Étude expérimentale des effets thermiques dans une garniture mécanique

Les performances et la fiabilité des garnitures mécaniques sont étroitement liées aux caractéristiques du film séparant les deux anneaux, le rotor et le stator, dont l’épaisseur doit être suffisante pour limiter le frottement et l’usure tout en maintenant la fuite à une valeur acceptable. Sous l’effet du frottement, résultant du cisaillement du film fluide et du contact des aspérités, des gradients de température sont générés au niveau des faces de la garniture et peuvent affectés de façon significative les performances de l’étanchéité. Les travaux précédents ont montrés que ces gradients peuvent atteindre quelques dizaines de degrés. Cet article présente une étude expérimentale du comportement thermique des garnitures sous différentes régimes de fonctionnement. Les expériences ont été réalisées sur une garniture réelle industrielle en utilisant des thermocouples implantés dans le grain fixe à différentes positions radiales et axiales afin d’obtenir une cartographie de la distribution de la température du stator. Les tests ont été réalisés à différentes valeurs de la vitesse de rotation et de la pression du fluide. L’amplitude des effets thermiques dépend fortement des conditions de fonctionnement. De manière générale, la température augmente avec la pression du fluide. Elle augmente également avec la vitesse lorsque le régime de lubrification est hydrodynamique. En revanche, en régime de lubrification mixte, la température peut diminuer lorsque la vitesse de rotation augmente

Rushes video summarization using a collaborative approach

This paper describes the video summarization system developed by the partners of the K-Space European Network of Excellence for the TRECVID 2008 BBC rushes summarization evaluation. We propose an original method based on individual content segmentation and selection tools in a collaborative system. Our system is organized in several steps. First, we segment the video, secondly we identify relevant and redundant segments, and finally, we select a subset of segments to concatenate and build the final summary with video acceleration incorporated. We analyze the performance of our system through the TRECVID evaluation

Active and supportive computer-mediated resources for student-to-student conversations

Communication is a central aspect of human learning. Using the Probability Inquiry Environment (PIE) as an example, we examine how external representations (both textual and iconic) mediate face-to-face conversations among students, and support productive mathematical discourse. We provide quantitative data that suggests that seventh grade students who used PIE learned some of the basic principles of probability. Two cases studies are that illustrate how communication supported by computer-mediated representations contributed to this success. The first case study demonstrates how the computer can actively prompt student conversations that lead to learning. The second case study examines how an animated graphical representation supported these productive conversations

How polymer additives reduce the pour point of hydrocarbon solvents containing wax crystals

We have investigated how four different pour point depressant (PPD) polymers affect the pour point transition in mixtures of a single pure wax in a solvent. We used either n-eicosane (C20), CH3(CH2)18CH3, n-tetracosane (C24), CH3(CH2)22CH3 or n-hexatriacontane (C36), CH3(CH2)34CH3 as the wax component with either n-heptane or toluene as the solvent component. For all wax–solvent combinations, the measured variation of wax solubility with temperature is well predicted by ideal solution theory. The variation of pour point temperature as a function of the overall wax concentration is quantitatively modelled using the idea that, for each overall wax concentration, the pour point occurs at a temperature at which a critical volume fraction ϕ* of wax crystals has precipitated. Close to the pour point temperature, extraction and examination of the wax crystals show they consist of polydisperse, irregularly-shaped platelets with axial ratios (h/d, where h is the plate thickness and d is the plate long dimension) in the range 0.005–0.05. It is found that the measured ϕ* values corresponding to the pour point transitions are weakly correlated with the wax crystal axial ratios (h/d) for all wax–solvent–PPD polymer combinations. These results indicate that the pour point transition occurs at a volume fraction larger than the value at which the volumes of rotation of the platelet crystals overlap, i.e., 2.5(h/d) < ϕ* < 11(h/d). PPD polymers work, in part, by increasing the wax crystal axial ratio (h/d), thereby increasing ϕ* and reducing the pour point temperature. Since the PPD's ability to modify the wax crystal shape relies on its adsorption to the crystal-solution surface, it is anticipated and observed experimentally that optimum PPD efficacy is correlated with the difference between the wax and the polymer solubility boundary temperatures. This finding and the mechanistic insight gained here provide the basis for a simple and rapid screening test to identify candidate species likely to be effective PPDs for particular wax systems

The effect of high [K(+)]o on spontaneous Ca(2+) waves in freshly isolated interstitial cells of Cajal from the rabbit urethra.

Interstitial cells of Cajal (ICC) act as putative pacemaker cells in the rabbit urethra. Pacemaker activity in ICC results from spontaneous global Ca(2+) waves that can be increased in frequency by raising external [K(+)]. The purpose of this study was to elucidate the mechanism of this response. Intracellular [Ca(2+)] was measured in fluo-4-loaded smooth muscle cells (SMCs) and ICC using a Nipkow spinning disk confocal microscope. Increasing [K(+)]o to 60 mmol/L caused an increase in [Ca(2+)]i accompanied by contraction in SMCs. Raising [K(+)]o did not cause contraction in ICC, but the frequency of firing of spontaneous calcium waves increased. Reducing [Ca(2+)]o to 0 mmol/L abolished the response in both cell types. Nifedipine of 1 μmol/L blocked the response of SMC to high [K(+)]o, but did not affect the increase in firing in ICC. This latter effect was blocked by 30 μmol/L NiCl2 but not by the T-type Ca(2+) channel blocker mibefradil (300 nmol/L). However, inhibition of Ca(2+) influx via reverse-mode sodium/calcium exchange (NCX) using either 1 μmol/L SEA0400 or 5 μmol/L KB-R7943 did block the effect of high [K(+)]o on ICC. These data suggest that high K(+) solution increases the frequency of calcium waves in ICC by increasing Ca(2+) influx through reverse-mode NCX

Dugesia species

25 p. : ill. ; 24 cm.Includes bibliographical references (p. 21-24)