Air Entrapment Research Articles

Singular air entrapment at vertical and horizontal supported jets: plunging jets versus hydraulic jumps

In plunging jets and at hydraulic jumps, large amounts of air bubbles are entrained at the impingement of the liquid jet into the receiving body. Air is entrapped and advected into a turbulent shear layer with strong interactions between the air bubble advection process and momentum shear flow. In this new physical study, air-water flow measurements were systematically repeated with identical inflow length, inflow depth and inflow velocity in a vertical supported jet (PJ) and a horizontal hydraulic jump (HJ). Detailed measurements were conducted with the same instrumentation. Both similarities and differences were observed between the two multiphase gas-liquid shear flows. Visual observations showed a key difference in the outer region, with a buoyancy-driven flow in the plunging jet with negligible void fraction, versus a strong recirculation motion with uncontrolled interfacial aeration in the hydraulic jump. Differences were also observed at the impingement perimeter, in terms of fluctuation frequencies and amplitudes, for identical inflow conditions. Both flow conditions yielded intense local singular air entrainment and close results were observed in terms of void fraction, bubble count rate, bubble chord sizes and interfacial area in the shear layer, in both the plunging jet and hydraulic jump. The transfer of momentum between impinging jet and receiving water, as well as the effect of buoyancy, were however affected by the flow geometry.

A compressible two-phase flow model for pressure oscillations in air entrapments following green water impact events on ships

A compressible two-phase flow model for pressure oscillations in air entrapments following green water impact events on ships

A molecular to macro level assessment of direct contact membrane distillation for separating organics from water

The removal of water-soluble organics from aqueous feeds is required in numerous practical applications, including bioresource processing, fermentation, and wastewater treatment. To this end, direct contact membrane distillation (DCMD) has been proposed as a separation technology. DCMD utilizes hydrophobic membranes – typically, comprising perfluorocarbons – which, when placed between a warm feed and a cold permeate, prevent mixing due to the robust entrapment of air inside the (membranes') pores. Thus, the membranes allow only pure water vapor to transport across, following the thermal gradient. Here, we assessed DCMD for separating organics from aqueous feeds in light of organic fouling by utilizing ethanol and perfluorodecyltrichlorosilane (FDTS) as the surrogate organic and hydrophobic coating, respectively. We investigated the adsorption of ethanol onto FDTS-grafted surfaces and membranes exposed to alcohol-water mixtures. Using the surface force apparatus, we found that the magnitude of hydrophobic forces between ultra-smooth FDTS-grafted mica surfaces in water-alcohol mixtures decreased with the increasing alcohol content. To simulate a practical DCMD scenario, we utilized FDTS-grafted polycarbonate membranes to separate a pure water reservoir from another containing 0.6 M NaCl and alcohol. For the 0% alcohol case, the membranes robustly separated the reservoirs for over a week, whereas even for ≥0.1% ethanol content, the membranes leaked within <5 h. After the leakage, the membranes’ hydrophobicity could not be recovered by rinsing them with pure water and blow-drying; a heat treatment at 363 K for 1 h proved to be successful, however. Our molecular dynamics simulations revealed that ethanol molecules in water got preferentially stabilized at the interfaces of water and hydrophobic surfaces. Furthermore, this stabilization is significantly enhanced at higher alcohol concentrations due to the emergence of II-D interfacial networks comprising adsorbed alcohol molecules. Thus, this micro to macro-scale assessment demonstrates that DCMD with hydrophobic membranes is not suitable for separating organics from water, even at low alcohol concentrations. We also compare the efficacies of apparent advancing and receding contact angles towards a reliable characterization of fouled surfaces.

Optimization of Device Deairing and Airless Connection Techniques for Cleveland Clinic Continuous-Flow Artificial Heart

Prevention of air introduction during mechanical circulatory support device implant is critical for a successful outcome. A substantial amount of air may be introduced into the circulation during pump to outflow port connection, which may become detrimental for optimal pump function and long-term survival. In this study, we evaluated techniques of seamless, airless connection of double-ended centrifugal pump (continuous-flow total artificial heart [CFTAH]) in vivo. The airless CFTAH connection techniques were evaluated in acute in vivo (n=2) CFTAH implants (Jersey calves, weight: 77.8 and 80.8 kg). Techniques consisted of pump priming with normal saline (Fig. 1 A) and application of specifically designed connection sleeve (Fig 1B). The silicone sleeve (0.3-0.5 mm thick) was prototyped using multilayer dip-coating technique over a 3D-printed mandrel. A suture line was added along the long axis, to enable easy removal. Deairing sleeve was pre-placed on each outflow port (conduit) and proximal port (port) ends (Fig.1 C). The present air was evacuated upward from an open line. The conduit and pump ports were connected within the sleeve (Fig.1 D), with air being fully excluded from circuit. The CFTAH was started at the minimum speed (2,200 rpm) after connection. The CFTAH pump were successfully primed with saline prior to implant to remove entrapped air. The completely seamless pump connection and exclusion of air from pump and outflow grafts has been found feasible. There were no clinical signs of air embolism in the pulmonary or systemic circulation observed during the experiment. The seamless connection of CFTAH to outflow grafts using specifically designed connection sleeve and its easy removal was demonstrated to be feasible. Device development is ongoing and will aim to address optimal geometry, size and material selection.

Mitigating cavitation erosion using biomimetic gas-entrapping microtextured surfaces (GEMS).

Cavitation refers to the formation and collapse of vapor bubbles near solid boundaries in high-speed flows, such as ship propellers and pumps. During this process, cavitation bubbles focus fluid energy on the solid surface by forming high-speed jets, leading to damage and downtime of machinery. In response, numerous surface treatments to counteract this effect have been explored, including perfluorinated coatings and surface hardening, but they all succumb to cavitation erosion eventually. Here, we report on biomimetic gas-entrapping microtextured surfaces (GEMS) that robustly entrap air when immersed in water regardless of the wetting nature of the substrate. Crucially, the entrapment of air inside the cavities repels cavitation bubbles away from the surface, thereby preventing cavitation damage. We provide mechanistic insights by treating the system as a potential flow problem of a multi-bubble system. Our findings present a possible avenue for mitigating cavitation erosion through the application of inexpensive and environmentally friendly materials.

A computational study on air entrapment and its effect on convective heat transfer during droplet impact on a substrate

This paper numerically investigates air entrapment behavior when a water droplet impacts on a solid substrate in the presence of ambient air. The dynamics of air entrapment is specifically described, in particular, retraction, contraction and toroid formation have been discussed. Volume-of-fluid method has been used in order to track the liquid-gas interface. The effect of surface wettability on the air entrapment and its evolution is studied. In line with experimental findings, we have observed a dimple formation on droplet surface at the impact point in the present study which has not been reported using a numerical study so far. It has been demonstrated using numerical study that the formation of dimple leads to the air entrapment. The surface wettability is demonstrated to have significant influence on the evolution of the air film into bubble. The processes during the evolution, such as inertial retraction, contraction and pinch-off are delayed on increasing the contact angle. Air bubble formed on the substrate gets detached from the substrate for low contact angle of 35° (hydrophilic surface) however, it remains attached to the substrate for higher contact angles: 90° and 120° (hydrophobic surface). Further, the influence of air entrapment on the heat transfer characteristics is studied. The deformation of the droplet as it approaches the substrate, influences the boundary layer on the substrate which eventually affects the wall heat flux. Post impact, the wall heat flux is primarily influenced by the liquid fraction and the air entrapped. In the region of only air, the wall heat flux is very low compared with that in the region of water or mixture of water and air. The present numerical results have been validated employing experimental results available in the literature.

Multiscale imaging approaches for simultaneously mapping distribution of multiple components in infant formula powders

To improve simultaneous characterization of structural and compositional features of infant formula powder the objectives of this study were to: 1) advance micro and macroscale fluorescence imaging methods to characterize the distribution of sugars, proteins and lipids in infant formula powder and 2) understand the impact of processing conditions and storage on the microstructure of infant formula powder. For micro and macroscale fluorescence imaging of infant formula, the samples were pre-stained with a trace amount of fluorescently labeled milk proteins, a fluorescent analogue of a simple sugar and a lipid specific chemical stain prior to spray drying. The labeled samples of infant formula powder from evaporator, collection chamber and cyclone units of a spray dryer were collected and analyzed using particle size measurements, scanning electron microscopy, confocal laser microscopy and widefield fluorescence macroscopy. In addition, the powder samples were also analyzed after storage for 6 months. The results of this study demonstrate that pre-staining of the infant formula powder eliminates the potential artifacts introduced by post-staining labeling of the infant formula powders using solvents. The results demonstrate that significant (p < 0.05) spatial co-localization of the fluorescence signal corresponding to the distribution of lipids, sugars and proteins in freshly prepared food powders analyzed using both micro and macroscale imaging. The results also illustrate the presence of void regions in food powders possibly due to entrapment of air during spray drying. The results also indicate separation of fluorescent sugar substrate and Nile Blue labeled lipids upon extended storage of food powders under ambient conditions. Overall, this study provides a novel labeling and imaging approaches to simultaneously image distribution of lipids, sugars and protein in a model food powder. The results also illustrate relatively high uniformity in distribution of macronutrients in a spray-dried powder.

The Impact of Capillary Trapping of Air on Satiated Hydraulic Conductivity of Sands Interpreted by X-ray Microtomography

The relationship between entrapped air content and the corresponding hydraulic conductivity was investigated experimentally for two coarse sands. Two packed samples of 5 cm height were prepared for each sand. Air entrapment was created by repeated infiltration and drainage cycles. The value of K was determined using repetitive falling-head infiltration experiments, which were evaluated using Darcy’s law. The entrapped air content was determined gravimetrically after each infiltration run. The amount and distribution of air bubbles were quantified by micro-computed X-ray tomography (CT) for selected runs. The obtained relationship between entrapped air content and satiated hydraulic conductivity agreed well with Faybishenko’s (1995) formula. CT imaging revealed that entrapped air contents and bubbles sizes were increasing with the height of the sample. It was found that the size of the air bubbles and clusters increased with each experimental cycle. The relationship between initial and residual gas saturation was successfully fitted with a linear model. The combination of X-ray computed tomography and infiltration experiments has a large potential to explore the effects of entrapped air on water flow.

Application of two-component pressure approach and Harten-Lax-van Leer (HLL) solver to model transient flow with regard to air entrapment.

Water distribution systems are basically designed to convey pressurized flow; however, in some situations such as the intermittent operation of the system, the network may experience a transition between free-surface and pressurized flow. On the other hand, combined sewer systems, designed basically for free-surface flow, may undergo pressurization due to extreme rainfalls. During transient flow, free-surface flow changes into a pressurized flow (and vice versa) which could be accompanied by intensive transient pressures causing structural damages to the system. In a pipe filling process, the existing air can be entrapped due to improper ventilation, intensifying the transient pressures in some cases. In this paper, the two-component pressure approach (TPA) and a Harten-Lax-van Leer Riemann solution are applied to model transient flow. The model is validated by comparing its results with the analytical solution of three simple examples, and then the model with an air chamber as the downstream boundary is used to simulate a literature experimental setup that includes air pocket entrapment. As the volume of the air pocket decreases, the errors of the model increase due to the inherent deficiency of the one-dimensional model. Furthermore, it is recommended to limit the Courant number to 0.5 for high acoustic wave speeds.

Mitigation of liquid sloshing in a rectangular tank due to slotted porous screen

Liquid sloshing inside a tank with a slotted porous screen at the center is studied based on numerical and experimental methods. Slotted screens with three different porosities (0.0964, 0.1968 and 0.3022) for two submergence depths of 1 and 2 cm have been considered. One of the main advantages of the slotted screens is that the resonance frequency of the sloshing tank can be altered and the sloshing-induced motion/load can be suppressed by energy dissipation across the porous screen. The complexities of slotted screens equipped in a sloshing tank are accompanied by wave breaking, jet formation and liquid fragmentations which are commonly seen phenomena across the porous screen. These violent free surface behaviors in a tank are studied based on numerical simulations using the incompressible turbulent model and compared with the experiments. For the numerical sloshing tank with porous screen, free surface elevation and pressure at the tank wall are in good agreement with the experimental results. The adopted numerical technique will be able to capture the nonlinear free surface wave profile, air entrapment and jet formation across the screen in agreement with the experiments. For the fully submerged screen, the lowest resonance period shifted slightly to higher values. The sloshing tank equipped with porous screen of 0.1968 for the fully submerged screen predicted lower values of the amplification factor and pressure at the tank wall compared to other cases.

Mesoscopic numerical investigation of dynamic mechanical properties of ice with entrapped air bubbles based on a stochastic sparse distribution mechanism

Hail ice inevitably contains entrapped air bubbles in nature. The dynamic mechanical properties of ice are highly affected by the dimension, distribution and amount of the entrapped air bubbles. The bubbles in ice neither could be manufactured by man-made or obtained from nature with controllable amount and distribution, nor simulated by classic FEM in mesoscale due to its small size. A stochastic algorithm has been proposed as the sparse distribution mechanism of air bubbles in ice to organize the air bubbles with controllable amount and agglomeration in numerical model. A mesoscopic numerical modeling method of ice with entrapped air bubbles has been developed by the meshfree ALE multi-material method. The effect of amount and agglomeration of the air bubbles in ice on dynamic mechanical properties has been investigated by simulated SHPB tests. The results show that the dynamic strengths of the ice with air bubbles are increase with the increasing of strain rates, nonlinear decrease with the increasing of volume ratios of the air bubbles. Both the volume ratios and distribution of air bubbles in ice affect the dynamic strengths at a low strain rates level; but the distribution rarely affects the dynamic strengths at a high strain rate level.

Drop impact on thin powder layers: pattern formation by air entrapment.

Impact of drops on thin powder layers displaces the powder particles radially outward producing shallow craters with thick rims, for example, as observed on dust layers on the floor. Here, we report that the patterns formed on thin powder layers by drop impact are not limited to such crater-like ones. Instead, depending upon the layer properties, disc or disc-plus-ring shaped patterns are formed at the impact point. We show that air entrapment and micro-bubble formation during the drop impact result in the formation of such patterns. Based on high-speed imaging, scaling analyses, and measurements with various liquids and powder layers, we propose a mechanism for the formation of such patterns. The phenomenon that we report can open further investigations on drop impact on the granular matter.

Clinical and Radiological Characteristics of Traumatic Pneumocephalus after Traumatic Brain Injury.

ObjectiveTraumatic pneumocephalus (TP) is a common complication of traumatic brain injury (TBI), which is characterized by the abnormal entrapment of air in the intracranial cavity after TBI to the meninges. The purpose of this study was to investigate the clinical and radiological characteristics related to TP associated with TBI.MethodsFrom January 2013 to March 2018, the data from 71 patients with TP after TBI were collected. Demographic and clinical characteristics were investigated and the distribution of TP was investigated as radiological characteristics. The author compared the demographic characteristics of TP to the data from the Korean Neurotrauma Data Bank System (KNTDBS).ResultsThere was a higher ratio of males in patients with TP compared with KNTDBS. The mean age was 48.4±20.5 years and the incidence was highest in those 41–60 years of age (42.3%). Surgical treatment was performed in 23.9% patients. The mortality associated with the TP was 4.2%. The most common cause of injury was a traffic accident (52.1%). TP was mainly located in the epidural space (53.5%) and showed a scatter pattern (60.6%). It was mostly located in the frontal and temporal area (81.7%). Skull fractures were observed in 40.8% in the temporal bone and 25.4% in the frontal bone.ConclusionThe author identified epidemiology, clinical, and radiological characteristics of TP associated with TBI. Although this study has many limitations, the author believes it is worthwhile as it examines various characteristics of TP, which previously had relatively little clinical interest.

Numerical simulation and optimization of shell mould casting process for leaf spring bracket

Although the shell mould casting process has a wide range of application in many fields, the prediction of casting defects is still a problem. In the present work, a typical leaf spring bracket casting of ZG310-570 was fabricated by shell mold casting. The finite element model and ProCAST software were utilized for simulating the filling and solidification processes of the casting; and the formation mechanism of the gas pore, and shrinkage porosity defects were analyzed. The results indicate that the gas pore and shrinkage porosity defects are formed due to air entrapment, insufficient feeding and non-sequential solidification. Subsequently, through changing the position of risers, adding a connecting channel between the risers, and setting blind risers at the U-shaped brackets, an optimized gating and feeding system was established to improve the quality of the casting. After optimization, the gas pore and shrinkage porosity defects of the leaf spring bracket casting are effectively eliminated. The experiment results with the optimized casting process are in good agreement with the numerical simulation, which verifies the validity of the finite element model in the shell mould casting.

Differences in apical vapor lock formation after sodium hypochlorite irrigation with and without surfactant using two needle types

Background: Endodontic irrigation with 5.25% sodium hypochlorite (NaOCl) solution using single side-vented and double side-vented needles is commonly used. Surfactant is added to reduce high surface tension of the NaOCl solution. Apical vapor lock, or air entrapment inside closed-end root canal system, lowers the efficacy of irrigants. Thus, the irrigants are hindered in penetrating the root canal system and can lead to risk of reinfection. Objective: The objective is to analyze the difference of 5.25% NaOCl solution with and without surfactant using two types of irrigation needle in the formation of apical vapor lock. Methods: Forty lower premolars were prepared and randomly divided into four groups (n = 10 per group) then irrigated with 5.25% NaOCl solution with and without surfactant using a single side-vented or double side-vented needle. Contrast medium was added so that the measurement of the apical vapor lock could be performed using a digital radiograph. Analysis was done with two-way analysis of variance and the Tukey method. Results: 5.25% NaOCl solution with a single side-vented needle showed a significant difference from 5.25% NaOCl solution with a double side-vented needle. 5.25% NaOCl solution with a surfactant using a single side-vented needle showed a significant difference from 5.25% NaOCl solution using a double side-vented needle, likewise NaOCl 5.25% using double side-vented needle group from NaOCl 5.25% with surfactant using single side-vented needle group. Conclusion: Minimal formation of an apical vapor lock resulted from the use of 5.25% NaOCl solution with a surfactant using a single side-vented needle.

Covalently Modulated and Transiently Visible Writing: Rational Association of Two Extremes of Water Wettabilities.

Anticounterfeiting measures are of ever-increasing importance in society, e.g., for securing the authenticity of and the proof of origin for medical drugs. Here, an arms race of counterfeiters and valid manufacturers is taking place, resulting in the need of hard-to-forget, yet easy-to-read out marks. Anticounterfeiting measures based on micropatterns-while being attractive for their need in not widely available printing methods while still being easily read out with fairly common basic optical equipment-are often limited by being too easy to be destroyed by wear or handling. Here, nature-inspired wettability is rationally exploited for developing an unprecedented anticounterfeiting method, where hidden information can be only identified under direct exposures to an aqueous phase or mist and disappears again on air-drying the interface. A chemically reactive and hierarchically featured dip coating, capable of spatially selective covalent modification with primary amine containing small molecules, is developed for abrasion-tolerant patterning interfaces with two extremes of water wettabilities, i.e., superhydrophilicity and superhydrophobicity. Arbitrary handwriting with glucamine followed by chemical modification with octadecylamine, provided "invisible" text on the synthesized interface. The glucamine-treated region selectively becomes optically transparent and superhydrophilic due to rapid infiltration of the aqueous phase on exposure to liquid water or mist. The remaining interface remains opaque and superhydrophobic due to metastable entrapment of air. The hidden text became transiently and reversibly visible by the naked eye under exposure to liquid water/mist. Furthermore, microchannel-cantilever spotting (μCS) is adopted for demonstrating well-defined chemical patterning on the microscale. These patterns are at the same time highly resistant against wear and scratching because of the bulk functionalization, retaining the wetting properties (and thus pattern readout) even on serious abrasion. Such a simple synthesis of spatially controlled, direct, and covalently modulated wettability could be useful for various applied and fundamental contexts.

Effect of Flow State of Pure Aluminum and A380 Alloy on Porosity of High Pressure Die Castings

Air entrapment defects prevent the heat treatment from improving the mechanical properties of die castings, which limits the die casting of high-performance components. The flow pattern of the filling process is complicated and experimental analysis is difficult in thin-walled complex die castings. In this study, we constructed a shock absorption tower to observe in real-time the filling process of pure aluminum and A380 aluminum alloy at different fast injection speeds. The degree of breakup of pure aluminum was larger than that of A380 during the filling process, which caused the porosity of pure aluminum to be greater than that of the A380 at each observation position. Re-Oh diagrams explained the difference in porosity between the two metals. The porosity in different regions was closely related to the flow state of aluminum liquid. In addition to porosity measurements, we specifically analyzed the relationship between the porosity of the flowback zone, the final filling zone, and the near-tail zone of cylinder. At the same injection velocity, the porosity at flowback zone was greater than that at the final filling position, the porosity at final filling position was larger than that at the near-tail zone of cylinder, and the final filling position changed as the injection velocity changed.

Continuous and ordered surface microtexturing on Cu and Ni-based alloys by novel electrochemical dissolution

The present study discusses the formation of surface micro-texture on flat samples of Cu-based and Ni-based alloys by two-step electrochemical dissolution. Prior to the electrochemical dissolution, laser marking only on the araldite coating has been carried out for effective dissolution of the selectively exposed metal surfaces along the laser cut regions. After second stage dissolution, the circular truncated conical grids with average base diameter of 210.7 μm for the Cu-based alloy and truncated square pyramid like grids with base width of 203.2 μm for the Ni-based alloy have formed. Similarly, maximum dissolution depths of 61.8 and 39.3 μm for the Cu-based and Ni-based alloys, respectively, could be observed. Moreover, excellent hydrophobicity of the textured Cu-based as well as Ni-based alloys correlating to grid dimensions along with air entrapment beneath the water droplet has been analysed with the help of Wenzel and Cassie-Baxter models. Nano-hardness of the as-received alloys as well as the textured alloy surfaces shows minor variation. Similarly, the micrographs of the flat surfaces of the as-received alloys and cross-sectional microstructure of the textured alloys have also portrayed little difference suggesting negligible heat or stress affected zone after dissolution process. Moreover, the textured surfaces have showed significant increment in surface area: ∼62.7% for the Cu-based alloy and ∼71.4% for the Ni-based alloy as compared to the flat specimens.

Experimental study on water entry of twin wedges

This article presents an experimental study on the water entry of twin wedges commonly used as sections of catamaran craft. A twin wedge is a combination of two single asymmetry wedges connected by so-called wet-deck. The single asymmetry wedge has a vertical side and a non-vertical side with a range of deadrise angles. This study is carried out on a set of twin wedges of 7°, 15° and 20° deadrise angles. For calibrating the experimental equipment, the twin wedges symmetric quality, tank depth effect, three-dimensional effect, repeatability and sampling rate effects are examined. The water entry experiments are conducted through a drop test method. Time histories of impact pressure are recorded for different deadrise angles, drop heights and space between two single wedges. The results show that the effect of space on the peak pressure is negligible. In water entry experiment, the pressure on the non-vertical side of an asymmetry wedge is considerably lower than that on the equivalent symmetry wedge. The vertical side of an asymmetry wedge is under the effect of air entrapment. It has much less pressure than the non-vertical side.

Reducing water entry impact loads on offshore marine structures by forced air entrapment

ABSTRACT In this study, the effect of air cushioning due to air entrapment on the water entry of cylinders is examined experimentally and numerically. Air entrapment is forced via corrugations on a cylinder. Emphasis is given to the early stages of penetration effecting the total impact forces. Jet flows, pileups and free surface elevation profiles are characterised and compared with the ones obtained with a smooth cylinder. At low Froude numbers (Fr < 3), while the jet flow does not separate and climbs up on the solid surface of the smooth cylinder, the flow separation occurs and a cavity is formed during the water entry of the corrugated cylinder. The maximum impact force is a function of the wetted length which depends on the flow separation point. The numerical results showed good visual agreement with the experiments. The impacts under air cushioning and early flow separation effects produced 30% smaller impact pressures.