Évaporation de gouttelettes polydispersées dans un écoulement de canal fortement turbulent : analyse de la formation du mélange diphasique par imagerie de fluorescence

Nous présentons ici une expérimentation modèle pour l'étude d'écoulements diphasiques turbulents en évaporation. L'étude s'intéresse à une situation où des gouttelettes pré-atomisées et dispersées par un écoulement turbulent chauffé s'évaporent et se mélangent. Le chargement massique en gouttelettes est pris suffisamment bas pour que l'on puisse considérer que l'on n'a pas de rétro-action du mouvement des particules sur l'écoulement porteur. L'écoulement est chauffé de façon à déclencher l'évaporation des gouttelettes. Enfin il n'y a pas combustion. Afin de mener à bien cette étude, un banc d'expérimentation modèle a été conçu pour générer un écoulement porteur fortement turbulent. Il s'agit d'un écoulement de canal présentant des caractéristiques de la turbulence homogène isotrope et des niveaux de turbulence élevés : 25 % dans la zone établie homogène isotrope, largement supérieurs à ceux obtenus à l'aide d'une turbulence de grille. La caractérisation de cet écoulement a été réalisée à l'aide de la Vélocimétrie par Images de Particules (PIV). Un système d'injection, utilisant un atomiseur passif à ultrasons, permet de créer un brouillard de gouttelettes qui sont rapidement dispersées dans l'écoulement. Les caractéristiques de ce brouillard sont également déterminées directement en sortie d'injecteur. Les gouttelettes ont des tailles suffisamment petites pour être toutes sensibles au moins aux grosses structures énergétiques, voire même aux échelles dissipatives. La polydispersion du brouillard est telle que tous les types d'interactions entre la turbulence et les gouttelettes sont possibles. L'étude de l'évaporation est réalisée à l'aide de la Fluorescence Induite par Laser (PLIF), qui permet de déterminer des concentrations massiques de vapeur. La PLIF étant une technique de mesures habituellement utilisée pour des écoulements monophasiques (liquide ou gaz), des traitements d'images ont du être mis au point pour séparer les signaux liés aux gouttelettes de ceux émis par la vapeur. L'analyse des images de fluorescence a été réalisée en deux temps. D'abord un comptage des gouttes a permis de s'intéresser à la répartition spatiale des gouttes. Il a été mis en évidence que si les gouttes sont en moyenne bien réparties dans toute la section de la veine, les répartitions instantanées sont tout sauf homogènes, et présentent de forts écarts par rapport à une distribution purement aléatoire. On en déduit l'existence d'amas dont la taille caractéristique peut varier en fonction des conditions de l'expérience. Ce point est à considérer car un groupement de gouttes est susceptible de freiner l'évaporation en concentrant la vapeur créée. Enfin, l'étude de l'évaporation pour un cas de référence a été réalisée. L'évolution du débit massique de vapeur généré est analysée et mise en relation avec des modèles simples d'évaporation. Les fluctuations de concentrations massiques permettent également de déterminer l'efficacité du mélange en présence d'évaporation. ABSTRACT : A model experiment for the study of evaporating turbulent two-phase flows is presented here. The study focuses on a situation where pre-atomized and dispersed droplets vaporize and mix in a heated turbulent flow. The liquid mass loading of the carrier flow is set sufficiently low to avoid two-way coupling where the droplets have an effect on the dynamic and the turbulence of the carrier flow. The carrier flow is heated in order to have vaporization of the droplets. Last, there is no combustion. For this study, a model experimental test bench was conceived to generate a highly turbulent carrier flow. It consists in a channel flow with caracteristics of homogeneous and isotropic turbulence where fluctuations' levels are very high. These levels range as high as 25 % in the established homogeneous and isotropic zone, which is much higher than those commonly obtained with a classical grid turbulence. The characterization of this gaz flow was realized with Particles Image Velocimetry (PIV). An injection system using a passive ultrasound atomizer allows the creation of a mist of small droplets which are quickly dispersed in the carrier flow. The caracteristics of this mist, namely the droplets mean diameters and velocities, are determined directly at the exit of the injector. The droplets have diameters small enough to be all sensitive to at least the biggest, most energetic turbulent structures. The smallest droplets are even sensitive to the smallest turbulent structures. The range of diameters is so large that pratically every kind of droplet behavior with regards to turbulence is possible. The study of evaporation is done through Planar Laser Induced Fluorescence (PLIF) which allows the determination of mass concentrations of vapor. PLIF is usually used on one-phase flows, be it liquid or gaz. Therefore, images treatments are done to separate fluorescence signals of droplets from signals due to vapor. Analysis of fluorescence images is made in two steps. First, droplets are being counted in order to characterize spatial repartition of droplets. While droplets are in average well dispersed throughout the channel, meaning there are no more droplets on the center of the channel than on the edge (or the opposite), the instantaneous repartitions are all but homogeneous. In fact, there are high deviations from a purely statistical distribution (represented by a Poisson law). Therefore, it has been concluded that there are droplets clusters in the flow whose caracteristic size may vary with the experimental conditions. This in turn may lead to the slowing of evaporation since mass fraction of vapor around droplets in a cluster will get higher. Eventually, the evaporation of a reference case was studied. The evolution of generated vapor mass flow rate is analyzed and compared with simple non-dimensional models for evaporation such as the D2 p law. The mass concentration fluctuations allow us to determine the efficiency of mixing during evaporatio

Simulation of mass and heat transfer in an evaporatively cooled PEM fuel cell

Evaporative cooling is a promising concept to improve proton exchange membrane fuel cells. While the particular concept based on gas diffusion layers (GDLs) modified with hydrophilic lines (HPILs) has recently been demonstrated, there is a lack in the understanding of the mass and heat transport processes. We have developed a 3-D, non-isothermal, macro-homogeneous numerical model focusing on one interface between a HPIL and an anode gas flow channel (AGFC). In the base case model, water evaporates within a thin film adjacent to the interfaces of the HPIL with the AGFC and with the hydrophobic anode GDL. The largest part of the generated water vapor leaves the cell via the AGFC. The transport to the cathode side is shown to be partly limited by the ab-/desorption into/from the membrane. The cooling due to the latent heat has a strong effect on the local evaporation rate. An increase of the mass transfer coefficient for evaporation leads to a transport limited regime inside the MEA while the transport via the AGFC is limited by evaporation kinetics

Hydrogen for electromobility : a promising energy carrier

Electromobility has received important attention in the last few years, but its perception by the public and decision makers is often limited to battery powered vehicles. Alternatives such as hydrogen fuel cells should however be taken into account, as their specific advantages (in particular short refueling times) make electro-mobility as a whole acceptable by a much broader public. Within the SCCER Mobility, PSI and ZHAW work on a novel fuel cell concept aiming at reducing the major limitation to the deployment of fuel cells: their cost

3-D simulation of water and heat transport processes in fuel cells during evaporative cooling and humidification

Evaporative cooling is a promising concept improve the efficiency and reduced costs of polymer electrolyte fuel cells (PEFCs) using modified gas diffusion layers with hydrophilic and hydrophobic lines. This concept has been demonstrated to simultaneously achieve cooling and membrane humidification in experiments. We have developed a 3-D numerical model of such an evaporative cooling cell to address remain questions from the experiments

Patterns of Bone Failure in Localized Prostate Cancer Previously Irradiated: The Preventive Role of External Radiotherapy on Pelvic Bone Metastases

Introduction: External beam radiation therapy (EBRT) can cure localized prostate cancer (PCa) by sterilizing cancer cells in the prostate gland and surrounding tissues at risk of microscopic dissemination. We hypothesized that pelvic EBRT for localized PCa might have an unexpected prophylactic impact on the occurrence of pelvic bone metastases.Material and Methods: We reviewed the data of 332 metastatic PCa patients. We examined associations between the number (≤5 vs. >5) and the location of bone metastases (in-field vs. out-of-field), which occurred at first relapse, and a previous history of EBRT for PCa (EBRT vs. No-EBRT).Results: One hundred and ten patients M0 at baseline were eligible. Fifty-six patients (51%) were in the No-EBRT group, and 54 patients (49%) in the EBRT group. The proportion of patients who developed >5 bone metastases in the bony pelvis was higher in the No-EBRT group vs. the EBRT group: 10 patients (18%) vs. 2 patients (4%), respectively (p = 0.02). By multivariate analysis EBRT was associated with a lesser occurrence of patients who had >5 bone metastases in the bony pelvis (OR = 0.17 [95%CI, 0.04–0.87], p = 0.03). Time to occurrence of bone metastases ≥5 years (OR = 0.10 [95%CI, 0.05–0.19], p < 0.01), prior curative prostate treatment (OR = 0.58 [95%CI, 0.36–0.91], p = 0.02), >5 bone metastases in bony pelvis (OR = 2.61 [95%CI, 1.28–5.31], p < 0.01), >5 bone metastases out of bony pelvis (OR = 1.73 [95%CI, 1.09–2.76], p = 0.02) were all predictive of overall survival.Conclusion: Previous pelvic EBRT for PCa is associated with a lower number of pelvic bone metastases, which is associated with better overall survival

Modelling the effects of using gas diffusion layers with patterned wettability for advanced water management in proton exchange membrane fuel cells

We present a macrohomogeneous two-phase model of a pro- ton exchange membrane fuel cell (PEFC). The model takes into account the mechanical compression of the gas diffusion layer (GDL), the two-phase flow of water, the transport of the gas species and the electrochemical reaction of the reactand gases. The model was used to simulate the behavior of a PEFC with a patterned GDL. The results of the reduced model, which considers only the mechanical compression and the two-phase flow, are compared to the experimental ex-situ imbibition data obtained by neutron radiography imaging. The results are in good agreement. Additionally, by using all the model features, a simulation of an operating fuel cell has been performed to study the intricate couplings in an operating fuel cell and to examine the patterned GDL effects. The model confirms that the patterned GDL design liberates the pre-defined domains from liquid water and thus locally increases the oxygen diffusivity.

Évaporation de gouttelettes polydispersées dans un écoulement de canal fortement turbulent (analyse de la formation du mélange diphasique par imagerie de fluorescence)

TOULOUSE-ENSEEIHT (315552331) / SudocSudocFranceF

3-D simulation of heat and water transport in PEFCs during evaporative cooling and humidification

Funded by the Swiss Competence Center for Energy Research (SCCER Mobility) and the Swiss Federal Office of Energy SFOE (contract number SI/501764-01).Evaporative cooling is a promising concept to increase the power density and reduce the complexity of polymer electrolyte fuel cell systems (PEFCs) by using gas diffusion layers (GDLs) modified with hydrophilic lines (HPL) [1]. While this concept has been demonstrated in experiments [2-3], a quantitative understanding of evaporative cooling and humidification is missing. Here we simulate the heat and water transport processes in part of a non-operating evaporative cooling cell using a 3-D macro-homogeneous model, which consists of a mem-brane electrode assembly (MEA) with one HPL in the anode GDL, sandwiched by flow chan-nel plates. In the base case simulation, water evaporates mostly in the HPL in contact with the gas flow and, to a smaller degree, with the hydrophobic part of the GDL. Almost all of the generated water vapour leaves the cell through the anode gas channel. The resulting membrane humidification (l) varies on the anode side being the highest below the hydrophilic line and ribs and the lowest below the gas channel, and decreases towards the cathode side. Evaporating rates are partly limited by the water evaporation transfer coefficient at values typically adopted for simulating evaporation in PEFC models. In contrast, at higher transfer coefficients, evaporation rates reach a plateau and, hence, become transport limited. The next step is to simulate the increasing humidification with more hydrophilic lines, and an oper-ating cell including electro-osmotic drag

3-D simulation of heat and water transport processes in PEFCs during evaporative cooling and humidification

Evaporative cooling (EC) is a promising concept to increase the power density and reduce the complexity of polymer electrolyte fuel cells (PEFCs) by using gas diffusion layers (GDLs) modified with hydrophilic lines (HPIL). While this concept has been demonstrated in recent experiments by use of a thermal test cell, a detailed understanding of the simultaneous cooling and humidification is missing. To close this gap, we have developed a non-isothermal, two-phase continuum model of an EC cell. This 3-D model consists of a membrane electrode assembly (MEA) with an anode GDL with of one HPIL. The MEA is sandwiched by flow channel plates that consist of one gas flow channel (GFC) and liquid water channel on the anode side and two GFCs on the cathode side. In the base case model, water evaporates mostly within a thin film at the anode GFC/HPIL interface and – to a smaller degree – at the interfaces of the HPIL with the hydrophobic anode GDL. The largest part of the generated water vapor leaves the cell through the anode GFC and only about a tenth reaches the cathode side. The membrane humidification varies on the anode side being the highest below the HPIL and ribs and the lowest below the anode GFC, and decreases towards the cathode side. The temperature drop due to latent heat of evaporation is the largest along the interface between the HPIL and anode GFC. As our model essentially simulates the first contact surface between the HPIL and anode GDL in gas flow direction, it represents an extreme case in comparison to the experiments in that higher local evaporation rates and heat fluxes and lower membrane humidification levels are simulated. The role of evaporation kinetics on the results is analyzed by varying the evaporation rate constant over several orders of magnitude. While a plateau is reached for the water vapor flux via the anode GDL and towards the cathode side at a sufficiently large evaporation rate constant, a much larger rate constant would be necessary to reach a transport limited regime in the anode GFC. This is shown to be challenging to resolve numerically due to increasingly sharper gradients within a thinner film along the borders of the HPIL. Future studies could couple this continuum model with a pore-scale simulation in combination with ex-situ experiments to improve the simulation of the two-phase flow in the modified GDL, which includes the upscaling of the Hertz-Knudsen-Schrage equation to the continuum level