Vertical Capillary Research Articles

Studies on a sealed-cell lambda-point device for use in low temperature thermometry

We report here the development of a sealed-cell lambda-point device which has been used to realize the lambda-point temperature of 4He, T λ =2.1768 K, to a precision of 0.04 mK with simple automatic control. With this device, one can produce and maintain T λ -plateaus for unlimited duration with no ascertained temperature drift. This unique performance is achieved by a design, which makes it possible to keep a HeI/HeII interface quasi-stationary inside a vertical variable-conductance capillary tube. The capillary connects a top HeII-cell and a bottom HeI-cell at its two ends, and acts as a sensitive heat switch which automatically adjusts a heat input to the bottom HeI-cell to compensate exactly the heat conducted out of the HeII-cell to the environment. The HeI/HeII interface thus stays at an equilibrium height determined by the magnitude of heat flow through the capillary and by the temperature difference along the capillary. The sharp difference between the thermal conductances of the interfacing HeII and HeI columns is the underlying physics of this self-adjusting function. Realization of T λ is achieved after appropriate corrections to the thermometer reading at the HeII-cell, in particular, by extrapolating to zero heat flow through the device.

1115 Forced Convective Boiling in a Capillary Tube

Flow boiling heat transfer in a vertical capillary tube, ID=1.45mm, is experimentally investigated using water for a sub-atmospheric and an atmospheric pressure condition in a range of mass flux from 20 to 100 kg/m^2s and a range of heat flux from 10 to 750 kW/m^2. The dominant flow pattern is found to be slug-annular and annular flow. It is found that heat transfer coefficient is not so dependent on heat flux, but dependent on quality. Therefore, heat transfer mechanism is primarily due to the convective boiling. The heat transfer coefficient is enhanced at a low heat flux and is underpredicted by Chen correlation. However, the correlation gives good prediction at a high heat flux.

Falling Slugs

The fall of viscous slugs in vertical capillary tubes is described. Deviations toward Poiseuille law are analyzed by taking into account the dissipation in menisci, together with the existence of a film behind the slug. Slugs are found to fall slower in dry tubes than in prewetted ones, which is quantitatively discussed in term of viscous friction. A criterion for the minimal length of the slug obeying the Poiseuille solution is finally derived.

The conditions of existence of autooscillations in a dropwise liquid flow through a vertical capillary

A model describing the dropwise liquid flow through a vertical capillary, taking into account the mutual influence of the drops, is developed, and the conditions of existence of an autooscillatory regime in this system are established. The period and amplitude of the autooscillations are described as functions of the system parameters.

Saturated-Liquid Viscosity of Ten Binary and Ternary Alternative Refrigerant Mixtures. Part I: Measurements

The saturated-liquid viscosities of 10 binary and ternary refrigerant mixtures composed of difluoromethane (R32), 1,1,1,2-tetrafluoroethane (R134a), pentafluoroethane (R125), and propane (R290) were measured in a sealed gravitational capillary viscometer with a straight vertical capillary from 245 to 345 K or to a maximum vapor pressure of 3 MPa. The maximum uncertainty of the measurements is estimated to be ±2.8% for the mixtures of R32 + propane. The largest contribution to the total uncertainty is that of the saturated liquid and vapor densities of the mixtures, which had to be estimated. Comparisons with viscosity data from the literature show agreement within the mutual uncertainty for all but one data set. Similar agreement is found with predicted viscosities using the extended-corresponding-states model in NIST Standard Reference Database 23 (REFPROP), except for the mixtures of the polar fluids R32 and R134a with the nonpolar propane. Their viscosity−composition dependences are strongly nonlinear.

Characteristics of leucocyte adhesion directly observed in flowing whole blood in vitro.

Use of whole blood in adhesion assays allows analysis of the rheological and haematological factors that may influence adhesion, and avoids the need for isolation procedures that may modify the properties of leucocytes. We have adapted an in vitro flow model to allow videomicroscopy of leucocytes fluorescently labelled with rhodamine 6G (20 microg/ml) in anticoagulated whole blood. Blood was perfused at a range of wall shear rates (35-280/s) through a vertical glass capillary with a rectangular cross-section (microslide) that had been coated with P-selectin (10 microg/ml). Nearly all adherent cells were rolling in blood that had been anticoagulated with buffered citrate, but 40-50% became immobilized when heparin or thrombin inhibitor (PPACK) were used. The efficiency of leucocyte adhesion decreased steadily during 1-4 h of blood storage. Smaller fluorescent cells (lymphocytes) adhered less efficiently than larger cells (granulocytes) and rolled faster. Adhesion decreased monotonically with increasing wall shear rate or stress, but the velocity of rolling varied little. Among healthy volunteer donors, adhesion correlated with blood leucocyte count, but did not vary significantly with natural variation in haematocrit, blood viscosity or red cell aggregation. In conclusion, we have characterized adhesion of leucocytes in flowing whole blood, identified key experimental variables and demonstrated that physical environmental factors can markedly influence adhesive behaviour.

Contact line dynamics near the pinning threshold: a capillary rise and fall experiment

We used video microscopy to study the pinning dynamics of air/water contact lines in vertical glass capillaries. Stick-slip behavior and avalanches are observed in tubes with rough interior walls and strong pinning forces. In tubes with smooth interior walls, we find that receding contact lines in falling water columns show no evidence of pinning, but advancing contact lines in rising water columns exhibit algebraic slow down. The measured value of the critical exponent beta varies from run to run, but it is always larger than unity. Furthermore, we find that the rise dynamics varies with the waiting time preceding the experiments. These observations led us to conclude that the wetting film on the surface and other microscopic changes in the slipping region near the contact line affect the macroscopic dynamics. We discuss the differences between the real system and the existing theories that might explain the results. We also present a brief review of other studies of contact line dynamics and a numerical study of a one-dimensional model.

615 垂直細管内等密度油・水混合流における油プラグの挙動(熱工学V)

615 垂直細管内等密度油・水混合流における油プラグの挙動(熱工学V)

The Wicking of Water in Yarn as Measured by an Electrical Resistance Technique

An apparatus was developed to study water-transport behaviour along yarns. The rise of the liquid water in the yarn triggered an electrical circuit which was coupled with a personal computer, and so the distance of water rise as a function of time was determined. To smooth the data, the method of least squares was applied and the theoretical equation of the flow through the vertical cylindrical capillary was used. Polypropylene filaments and polyester yarns in various forms were examined. The rate constants and the relations between vertical water transport and time in different yarns are reported.

Breakup of drops and bubbles translating through cylindrical capillaries

We examine the shape deformation and breakup of air bubbles and viscous drops moving through vertical cylindrical capillaries under the action of pressure and/or buoyancy forces. Experimental observations of fluid particle shape are reported over a wide range of particle sizes and capillary numbers in a variety of two-phase systems. Four different modes of breakup are identified, and the critical conditions for the onset of various modes are examined. It is found that buoyancy forces can have a stabilizing effect on the breakup mechanism observed by Olbricht and Kung [Phys. Fluids 4, 134, (1992)] for low viscosity-ratio drops, wherein a growing indentation at the trailing end of the drop develops into a penetrating jet of outer phase fluid.

Anisotropic Properties of the LC Surface Tension

We have proposed a measurement technique for detection of the anisotropy of the surface tension (ST). A flat vertical capillary of glass substrates with transparent electrodes is split into 5 vertical sections. In each section a different LC orientation is set, such as planar, homeotropic, and homeoplanar. The open capillary end is placed into a vessel with LC. In each section the LC has a special level to ensure measurement of ST coefficients, while by varying temperature and applying driving voltage one can study how the ST is affected. The main results are:1)The smallest rise(and the lowest ST) is observed for the homeotropic orientation, the highest for planar orientation, and average for the homeoplanar orientation. 2) Heating of the LC up to 30 to 50°C above isotropic point or application of orienting fields do not qualitatively change the picture of distribution of the rise levels. Interpretation of the results obtained and the ST coefficients for known LC are given.

Architecture of blood vessels in human fetal gastric corpus: a corrosion casting study

Vascular architecture of the gastric corpus was investigated in 16-24 wk human fetuses using a corrosion casting technique and the scanning electron microscopy. The general distribution of blood vessels seen in adults has already been established in the fetus, with three major vascular plexuses located in the serosa, submucosa and mucosa. The serosal plexus, supplied and drained by large extramural vessels, contained anastomosing, arcade-like arrays of arteries and veins with their branches piercing the muscularis and communicating with the compact submucosal plexus. Vertical arterioles and capillaries were sent by submucosal arteries to supply a very dense capillary plexus which surrounded the gastric pits and consisted of wide, sinusoidal vessels showing morphological manifestations of angiogenesis by intussusceptive growth. The plexus was drained by vertical venules emptying into submucosal veins. In contrast to the richly vascularized upper half of the mucosa, the lower half showed a relative paucity of blood vessels, probably due to the thinness of the fetal mucosa allowing an effective diffusion of oxygen and nutrients from the upper half. Neither arteriovenous anastomoses, nor end-arteries were found in the fetal stomach. Results of this study support one of the two existing models of mucosal vascularization in the human stomach: i.e. the model postulating the presence of short and long arterioles and two distinct, albeit interconnected capillary networks in the upper and lower zones of the mucosa respectively. In human fetuses, the latter network is absent; it probably develops by remodelling of the preexisting vertical capillaries in the last phase of pregnancy, prior to the onset of gastric gland function.

Interdiffusion and Convection in High Magnetic Fields

In liquid diffusion measurements, the main problem is the additional mass transport by convection. In the system indium-tin, convection caused by density gradients in horizontal and vertical capillaries was characterised, and the magnetohydrodynamic damping was investigated experimentally. While the magnetic field effect on diffusion experiments in vertical capillaries was relatively minor, the damping effect of the magnetic field could be clearly demonstrated in horizontal capillaries. Comparing these experiments with experiments in microgravity it can be seen that in the investigated system convection was practically completely suppressed by transversal magnetic fields higher than 3T. In electrically conducting crucibles, the sample was mixed by Lorentz forces on account of thermoelectric currents.

Viscosity measurements of ammonia, R32, and R134a. Vapor buoyancy and radial acceleration in capillary viscometers

The saturated liquid viscosity of ammonia (NH3) and of the hydrofluorocarbons, difluoromethane (CH2F2, R32) and 1,1,1,2-tetrafluoroethane (CF3–CH2F, R134a), was measured in a sealed gravitational viscometer with a straight vertical capillary. The combined temperature range was from 250 to 350 K. The estimated uncertainty of the ammonia measurements is ±3.3 and ±2 to 2.4% for the hydrofluorocarbons with a coverage factor of two. The results are compared with literature data which have been measured with capillary viscometers of different design. Agreement within the combined experimental uncertainty is achieved when some of the literature data sets are corrected for the vapor buoyancy effect and when a revised radial acceleration correction is applied to data which were obtained in viscometers with coiled capillaries. An improved correction for the radial acceleration is proposed. It is necessary to extend inter-national viscometry standards to sealed gravitational capillary instruments because the apparent inconsistencies between refrigerant viscosity data from different laboratories cannot be explained by contaminated samples.

Pressure-Driven Motion of Drops and Bubbles through Cylindrical Capillaries: Effect of Buoyancy

We examine the motion and deformation of air bubbles and viscous drops through vertical cylindrical capillaries in the presence of an imposed pressure-driven flow. Experimental measurements of the terminal velocity of drops and bubbles are reported for a wide range of drop sizes in a variety of two-phase systems, and the steady drop shapes are quantitatively characterized using digital image analysis. In contrast to the pressure-driven motion of neutrally-buoyant drops, the relative mobility of a buoyant drop is not a monotonically increasing function of capillary number. The relative mobility is enhanced as the buoyancy force becomes more dominant compared to surface tension, or as the drop fluid becomes less viscous relative to the suspending fluid. However, there is a limiting value of the Bond number beyond which the relative mobility becomes insensitive to the value of the Bond number. Similarly, the thickness of the liquid film surrounding large drops increases rapidly with increasing Bond number, but eventually approaches a constant value as the Bond number exceeds the limiting value. This limiting value of the Bond number is found to be a decreasing function of capillary number. When buoyancy and pressure forces act in the same direction, increasing the Bond number is found to delay the formation of a re-entrant cavity at the trailing end of the drop.

Density Oscillator: Analysis of Flow Dynamics and Stability

Two open containers filled with fluids of different density and connected by a vertical capillary tube can approach their thermodynamical equilibrium via an oscillatory flow. The temporal evolution of flow velocities was measured quantitatively. Data on the critical values determining the flow reversal are extracted yielding a simple dependence of the critical level of heavy fluid as a function of the density ratio and the length of the capillary. A novel theoretical description of the system is presented which is based on a one-fluid model and a steady-state approximation for a two-dimensional flow. Theoretical results are in good agreement with the experimental data.

Dynamics of Contact Line Pinning in Capillary Rise and Fall

The dynamics of water columns rising and falling in vertical glass capillaries are studied near the pinning threshold. In tubes with rough interior walls, the column height $h$ as a function of time $t$ exhibits stick-slip behavior. In tubes with smooth walls, falling experiments show no pinning effects, but rising experiments exhibit critical behavior with nonunique exponents. We argue that this is due to the fact that fluid has to be transported in a microscopic wetting film which does not have a stationary structure. Similar restrictions can affect avalanche dynamics in other systems.

Dynamic simulation of free surfaces in capillaries with the finite element method

The mathematical formulation of the dynamics of free liquid surfaces including the effects of surface tension is governed by a non-linear system of elliptic differential equations. The major difficulty of getting unique closed solutions only in trivial cases is overcome by numerical methods. This paper considers transient simulations of liquid–gas menisci in vertical capillary tubes and gaps in the presence of gravity. Therefore the CFD code FIDAP 7.52 based on the Galerkin finite element method (FEM) is used. Calculations using the free surface model are presented for a variety of contact angles and cross-sections with experimental and theoretical verification. The liquid column oscillations are compared for numerical accuracy with a mechanical mathematical model, and the sensitivity with respect to the node density is investigated. The efficiency of the numerical treatment of geometric non-trivial problems is demonstrated by a prismatic capillary. Present restrictions limiting efficient transient simulations with irregularly shaped calculational domains are stated. © 1998 John Wiley & Sons, Ltd.

Thermocapillary Effects on the Wetting Characteristics of a Heated Curved Meniscus

An investigation of thermocapillary effects on a heated, evaporating meniscus, formed by a wetting liquid in a vertical capillary tube, has been conducted. Experiments were conducted to primarily observe how the wetting characteristics of the working fluid (pentane) are affected by the dynamics associated with the heating of and evaporation from a meniscus. The results have demonstrated that interfacial thermocapillary stresses arising from liquid-vapor interfacial temperature gradients can noticeably degrade the ability of the liquid to wet the pore.

Drop formation in viscous flows at a vertical capillary tube

Drop formation at the tip of a vertical, circular capillary tube immersed in a second immiscible fluid is studied numerically for low-Reynolds-number flows using the boundary integral method. The evolution and breakup of the drop fluid is considered to assess the influences of the viscosity ratio λ, the Bond number ℬ, and the capillary number 𝒞 for 10−2⩽λ⩽10, 10−2⩽C⩽1, and 0.1⩽B⩽5. For very small λ, breakup occurs at shorter times, there is no detectable thread between the detaching drop and the remaining pendant fluid column, and thus no large satellite drops are formed. The distance to detachment increases monotonically with λ and changes substantially for λ>1, but the volume of the primary drop varies only slightly with λ. An additional application of the numerical investigation is to consider the effect of imposing a uniform flow in the ambient fluid [e.g., Oguz and Prosperetti, J. Fluid Mech. 257, 111 (1993)], which is shown to lead to a smaller primary drop volume and a longer detachment length, as has been previously demonstrated primarily for high-Reynolds-number flows.