Experimental Analysis of the Fluid Flow in the Flat Plate Pulsating Heat Pipe Under Microgravity Conditions

An experimental study of a flat plate pulsating heat pipe has been performed under various gravity conditions during ESA 69th parabolic flight campaign. A molybdenum plate, with 14 milled rectangular channels with cross-section of 3×3 mm2, was covered with a sapphire window, and tested in vertical position with ethanol as working fluid. If operation in normal and hyper-gravity conditions were characterized by nucleate boiling regime, FP-PHP working like in looped thermosyphon mode, transition into microgravity was accompanied by a flow pattern change into slug/plug regime with thin film evaporation, due to absence of buoyancy forces. Hydrodynamic instabilities, accompanied with short-term periods of emergence of nucleate boiling under microgravity, were observed during several parabolas. Formations of long vapor slugs in the channel lead to thin film evaporation. Combined optical and infrared visualizations showed velocity and amplitude increase of the menisci motions

Experimental Analysis of the Fluid Flow in the Flat Plate Pulsating Heat Pipe Under Microgravity Conditions

An experimental study of a flat plate pulsating heat pipe has been performed under various gravity conditions during ESA 69th parabolic flight campaign. A molybdenum plate, with 14 milled rectangular channels with cross-section of 3×3 mm2, was covered with a sapphire window, and tested in vertical position with ethanol as working fluid. If operation in normal and hyper-gravity conditions were characterized by nucleate boiling regime, FP-PHP working like in looped thermosyphon mode, transition into microgravity was accompanied by a flow pattern change into slug/plug regime with thin film evaporation, due to absence of buoyancy forces. Hydrodynamic instabilities, accompanied with short-term periods of emergence of nucleate boiling under microgravity, were observed during several parabolas. Formations of long vapor slugs in the channel lead to thin film evaporation. Combined optical and infrared visualizations showed velocity and amplitude increase of the menisci motions

Étude analytique et expérimentale des transferts de masse et de chaleur dans les caloducs oscillants pour des applications de vols hors gravité

L'intégration de dispositifs électroniques de haute puissance et dissipant des centaines de watts par centimètre carré au sein des systèmes spatiaux provoque une hausse significative des besoins en systèmes de refroidissement de haute performance, légers et efficaces. En raison de ces exigences spécifiques, l’utilisation des systèmes de refroidissement diphasiques représentent un grand intérêt pour ces applications spatiales. Les caloducs oscillants, dispositifs passifs et simples de transfert de chaleur diphasique sont devenus une excellente solution pour les systèmes de management thermique spatiaux. Actuellement, le fonctionnement de ce type d'échangeurs de chaleur n'est pas entièrement étudié, notamment en conditions de microgravité. C'est pourquoi, dans le cadre des projets successifs INWIP et TOPDESS de l'ESA, l'évaluation des performances des caloducs oscillants plats et des spécificités opérationnelles sous différentes conditions de gravité a été défini comme principal objectif du présent travail de thèse.Les études expérimentales menées au cours de ce travail ont porté sur la caractérisation thermique des différents caloducs oscillants testés au sol et au cours de quatre campagnes de vols paraboliques de l'ESA. Des études paramétriques des performances de ces systèmes ont été effectuées en configuration verticale et horizontale, en utilisant différents fluides de travail (mélanges alcool-eau, fluides remouillants ou solutions de surfactant), différentes températures de condenseur, plusieurs concentrations de gaz non condensables et différents niveaux de gravité (terrestre, martien, lunaire, hyper et microgravité). L'amélioration des performances, parfois inattendues, ont été observées sous certaines conditions opératoires. Les études analytiques de l'écoulement diphasique dans le caloduc oscillant, concernant notamment le passage de l'arrêt de fonctionnement aux oscillations de forte amplitude (« phases de réactivation ») ont été réalisées grâce à des visualisations haute fréquence. Pour finir, les critères de transition d'écoulement du régime bulles/bouchons au régime semi-annulaire/annulaire a été étudiée dans le cadre d’une l'analyse non-dimensionnelle des principaux paramètres de l’écoulement.Integration of high power and performance electronic devices in modern spacecraft systems with heat generation of hundreds of watts per square centimeter crucially increases needs in high performance, low weight, energy efficient and reliable thermal management systems. Due to these specific demands, two-phase heat rejection devices are of major interest for space applications. The Pulsating Heat Pipes, as simple passive two-phase heat transfer devices, have become an excellent solution for space cooling systems. Today, operation of this kind of heat transfer devices is not fully studied, especially in microgravity. So, in the context of ESA consecutive projects INWIP and TOPDESS, evaluation of the flat plate pulsating heat pipe performances and operational peculiarities under different gravity conditions has been defined as the main aim of the present PhD work. Experimental studies conducted during this work concerned the thermal characterization of different flat plate pulsating heat pipes tested on ground and during four ESA Parabolic Flight Campaigns. Parametric investigations have been carried out for vertical and horizontal orientations of the device using different working fluids (including alcohol aqueous solutions, self-rewetting fluids and surfactant solutions), condenser temperatures, non-condensable gases concentrations and gravity levels (Earth, Martian, Lunar, hyper and microgravity). Obvious, and sometimes unexpected, performances improvement has been observed under certain conditions. Analytical studies of the two-phase flow inside the FPPHP, especially concerning the transition from stopover to high-amplitude oscillations (fluid flow “re-activation phases”) have been done thanks to high-frequency visualizations. Finally, the flow transition criterion, from the slug-plug flow regime to semi-annular/annular flow, was studied thanks to dimensionless analyzes considering the main parameters of the flow

Étude analytique et expérimentale des transferts de masse et de chaleur dans les caloducs oscillants pour des applications de vols hors gravité

Integration of high power and performance electronic devices in modern spacecraft systems with heat generation of hundreds of watts per square centimeter crucially increases needs in high performance, low weight, energy efficient and reliable thermal management systems. Due to these specific demands, two-phase heat rejection devices are of major interest for space applications. The Pulsating Heat Pipes, as simple passive two-phase heat transfer devices, have become an excellent solution for space cooling systems. Today, operation of this kind of heat transfer devices is not fully studied, especially in microgravity. So, in the context of ESA consecutive projects INWIP and TOPDESS, evaluation of the flat plate pulsating heat pipe performances and operational peculiarities under different gravity conditions has been defined as the main aim of the present PhD work. Experimental studies conducted during this work concerned the thermal characterization of different flat plate pulsating heat pipes tested on ground and during four ESA Parabolic Flight Campaigns. Parametric investigations have been carried out for vertical and horizontal orientations of the device using different working fluids (including alcohol aqueous solutions, self-rewetting fluids and surfactant solutions), condenser temperatures, non-condensable gases concentrations and gravity levels (Earth, Martian, Lunar, hyper and microgravity). Obvious, and sometimes unexpected, performances improvement has been observed under certain conditions. Analytical studies of the two-phase flow inside the FPPHP, especially concerning the transition from stopover to high-amplitude oscillations (fluid flow “re-activation phases”) have been done thanks to high-frequency visualizations. Finally, the flow transition criterion, from the slug-plug flow regime to semi-annular/annular flow, was studied thanks to dimensionless analyzes considering the main parameters of the flow.L'intégration de dispositifs électroniques de haute puissance et dissipant des centaines de watts par centimètre carré au sein des systèmes spatiaux provoque une hausse significative des besoins en systèmes de refroidissement de haute performance, légers et efficaces. En raison de ces exigences spécifiques, l’utilisation des systèmes de refroidissement diphasiques représentent un grand intérêt pour ces applications spatiales. Les caloducs oscillants, dispositifs passifs et simples de transfert de chaleur diphasique sont devenus une excellente solution pour les systèmes de management thermique spatiaux. Actuellement, le fonctionnement de ce type d'échangeurs de chaleur n'est pas entièrement étudié, notamment en conditions de microgravité. C'est pourquoi, dans le cadre des projets successifs INWIP et TOPDESS de l'ESA, l'évaluation des performances des caloducs oscillants plats et des spécificités opérationnelles sous différentes conditions de gravité a été défini comme principal objectif du présent travail de thèse.Les études expérimentales menées au cours de ce travail ont porté sur la caractérisation thermique des différents caloducs oscillants testés au sol et au cours de quatre campagnes de vols paraboliques de l'ESA. Des études paramétriques des performances de ces systèmes ont été effectuées en configuration verticale et horizontale, en utilisant différents fluides de travail (mélanges alcool-eau, fluides remouillants ou solutions de surfactant), différentes températures de condenseur, plusieurs concentrations de gaz non condensables et différents niveaux de gravité (terrestre, martien, lunaire, hyper et microgravité). L'amélioration des performances, parfois inattendues, ont été observées sous certaines conditions opératoires. Les études analytiques de l'écoulement diphasique dans le caloduc oscillant, concernant notamment le passage de l'arrêt de fonctionnement aux oscillations de forte amplitude (« phases de réactivation ») ont été réalisées grâce à des visualisations haute fréquence. Pour finir, les critères de transition d'écoulement du régime bulles/bouchons au régime semi-annulaire/annulaire a été étudiée dans le cadre d’une l'analyse non-dimensionnelle des principaux paramètres de l’écoulement

Novel Infrared Approach for the Evaluation of Thermofluidic Interactions in a Metallic Flat-Plate Pulsating Heat Pipe

A novel and advanced analysis tool, based on the resolution of the inverse heat conduction problem, is used to evaluate wall-to-fluid heat fluxes in a metallic flat-plate pulsating heat pipe. The device under analysis is made of copper and formed by 16 channels having a squared section of 3 × 3 mm2 and filled with a water–ethanol mixture (20 wt.% of ethanol) with a volumetric filling ratio of 50%. One flat side of the device is externally coated with a highly emissive paint to perform temperature measurements by means of a medium-wave infrared camera. The acquired infrared maps are first processed by a three-dimensional Gaussian filter and then used as inputs for the inverse approach for the evaluation of heat fluxes locally exchanged between the fluid and the thin walls of each channel. The suggested procedure is successfully validated by means of synthetic data. The resulting space–time heat flux distributions are therefore statistically investigated in terms of amplitude and space–time variations, providing quantitative references for the identification of two-phase flow regimes. These unique data give an evaluation of the local heat transfer behavior, which is essential to provide empirical values for the numerical models of pulsating heat pipes

Novel Infrared Approach for the Evaluation of Thermofluidic Interactions in a Metallic Flat-Plate Pulsating Heat Pipe

A novel and advanced analysis tool, based on the resolution of the inverse heat conduction problem, is used to evaluate wall-to-fluid heat fluxes in a metallic flat-plate pulsating heat pipe. The device under analysis is made of copper and formed by 16 channels having a squared section of 3 × 3 mm2 and filled with a water–ethanol mixture (20 wt.% of ethanol) with a volumetric filling ratio of 50%. One flat side of the device is externally coated with a highly emissive paint to perform temperature measurements by means of a medium-wave infrared camera. The acquired infrared maps are first processed by a three-dimensional Gaussian filter and then used as inputs for the inverse approach for the evaluation of heat fluxes locally exchanged between the fluid and the thin walls of each channel. The suggested procedure is successfully validated by means of synthetic data. The resulting space–time heat flux distributions are therefore statistically investigated in terms of amplitude and space–time variations, providing quantitative references for the identification of two-phase flow regimes. These unique data give an evaluation of the local heat transfer behavior, which is essential to provide empirical values for the numerical models of pulsating heat pipes

Experimental investigation of a coil shaped deployable pulsating heat pipe

The development of foldable heat transfer devices is currently a quite intriguing technological challenge in the field of thermal management. Due to its simple structure (i.e. capillary tube without any wick structure inside), the Pulsating Heat Pipe is one of the most promising candidate to be applied to foldable devices. A possible solution is to realize a PHP with the adiabatic section formed as a coil. In this case the adiabatic section is very long and the performance of the PHP can be largely affected. This paper presents the results of the comparison between two twin PHPs built with the same geometry but different adiabatic section shapes (planar and coil). A full thermal characterization of the planar assessment for different working fluids has been performed: HFE-7000 (filling ratio: 70%) shows the best thermal behavior in horizontal orientation. On the other hand, the coiled PHP has been tested under different orientations and folding configurations (different mutual locations of the evaporator and condenser with respect to gravity) and compared with the planar reference case. The coiled PHP in some configurations seems to present worse thermal performances than the planar one

Toward Low Earth Orbit (LEO) Applications: the Scientific Journey of the "Space Pulsating Heat Pipe" Experiments

International audienceThis paper shortly summarises the experimental results obtained since 2011 by a large European academic consortium for the scientific conceptualisation, the definition of the technical requirements, the generation of experimental data, and the validation of a numerical code, for the Pulsating Heat Pipes (PHP) experiment on the International Space Station (ISS). The PHP is a passive, wickless thermal device, whereby a two-phase fluid, forming liquid plugs and vapour slugs, moves with a pulsating or circulating motion inside a meandering tube or channel. The PHP may have a very broad range of geometries (flat, tubular, 3D structured), it can dissipate heat from large areas, and it can be suitable for high power applications with low/medium heat fluxes. PHP functioning is based on the capillary effect, which provides the existence of liquid plugs completely filling the channel cross-section, in a way that any expansion or contraction of the vapour slugs will naturally generate a movement of the fluid along the channel axis. For this, it is important that the channel has a cross-section size below a given threshold, which depends on the liquid surface tension and (for a static fluid) on the gravity acceleration. In space, when only residual accelerations are acting, such a static size threshold is virtually infinite, while a finite dynamic threshold exists even in the absence of gravity. The concept of a "Space PHP" was originally developed in 2014 by the team, and from then 17 Parabolic Flight Campaigns (PFC) and 3 Sounding Rocket (SR) experiments have been carried out to generate the data for the preparation of an experiment targeting a Low Earth Orbit (LEO) mission. Both a tubular and a flat plate PHP have been successfully tested in reduced gravity and on ground, by using different combinations of fluids and building materials. The need for having an experiment on a LEO environment is mainly because, during a PFC, only 22sec of reduced gravity are possible, which is a period below the characteristic time for reaching a steady state condition for almost all of the tested devices. Instead, a steady state was reached using the SR campaigns: in this case however, only one experimental condition was achievable, and long-duration data of the PHP performance still remains beyond reach. Several measurement methodologies have been used to characterise the Space PHP, like infrared analysis, high-speed camera visualisation techniques, with data processed with different techniques, from wavelets to inverse heat transfer problem solution. The results clearly showed that PHPs are very interesting for space applications due to their simplicity of construction, the capacity to transfer heat up to several hundred watts, a high power/weight ratio, their geometrical adaptability, and, in particular, the Space PHP will be a breakthrough technology for space thermal management