of the proposed set-up. A parallel proposal for parabolic flights on this subject is currently submitted to ESA.
Finally, the models and experimental techniques developed for CIMEX1 are also expected to be useful as far as several other objectives of the CIMEX program are concerned : the evaporation of drops in air (CIMEX4b), the enhancement of thermo-capillary convection in the vicinity of boiling bubbles (CIMEX4a) and in heat-pipe micro-regions (CIMEX2a) due to dissolved and non-condensable gases, the effect of the latter on the efficiency of heat pipes and capillary-pumped loops, …
2. CIMEX2 : The CIMEX2 experiments, originally proposed by Prof. P. Stephan in TUD-Darmstadt, concern the study of evaporating contact lines (CIMEX2a) and of advanced capillary structures (CIMEX2b). As far as CIMEX2a is concerned, it has been shown by P. Stephan that micro-region effects (Van der Waals forces, curvature effects, chemical non-equilibrium at the interface, …) are of uttermost importance since most of the evaporation rate (about 50%) occurs very near the contact line of the liquid surface with the heater. In strong collaboration with the TUD team, ULB will develop an experimental set-up allowing to observe evaporating contact line instabilities with a Schlieren device. In addition, ULB will contribute to the modeling and interpretation of the ground and micro-gravity experiments, by incorporating several effects (non-condensable gas and Marangoni effects, improved disjoining potentials, …) into the models previously developed by P. Stephan.
As far as CIMEX2b is concerned, TUD has proposed new capillary structures aimed to maximize the heat transfer coefficient (hence, the efficiency of heat pipes) by the addition of micro-grooves orthogonal to the main grooves (used in current heat pipes, to bring back liquid from the condenser). This will be tested for a flat plate during the DAGOBERT experiment, now foreseen for the FOTON-M2 flight (after the unfortunate disaster of FOTON-M1). In parallel, ULB and ULg will be studying evaporation in porous capillary structures, both theoretically and experimentally, in order to optimise the geometries of heat pipes and capillary-pumped loops used by EHP, which should allow to reduce the number of prototypes to be tested. In a first stage, this requires the development of models of evaporation in porous structures allowing the positioning of the evaporating interface (within or outside the porous medium) versus the applied heat flux. In turn, these models will be validated by direct comparisons with experiments in model porous media (packing of glass beads, or Hele-Shaw cells in relation with CIMEX1-2D) performed in ULB.
Satellite(s) or flight opportunity(ies):
- Fluid Science Laboratory on the International Space Station
Field of research:
Physical Science: Fluid Physics
