A study on self-rewetting fluids for heat transfer in microgravity

In a number of water alcohols solutions the surface tension is an increasing function of temperature. Such mixtures are generally referred as self-rewetting fluids because convection driven by a reverse Marangoni effect is direct from cold to hot region along liquid-vapour interface. Within two phase heat transfer device, like heat pipes, liquid condensed is spontaneously driven to the hot evaporator improving the thermal performances and increasing the maximum heat flux. The research activities focused on numerical simulation and experimental laboratory for thermal characterization of heat pipe filled with self-rewetting fluids. Measurements of thermo-physical properties, in particular surface tension, have been carried out for several binary and ternary mixtures. A comparison between heat pipe filled ordinary and self-rewetting fluids has been carried out. Flow visualization of self-rewetting fluids has been obtained using optical diagnostic systems

Next Generation Ceramic Composites for Combustion Harsh Environment and Space

The main purpose of C³HARME is the design, development, manufacturing and testing of a new class of Ceramic Matrix Composites based on ultra-high temperature ceramic matrices reinforced with SiC or C fibers suitable for application in severe aerospace environments. The C³HARME consortium is composed of 3 academic institutions, 3 research centers, 3 large companies and 3 SMEs based in 6 different countries. The consortium ensures a critical mass of scientific expertise and excellence in key areas (materials science, engineering, process technology, material modelling, processing industrial scale up). The project, started on 1 June 2016, will run for 48 months and it has been funded for a total of 8M€

Recent development on heat pipes for ground and space applications

Two-phase flow and thermal physics are extensively investigated in microgravity environment with the continuous motivation of improving heat transfer devices in space as well as in terrestrial applications. For this purpose a dedicated hardware, called Thermal Platform (TP1), is under development by the European Space Agency to be implemented in the Fluid Science Laboratory (FSL) on board the International Space Station (ISS). Innovative heat pipes with enhanced performances and other heat transfer related phenomena, will be investigated in microgravity conditions by using advanced working fluids, i.e. special mixture called self rewetting fluids, materials and optimized geometries. Self Rewetting fluids means binary or multi-component mixtures having an unusual surface tension behavior. For these mixtures the reverse Marangoni effect along liquid-vapour interfaces is responsible for relatively strong surface tension-driven flows towards the hot region. One of the most interesting applications of this effect is spontaneous liquid inflow towards hot spot or dry patch on the heater surface of heat pipe or similar evaporation-based heat transfer devices. This article focused on the experimental insert SELf rewetting fluids for thermal ENErgy management (SELENE) which is aimed to study the fundamental physics and the evaporative heat and mass transfer from the free surface of pure liquids and self rewetting fluids in a partially transparent V-shaped groove channel configuration. The paper discusses the different aspects of the experiment in microgravity conditions, where surface tension-driven phenomena dominate fluid behavior. The role of thermo-capillary effect on two phase heat transfer devices in pure liquids has been investigated on ground with a breadboards apparatus based on a single V-groove. Transparent configuration allows flow visualization and optical diagnostic techniques to be used. The working fluids are an environmentally sustainable engineered liquid (Methoxy-nonafluorobutane, Novec 7100), whose low freezing point makes it ideal for use in space applications, and water. Special attention is focused on the role of wetting properties. The experimental results are explained on the basis of numerical models including evaporation, heat and mass transfer from the liquid/vapour interface and Marangoni effects

Self-induced Marangoni flow in evaporating alcoholic solutions

The self-induced Marangoni convection in alcoholic solutions is the subject of the present experimentalinvestigation. Pure ethanol and its mixtures with 5%, 10% and 20% in weight of water are presented anddiscussed. In particular, Marangoni flow in horizontal pipes from 100 to 1000 lm inner diameter is studied.Vortex spinning frequency, average particle tracers velocity and evaporation rate are measured anddiscussed. The evaporation rate increases and the evaporation flux decreases at bigger tube sizes in linewith previous investigations; pure ethanol has higher evaporation rate and flux than ethanol/water mixtures.The spinning frequency and the average tracer particles velocity decrease for increasing water contentin the mixtures. All of these findings are due to evaporative cooling effect which is higher at themeniscus wedge (where the triple-line region is found) than at the meniscus center; this causes a differencein temperature between the wedge and the center that generates a gradient of surface tension drivingvigorous Marangoni convection, that has been reported and analyzed. The experimental results areexplained on the basis of a numerical model including evaporation, vapor diffusion, heat and mass transferfrom the liquid to the surrounding ambient and the Marangoni effects.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Thermo-chemical surface instabilities of SiC-ZrB 2 ceramics in high enthalpy dissociated supersonic airflows

The response of three different SiC-ZrB2 ceramics obtained by hot-pressing was studied at typical conditions of thermal protection systems of a re-entry spacecraft. Button-like lab-scale demonstrators were manufactured and tested in high enthalpy dissociated supersonic airflows using an arc-jet ground facility. Under severe aero-heating of up to 21MJ/kg of specific total enthalpy and 3.5 MW/m2 of (cold-wall) heat flux the SiC-ZrB2 UHTC buttons endured rather well, though thermo-chemical surface instabilities started taking place for side wall surface temperatures of some buttons above 2050K. The experimental determinations of the surface temperature, correlated to the microstructure changes occurred during testing, allowed to interpret the observed phenomena. Potentials and limits of the oxidation-resistant SiC-ZrB2 system to withstand such extreme conditions were outlined

Surface tension-driven flows in evaporative two-phase systems in microgravity conditions

This article deals with numerical research activities in preparation of the experiment "SELf rewetting fluids for thermal ENErgy management" (SELENE). This research program foresees the development of a dedicated hardware (Thermal Platform) for microgravity experiments in the Fluid Science Laboratory (FSL) on board the International Space Station (ISS). The primary objective of the research is to investigate multiphase flows in special heat transfer fluids, called self-rewetting fluids, including binary or multicomponent mixtures with unusual surface tension behaviour. For such mixtures, the reverse Marangoni effect at liquid-vapour interfaces is responsible for relatively strong surface tension-driven flows towards the hot region of the interface. One of the most interesting applications of this effect is spontaneous liquid inflow towards hot spot or dry patch on the heater surface of heat pipe or similar evaporation-based heat transfer devices. Self-rewetting mixtures prevent the liquid film dry-out and increase the heat transfer performances of the system. In this paper numerical modelling of the capillary/surface tension driven flows is given, including numerical modelling of the breadboard apparatus. The different aspects of the experiment in microgravity conditions, were surface tension-driven phenomena dominating the fluid behaviour are discussed.SCOPUS: cp.pinfo:eu-repo/semantics/publishe

Surface tension-driven flow in wickless heat pipes with self-rewetting fluids

Surface tension-driven effects in wickless heat pipes with aqueous solutions of long-chain alcohols are investigated. Flow visualizations and numerical simulation of bubbles behaviour and boiling pattern in transparent capillaries show the potential advantages of ‘‘self-rewetting” fluids, i.e. liquid solutions with a non-linear dependence of the surface tension with temperature. Surface tension measurements at different temperatures have been carried out also for a number of ternary aqueous solutions with relatively low freezing points. Some of them interestingly exhibit the same anomalous positive surface tension gradient with temperature as binary self-rewetting solutions. These results may open new horizons toward the development of more efficient heat transfer devices for different applications