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[en] In cryogenic heat pipes, special attention must be paid to the capillary structure, since the capacity of these pipes is already limited by the adresse properties of the liquid alone, i.e. low surface tension, evaporation heat and thermal conductivity, high viscosity. For a known surface tension of the liquid the available capillarity is determined by the used capillary structure. The exact influence of the configuration of the capillary structure, which is of special importance in low-power cryogenic heat pipes (whose efficiency is always low) has not yet been fully studied for arteries and grooves. The authors therefore experimented with various kinds of arteries and grooves, measured their capillarity and established formulae to calculate the capillary force from this. (orig.)
[de]Der Kapillarstruktur muss bei kryogenen Waermerohren besondere Beachtung geschenkt werden, da die Leistungsfaehigkeit schon durch die schlechten Eigenschaften der Fluessigkeit - niedrige Oberflaechenspannung, Verdampfungswaerme und Waermeleitfaehigkeit, hohe Viskositaet - stark eingeschraenkt ist. Bei bekannter Oberflaechenspannung der Fluessigkeit wird die verfuegbare Kapillarkraft aus der verwendeten Kapillarstruktur bestimmt. Der exakte Einfluss der geometrischen Groessen der Kapillarstruktur, der besonders bei den ohnehin niedrigen Leistungen der kryogenen Waermerohre von Bedeutung ist, wurde fuer Arterien und Rillen noch nicht hinreichend genau untersucht. Daher wurden von uns verschiedene Arterien und Rillen hergestellt, ihre Kapillarkraft gemessen und daraus Formeln zur Berechnung der Kapillarkraft abgeleitet. (orig.)
[en] The capillary rise/fall of a liquid within a thin capillary tube is described by the well-established Jurin's law. The liquid reaches an equilibrium height/depth as the capillary pressure is balanced by the hydrostatic pressure. When the adhesion force at the three-phase contact line is counteracted by the liquid weight, the liquid column also stabilizes. Both descriptions evidently coincide although they are based on different physical quantities. Most undergraduate textbooks do not mention why these two routes draw two identical results; besides, the proofs become conceptually incorrect. We review the underlying ideas in Jurin's law and the two possible derivations.
[en] Here the authors study the effect of capillarity on a microscopic problem related to foam flow in porous media: the transport of a bubble or drop through a constricted capillarity tube. The aim is to study the snap off process which occurs during the formation of a foam in a porous media. The authors illustrate the dynamics of the bubble for different initial data and for several different sets of the physical parameters
[en] A recent experiment showed that cylindrical segments of water filling a hydrophilic stripe on an otherwise hydrophobic surface display a capillary instability when their volume is increased beyond the critical volume at which their apparent contact angle on the surface reaches 90 deg. (Gau et al 1999 Science 283 46-9). Surprisingly, the fluid segments did not break up into droplets-as would be expected for a classical Rayleigh-Plateau instability-but instead displayed a long-wavelength instability where all excess fluid gathered in a single bulge along each stripe. We consider here the dynamics of the flow instability associated with this setup. We perform a linear stability analysis of the capillary flow problem in the inviscid limit. We first confirm previous work showing that all cylindrical segments are linearly unstable if (and only if) their apparent contact angle is larger than 90 deg. We then demonstrate that the most unstable wavenumber for the surface perturbation decreases to zero as the apparent contact angle of the fluid on the surface approaches 90 deg, allowing us to re-interpret the creation of bulges in the experiment as a zero-wavenumber capillary instability. A variation of the stability calculation is also considered for the case of a hydrophilic stripe located on a wedge-like geometry.
[en] This study examines the effect of pore throat size and injection flowrate on the values of the pore-scale capillary number, the Newtonian-fluid capillary number and the apparent capillary number (Nc1, Nc2 and Nc3, respectively) and their sensitivity to change in high-capillary-number flow through porous media, which occurs in polymer-assisted dilute surfactant flooding (PADSF). Additionally, the influence of pore throat size and injection flowrate on oil recovery at breakthrough and at the end of displacement (ultimate) and the relationship between the effective shear rate γeff and the porous medium-dependent shift factor α are discussed. The results indicated that Nc2 was the smallest and Nc3 was the largest value. The difference between Nc2 and Nc3 is due to the increase in apparent viscosity of the polymer-contained surfactant solution during the flow through porous media and the change in Nc3 should be utilized to characterize the macroscopic behavior of the PADSF. Generally, the decrease in pore throat size and the increase in injection flowrate caused an increase in the ultimate oil recovery and Nc3. Moreover, the oil recovery at breakthrough decreased with an increase in pore throat size and injection flowrate. Finally, the rate of change of γeff, with change in α, increased almost uniformly with a decrease in pore throat size and an increase in injection flowrate.
[en] A method is described for measuring the magnetorheological characteristics of ferrosuspensions by means of a capillary rheometer by use of measurement section in the form of a helix. This configuration permits liberal variation of the fundamental technical characteristics (diameter and length of the capillary) while imparting overall dimensions to the measurement section such that practical homogeneous magnetic field patterns can be utilized. It is noted that because of the orientation effect, the viscosity of the ferrosuspension in the inlet and exit ducts will be much smaller than the viscosity in the working section, thereby enhancing the measurement accuracy and simplifying the measurement of the pressure differential. Principal measurement errors associated with the proposed experimental scheme are analyzed. 6 references
[en] The motion of an air–fluid interface through an irregularly coated capillary is studied by analyzing the Lucas–Washburn equation with inertia, viscosity and a random capillary force. Below a critical velocity, the front enters a strongly intermittent dynamic regime, as recently observed in experiments. Analytical estimates for the average asymptotic front trajectory and pinning length distribution are obtained, and a numerical procedure for predicting quantities of experimental interest is also illustrated. (letter)
[en] Molecular transport through nanopores has recently received considerable attention as a result of advances in nanofabrication and nanomaterial synthesis technologies. Surprisingly, water transport investigations through carbon nanochannels resulted in two contradicting observations: extremely fast transport or rejection of water molecules. In this paper, we elucidate the mechanism of impeded water vapor transport through the interstitial space of aligned multiwalled carbon nanotubes (aligned-MWCNTs)—capillary condensation, agglomeration, reverse capillary flow, and removal by superhydrophobicity at the tip of the nanotubes. The origin of separation comes from the water’s phase change from gas to liquid, followed by reverse capillary flow. First, the saturation water vapor pressure is decreased in a confined space, which is favorable for the phase change of incoming water vapor into liquid drops. Once continuous water meniscus is formed between the nanotubes by the adsoprtion and agglomeration of water molecules, a high reverse Laplace pressure is induced in the mushroom-shaped liquid meniscus at the entry region of the aligned-MWCNTs. The reverse Laplace pressure can be significantly enhanced by decreasing the pore size. Finally, the droplets pushed backward by the reverse Laplace pressure can be removed by superhydrophobicity at the tip of the aligned-MWCNTs. The analytical analysis was also supported by experiments carried out using 4 mm-long aligned-MWCNTs with different intertube distances. The water rejection rate and the separation factor increased as the intertube distance decreased, resulting in 90% and 10, respectively, at an intertube distance of 4 nm. This mechanism and nanotube membrane may be useful for energy-efficient water vapor separation and dehumidification. (paper)
[en] The authors found that an employment of crushed scallop shells in the lower layer instead of gravel greatly helps to reduce the transport of the finer particles in the upper layer into the lower layer, and to provide an effective function of capillary barriers. In this study, the diversion lengths are measured in the field experiments using crushed shells instead of gravel in the lower layer, and compared it with the diversion lengths of the inclined capillary barrier having cover soil. As the result, the authors make clearly the diversion length of the inclined capillary barrier having cover soil. (author)
[en] Highlights: • Evaporator temperature regulation of a capillary pumped loop is proposed. • Reduced models are built via Modal Identification Method from in situ measurements. • The model-based control is achieved through a state feedback loop. • The study is conducted for two types of disturbance: a smooth one and a sharp one. • Results show the disturbance rejection for the sine signal. - Abstract: This study focuses on the regulation of a two-phase capillary pumped loop. The aim is to adjust the temperature and therefore the pressure in the reservoir in real time in order to reject the disturbance which occurs in the evaporator due to high-power cycles of electronic equipment. Regulation is achieved through a state feedback approach, which appeared to be very efficient for other fields of application. A very fast model of the thermal behavior of the loop is required. Low order models are elaborated via the Modal Identification Method. This model, built from in situ measurements, is valid around a nominal operating point where we can consider the behavior of the loop linear. A Kálmán filter is introduced in order to estimate the state of the system in real time. Two types of disturbance are considered: a slow and a fast one, in order to simulate a real power cycle. Results show that the control of the evaporator temperatures, adjusting the reservoir temperature, is quite satisfying when the disturbance variations are smooth in time. In this case, the objective, which is to limit the temperature variations at the evaporator, in order to avoid deterioration of the high-power electronics, is reached.