Numerical investigation of boiling

In this work, we study different phenomena that occur during nucleate boiling. We numerically investigate boiling using two phase flow direct numerical simulation based on a level set / Ghost Fluid method. This method allows us to follow the interface and to make accurate geometric calculation as for bubble curvature. Nucleate boiling on a plate is not only a thermal issue, but also involves multiphase dynamics issues at different scales and at different stages of bubble growth. As a consequence, we divide the whole problem and investigate separately the different phenomena considering their nature and the scale at which they occur. First we analyse the boiling of a static bubble immersed in an overheated liquid. We perform numerical simulations at different Jakob numbers in the case of strong discontinuity of density through the interface. These simulations permit us to estimate the accuracy of our numerical method dealing with phase change in the context of two phase flow direct numerical simulation. The results show a good agreement between numerical bubble radius evolution and the theoretical evolution found by Scriven(1959). The validation of our code for the Scriven case allows to pursue our study by focusing on the phenomena that take place in the particular case of an interaction with a wall. This interaction is characterised by the angle formed between a solid and a fluid interface, named the contact angle. We implement a method that makes it possible for a droplet, to reach, in the case of a static contact angle, a steady state corresponding to a theoretical equilibrium. Besides this method enables to take into account the contact angle hysteresis model, which considers different angles whether the contact line is advancing or recoiling. We perform simulations of the spreading of a liquid droplet impacting on a plate, and we compare the maximum spreading diameter and the advancing and receding droplet behaviour of our numerical results with the experimental data Son and Lee (2010) have reported

Pool boiling in microgravity with a single specie system

Pool boiling experiments in microgravity on the small copper plate of 1cm² have been performed in the SOURCE 2 experiment aboard the sounding Rocket Maser 12 launched on February 13th, 2012. The SOURCE 2 experiment is a small-scale tank devoted to the study of heat and mass transfers with a liquid refrigerant HFE7000 pressurised with its vapour. SOURCE 2 (SOUnding Rocket Compere Experiment) was developed in the frame of a French German space programme COMPERE (on the behaviour of propellant in launcher tanks) managed by CNES and DLR and a MAP ESA Project "Multiscale Analysis of boiling". During the 6 minutes of the flight different physical phenomena were studied by our partners: ZARM, University of Bremen, Air Liquide and Astrium. The boiling experiment was performed in a 6 cm diameter and 28 cm long cylindrical tank partly filled with a refrigerant Novec HFE7000 with a low boiling point (34°C at 1 bar) and pressurized by its own vapour. The heating element used for the boiling investigation consisted of an electrical resistance heated by Joule effect in contact with a flux-meter and a copper plate with a thickness of 40 micrometres. The flux-meter was equipped with two thermocouples. It was then possible to measure at the same time the heat flux transmitted to the liquid and the wall temperature. The liquid temperature above the heater was measured by 5 micro thermocouples located in the vicinity of the wall. Images of the boiling phenomenon were recorded by a video camera through the transparent cylindrical wall of the tank. SOURCE 2 is in the continuity of the SOURCE 1 experiment, which flew successfully on Maser 11 on 15 May 2008 (Kannengieser et al. 2010). The experiment scenario was similar to the present case. However, in SOURCE 1, liquid HFE-7000 was pressurized by gaseous nitrogen. The lateral glass wall was preheated and a strong evaporation took place at the free surface in the vicinity of the wall. Due to the presence of nitrogen a strong Marangoni convection occurred at the free surface enhancing nitrogen dissolution in the liquid phase. Then during the boiling experiment, the bubble growth was due to liquid vaporization and nitrogen desorption. Marangoni convection also occurred at the bubble interface leading to a capillary force pushing the bubble to the heated wall. In the SOURCE 2 experiment, a single specie configuration is studied (liquid/vapour HFE7000). This changes the thermo-hydraulic behaviour of the system. Since no Marangoni convection kept the bubble in contact with the heated wall, a primary bubble detached and grew by feeding itself with the smaller bubbles formed over the heated surface. The change in the bubble size is only due to vaporization. Then the measurements of the heat flux transferred to the fluid by the heater could be directly correlated to the amount of vapour production (balance of evaporation and re-condensation) that could be evaluated from visualization and image processing. When the rocket was launched, the test cell was empty. The top part and the lateral glass wall of the test cell were preheated before launch. Just before the microgravity period, the tank filling began, liquid HFE7000 at 25 °C was injected in the tank by the pump and pressurized by hot vapour HFE7000. At the beginning of the boiling experiment, the tank pressure was reduced to 1.5 bars and the heating of the heater element was switch on and boiling started. During this period, it was planed to study boiling with high sub-cooled liquid then to decrease the pressure to 1 bar to study saturated boiling. However while pressurization with hot vapour HFE7000, strong condensation occurred that rapidly warmed up HFE7000 up to 40°C during boiling study. As a consequence, at 1.5 bars, the sub-cooling was limited to 5 degrees. Then the tank pressured was reduced to 1.3 bars to investigated boiling in saturated conditions. On the other hand, before launch, tests were performed on ground and boiling curves were plotted. In this paper we will attend to compare the results obtained in micro-gravity and in 1G

Yielding and flow in adhesive and non-adhesive concentrated emulsions

The nonlinear rheological response of soft glassy materials is addressed experimentally by focusing on concentrated emulsions where interdroplet attraction is tuned through varying the surfactant content. Velocity profiles are recorded using ultrasonic velocimetry simultaneously to global rheological data in the Couette geometry. Our data show that non-adhesive and adhesive emulsions have radically different flow behaviors in the vicinity of yielding: while the flow remains homogeneous in the non-adhesive emulsion and the Herschel-Bulkley model for a yield stress fluid describes the data very accurately, the adhesive system displays shear localization and does not follow a simple constitutive equation, suggesting that the mechanisms involved in yielding transitions are not universal.Comment: 4 pages, 5 figures, to appear in Physical Review Letter

A border opening onto numerous geopolitical issues

On September 3rd 2002, the Yanbian Korean Autonomous Prefecture celebrated the fiftieth anniversary of its founding. Yanbian is located in the province of Jilin; of China’s thirty autonomous prefectures, it is the only one in the northeast (1). It is the largest administrative entity in the province, covering an area of 42,700 square kilometres; it is subdivided into six municipalities, Yanji, Longjing, Helong, Tumen, Dunhua and Hunchun, and two counties, Antu and Wangqing (see map). As the l..

La préfecture autonome des Coréens de Yanbian : une ouverture frontalière aux multiples enjeux géopolitiques

Le 3 septembre 2002, la préfecture autonome des Coréens de Yanbian célébrait le cinquantenaire de sa création. Située dans la province du Jilin, Yanbian est la seule des trente préfectures autonomes, que compte la République populaire de Chine, à se situer dans le Nord-Est du pays. D’une superficie de 42 700 kilomètres carrés, elle est l’unité administrative la plus vaste du Jilin ; elle est subdivisée en six municipalités, Yanji, Longjing, Helong, Tumen, Dunhua et Hunchun, et en deux distri..

Spatio-temporal dynamics of wormlike micelles under shear

Velocity profiles in a wormlike micelle solution (CTAB in D2O) are recorded using ultrasound every 2 s after a step-like shear rate into the shear-banding regime. The stress relaxation occurs over more than six hours and corresponds to the very slow nucleation and growth of the high-shear band. Moreover, oscillations of the interface position with a period of about 50 s are observed during the growth process. Strong wall slip, metastable states and transient nucleation of three-band flows are also reported and discussed in light of previous experiments and theoretical models.Comment: 4 pages, 5 figures, submitted to Phys.Rev.Let

Bill Hayton, The South China Sea: The Struggle for Power in Asia

Publié en 2014, cet ouvrage de Bill Hayton sur la mer de Chine du Sud se démarque d’une bonne partie de la littérature récente consacrée à cet espace maritime, et ce au moins pour deux raisons. D’abord parce qu’il s’agit d’une monographie rédigée par un seul auteur, dont le mérite est de retracer dans ses grandes lignes une histoire de la mer de Chine du Sud, de la préhistoire au début des années 2010, et d’analyser les conflits territoriaux et les rivalités de puissance dont cet espace est l..

Éditorial

Les multiples tensions qui ont cours dans les mers de Chine depuis la fin des années 2000, qu’elles se manifestent par l’inflexibilité des discours relatifs aux droits de souveraineté ou par des provocations et accrochages en mer, ont rendu nécessaire d’analyser de manière plus approfondie les politiques maritimes et étrangères des États impliqués. Ces analyses sont d’autant plus importantes que les rapports de force s’expriment depuis quelques années avec plus de vigueur, en mer de Chine du Sud comme en mer de Chine orientale. En témoignent les actions plus fermes prises par les États comme la construction par la Chine d’infrastructures civiles et militaires dans les îles Spratleys, la nationalisation des îles Senkaku/Diaoyu par le Japon en septembre 2012 ou encore la procédure d’arbitrage lancée par les Philippines en janvier 2013. Plus récemment, entre octobre 2015 et mai 2016, les États-Unis ont organisé trois opérations de liberté de navigation, rappelant qu’au-delà des litiges territoriaux entre pays voisins la situation géopolitique dans les mers de Chine est surtout le résultat de la rivalité stratégique sino-américaine. Sans viser l’exhaustivité, ce dossier se propose d’étudier quelques pans de la politique de la Chine en direction des mers de Chine. […] Les quatre articles qui structurent ce dossier viennent […] éclairer chacun à leur façon la politique chinoise des mersde Chine. Il y est question d’enjeux de sécurité, de stratégies d’acteurs, decoopération et de puissance navale.