Excess Bubbles Research Articles

Crystallization fouling of turbo-structured tubes during nucleate pool boiling

The superiority of enhanced boiling on structured heat transfer surfaces over plain surfaces is based on excessive bubble generation and behavior as well as the latent heat of evaporation. The generated bubbles with turbulence of large scale would give rise to profoundly higher heat transfer coefficients. Meanwhile, the very same reasons of bubble formation and behavior may lead to accelerate the fouling of these surfaces for aqueous solutions with inverse solubility. The present study characterizes the fouling behaviour of turbo structured tubes under nucleate pool boiling. Several experiments were conducted for heat fluxes ranging from 50 to 250 kW/m2 and a concentration of 1.6 g/L of CaSO4 aqueous solution. The investigated structured tubes were re-entrant cavity tubes referred to as the Turbo-B type which were compared with plain tubes and low finned tubes of different fin heights and densities. Of these investigated tubes, the Turbo-B showed preferable anti-fouling behaviour. Based on a comparison of anti-fouling performance of the test tubes, the Turbo-B reduced fouling resistance up to ∼27 % and ∼83 % than the finned test tubes of 19 fpi (fins per inch) and 40 fpi, respectively. Most interestingly, the deposition of turbo tubes was of powdery and frail structure which could easily be cleaned. Moreover, the deposit was only formed on top of these tubes while no sign of deposition was observed within the cavities. The deposit structure for plain tubes, on the contrary, was a hard, thick and homogeneous layer under similar operating conditions as those for the turbo tubes.

Assessment of computational method for highly subcooled flow boiling in a horizontal channel with one-sided heating and improvement of bubble dispersion

This study employs computational and experimental methods to explore local hydrodynamic and heat transfer characteristics of subcooled horizontal flow boiling of FC-72 in a 2.5-mm × 5.0-mm rectangular channel heated along its 2.5-mm bottom wall. Limited studies have been conducted investigating the limit of the multiphase model and the way to improve the dynamic interfacial behavior between phases. The 3D CFD solver is constructed in ANSYS Fluent in which the Volume of Fluid (VOF) model is combined with the shear stress transport (SST) k-ω turbulent model, a surface tension model, and interfacial phase change model, along with a model for shear-lift force and bubble dispersion. Discussed are simulation results for void fraction distribution, pressure drop, and local fluid and wall temperatures. The simulation results are compared to experimental data to validate the solver's ability to predict the asymmetrically heated horizontal flow configuration. Body force is shown to have an appreciable impact on both predicted and measured results, the outcome of bubbles along the heated wall migrating toward the top adiabatic wall and causing asymmetry in fluid velocity and temperature within the cross-sectional area and along the channel. These asymmetrical effects are quite pronounced at low mass velocities, where the upstream fully developed liquid boundary layer is disrupted by bubble formation along the heated bottom wall. Predictions are presented for simulations, including shear-lift force with and without bubble collision dispersion force. The shear-lift force is shown to govern mostly the dynamic behavior of small bubbles stuck on the heated bottom wall and therefore has a greater impact on both heat transfer and heated wall temperature. Absent bubble collision dispersion force, the simulations show excessive bubble coalescence creating inordinate vapor pockets along the adiabatic wall. But by including this force, coalescence of densely packed bubbles in the bulk region is significantly inhibited, with larger bubbles even incurring additional breakup into smaller bubbles and culminating in far less vapor accumulation along the top wall. Including the bubble collision dispersion force is shown to yield better agreement with experimental interfacial behavior along the channel. The mean deviation and error of wall temperature across cases are 2.2 °C and 3.34%.

1980년대 후반 증시호황기 ‘개미’의 탄생과 시련

This paper examines Korea’s stock market, which soared after the three low boom in the late 1980s, and the emergence of the first large-scale individual investors called Ants. The government implemented various stimulus measures to use this as an opportunity to nurture the stock market. Among them, many ordinary investors have also stepped into the stock market through the public offering of the People’s Stock Share Program that is for the common people. It was the first large-scale individual investor to appear in the history of the Korean stock market, and it was the beginning of a mass investment society. However, stock prices have plummeted since the end of 1989, and ordinary investors have failed. The stock price crash was largely responsible for boosting the stock market’s excessive bubble by the government’s aggressive policy of volume, and for the companies of the pursuit of maximum profits regardless of the means. In particular, while promoting the “stock market for the people,” the government firmly supported the pro-capital policy whenever the contradiction of the stock market policy between the capital and the public was amplified.

Validation of damage on vascular endothelial cells under ultrasound exposure according to adhered situation of bubbles

We investigated the viability of vascular endothelial cells with the existence of lipid bubbles under ultrasound exposure. First, we estimated the various situations of bubbles on the cells including either adhesion, floating, or both of them using not only image analysis but also an experiment to retain the cells in flow. Then we examined the viability measurement of the cells using the ultrasound conditions with the frequency of 3 MHz, a maximum sound pressure of 400 kPa pp, and a maximum irradiation time of 60 s. We found that the floating bubbles caused more damage on the cells rather than the adhered bubbles. Because insufficient adhesion of bubbles might cause damage by floating bubbles, we consider that the adhered bubbles were protective of cells against floating bubbles. However, excessive bubbles with a higher concentration than the saturation also might cause damage by destructing both the floating and adhered bubbles.

A Review of On-Site Carwash Wastewater Treatment

In recent years, people’s environmental awareness has increased. The high density of the urban population has caused a considerable increase in the demand for car washing services, which has created large quantities of car wash wastewater. The main pollutants in car wash wastewater are detergents, dirt, oil, and grease. Untreated wastewater released into rainwater sewer systems or other water bodies may pollute the water and generate excessive bubble foams, which negatively affects urban appearance. Car washes are divided into mechanical car washes and manual or self-service car washes. In general, car washes have a small operation and scale, occupy limited land, and cannot afford wastewater treatment costs. Therefore, most car washes are not equipped with wastewater treatment facilities. Consequently, the discharge of wastewater from car washes negatively affects the water quality in the surrounding environment and results in wasteful use of water resources. This study reviewed 68 research papers on the quality, treatment techniques, treatment costs, and treatment effectiveness of car wash wastewater to provide a reference for car wash operators to contribute to the preservation of water resources. We found that there is a higher chance of recycling car wash wastewater when combing two different techniques for car wash wastewater treatment.

Literature Review of Real Estate Bubble Measurement Methods

The real estate industry is the pillar industry of China’s economic development, and the excessive bubble in its market 
 will seriously affect the normal development of the national economy. The rapidly expanding real estate bubble has laid many 
 hidden dangers for the outbreak of the financial crisis. Therefore, scientific analysis of the bubble in the real estate market and 
 accurate measurement of the degree of market bubble are the prerequisites for taking appropriate corresponding measures. This 
 article compares and evaluates the different measurement methods of real estate bubbles by domestic and foreign scholars, and 
 puts forward reasonable suggestions.

Fluidization behavior of graphitized glassy particles in a fluidized carbon bed cooling process for investment casting

Fluidized Carbon Bed Cooling (FCBC) is an innovative investment casting process for directional solidification of superalloy components. It takes advantage of a fluidized bed with a base of small glassy carbon beads for cooling and other low-density particles that form an insulating layer by floating to the bed surface. This so-called “Dynamic Baffle” protects the fluidized bed from the direct heat input from the high-temperature heating zone and provides the basis for an improved bed microstructure. The prerequisites for a stable casting process are stable fluidization conditions where neither collapse of the bed nor particle blow out at excessive bubble formation occur.This work aimed to investigate the fluidization behavior of spherical carbon bed material in argon and air at temperatures between 20 to 350 °C. Systematic studies at reduced pressures using the FCBC prototype device were performed to understand the stable fluidization conditions at all stages of the investment casting process. The particle shape factor and size distribution characterization and the measurement of the powder’s minimum fluidization velocity and bed voidage show that this material can be fully utilized as a cooling and buoyancy medium during the FCBC process.

Awareness of Crash Risk Improves Kelly Strategies in Simulated Financial Time Series

We simulate a simplified version of the price process including bubbles and crashes proposed in Kreuser and Sornette (2018). The price process is defined as a geometric random walk combined with jumps modelled by separate, discrete distributions associated with positive (and negative) bubbles. The key ingredient of the model is to assume that the sizes of the jumps are proportional to the bubble size. Thus, the jumps tend to efficiently bring back excess bubble prices close to a “normal” or fundamental value (“efficient crashes”). This is different from existing processes studied that assume jumps that are independent of the mispricing. The present model is simplified compared to Kreuser and Sornette (2018) in that we ignore the possibility of a change of the probability of a crash as the price accelerates above the normal price. We study the behaviour of investment strategies that maximize the expected log of wealth (Kelly criterion) for the risky asset and a risk-free asset. We show that the method behaves similarly to Kelly on Geometric Brownian Motion in that it outperforms other methods in the long-term and it beats classical Kelly. As a primary source of outperformance, we determine knowledge about the presence of crashes, but interestingly find that knowledge of only the size, and not the time of occurrence, already provides a significant and robust edge. We then perform an error analysis to show that the method is robust with respect to variations in the parameters. The method is most sensitive to errors in the expected return.

Super-Exponential RE bubble model with efficient crashes

ABSTRACTWe propose a dynamic Rational Expectations (RE) bubble model of prices, combining a geometric random walk with separate crash (and rally) discrete jump distributions associated with positive (and negative) bubbles. Crashes tend to efficiently bring back excess bubble prices close to a “normal” process. Then, the RE condition implies that the excess risk premium of the risky asset exposed to crashes is an increasing function of the amplitude of the expected crash, which itself grows with the bubble mispricing: hence, the larger the bubble price, the larger its subsequent growth rate. This positive feedback of price on return is the archetype of super-exponential price dynamics. We use the RE condition to estimate the real-time crash probability dynamically through an accelerating probability function depending on the increasing expected return. After showing how to estimate the model parameters, we obtain a closed-form approximation for the optimal investment that maximizes the expected log of wealth (Kelly criterion) for the risky bubbly asset and a risk-free asset. We demonstrate, on seven historical crashes, the promising outperformance of the method compared to a 60/40 portfolio, the classic Kelly allocation, and the risky asset, and how it mitigates jumps, both positive and negative.

Preparation and Properties of YAG Porous Ceramics via the Casting Method

YAG has many excellent features and therefore has a wide range of applications. Preparation and properties of YAG porous ceramics via the casting method is investigated. Through analysis and discussion, the following conclusions can be obtained. With the increase of the content of foaming agent, in the foaming process of the slurry, bubbles generated by the foaming agent and water are gradually increased, the more pores are left in the body after casting, and when the blowing agent is too much, excessive bubbles are generated, which can lead to a very low strength of the green body and can not be formed and operated later. As the ratio of water to material increases, the porosity of the porous material increases. This is because when the blowing agent is constant, the amount of water required for foaming is constant, and the amount of foam generated is also constant. When the content of foaming agent and the ratio of water to material is both 1, the porosity and compressive strength is better.

Wenzhou Real Estate Bubble and Rupture Risk Warning

It is known to us all that the excessive bubble in the real estate market can seriously affect the normal development of the national economy, which could cause the hidden dangers of financial crisis. In 2011, A wave of defaults destabilized markets across Wenzhou, which punctured the property price bubble and caused a painful lesson for the regional economy. Therefore, this paper reviews the trend of housing price in Wenzhou from 2006 to 2015, and analyzes the causes of house bubble. In addition, the adjustment process of Wenzhou real estate is of typical significance, which could provide references for current market trend.

Super-Exponential RE Bubble Model with Efficient Crashes

We propose a dynamic Rational Expectations (RE) bubble model of prices with the intention to exploit it for and evaluate it on optimal investment strategies. Our bubble model is defined as a geometric Brownian motion combined with separate crash (and rally) discrete jump distributions associated with positive (and negative) bubbles. We assume that crashes tend to efficiently bring back excess bubble prices close to a “normal” or fundamental value (“efficient crashes”). Then, the RE condition implies that the excess risk premium of the risky asset exposed to crashes is an increasing function of the amplitude of the expected crash, which itself grows with the bubble mispricing: hence, the larger the bubble price, the larger its subsequent growth rate. This positive feedback of price on return is the archetype of super-exponential price dynamics, which has been previously proposed as a general definition of bubbles. Our bubble model also allows for a sequence of small jumps or long-term corrections. We use the RE condition to estimate the realtime crash probability dynamically through an accelerating probability function depending on the increasing expected return. After showing how to estimate the model parameters, we examine the optimal investment problem in the context of the bubble model by obtaining an analytic expression for maximizing the expected log of wealth (Kelly criterion) for the risky asset and a risk-free asset. We also obtain a closed-form approximation for the optimal investment. We demonstrate, on seven historical crashes, the promising outperformance of the method compared to a 60/40 portfolio, the classic Kelly allocation, and the risky asset, and how it mitigates jumps, both positive and negative.

数値解析に基づくケーソン背後の津波越流の再現と被覆材の安定性の検討

Tsunami overflowing a breakwater is reproduced by a numerical model to evaluate the stability of armor units covering the harbor-side of the rubble mound. This model solves the overflow jet above the water surface and the harbor-side flow field separately to avoid entrainment of excessive bubbles. Using this model, the applicability of the Isbash formula for the stability of armor stones and concrete blocks against tsunami overflow is examined. The relationship between the Isbash number and the degree of movement of armor stones is clarified. In the case of concrete blocks, the Isbash number of the stability limitation varied with the thickness of the overflow jet. The Isbash formula by C.E.R.C. (1977) tends to overestimate the effect of slope angle, when applied to concrete blocks. A stability prediction method for armor blocks based on fluid forces acting on each armor block is presented as well.

Challenges Ahead in International Economic Law

Challenges Ahead in International Economic Law

A DNS study of laminar bubbly flows in a vertical channel

Direct numerical simulations are used to examine laminar bubbly flows in vertical channels. For equal size nearly spherical bubbles the results show that at steady state the number density of bubbles in the center of the channel is always such that the fluid mixture there is in hydrostatic equilibrium. For upflow, excess bubbles are pushed to the walls, forming a bubble rich wall-layer, one bubble diameter thick. For downflow, bubbles are drawn into the channel center, leading to a wall-layer devoid of bubbles, of a thickness determined by how much the void fraction in the center of the channel must be increased to reach hydrostatic equilibrium. The void fraction profile can be predicted analytically using a very simple model and the model also gives the velocity profile for the downflow case. For the upflow, however, the velocity increase across the wall-layer must be obtained from the simulations. The slip velocity of the bubbles in the channel core and the velocity fluctuations are predicted reasonably well by results for homogeneous flows.

The feasibility of doppler monitoring during EVA

During extravehicular activities (EVA) outside the spacecraft, astronauts have to work under reduced pressure in a space suit. This pressure reduction induces the risk of decompression sickness (DCS) by the formation of gas bubbles from excess nitrogen dissolved in the organism by breathing air at normal pressure. Under laboratory conditions the gas bubbles moving in the blood stream can be detected by the non-invasive ultrasonic Doppler method. By early detection of excessive bubble formation the development of DCS symptoms may be prevented by early application of preventative measures. The method could also be useful when applied in the space suit in order to compare the results of laboratory tests with operational results, because there is a discrepancy according to the DCS risk of laboratory experiments and actual EVA missions, where no symptoms have been reported yet. A prototype Doppler sensor has been developed and implemented in the Russian Orlan suit. To investigate the feasibility of this method under simulated space conditions, the equipment has been used in a series of 12 thermovacuum chamber tests with suited subjects, where intravenous bubble formation was compared to unsuited control experiments. In more than 50% of the suited tests good Doppler recordings could be achieved. In some cases with unsatisfying results the signal could be improved by breathholding. Although the results do not yet allow any conclusion about a possible difference between suited and unsuited subjects due to the small number of tests performed, the method proved its feasibility for use in EVA suits and should be further developed to enhance the safety of EVA procedures.

ENHANCED POOL BOILING HEAT TRANSFER WITH SURFACE ATTACHMENT

Abstract A pool boiling experiment with either water or Freon-113 was conducted to investigate nucleate boiling from a heated wall with either a spherical or a cylindrical attachment. The result revealed that nucleate boiling can be enhanced by applying a horizontal cylindrical attachment to a vertical heated wall, owing to the favorable thermal environment characterized by a small-gradient liquid temperature profile within the restricted regions between the attachment and the heated wait. Nucleate boiling is enhanced in terms of a lower wall superheat required for incipient boiling and more bubbles generated than from an open heated wall. As a result of the enhanced nucleate boiling, heal transfer of the vertical heated wall above the attachment was improved due to excessive bubbles moving upward along the heated wall, causing removal of the thermal layer near the wall and evaporation of the thin liquid film between the bubbles and the wall. The boiling curve hysteresis with Freon-113 was significantly r...

Operation and failure of a hairpin nucleate generator on a bubble memory chip

The dynamic operation as well as the failure modes of a hairpin nucleate bubble generator operating at 125 kHz has been studied using a high-speed optical sampling system. The hairpin generator loop consists of a 6.8-μm wide 4.2-μm gap conductor located under a 20-μm period 110° chevron propagation pattern. In stable operation in the center of the operating range, bubbles are nucleated along the hairpin conductor preserving a U shape that reflects the shape of the hairpin conductor. After nucleation, the gap of the conductor fills with a long wide domain. This long domain then shrinks to a stable position under the propagation elements with a wall velocity of 40 m/s. All velocities observed are consistent with velocities observed on free bubbles using radial expansion, except for the growth in length after nucleation during the generate pulse, where the domain grows about three times faster than would be expected. This difference is attributed to the in-plane component of the generate current field that is very nonuniform through the thickness of the sample. The failure mode for a low or short generate pulse is multibubble generation caused by nonuniform nucleation along the length of the conductors of the hairpin loop. For certain pulse characteristics at this low end of the operating range, a stable U-shaped domain is nucleated only to be cut into two domains at the end of the generate pulse as the drive field reverses the poles on the nearest chevron stack from what it was during the pulse. The failure mode at the upper end of the operating range also results in excess bubbles. Here the mechanism can be identified as the development of a wavy wall as it moves rapidly under the conductor element in the presence of a large in-plane field caused by the generate current. This distortion then increases as the domain shrinks, resulting in nobs that are pinched off to create excess bubbles. The distortion is similar to that seen during the collapse of highly expanded stripes. This failure can be cured by increasing the fall time of the generate pulse to a least 100 ns.

The measurement of the dynamic surface tension of manoxol OT solutions for freshly formed surfaces

The excess bubble pressure method has been extended by use of stroboscopic counting to cover surface ages of between 10 and 200 milliseconds and used to measure the dynamic surface tensions of Manoxol OT solutions in this time range.