Etude expérimentale de la rupture de gouttes dans un écoulement turbulent

Dans ce travail, une étude expérimentale du phénomène de rupture de gouttes dans un écoulement turbulent en aval d'une restriction de conduite est proposée. Une analyse statistique de la rupture d'une goutte isolée a été effectuée à l'échelle d'un volume d'écoulement fini en aval de l'orifice. Un nombre de Weber global, basé sur la perte de charge maximale à travers l'orifice, est un paramètre pertinent pour décrire la probabilité de rupture, le nombre de fragments moyen ou encore le diamètre moyen des gouttes-filles. A l'échelle locale, en revanche, la distribution hétérogène des lieux de rupture révèle un effet important du nombre de Reynolds. La caractérisation du champ hydrodynamique local de l'écoulement en aval de l'orifice par PIV rapide confirme l'existence de zones de ruptures localisées, associées aux maxima des contraintes turbulentes à l'échelle de la goutte. Afin d'analyser les mécanismes de rupture dans le champ turbulent, la déformation de la goutte le long de sa trajectoire est simulée à l'aide du modèle linéaire de Rayleigh-Lamb, dans lequel le forçage externe est représenté par la valeur locale instantanée du Weber turbulent. L'analyse statistique de la réponse dynamique de la goutte a permis d'identifier un critère de rupture basé sur un seuil de déformation critique, généralisable à l'ensemble des conditions opératoires. Cette approche multi-échelle a été reconduite pour étudier la rupture d'une goutte en milieu dense. La diminution de la probabilité de rupture en fonction de la concentration est liée à des modifications de la distribution des Weber turbulents en aval de l'orifice. ABSTRACT : This work presents the drop breakup phenomenon in a turbulent flow induced by a cross-section restriction in a pipe. A global analysis of single-drop breakup, in a finite volume downstream of the orifice, has allowed deriving statistical quantities such as the break-up probability and the daughter-drop distribution. These parameters are function of a global Weber number based on the maximal pressure drop through the orifice. At a local scale, the locations of breakup events are distributed heterogeneously and depend on the flow Reynolds number. The local hydrodynamic study in downstream of the orifice, which has been done by using Particle Image Velocimetry, reveals the specific breakup zones. Otherwise, this analysis has proved that the turbulence is the predominant external stress at the drop scale. The relation between drop deformation and the external stress along the trajectory has been simulated numerically by the response of a damped oscillator to the locally measured instantaneous turbulence forcing. The results of statistical analysis have allowed to introduce a breakup criterion, based on a unique deformation threshold value for all experiments. This multiscale approach has been conducted to study drop breakup mechanisms in a concentrated dispersion. The breakup probability decrease with the increase of dispersed phase concentration, which influences the turbulent Weber number distribution in downstream of the orifice

Etude expérimentale de la rupture de gouttes dans un écoulement turbulent

Dans ce travail, une étude expérimentale du phénomène de rupture de gouttes dans un écoulement turbulent en aval d'une restriction de conduite est proposée. Une analyse statistique de la rupture d'une goutte isolée a été effectuée à l'échelle d'un volume d'écoulement fini en aval de l'orifice. Un nombre de Weber global, basé sur la perte de charge maximale à travers l'orifice, est un paramètre pertinent pour décrire la probabilité de rupture, le nombre de fragments moyen ou encore le diamètre moyen des gouttes-filles. A l'échelle locale, en revanche, la distribution hétérogène des lieux de rupture révèle un effet important du nombre de Reynolds. La caractérisation du champ hydrodynamique local de l'écoulement en aval de l'orifice par PIV rapide confirme l'existence de zones de ruptures localisées, associées aux maxima des contraintes turbulentes à l'échelle de la goutte. Afin d'analyser les mécanismes de rupture dans le champ turbulent, la déformation de la goutte le long de sa trajectoire est simulée à l'aide du modèle linéaire de Rayleigh-Lamb, dans lequel le forçage externe est représenté par la valeur locale instantanée du Weber turbulent. L'analyse statistique de la réponse dynamique de la goutte a permis d'identifier un critère de rupture basé sur un seuil de déformation critique, généralisable à l'ensemble des conditions opératoires. Cette approche multi-échelle a été reconduite pour étudier la rupture d'une goutte en milieu dense. La diminution de la probabilité de rupture en fonction de la concentration est liée à des modifications de la distribution des Weber turbulents en aval de l'orifice. ABSTRACT : This work presents the drop breakup phenomenon in a turbulent flow induced by a cross-section restriction in a pipe. A global analysis of single-drop breakup, in a finite volume downstream of the orifice, has allowed deriving statistical quantities such as the break-up probability and the daughter-drop distribution. These parameters are function of a global Weber number based on the maximal pressure drop through the orifice. At a local scale, the locations of breakup events are distributed heterogeneously and depend on the flow Reynolds number. The local hydrodynamic study in downstream of the orifice, which has been done by using Particle Image Velocimetry, reveals the specific breakup zones. Otherwise, this analysis has proved that the turbulence is the predominant external stress at the drop scale. The relation between drop deformation and the external stress along the trajectory has been simulated numerically by the response of a damped oscillator to the locally measured instantaneous turbulence forcing. The results of statistical analysis have allowed to introduce a breakup criterion, based on a unique deformation threshold value for all experiments. This multiscale approach has been conducted to study drop breakup mechanisms in a concentrated dispersion. The breakup probability decrease with the increase of dispersed phase concentration, which influences the turbulent Weber number distribution in downstream of the orifice

On single bubble mass transfer in a volatile liquid

We consider single bubble mass transfer of an non-condensible gas into a volatile liquid phase in indus- trial conditions, as observed for example in hydrocarbons liquid phase oxidation processes. Instantaneous bubble size, shape and velocity are measured using image processing with a particle tracking method. The mass transfer rate nitrogen into hot and pressurized liquid cyclohexane is deduced from the bubble volume decrease rate and is compared to literature correlations valid under isothermal conditions. Experiments are performed in a pressurized reactor for P = 20 bar, 30° ≤ T ≤ 150°C and bubble Reynolds number Re = O(10—100). The analysis of bubble rise dynamics shows that the gas-liquid system studied can be considered as a clean system. The mass transfer results are found to follow isothermal correlations predictions excepted for ambient temperature for which liquid evaporation in bubbles is shown to be coupled with mass transfer. This phenomena seems to be a consequence of having a high Lewis number

Etude expérimentale de la rupture de gouttes dans un écoulement turbulent

Ce travail concerne la rupture de gouttes dans un écoulement turbulent généré en aval d'une restriction de conduite. Un dispositif expérimental spécifique produit un écoulement dispersé liquide-liquide dans une conduite verticale. L'utilisation de la trajectographie par vidéo rapide a permis d'effectuer une analyse globale de la rupture d'une goutte isolée : des données statistiques, comme la probabilité de rupture, la distribution de taille des gouttes filles, ont été dérivées des observations expérimentales. Une étude de l'hydrodynamique locale en aval de l'orifice, via la vélocimétrie par images de particules, a mis en évidence la nature de la contrainte exercée par la phase porteuse sur la goutte responsable de la rupture. Le rôle de la dynamique propre de la goutte sur la rupture face à cette contrainte a été identifié grâce à la simulation numérique. Les mécanismes de la rupture de gouttes en écoulement concentré ont finalement été étudiés, expérimentalement et numériquementThis work presents the drop breakup phenomenon in a turbulent flow induced by a cross-section restriction in a pipe. A global analysis of single-drop breakup, in a finite volume downstream of the orifice, has allowed deriving statistical quantities such as the break-up probability and the daughter-drop distribution. These parameters are function of a global Weber number based on the maximal pressure drop through the orifice. At a local scale, the locations of breakup events are distributed heterogeneously and depend on the flow Reynolds number. The local hydrodynamic study in downstream of the orifice, which has been done by using Particle Image Velocimetry, reveals the specific breakup zones. Otherwise, this analysis has proved that the turbulence is the predominant external stress at the drop scale. The relation between drop deformation and the external stress along the trajectory has been simulated numerically by the response of a damped oscillator to the locally measured instantaneous turbulence forcing. The results of statistical analysis have allowed to introduce a breakup criterion, based on a unique deformation threshold value for all experiments. This multiscale approach has been conducted to study drop breakup mechanisms in a concentrated dispersion. The breakup probability decrease with the increase of dispersed phase concentration, which influences the turbulent Weber number distribution in downstream of the orifice.TOULOUSE-ENSIACET (315552325) / SudocSudocFranceF

Experimental study of mass transfer in a dense bubble swarm

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Ultrasound-detected tibial artery calcification as a marker of cardiovascular and lower-limb risk in asymptomatic patients with type-2diabetes

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Vascular ultrasound for cardiovascular risk stratification in asymptomatic patients with type-2 diabetes

International audienceBackground Cardiovascular diseases are among the main longterm complications of type-2 diabetes (T2D). Several tools have been proposed to estimate the cardiovascular (CV) risk in patients with T2D, but their prognostic value requires further investigation. We aimed to identify new independent vascular markers to predict CV events in patients with T2D, and their incremental value compared to the Swedish National Diabetes Register (NDR) risk score.Methods A retrospective cohort study was conducted on 1332 asymptomatic patients with T2D, free from prior CV event, assessed for CV work-up, including 12-lead ECG, ankle-brachial index and carotid and femoral bifurcations ultrasound to detect plaque in any of the 4 territories (right and left carotid and femoral arteries). The extent of atherosclerosis was rated from 0 to 4 and recorded as atherosclerosis burden score (ABS). Patients were followed up to 5 years and the occurrence of CV events (cardiovascular death, stroke/transient ischemic attack, myocardial infarction, unstable angina and cardiac revascularisation) was recorded.Results A total of 82 patients (6.2%) experienced a cardiovascular event, including 34 (2.6%) with myocardial infarction, 18(1.4%) with cardiac revascularisation and 17 (1.3%) with stroke. The independent determinants of these events were: male gender (HR = 1.81 [1.13—2.88], P=0.013) and ABS ≥2 (HR = 1.98 [1.21—3.25], P=0.007). The NDR risk score performed poorly to predict cardiovascular events (area under the curve = 0.56 [0.49—0.63], P=0.11), whereas screening for atherosclerotic plaques provided significant incremental prognostic value over the NDR score (model2 increase: +231%, P=0.002).Conclusion Duplex ultrasonography for atherosclerotic plaques improve the estimation of CV prognosis on top of clinical data. Because of its high availability and safety, it could be routinely used to stratify the cardiovascular risk and implement the guidelines-based interventions. Disclosure of interest The authors declare that they have no competing interest

PF4 Immunoassays in Vaccine-Induced Thrombotic Thrombocytopenia

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Factor IX assays in treated hemophilia B patients

International audienceReplacement therapy with plasma-derived or recombinant FIX (pdFIX or rFIX) concentrates is the standard of treatment in patients with hemophilia B. The method predominantly used for measuring factor IX (FIX:C) levels is the one-stage clotting assay (OSA) but this method depends on the activated partial thromboplastin time (APTT) reagent and the coagulation analyzer used, and wide variations in the measurements of FIX recovery have been reported with some factor concentrates. The French study group on the biology of hemorrhagic diseases (a collaborative group of the GFHT and MHEMO network), presents a review of the literature and proposals for the monitoring of FIX:C levels in treated hemophilia B patients. The use of OSA calibrated with a plasma reference tested against the current FIX WHO International Standard is recommended for the monitoring of patients treated with pdFIX or rFIX. Chromogenic substrate assays (CSA) are adequate for the monitoring of patients treated with Rixubis®, but data available for Benefix® are currently too limited. For extended half-life rFIX (EHL-rFIX), large discordances in the FIX:C levels measured were evidenced, depending on the method and reagents used. Great attention is therefore required for measuring FIX:C levels by OSA in patients substituted by EHL-rFIX. Commercial kits for CSA are not equivalent, and although potentially useful, they are not validated for all EHL-rFIX. Most of recent studies reported data obtained with spiked plasmas, which deserve to be confirmed on plasma samples collected in treated patients