Bubble Height Research Articles

Effects of contact angle hysteresis on bubble dynamics and heat transfer characteristics in saturated pool boiling

As an inherent physical phenomenon of the contact-line motion, the contact angle hysteresis (CAH) has an important effect on dynamic behaviors of bubble/bubbles from a solid surface. In this study, boiling heat transfer considering the CAH is investigated by employing a phase change lattice Boltzmann (LB) model. Effects of the CAH on the bubble/bubbles dynamics during the boiling process and the boiling curves for both the hydrophilic and hydrophobic hysteresis windows are studied in detail. It is found that the bubble base, bubble height and phase change rate increase as the increase of the width of hysteresis window. On the other hand, the pinning of three-phase contact line is observed during the bubble growth processes. And the pinning times increase with the contact angle hysteresis. For the hydrophilic hysteresis window, the bubble completely departs from the heater if a small range of the hysteresis window is considered, while a residual bubble is left when the bubble departs from the heater in the cases of the hysteresis window larger than an approximate critical value of 45°. For the hydrophobic hysteresis window, a residual bubble can be observed regardless of the level of the contact angle hysteresis. The numerical results also indicate that the CAH affects the position of bubbles coalescence. The bubbles merge above the heater for the hydrophilic hysteresis window with a small level of hysteresis. While the bubbles merge on the heater for the hydrophilic hysteresis window with a large level of hysteresis as well as for the hydrophobic hysteresis window. Moreover, as the increase of the width of the hysteresis window, the critical heat flux (CHF) as well as the Leidenfrost temperature (the minimum temperature for film boiling) decrease, and the transition boiling regimes become shorter. Finally, we present the fitting equations between the width of the hysteresis window and the CHF/Leidenfrost temperature. The results show that the CHF and the Leidenfrost temperature decrease linearly as the width of the hysteresis window increases.

Risk Mitigation of Plunger-Stopper Displacement Under Low Atmospheric Pressure by Establishing Design Space for Filling-Stoppering Process of Prefilled Syringes: A Design of Experiment (DoE) Approach

There is a concern that low atmospheric pressure typically encountered during shipment could result in plunger-stopper displacement in prefilled syringes impacting sterility and container closure integrity (CCI) of drug product.1 In this work, following DoE principles we first investigated the impact of filling and stoppering operating parameters on creation of bubble height as performance parameters among others in nominal 1 mL and 2.25 mL Type I glass prefilled syringes (PFSs) with staked needle and rigid needle shield (RNS). Bubble height ranging from <2.0 mm to >15.0 mm were produced in syringes by filling water and vacuum stoppering at operating vacuum pressure ranging from 400 mbar to 950 mbar using a pilot scale filling-stoppering machine. We found that for a particular nominal fill volume in prefilled syringe, as the stoppering vacuum pressure increased, bubble height decreased resulting in plunger-stopper placed closer to the fill level. Subsequently, syringes with varying bubble size were exposed to reduced atmospheric pressure ranging from 628 Torr to 293 Torr bracketing the low pressure recommended by ASTM D4169 standard to qualify shipping containers for transportation of drug products. We found inverse linear correlation between bubble height and plunger-stopper displacement under low atmospheric pressure. However, plunger-stopper displacement increased exponentially as atmospheric pressure decreased. The results suggest that air bubble size in filled glass syringes should be minimized in order to mitigate sterility and container closure integrity (CCI) risk to drug product in prefilled syringes.

Generation of hydrogen bubble in biodiesel—Influence of non-uniform electric field

With regard to improving the blending performance of biodiesel with hydrogen, it is important to generate smaller hydrogen bubbles in biodiesel at low energy consumption. In this context, a strong non-uniform direct current (DC) electric field has been created between a needle-ring electrode configuration, which is used to study the effect of EHD on hydrogen bubble behaviors in biodiesel considering the various gas flow rates and applied voltages. The special geometry of gas cones similar to the Taylor cone in liquid droplets under an electric field is demonstrated. The hydrogen bubble features—which are characterized by the bubble heights, frequencies, velocities, bubble sizes, and shapes—show that the bubble geometry and dynamic characteristics in the electric field are performed in manners different from those reported in the literature. It is found that the hydrodynamic behavior of the bubbles is also modified by the EHD effect. The distributions of bubble size in different cases follow the Gaussian distribution. Small bubbles are more likely to be associated with high separation velocities. Further, the diversity of bubble shapes at the separation is demonstrated, and two regimes of impact mechanism based on the surface tension and electric forces are revealed.

Dynamics of bubble growth under a heated substrate

This paper investigates the growth dynamics of a vapor-gas bubble pressed against a heating plate by the buoyancy force. The shadow method was used to capture images, which were then automatically processed to calculate the size of the bubble. As expected, the bubble dynamics significantly depends on the heating power. It was found that the ratio of bubble diameter to bubble height increases as it grows.

Surface Roughness Impact on Boundary Layer Transition and Loss Mechanisms Over a Flat-Plate Under a Low-Pressure Turbine Pressure Gradient

Abstract An experimental study has been conducted to investigate the effects of surface roughness on the profile loss of a flat-plate with a contoured wall. All of the measurements have been conducted for the suction side pressure gradient of a high-lift low pressure turbine airfoil at the fixed freestream turbulence intensity (Tu) of 3.2% under Reynolds numbers of Rec = 1. 2 · 105 (cruise) and Rec = 5.2 · 105 (take-off). The time-resolved streamwise and wall-normal velocity fields for three different surface roughness values of Ra/C · 105 = 0.065, 4.417, and 7.428 have been measured with a 2D hot-wire probe. At the take-off condition (Rec = 5.2 · 105), attached flow transition occurs, and increased surface roughness increases the loss. For all of the surfaces, momentum deficits in the laminar to early transition region (γ ≈ 0.05) are similar. For the intermediate transition (γ ≈ 0.5), increased roughness reduces the Reynolds stress and accelerates the breakdown of large-scale turbulent spots into small-scale turbulent eddies. Therefore, turbulent energy and momentum deficit are decreased for rough surfaces. For the late transition (γ &amp;gt; 0.9), transitional boundary layers become similar to turbulent boundary layers, and increased surface roughness increases turbulent mixing, boundary layer thickness, and, hence, the momentum deficit. On the other hand, at the cruise condition (Rec = 1.2 · 105), separated flow transition occurs and increased surface roughness decreases loss. Since the portion of turbulent flow is relatively small, the overall profile loss is mainly determined by the momentum deficit generated during transition. Increased roughness decreases the maximum height and length of the separation bubble but does not affect the separation bubble onset location. The beneficial effects of increased surface roughness on the profile loss appear in the separated shear layer and reattachment. Increased surface roughness increases turbulent mixing in the separated shear layer, reducing the shear layer thickness and momentum deficit. In addition, increased surface roughness reduces the length scale and turbulence intensity of the shed vortices. Consequently, turbulent mixing and momentum deficit during the reattachment of boundary layers are decreased, resulting in a lower profile loss.

Numerical simulation and optimization of a cold model of a flat membrane bioreactor air scouring for membrane fouling control

Aeration scouring is currently the most common membrane fouling control technology. The optimization of aeration parameters, such as the aeration aperture, aeration pore spacing, and aerator layout height, can improve the efficiency of aeration and reduce the energy consumption of aeration. However, the simulation results cannot guide engineering practices well due to the limitations in the model precision of computational fluid dynamics. On this basis, the study compared and selected the computational fluid dynamics turbulence model. The DES model was used to study the influence of aeration parameters on membrane fouling control. The study showed that the DES turbulence model had a higher precision and accuracy in predicting the average shear force and the instantaneous shear force distribution on the membrane surface than the k-ε turbulence model. The average error of the DES turbulence model simulation of a bubble height was only 1.33%, which was 14.33% lower than the 15.66% of k-ε. An aeration pore spacing of 20 mm, a height of 100 mm, and an aeration aperture of 2.0 mm were the best layout parameters. The degree of influence of the aeration aperture, the aeration pore spacing, and the height of the aerator layout on the shear force of the membrane surface decreased sequentially. The influence of each interaction on the average shear force of the membrane surface was negligible. Among these parameters, the main reason for the membrane surface shear force was the change in the flow velocity caused by aeration. By calculating the fitting equation and the critical value of the shear force, the critical value of the liquid velocity on the membrane surface that could remove membrane fouling was 0.232 m/s. Although the numerical simulation and optimization of a flat membrane bioreactor air scouring for membrane fouling control were carried out with pure water, it still plays a significant impact on the actual membrane fouling control. These results could guide the optimization design of aeration parameters for flat membrane bioreactors and provide a reference for the analysis of aeration scour control membrane pollution control research.

Nonlinear exciton drift in piezoelectric two-dimensional materials

The noncentrosymmetric nature of single-layer (SL) transition-metal dichalcogenides (TMD) manifests itself in the finite piezoelectricity and valley-Zeeman coupling. We microscopically model nonlinear exciton transport in nanobubbles of SL TMDs. Thanks to the giant piezoelectric effect, we obtain an enormous internal electric field, ${E}_{\mathrm{piezo}}\ensuremath{\sim}{10}^{7}\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{m}$, resulting in a built-in dipole moment of excitons. We demonstrate that the piezo-induced dipole-dipole interaction provides a novel channel for the nonlinear exciton transport distinct from the conventional isotropic funneling of excitons and leads to the formation of a hexagon-shaped exciton droplet on the top of circularly symmetric nanobubbles. Moreover, we found that the hexagonal distribution of exciton density is preserved even for strongly elliptic nanobubbles. The effect is tunable via the bubble-size dependence of the piezoelectric field ${E}_{\mathrm{piezo}}\ensuremath{\sim}{h}_{\mathrm{max}}^{2}/{R}^{3}$ with ${h}_{\mathrm{max}}$ and $R$ being the bubble height and radius, respectively.

The Impact of Aerosol Vertical Distribution on a Deep Convective Cloud

This study investigates the effects of aerosol vertical distribution on a deep convective cloud system. We intend to elucidate the mechanisms for aerosols entering the cloud from different heights, and how they affect cloud microphysics and precipitation. A thermal bubble is released at 1.5 km initially to run an idealized case using the Weather Research and Forecast (WRF) model. The aerosol layer with high concentration was initially put at different altitudes in the model to study the mechanisms and the number of aerosols entering the cloud. It was found that there are three mechanisms for aerosols from different heights to enter the cloud, depending on their relative height with the thermal bubble. Aerosols from lower altitudes (below 1 km) enter the cloud through pumping, while aerosols from higher altitudes (2–3 km, 3–5 km) enter the cloud through entrainment. Both mechanisms lead to low cloud condensation nuclei (CCN) concentration in the cloud. Only aerosols from intermediate altitudes (1–2 km), which is the same as the initial height of the thermal bubble, enter the cloud mainly by ascending with the bubble and lead to high CCN concentration in the cloud. The differences in activated CCN concentration affect the microphysical processes and precipitation remarkably. For the simulations with an initial aerosol layer at 1–2 km and 0–5 km, aerosols can enter the cloud more efficiently than the other four simulations. More activated CCNs in these two simulations lead to more graupels with smaller sizes at higher altitudes, which delays the precipitation but makes the precipitation last longer. However, the accumulated precipitation is similar in all six simulations, no matter what aerosol vertical distribution is like. The results in this study indicate that the altitude of aerosol layers determines the mechanisms for aerosols entering clouds, CCN concentration in the cloud, and to what extent the cloud microphysical processes and precipitation are affected.

On cointegration and cryptocurrency dynamics

This paper aims to model the joint dynamics of cryptocurrencies in a nonstationary setting. In particular, we analyze the role of cointegration relationships within a large system of cryptocurrencies in a vector error correction model (VECM) framework. To enable analysis in a dynamic setting, we propose the COINtensity VECM, a nonlinear VECM specification accounting for a varying systemwide cointegration exposure. Our results show that cryptocurrencies are indeed cointegrated with a cointegration rank of four. We also find that all currencies are affected by these long term equilibrium relations. The nonlinearity in the error adjustment turned out to be stronger during the height of the cryptocurrency bubble. A simple statistical arbitrage trading strategy is proposed showing a great in-sample performance, whereas an out-of-sample analysis gives reason to treat the strategy with caution.

Numerical investigation into rotational augmentation with passive vortex generators on the NREL Phase VI blade

Passive vortex generators (VGs) have been widely used on wind turbines to improve the aerodynamic power. Nevertheless, most studies of VGs are conducted on flat plates and airfoils rather than rotating blades. The effect of VGs on rotational blade flow therefore remains unclear. This paper presents a comparative study between the NREL S809 airfoil flow and NREL Phase VI blade flow, to highlight rotational augmentation with VGs. There are 36 pairs of rectangular VGs installed at about 20% c between 25% and 50% spans of the blade. Fully-resolved RANS simulations are used to identify the airfoil and blade flow characteristics with VGs. Although VGs greatly increase the maximum lift coefficient of airfoil by almost 60%, they cause unexpected decreases in aerodynamic forces on the inboard blade. This is mainly because VGs can reduce the radial flow and thus undermine rotational augmentation. Furthermore, increasing VG size amplifies the VG effect on radial flow and enlarges the separation bubble at 30% span from 43% c to 57% c in height. Positioning VGs in a skewed layout, however, effectively diminishes this negative VG effect and reduces the separation bubble height to 28% c. These findings indicate a huge difference between the airfoil flow and blade flow with VGs due to rotational effects. This work might deepen the understanding of rotational augmentation with VGs on a rotating blade.

Experimental investigation on bubble coalescence regimes under non-uniform electric field

In this study, the characteristics of interactions between bubbles in ethanol were examined using a needle-ring electrode rig in the presence of a direct current electric field, using high-speed photography and with a focus on the effect of the electric field strength on the bubble coalescence. The results show bubble coalescence associated with four regimes in the manipulated parameter regions-namely, the coalescence of growing bubbles, single coalescence of rising bubbles, secondary coalescence of rising bubbles, and multiple coalescences of bubbles. The bubble interaction dynamics-which are characterized by the bubble trajectories, height positions, inclination angles, and bubble sizes-show that the bubble approach, collision, and coalescence in the electric field proceed in ways different from those reported in the literature. Furthermore, the cross-sectional average bubble diameter values corresponded to the bubble coalescence regimes. In addition, the increase in the Weber number (We) and electrical Bond number (BoE) gives rise to the bubble coalescence frequency. The We works to strengthen the wake induction, while the BoE works to accelerate the generation of smaller bubbles.

Numerical study and buoyancy–drag modeling of bubble and spike distances in three-dimensional spherical implosions

High-resolution three-dimensional implicit large eddy simulations of implosion in spherical geometries are presented. The growth of perturbations is due to Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities and also to geometric convergence and compression effects. RM and RT instabilities have been studied extensively in planar configurations, but there are comparatively few studies on spherical geometries. Planar geometries lack the effect of convergence that changes the morphology and growth of perturbations in spherical geometries. This paper presents a study of turbulent mixing in spherical geometries considering different narrowband (NB) and broadband multimode initial perturbations and examines several quantities including the evolution of the integral mixing layer width and integral bubble and spike heights using novel integral definitions. The growth of the bubble and spike is modeled using a Buoyancy–Drag (BD) approach that is based on simple ordinary differential equations to model the growth of the turbulent mixing layer. In a recent study, Youngs and Thornber [“Buoyancy-drag modelling of bubble and spike distances for single-shock Richtmyer-Meshkov mixing,” Physica D 410, 132517 (2020)] constructed modifications to the BD equations to take into account the early stages of the mixing process that are dependent on the initial conditions. Those modifications are shown to be important to obtain correct results. The current study adopted the same modifications and adapted the BD equations to the spherical implosion case. The results of the BD model are compared with those of different initial NB cases that include different initial amplitudes and wavelengths of the perturbations, for validation purposes. The predictions from the new BD model are in very good agreement with the numerical results; however, there exist some limitations in the accuracy of the model, in particular the use of the interface position and fluid velocity from one-dimensional data.

Influence of helium ion irradiation on the morphology and microstructure of CN-G01 beryllium

Beryllium is considered as a candidate of ITER first-wall (FW) armor and neutron multiplier in fusion reactors. To assess the irradiation resistance of CN-G01 beryllium, which has been accepted as an alternative ITER-grade beryllium in China, 180 keV helium ions with fluences of 1.0 × 1017 ions cm−2, 5.0 × 1017 ions cm−2 and 1.0 × 1018 ions cm−2 were implanted at room temperature. The theoretical simulation of energy loss, damage distribution and helium concentration were proceeded by SRIM. In this paper, we report the experimental exploration of ion fluence effect on surface morphology and microstructure of irradiated samples. Field Emission Scanning Electron Microscope (FESEM) analysis depicted the formation and growth of helium bubbles at different irradiation ion fluences. No obvious exfoliation or cavities was observed on the surface at all ion fluences, suggesting a reliable radiation resistance of CN-G01 beryllium. Atomic force microscopy (AFM) morphology showed that the maximum height of bubbles was 47.8 nm. Surface roughness values increased slightly due to the formation of defects and bubbles on the irradiated beryllium surface. Nevertheless, the structural analysis demonstrated by grazing incidence x-ray diffraction (GIXRD), indicated an obvious preferred orientation on (101) peak at various ion fluences. Annihilation of defects caused by a small rise of the localized temperature could explain the increasing intensity of diffraction peak (101).

Experimental Study on Time Evolution of Shock Wave and Turbulent Boundary Layer Interactions

In this paper, a test system based on the Nano-tracer Planar Laser Scattering (NPLS) technique for studying time evolution of unsteady flow structures was finished. Based on this system, the experimental study on the interactions between the incident shock wave and the turbulent boundary layer of the incoming wall was performed. The experiments were performed in a Mach 3.4 supersonic low-noise wind tunnel at the unit Reynolds number of 6.30 × 106/m-1. For the first time, five frames of temporal-correlated fine structure images of transient flow field with shock wave and the turbulent boundary layer interactions (SWTBLI) were obtained under the experimental conditions, and the spatiotemporal evolution characteristics of the flow structure were analyzed. At the same time, the flow field characteristics of temporal-correlated images were studied when the density boundary layer thickness of incoming turbulent layer is δ1 = 0.55δ, δ2 = 0.72δ, δ3 = 0.87δ respectively. During the development of vortex structure in the boundary layer from turbulent boundary layer to separation bubble, the oscillation interval distribution law of induced shock wave was summarized, and the group velocity of vortex structure development in the boundary layer and the relationship between boundary layer thickness and physical space size growth law of separation bubble under different incoming turbulent boundary layer thicknesses were obtained. The results also show that with the increase of the incoming boundary layer thickness, the group velocity in the development process of vortex structure in the turbulent boundary layer does not change significantly. As the thickness of the boundary layer entering the separation bubble increases, the overall growth height of the separation bubble also increases.

Adaptive separation control of a laminar boundary layer using online dynamic mode decomposition

Abstract

Contact Pressures Between the Rearfoot and a Novel Offloading Insole: Results From a Finite Element Analysis Study.

Pressure offloading is critical to diabetic foot ulcer healing and prevention. A novel product has been proposed to achieve this offloading with an insole that can be easily modified for each user. This insole consists of pressurized bubbles that can be selectively perforated and depressurized to redistribute weight to the nonulcer region of the foot. However, the effect of the insole design parameters, for example, bubble height and stiffness, on offloading effectiveness is unknown. To this end, a 3-dimensional finite element model was developed to simulate contact between the rearfoot and insole. The geometry of the calcaneus bone and soft tissue was based on the medical images of an average male patient, and material properties and loading conditions based on the values reported in the literature were used. The model predicts that increasing bubble height and stiffness leads to a more effectively offloaded region. However, the model also predicts that increasing stiffness leads to increasing contact pressures on the surrounding soft tissue. Thus, a combination of insole design parameters was determined, which completely offloads the desired region, while simultaneously reducing the contact pressure on the surrounding soft tissue. This design is expected to aid in diabetic foot ulcer healing and prevention.

Unsteady flow control of a plane diffuser based on a Karman-vortex generator

Implicit large-eddy simulation is employed to simulate the flow in an asymmetric plane diffuser at Re=9000. Flow separation exists near the throat and evolves to large-scale, unsteady separation in the expansion section and the downstream region. An unconventional flow control method, namely, a cylindrical Karman-vortex generator (KVG) with different sizes and locations that induces periodic spanwise vortex shedding, is set upstream of the throat to suppress the flow separation. An appropriately designed KVG can enhance the mixing of the outer flow and the low energy fluid near the wall region by the periodic shedding Karman-vortices, and effectively reduce the separation bubble size. For the present optimal case, the length and height of the separation bubble are decreased 50.4% and 90.9%, respectively. The static pressure recovery coefficient is also increased by about 50%. Moreover, the velocity and total pressure distributions at the end of the expansion section are more uniform with lower fluctuation in the case with KVG installed. An optimal KVG diameter DK is suggested to be 3–4% of the expansion section length LE. The gap ratio to the lower wall G/DK and the length ratio to the throat Lt/DK are suggested to be 2.0–3.0 and 5.0–10.0, respectively.

Experimental study of bubble formation process on the micro-orifice in mini channels

A visualized experimental system is designed and constructed to study the bubble behaviors on the micro-orifice in liquid layer. The test section is performed in a rectangular duct of 0.27 m × 0.006 m × 0.012 m, and the air is injected from an orifice with 0.49 mm in diameter. The gas flow rate is in the range of 0.3–60 ml/min, while the corresponding Reynolds number is ranged from 0.87 to 173.18. The effect of gas flow rate on bubble behaviors and bubble coalescence characteristics are investigated systematically. The results state that the bubble height, the radius of curvature at the apex and the bubble detachment volume show little variation with the gas flow rate when the gas flow rate is less than 24 ml/min. While the bubble contact line diameter increases firstly and then decreases. During the multi-bubble regimes stage, the detachment volume of both leading bubbles and trailing bubbles increase with the Weber number. The time interval between two adjacent bubbles or coalescence cycles is almost 0 when Weber number is above 18.27.

Effect of surfactant redistribution on the flow and stability of foam films.

The viscous froth model for two-dimensional (2D) dissipative foam rheology is combined with Marangoni-driven surfactant redistribution on a foam film. The model is used to study the flow of a 2D foam system consisting of one bubble partially filling a constricted channel and a single spanning film connecting it to the opposite channel wall. Gradients of surface tension arising from film deformation induce tangential flow that redistributes surfactant along the film. This redistribution, and the consequent changes in film tension, inhibit the structure from undergoing a foam-destroying topological change in which the spanning film leaves the bubble behind; foam stability is thereby increased. The system's behaviour is categorized by a Gibbs-Marangoni parameter, representing the ratio between the rate of motion in tangential and normal directions. Larger values of the Gibbs-Marangoni parameter induce greater variation in surface tension, increase the rate of surfactant redistribution and reduce the likelihood of topological changes. An intermediate regime is, however, identified in which the Gibbs-Marangoni parameter is large enough to create a significant gradient of surface tension but is not great enough to smooth out the flow-induced redistribution of surfactant entirely, resulting in non-monotonic variation in the bubble height, and hence in foam stability.

Effects of compressibility and Atwood number on the single-mode Rayleigh-Taylor instability

In order to study the effect of compressibility on Rayleigh-Taylor (RT) instability, we numerically simulated the late-time evolution of two-dimensional single-mode RT instability for isothermal background stratification with different isothermal Mach numbers and Atwood numbers (At) using a high-order central compact finite difference scheme. It is found that the initial density stratification caused by compressibility plays a stabilizing role, while the expansion-compression effect of flow plays a destabilizing role. For the case of small Atwood number, the density difference between the two sides of the interface is small, and the density distribution of the upper and lower layers is nearly symmetrical. The initial density stratification plays a dominant role, and the expansion-compression effect has little influence. With the increase in the Atwood number, the stabilization effect of initial density stratification decreases, and the instability caused by the expansion-compression effect becomes more significant. The flow structures of bubbles and spikes are quite different at medium Atwood number. The effect of compressibility on the bubble velocity is strong at large At. The bubble height is approximately a quadratic function of time at potential flow growth stage. The average bubble acceleration is nearly proportional to the square of Mach number at At = 0.9.