Search Results (17)

The application of electric fields during pool boiling heat transfer has demonstrated significant potential to enhance thermal performance. However, research on boiling heat transfer enhancement under pulsed electric fields remains insufficient. To further improve pool boiling efficiency, this study systematically investigates the effects of pulsed electric fields, uniform electric fields, and electric field-free conditions on heat transfer performance using the Lattice Boltzmann Method (LBM). The results show that, compared with the uniform electric field and electric field-free condition, the pulsed electric field resulted in the smallest bubble detachment diameter and detachment period, with a higher heat flux density on the wall and the best heat transfer enhancement effect. Under the pulsed electric field, the electric force undergoes abrupt changes at the beginning and end of each pulse peak, exerting greater compression on the bubble base. Simultaneously, this leads to accelerated gas rise inside the bubble, bubble stretching, and contraction of the bottom phase boundary. There exists an optimal pulse frequency that minimizes the bubble detachment period and diameter, resulting in the best wall heat transfer enhancement effect. The effective areas for enhanced boiling heat transfer by pulsed electric fields are the bubble base and the “V”-shaped region connected to the bubble bottom. Full article

Extracorporeal gas exchange therapies evolved considerably within the first three–four decades of their appearance, and have since reached a mature stage, where minor alterations and discrete fine-tuning might offer some incremental improvement. A different approach is introduced here, making use of modern, purely diffusive membrane materials, and taking advantage of the elevated concentration gradient ensuing from gas pressure buildup in the gas chamber of the oxygenator. An assortment of silicone membrane gas exchangers were tested in vitro as per a modified protocol in pursuance of assessing their gas exchange efficiency under both regular and high-pressure aeration conditions. The findings point to a stark performance gain when pressurization of the gas compartment is involved; a 40% rise above atmospheric pressure elevates oxygen transfer rate (OTR) by nearly 30%. Carbon dioxide transfer rate (CTR) does not benefit as much from this principle, yet it retains a competitive edge when higher gas flow/blood flow ratios are employed. Moreover, implementation of purely diffusive membranes warrants a bubble-free circulation. Further optimization of the introduced method ought to pave the way for in vivo animal trials, which in turn may potentially unveil new realms of gas exchange performance for therapies associated with extracorporeal circulation. Full article

Exposure to high altitude results in hypobaric hypoxia, leading to physiological changes in the cardiovascular system that may result in limiting symptoms, including dyspnea, fatigue, and exercise intolerance. However, it is still unclear why some patients are more susceptible to high-altitude symptoms than others. Hypoxic simulation testing (HST) simulates changes in physiology that occur at a specific altitude by asking the patients to breathe a mixture of gases with decreased oxygen content. This study aimed to determine whether the use of transthoracic echocardiography (TTE) during HST can detect the rise in right-sided pressures and the impact of hypoxia on right ventricle (RV) hemodynamics and right to left shunts, thus revealing the underlying causes of high-altitude signs and symptoms. A retrospective study was performed including consecutive patients with unexplained dyspnea at high altitude. HSTs were performed by administrating reduced FiO₂ to simulate altitude levels specific to patients’ history. Echocardiography images were obtained at baseline and during hypoxia. The study included 27 patients, with a mean age of 65 years, 14 patients (51.9%) were female. RV systolic pressure increased at peak hypoxia, while RV systolic function declined as shown by a significant decrease in the tricuspid annular plane systolic excursion (TAPSE), the maximum velocity achieved by the lateral tricuspid annulus during systole (S’ wave), and the RV free wall longitudinal strain. Additionally, right-to-left shunt was present in 19 (70.4%) patients as identified by bubble contrast injections. Among these, the severity of the shunt increased at peak hypoxia in eight cases (42.1%), and the shunt was only evident during hypoxia in seven patients (36.8%). In conclusion, the use of TTE during HST provides valuable information by revealing the presence of symptomatic, sustained shunts and confirming the decline in RV hemodynamics, thus potentially explaining dyspnea at high altitude. Further studies are needed to establish the optimal clinical role of this physiologic method. Full article

We review the mechanisms driving the ionized gas outflows in radio-quiet (RQ) AGN. Although it constitutes ∼90% of the AGN population, what drives these outflows in these AGNs remains an open question. High-resolution imaging and integral field unit (IFU) observation is key to spatially resolving these outflows, whereas radio observations are important to comprehend the underlying radiative processes. Radio interferometric observations have detected linear, collimated structures on the hundreds of pc scale in RQ AGN, which may be very similar to the extended radio jets in powerful galaxies. Proper motions measured in some objects are sub-relativistic. Other processes, such as synchrotron radiation from shock-accelerated gas around the outflows could give rise to radio emissions as well. Near the launching region, these outflows may be driven by the thermal energy of the accretion disk and exhibit free–free emission. IFU observations on the other hand have detected evidence of both winds and jets and the outflows driven by them in radio-quiet AGN. Some examples include nearby AGN such as Mrk 1044 and HE 1353-1917. An IFU study of nearby (z $<0.06$) RQ AGN has found that these outflows may be related to their radio properties on <100 pc scale, rather than their accretion properties. Recent JWST observations of RQ AGN XID 2028 have revealed that radio jets and wind could inflate bubbles, create cavities, and trigger star formation. Future high-resolution multi-wavelength observations and numerical simulations taking account of both jets and winds are hence essential to understand the complex interaction between radio-quiet AGN and the host from sub-pc to kpc scales. Full article

Vapor bubbles are widely concerned in many industrial applications. The deformation and collapse of a vapor bubble near a free surface after being heated and raised from the bottom wall are investigated in this paper. On the basis of smoothed particle hydrodynamics (SPH) and the van der Waals (VDW) equation of state, a numerical model of fluid dynamics and phase change was developed. The effects of fluid dynamics were considered, and the phase change of evaporation and condensation between liquid and vapor were discussed. Quantitative and qualitative comparisons between our numerical model and the experimental results were made. After verification, the numerical simulation of bubbles with the effects of the shear viscosity η_s and the heating distance L were taken into account. The regularity of the effect of the local Reynolds number (Re) and the Ohnesorge number (Oh) on the deformation of vapor bubbles is summarized through a further analysis of several cases, which can be summarized into four major patterns as follows: umbrella, semi-crescent, spheroid, and jet. The results show that the Re number has a great influence on the bubble deformation of near-wall bubbles. For Re > 1.5 × 10² and Oh < 3 × 10⁻⁴, the shape of the bubble is umbrella; for Re < 5 × 10⁰ and Oh > 10⁻³, the bubble is spheroidal; and for 5 × 10⁰ < Re < 1.5 × 10², 3 × 10⁻⁴ < Oh < 10⁻³, the bubble is semi-crescent. For liquid-surface bubbles, the Re number effect is small, and when Oh > 5 × 10⁻³, the shape of the bubble is jet all the time; there is no obvious difference in the bubble deformation, but the jet state is more obvious as the Re decreases. Finally, the dynamic and energy mechanisms behind each mode are discussed. The bubble diameter, bubble symmetry coefficient, and rising velocity were analyzed during their whole processes of bubble growth and collapse. Full article

Path instability of a rising bubble is a complex phenomenon. In many industrial applications, bubbles encounter walls, and the interactions between the bubbles and the wall have a significant impact on flow physics. A single bubble rising near a vertical wall was experimentally observed to follow a bouncing trajectory. To investigate the near-wall dynamics of rising bubbles, 3D numerical simulations were performed based on the volume of fluid (VOF) method using the open source solver OpenFOAM. The effect of wall proximity and surface tension on the bubble trajectory was investigated. Previous studies have focused on the near-wall rising dynamics of bubbles for higher Eotvos numbers (Eo) and varied the Galilei number (Ga). The physical properties of the flow were chosen such that the free-rising bubble lies in the rectilinear regime. The Ga number was fixed and the Eo number was varied to analyze its effect on the bubble’s rising trajectory. It was found that the presence of the wall increases the drag experienced by the bubble and induces an early transition from rectilinear to a planar zigzagging regime. We identify the maximum wall distance and the critical Eo number for the bubble to follow a bouncing trajectory. The amplitude, frequency and wavelength of the bouncing motion are independent of the initial wall distance, but they decrease with decreasing surface tension. Full article

The most concerning adverse effects of thrombolytic agents are major bleeding and intracranial hemorrhage due to their short half-life, low fibrin specificity, and high dosage. To alleviate bleeding side effects during thrombolytic therapy which would bring about the risk of aggravation, we try to find a novel biodegradable delivery nanosystem to carry drugs to target the thrombus, reduce the dosage of the drug, and system side effects. A novel urokinase/poly-α, β-d, l-aspartyl-Arg-Gly-Asp-Ser complex (UK/PD-RGDS) was synthesized and simply prepared. Its thrombolytic potency was assayed by the bubble-rising method and in vitro thrombolytic activity by the thrombus clot lysis assay separately. The in vivo thrombolytic activity and bleeding complication were evaluated by a rat model of carotid arteriovenous bypass thrombolysis. The thrombolytic potency (1288.19 ± 155.20 U/mg) of the UK/PD-RGDS complex nano-globule (18–130 nm) was 1.3 times that of commercial UK (966.77 ± 148.08 U/mg). In vivo, the UK/PD-RGDS complex (2000 IU/kg) could reduce the dose of UK by 90% while achieving the equivalent thrombolysis effect as the free UK (20,000 IU/kg). Additionally, the UK/PD-RGDS complex decreased the tail bleeding time compared with UK. The organ distribution of the FITC-UK/PD-RGDS complex was explored in the rat model. The UK/PD-RGDS complex could provide a promising platform to enhance thrombolytic efficacy significantly and reduce the major bleeding degree. Full article

The diffusion of carbon dioxide (CO${}_2$) and ethanol (EtOH) is a fundamental transport process behind the formation and growth of CO${}_2$ bubbles in sparkling beverages and the release of organoleptic compounds at the liquid free surface. In the present study, CO${}_2$ and EtOH diffusion coefficients are computed from molecular dynamics (MD) simulations and compared with experimental values derived from the Stokes-Einstein (SE) relation on the basis of viscometry experiments and hydrodynamic radii deduced from former nuclear magnetic resonance (NMR) measurements. These diffusion coefficients steadily increase with temperature and decrease as the concentration of ethanol rises. The agreement between theory and experiment is suitable for CO${}_2$. Theoretical EtOH diffusion coefficients tend to overestimate slightly experimental values, although the agreement can be improved by changing the hydrodynamic radius used to evaluate experimental diffusion coefficients. This apparent disagreement should not rely on limitations of the MD simulations nor on the approximations made to evaluate theoretical diffusion coefficients. Improvement of the molecular models, as well as additional NMR measurements on sparkling beverages at several temperatures and ethanol concentrations, would help solve this issue. Full article

This paper presents a computational study on bottom gas injection in a cylindrical tank. The bubble formation at submerged orifices, bubble rising, and interactions between bubbles and bubbles with the free surface were studied using the conservative level set method (CLSM). Since the gas injection is an important technique in various fields and this process is quite complicated, the scenario was chosen to quantify the efficacy of the CLSM to describe the gas-liquid complex interactions with fast changes in the surface tension force and buoyancy force. The simulation accuracy is verified with the grid convergence index (GCI) approach and Richardson Extrapolation (RE) and is validated by comparing the numerical results with experimental observations, theoretical equations, and published data. The results show that the CLSM accurately reproduces the bubble formation frequency, and that it can handle complicated bubble shapes. Moreover, it captures the challenging phenomena of interaction between bubbles and free surface, the jet of liquid produced when bubbles break through the free surface, and the rupture of the film of liquid. Therefore, the CLSM is a robust numerical technique to describe gas-liquid complex interactions, and it is suited to simulate the gas injection operation. Full article

Microbubbles with slow rising speed, higher specific area and greater oxygen dissolution are desired to enhance gas/liquid mass transfer rate. Such attributes are very important to tackle challenges on the low efficiency of gas/liquid mass transfer that occurs in aerobic wastewater treatment systems or in the aquaculture industries. Many reports focus on the formation mechanisms of the microbubbles, but with less emphasis on the system optimization and assessment of the aeration efficiency. This work assesses the performance and evaluates the aeration efficiency of a porous venturi-orifice microbubble generator (MBG). The increment of stream velocity along the venturi pathway and orifice ring leads to a pressure drop (P_atm > P_abs) and subsequently to increased cavitation. The experiments were run under three conditions: various liquid velocity (Q_L) of 2.35–2.60 m/s at fixed gas velocity (Q_g) of 3 L/min; various Q_g of 1–5 L/min at fixed Q_L of 2.46 m/s; and free flowing air at variable Q_Ls. Results show that increasing liquid velocities from 2.35 to 2.60 m/s imposes higher vacuum pressure of 0.84 to 2.27 kPa. They correspond to free-flowing air at rates of 3.2–5.6 L/min. When the system was tested at constant air velocity of 3 L/min and under variable liquid velocities, the oxygen dissolution rate peaks at liquid velocity of 2.46 m/s, which also provides the highest volumetric mass transfer coefficient (K_La) of 0.041 min⁻¹ and the highest aeration efficiency of 0.287 kgO₂/kWh. Under free-flowing air, the impact of Q_L is significant at a range of 2.35 to 2.46 m/s until reaching a plateau K_La value of 0.0416 min⁻¹. The pattern of the K_La trend is mirrored by the aeration efficiency that reached the maximum value of 0.424 kgO₂/kWh. The findings on the aeration efficiency reveals that the venturi-orifice MBG can be further optimized by focusing on the trade-off between air bubble size and the air volumetric velocity to balance between the amount of available oxygen to be transferred and the rate of the oxygen transfer. Full article

Among various carbon capture and storage technologies to mitigate global warming and ocean acidification due to greenhouse gases, ocean geological storage is considered the most feasible for Korea due to insufficient inland space to store CO₂. However, the risk of CO₂ leakage and the behavior and environmental effects of the leaked CO₂ need to be assessed for its successful implementation. Therefore, the behavior of CO₂ bubbles/droplets dissolving into the surrounding seawater and the diffusion of dissolved CO₂ by ocean flows should be accurately predicted. However, finding corresponding research has been difficult in Korea. Herein, the behavior and convection-diffusion of CO₂ that was assumed to have leaked from the seafloor near the southeastern coast of Korea were numerically predicted using a multi-scale ocean model for the first time. In the simulation region, one of the pilot projects of CO₂ ocean geological storage had started but has been temporarily halted. In the ocean model, hydrostatic approximation and the Eulerian–Lagrangian two-phase model were applied for meso- and small-scale regions, respectively. Parameters for the simulations were the leakage rate and the initial diameter of CO₂. Results revealed that all leaked and rising CO₂ bubbles were dissolved into the seawater before reaching the free surface; further, the change in the partial pressure of CO₂ did not exceed 500 ppm during 30 days of leakage for all cases. Full article

Many researchers have focused on multi-phase reactor development for improving mass transfer performance. However, solid particle addition in gas–liquid contactor for better oxygen mass transfer performance is still limited. Hence, this study aims to analyze the relative effect of different types of local solid media on the bubble hydrodynamic characteristics towards mass transfer enhancement in bubble columns (BCR) and airlift reactors (ALR). This was investigated by varying solid media types (ring, sphere, cylinder, and square), solid loadings (0%–15%), and superficial gas velocities (Vg) (2.6–15.3 × 10⁻³ m/s) in terms of the bubble hydrodynamic and oxygen mass transfer parameters. The result showed that bubble size distribution in BCR and ALR with additional plastic media was smaller than that without media addition, approximately 22%–27% and 5%–29%, respectively, due to the increase of the bubble breaking rate and the decrease of the bubble rising velocity (U_B). Further, adding media in both reactors significantly decreased the U_B value. Since media increased flow resistance, resulting in decreased liquid velocity, it can also be the moving bed to capture or block the bubbles from free rising. Therefore, oxygen mass transfer performance was investigated. The oxygen transfer coefficient (K_La) in BCR with solid media addition was enhanced up to 31%–56% compared to a non-addition case, while this enhancement was greater at higher solid loading due to its higher effective surface, resulting in a higher bubble break-up rate compared to the lower loading. In ALR, up to 38.5% enhanced K_La coefficient was archived after adding plastic media over the non-addition case. In conclusion, ring and cylinder media were found to be the most significant for improving K_La value in BCR and ALR, respectively, without extra energy. Full article

The formation of polycondensation-type thermoset resins from natural reactants such as citric and glutaric acid, as well as 1,3-propanediol and glycerol, was studied. Monitoring the mass loss by thermogravimetric analysis (TGA) allowed the rate constants of the esterification to be calculated, which were in the order of 7·10⁻⁵ s⁻¹ for glutaric acid and approximately twice as high for citric acid. However, the combination citric acid/glycerol was previously reported to froth up at high conversions, giving rise to foams, which makes the preparation of compact engineering composites challenging. In light of this, we observed that shredded poultry feathers not only increased the conversion and the reaction rate of the combination citric acid/glycerol, but increasing the amount of feathers continuously decreased the number of visible bubbles. The addition of 20 wt% of feathers completely prevented the previously reported frothing and gave rise to compact materials that were macroscopically free of defects. Besides this, the addition of feathers also improved the fire-retardant properties. The tensile properties of the first specimens are still rather low (σ = 11.6 N/mm², E = 750 N/mm²), but the addition of poultry feathers opens a new path for green thermoset resins. Full article

Interfacial bubbles are unintentionally created during the transfer of atomically thin 2D layers, a required process in the fabrication of van der Waals heterostructures. By encapsulating a WSe₂ monolayer in hBN, we study the differing photoluminescence (PL) properties of the structure resulting from bubble formation. Based on the differentiated absorption probabilities at the bubbles compared to the pristine areas, we demonstrate that the visibility of the bubbles in PL mapping is enhanced when the photoexcitation wavelength lies between the n = 1 and n = 2 resonances of the A-exciton. An appropriate choice of detection window, which includes localized exciton emission but excludes free exciton emission, further improves bubble imaging capability. The interfacial position dependence of the bubbles, whether they are located above or below the WSe₂ monolayer, gives rise to measurable consequences in the PL shape. Full article

A two-phase bubbly flow is often found in the process industry. For the efficient operation of such devices, it is important to know the details of the flow. The paper presents a numerical simulation of the rising bubble in a stagnant liquid column. The interFOAM solver from the open source Computational Fluid Dynamics (CFD) toolbox OpenFOAM was used to obtain the necessary data. The constant and dynamic computational grids were used in the numerical simulation. The results of the calculation were compared with the measured values. As expected, by using the dynamic mesh, the bubble trajectory was closer to the experimental results due to the more detailed description of the gas–liquid interface. Full article