액체 온도 변화에 따른 평판 충돌 액적의 거동에 관한 연구

A new type of atomizer has been developed to control mean droplet diameter over a wide range of liquid flow rates; this newly developed atomizer of Y-jet type employs a fluid amplifier to control the liquid flow rate. The liquid flowing into the atomizer is distributed by a fluid amplifier into two ports with different inner diameters. During the present experiments, the liquid flow rate was varied over a turndown ratio of 7, while the air flow rate was kept constant. Generally, the mean droplet diameter is greatly influenced by the atomizing air velocity, and the mean diameter decreases steadily as the atomizing air velocity increases. In the proposed method, to keep the mean diameter constant, the liquid flowing into smaller ports where high-speed air passes is greatly increased with total liquid flow rate. On the other hand, liquid flowing into larger ports, where low-speed air passes through, is gradually increased. Consequently, the mean droplet diameter changes only 10 μm for the optimum design over the turndown ration of 7, whereas the mean diameter changes 30 μm without the fluid amplifier.

Summary Dynamics of droplet impact on solid surfaces is of great importance in many engineering applications such as ink-jet printing and rapid spray cooling of hot surfaces. In this study, the lattice kinetic scheme based on the lattice Boltzmann method for immiscible two-phase fluids [Inamuro (2006)] is applied to such micro-fluid problems. The present method enables us to perform stable calculations of two-phase flows with large density ratios of up to 1000. The boundary condition based on the wetting potential that is calculated according to a prescribed static contact angle is used on solid surfaces [Briant, Papatzacos, and Yeomans (2002)]. First, the dynamic behavior of a single droplet impinging on a horizontal wall is investigated for various Weber numbers. The dynamic contact angle, the contact line velocity, and the wet length are calculated, and found to be in good agreement with available experimental data. Next, the collision of a falling droplet with a stationary droplet on a solid surface is investigated. The behavior of the droplets and the mixing process during their collision are simulated for various impact velocities and several static contact angles on the solid surface. It is seen that mixing occurs around the rim of the coalescent droplet owing to the circular flows. Also, the relation between the mixing rate of the primary coalescent droplet and Weber number is obtained. Finally, adherence and bouncing of liquid droplets impacting on dry surfaces are examined for several Reynolds numbers based on the droplet diameter. These calculated results indicate that the present method can be a promising numerical approach to investigation of microscale droplet dynamics on solid surfaces.

The dynamic behavior of liquid mercury and water droplets colliding with a solid surface were precisely examined as part of a fundamental study for investigating the behavior of a liquid microjet emitted as a result of a cavitation bubble collapse in mercury. Liquid droplets were collided with a quartz plate by the free-fall method, and the colliding and spreading behaviors of the liquids were observed using a high-speed video camera. In the case of mercury droplets, the spreading, recoiling, and jump-up phenomena resulting from the high surface tension of mercury were observed. The ratio of the maximum spreading diameter,Dmax, to the initial droplet diameter,D0, was investigated by parametrically changing the colliding velocity and tilt-angle of the quartz plate. The result showed that the ratioDmax/D0was well correlated with the Weber number, which is defined as a function of the colliding velocity and surface tension, independent of the liquid considered.

This article introduces two dimensionless positive geometric parameters that characterize the shape of a liquid droplet on a flat solid surface, which formed by the surface tension. The first parameter, “shape coefficient” K, is defined by the ratio of volume to surface and is always >3 (3 is the space dimension). The second parameter, “holding limit” κ0, is defined by the fraction of osculating surface and K and is <1. The ratio of the surface tension energy of a droplet attached to a substrate in zero gravity to the energy of the same droplet floating in zero gravity is presented through these parameters as 1-(K-3)(κ0-κ)/3(1-κ0), where the material parameter κ (which appears in the Young equation κ=cosθ) indicates the decrease in liquid surface tension by the solid The relative energy of the surface tension, K and κ0, are explicitly expressed for a droplet of an elliptical rounded segment (ERS) shape through its eccentricity e, relative height χ, and relative rounding radius η. It is shown that the Young equation is a self-consistent (i.e., leading to η=0) minimum condition of the energy only in the spherical (e=0) case. The rounding, either inner or outer, is specified by the legs of a triangle with zero angles and the median as a slope line. The main result obtained is the proof that the outer rounded ERS weighty droplets with inflection points, due to weight and hydrostatic forces, cannot exist if their radii larger than 2-4 capillary length. This proscription is absent in zero gravity.

To determine whether solar load distribution pattern on a solid nondeformable ground surface is the product of contact erosion and is the mirror image of load distribution on a deformable surface in horses. 30 clinically normal horses. Solar load distribution was compared among 25 clinically normal horses during quasistatic loading on a solid nondeformable surface and on a highly deformable surface. Changes in solar load distribution patterns were evaluated in 5 previously pasture-maintained horses housed on a flat nondeformable surface. Changes in solar load distribution created by traditional trimming and shoeing were recorded. Unshod untrimmed horses had a 4-point (12/25, 48%) or a 3-point (13/25, 52%) wall load distribution pattern on a flat solid surface. Load distribution on a deformable ground surface was principally solar and located transversely across the central region of the foot. Ground surface contact areas on solid (24.2 +/- 8.62 cm2) and deformable (69.4 +/- 22.55 cm2) surfaces were significantly different. Maintaining unshod horses on a flat nondeformable surface resulted in a loss of the 3- and 4-point loading pattern and an increase in ground surface contact area (17.9 +/- 2.77 to 39.9 +/- 12.77 cm2). Trimming increased ground surface contact area (24.2 +/- 8.60 to 45.7 +/- 14.89 cm2). In horses, the solar surface is the primary weight-loading surface, and deformability of ground surface may have a role in foot expansion during loading. Increased surface area induced by loading on deformable surfaces, trimming, and shoeing protects the foot.

Pulsed laser ablation of a Si substrate in cooled KOH solution and a water-confined region is investigated. The liquid temperature is varied from room temperature down to 4°C. For laser ablation in water, the material removal rate reduces slightly as liquid temperature increases. However, the material removal rate for laser ablation in KOH solution is twice as high as that in the water at a cooled liquid environment (temperature ∼10°C). This enhancement results from a strong chemical interaction between the KOH solution and Si materials around the ablated crater during fast cooling of the substrate surface from the vaporization temperature after the laser ablation. It is also found that the material removal rate is reduced greatly in the KOH solution as the liquid temperature increases. It is because at a liquid temperature above 20°C, the KOH solution starts to interact strongly with Si when the two materials are in contact. The formation of Si(OH)x chemical debris blocks the incident laser beam to reach the Si surface and reduces the laser ablation effect. This result of the investigation is applied to set up a cooled KOH flow system for wafer thinning to thin substrate thickness down to 50 μm.

Spray of gelled propellants from an impinging-jet injector under different temperatures

The stability of underwater bubbles is important to many natural phenomena and industrial applications. Since stability analyses are complex and influenced by numerous factors, they are often performed on a case-specific basis, with most being qualitative. In this work, we propose a unified and quantitative criterion for evaluating bubble stability by analysing its free energy. This criterion is broadly applicable across various bubble sizes (from nanometres to macroscale) and contact conditions (suspended, attached and trapped bubbles) on surfaces with diverse chemical (hydrophilic and hydrophobic) and morphological (flat and structured solid surfaces) features. This criterion not only applies to the classical stable bubble mode, which depends on contact line pinning at the tips of surface structures, but also predicts a new mode where the synergy between the geometry and wettability of the sidewalls maintains the bubble’s stable state. The contact line can spontaneously adjust its position on the solid surface to maintain pressure balance, which enhances bubble adaptability to environmental changes. A geometric standard for solid surfaces supporting this new stable state is raised, following which we realise the optimisation of solid surface geometries to control the stability of gas bubbles. This work not only provides a universal framework for understanding underwater bubble stability, but also opens avenues for applications.

This paper is concerned with numerical simulations of the deformation behavior of a liquid droplet impinging on a flat solid surface, as well as the flow field inside the droplet. In the present situation, the case where a droplet impinges on the surface at room temperature with a speed in the order of a few [m/s], is treated. These simulations were performed using the MAC-type solution method to solve a finite-differencing approximation of the Navier-Stokes equations governing an axisymmetric and incompressible fluid flow. For the first case where the liquid is water, the liquid film formed by the droplet impinging on the solid surface flows radially along it and expands in a fairly thin discoid-like shape. Thereafter, the liquid flow shows a tendency to stagnate at the periphery of the circular film, with the result that water is concentrated there is a doughnut-like shape. Subsequently, the water begins to flow backwards toward the center where it accumulates in the central region. For the second case where a n-heptane droplet impinges the surface, the film continues to spread monotonically up to a maximum diameter and there is no recoiling process to cause a backwards flow towards the central region. In this study the whole deformation process was investigated from numerical as well as experimental points of view. We find that the results obtained by the present mathematical model give fairly good agreement with the experimental observations. The effects of the viscous stresses and the surface tension on the deformation process of the droplets are estimated and discussed from a practical standpoint.

In a first set of experiments, observations of anisotropic wetting of different orientations of a macroscopic (milimeter-sized) solid single crystal of Pb by pure liquid Pb have been made at the melting point. It was found that the surface orientations which undergo premelting transitions below the bulk melting point are perfectly wetted by the liquid, whereas the {100} and {111} facets, which do not premelt, are only partially wetted. On those surfaces, wetting improves with decreasing atomic density of the surface. In a second set of observations on Pb-Ga alloys, wetting of a mezoscopic (micronsized) solid single crystal of Pb by liquid Ga was studied in a scanning Auger microprobe. At relatively low temperatures, below the premelting of the solid Pb surfaces, liquid Ga droplets are found to reside only at the {210} orientations of Pb surface. As the temperature is raised above that where most Pb surface orientations undergo premelting, the anisotropic distribution of Ga droplets disappears. In both cases, the anisotropy of wetting is driven by the anisotropy of surface energy of the solid. The degree of wetting is shown to be strongly correlated to the degree of premelting at the solid surface.

Study of the influence of interfacial waves on heat transfer in turbulent falling films

One of the problems in the storage of cryogenic liquids is the increase in pressure in the closed container, A considerable quantity of heat may be transferred into the container because of the large temperature difference between the ambient temperature outside the container and the temperature of the cryogenic liquid. The increase in pressure in a closed container due to this heat leak may be calculated from the increase in liquid temperature, obtained from the heat-leak rate, and the vapor pressure-temperature equilibrium relation. Experience has shown, however, that the container pressure may be considerably greater than the calculated pressure based on an average liquid temperature. The difference between the actual pressure and that calculated from an average liquid temperature has been attributed to the development of temperature gradients in the liquid, O. A, Hansen,1 Linde Air Products, Inc., and M. D. Andonian,2 Cambridge Corporation, referred to the presence of liquid-temperature strata in their papers presented at the 1956 Cryogenic Engineering Conference.

In the deep geological situation, the gypsum is almost always associated with high temperature and the other mineral substances such as sodium chloride. And, hot saline brine will greatly affect the strength and deformability of gypsum by influencing the petrographic characteristics such as microstructure, granular minerals, composition content and morphology. To quantify the effect of brine temperature and concentration on fracture toughness and fracture characteristics of gypsum, a serious of tests, including cracked straight through Brazilian disc (CSTBD) tests and scanning electron microscopy (SEM), were carried out. The results demonstrated that, when at a given crack inclination angle, the fracture peak of gypsum decreases with the increase of liquid temperature. When at a given liquid temperature, due to common-ion effect, the fracture peak first decrease then increase with the increase of brine concentration. In all these cases, due to the effect of friction coefficient, the fracture peak first decreases then increases with the increase of prefabricated crack angle. From SEM morphology results, the microcrack density increases with liquid temperature increasing, whereas first increase then decrease with brine concentration increasing. Subsequently, by adopting the approach of back-propagation neural network, the effective FPZ at different conditions were obtained. It was found that the size of effective FPZ is closely dependent on brine temperature and concentration.

The conditions to produce liquid droplets of uniform size were investigated experimentally. The number of droplets produced and the region which allowed production of uniform-size droplets were related with the operating conditions, design parameters of nozzle and physical properties of liquid. It was confirmed that the number of liquid droplets produced is equal to the vibration frequency within the uniform region, and the diameter of a liquid droplet which is equivalent to a sphere was calculated by Dabora''s equation. The region producing uniform droplets was not affected by the appropriate amplitude of nozzle vibration, but was affected by nozzle diameter, physical properties of liquid and frequency. Nondimensional empirical equations for their prediction were obtained. It was found that the range of liquid velocity of the uniform region corresponds to the range of the forced oscillating smooth jet from a vibrating nozzle. Uniform droplets were produced from the forced oscillating smooth jet which is vibrated at an appropriate amplitude and frequency.

Liquid dispersion and gas absorption in a multi-stage high-speed disperser