A study of the mechanism of flashing flow by experiment and theoretical analysis

Characteristics of bubble nucleation and growth are critical for its application. It is affected by several factors including viscosity, surface tension and temperature. However, the effect of pressure on bubble nucleation and growth has been underreported, although it processes significant effect on above characteristics. In this work, a micro copper electrode is etched on a slab covered with copper to produce bubble on the surface by current input. The nucleation time of bubble is measured under different heat flux and system pressures. The nucleation and growth processes are recorded with a high speed camera in order to discuss the effects of heat flux and system pressure on bubble characteristics. The experiment results indicate that the micro electrode with higher heat flux produces more thermal energy, which makes the time of bubble nucleation shorter and the speed of bubble growth faster. Higher system pressure causes the increase of the critical nucleation temperature and also baffles the bubble nucleation and growth. Bubble growth includes the stages of rapid growth and dynamic equilibrium, with the speed being from fast to slow. In the former part of rapid growth, heat flux plays a dominant role in bubble growth. While the effect of system pressure on bubble growth becomes significant in the latter part of rapid growth. Both the nucleation time and bubble growth agree well with the theoretical analysis. The obtained results help to accurately control bubble nucleation and growth required in different application.

Electrolyser cell performance is usually limited by transport and ohmic losses due to evolution of gas bubbles on the electrode surfaces especially at higher operating current densities. In the advancements of electrolyser cell design, the electrolyser with zero-gap cell design performs at higher current densities over the conventional design and also utilizing of economic and abundantly available materials (such as Ni and porous carbon). However, the resistances from the electrolyte and gas bubbles on the back surface of electrodes still limit the full potential performance of the zero-gap cell electrolyser. This work addressed the gas bubble resistances by providing additional insight into the gas bubble nucleation, growth and detachment at the interface of electrode and gas diffusion layer (GDL) by a computational approach. Due to the lower surface energy at interfaces, the probability of nucleation of gas bubbles is energetically favorable. The bubble nucleates and grows with contact of two solid surfaces (electrode and GDL strut) provided that there is a state of supersaturation of gas in the electrolyte. The nucleation of bubbles at the interface is detrimental to the performance of electrolyser, where the bubbles have stronger adhesion to the interface surfaces and delay the detachment time. Hence the magnitude of buoyancy force is high in relative to detachment of bubbles at non-interface sites. Using openFoam, we investigated the effects of interface surface texture on the detachment time of bubbles. The porous layer (electrode) and the GDL struts are considered orthogonal to each other, the porous layer architectures were considered to be cubic and to be a primitive triply periodic minimal surface (TPMS), respectively, both with an area of 1 mm2 and thickness of 200 µm. The volume fraction of the cubic and primitive TPMS was chosen to be 0.6. The GDL struts interface with the considered porous layers, the GDL architecture was chosen as an open triangular block with sides of rectangular and cylindrical shapes of different thickness of 70 µm, 100 µm and 150 µm, respectively. The dimensions chosen are representative of electrolyser systems, in the multiphase simulations, the nucleation of bubble at multiple sites of the interface geometries was studied. The spherical shape and growth of the bubble was defined by the parabolic inlet at the interfaces (nucleation sites), where the growth rate of bubble was mimicked by inducing the mass flow rate of gas through the interface. The growth profile of the bubble chosen in the simulation corresponds to operating current density and supersaturation of the water (electrolyte) with dissolved gas (oxygen). The geometric volume of fluid (VOF) method was implemented to track the advection of the interface of water and gas bubble. The outcomes of the simulation studies portray the time scales of growth and detachment of bubbles with emphasis on critical bubble radius at the interfaces. These results were compared with a relative study of critical radius of bubble and bubble detachment times at non-interface sites. Further, the effect of temperature and pressure in the simulation studies was implemented by the variation in surface tension of water and oxygen. Contact angles of the electrode and GDL surfaces are varied from aerophilic to aerophobic (contact angle of bubble to solid surface in aqueous media), which induces a decrease in adhesion strength of the bubble. Due to the convergence issues, it is necessary to run the simulations at very low time steps (10-6 to 10-12 s), which is computationally expensive. The implementation of a piece-wise linear interface reconstruction of the distance function (PLIC-RDF) improved the time frames.The analysis and insights stress the need of surface engineering of the electrode and GDL interfaces for enhanced detachment time of gas bubbles. The work-flow we described in this work can be extended to study other complex interfaces of GDL and electrode, that paves the way to design water electrolyser system to perform at higher operating current densities with low losses. Figure 1

We present a new hydrothermal moissanite cell for in situ experiments at pressures up to 1000 bar and temperature to 850 °C. The original moissanite cell presented by Schiavi et al. (2010) was redesigned to allow precise control of fluid pressure. The new device consists of a cylindrical sample chamber drilled into a bulk piece of NIMONIC 105 super alloy, which is connected through a capillary to an external pressure control system. Sealing is provided by two gold gasket rings between the moissanite windows and the sample chamber. The new technique allows the direct observation of various phenomena, such as bubble nucleation, bubble growth, crystal growth, and crystal dissolution in silicate melts, at accurately controlled rates of heating, cooling, and compression or decompression. Several pilot experiments on bubble nucleation and growth at temperature of 715 °C and under variable pressure regimes (pressure oscillations between 500 and 1000 bar and decompression from 800 to 200 bar at variable decompression rates) were conducted using a haplogranitic glass as starting material. Bubble nucleation occurs in a short single event upon heating of the melt above the glass transition temperature and upon decompression, but only during the first 100 bar of decompression. New bubbles nucleate only at a distance from existing bubbles larger than the mean diffusive path of water in the melt. Bubbles expand and shrink instantaneously in response to any pressure change. The bubble-bubble contact induced during pressure cycling and decompression does not favor bubble coalescence, which is never observed at contact times shorter than 60 s. However, repeated pressure changes favor the diffusive coarsening of larger bubbles at the expense of the smaller ones (Ostwald ripening). Experiments with the haplogranite show that, under the most favorable conditions of volatile supersaturation (as imposed by the experiment), highly viscous melts are likely to maintain the packing of bubbles for longer time before fragmentation. In-situ observations with the new hydrothermal moissanite cell allow to carefully assess the conditions of bubble nucleation, eliminating the uncertainty given by the post mortem observation of samples run using conventional experimental techniques.

The experimental and numerical relation between the solubility, diffusivity and bubble nucleation of supercritical CO2 in Polystyrene via visual observation apparatus

Experimental study of bubble growth in Stromboli basalt melts at 1 atm

Comparison of bubble growth process within a superheated water droplet and in superheated water due to rapid depressurization

Initial bubble growth in polymer foam processes

A single hydrogen bubble generated at an electrode surface during water electrolysis is simulated via the volume of fluid (VOF) multiphase flow model to capture the details of the interface evolution and the mass transfer that occurs at the interface. The hydrogen bubble that grows at the electrode is driven by supersaturation of the dissolved hydrogen in the liquid. Two models are used to calculate the gas‐liquid interface mass transfer coefficient. The bubble growth from experimental results agrees closely with theoretical predictions. In addition, the mass transfer of dissolved hydrogen from the electrode surface to the bulk liquid is evaluated during the bubble nucleation and growth stages. During the nucleation stage, the mass transfer coefficient is < 5.1 × 10−5 m/s. Once the bubble embryo is formed, the mass transfer greatly increases. Before the bubble releases, the mass transfer coefficient reaches 2.8 × 10−4 m/s. More detailed information about the bubble growth is presented, including bubble‐induced convection and the concentration distribution of dissolved hydrogen around the growing bubble. The results indicate that the VOF method is suitable and reliable for simulating bubble behaviour during electrolysis or other electrochemical reactions that involve gas bubble desorption.

Forced convective flow boiling in microchannels is characterized by the nucleation and rapid growth of vapor bubbles in confined geometries. Experimental studies of these flows have been limited to the measurement of wall temperature, inlet liquid velocity, inlet and outlet pressures, and high speed imaging forcing analysts to infer the conditions inside the channel from measured external values. The present study examines the evolution of the pressure field during bubble growth prior to bubble departure using a one-dimensional fully compressible Lagrangian-Eulerian model. Numerical results for a single bubble growing from a nucleation site for both constant pressure and constant volumetric flow rate conditions demonstrate the magnitude of the pressures generated and bound the magnitude of the reflected pulses from the channel ends. The reflected pulses can locally decrease the pressure in the channel below the levels predicted by incompressible models. Additional simulations predict nucleation and growth of bubbles at sites that would be inactive if liquid compressibility is neglected. The results indicate the acoustic characteristics of microchannels for flow boiling can not be neglected and will be important in the optimization of microchannel designs.

Influence of Bubbles on the Energy Conversion Efficiency of Electrochemical Reactors

Experimental and simulation study of the physical foaming process using high-pressure CO2

Bubble nucleation and growth were investigated in low density polyethylene (LDPE) using nitrogen (N2). The solubility and diffusivity of N2 in low density polyethylene were determined by a magnetic suspension balance (MSB) system. The bubble nucleation and growth during foaming was assessed using a visualization batch foaming system. It was observed that the increase in temperature and pressure decrement, decreased the bubble density, respectively. The bubble size during the foaming was also studied through numerical model and the effect of various simulation variables on the bubble growth was investigated. It was concluded that the numerical model could predict the foaming process. Moreover, according to the bubble pressure profile, the N2diffusion are formative factors in controlling the bubble growth.

The growth of helium bubbles impacts structural integrity of materials in nuclear applications. Understanding helium bubble nucleation and growth mechanisms is critical for improved material applications and aging predictions. Systematic molecular dynamics simulations have been performed to study helium bubble nucleation and growth mechanisms in Fe70Ni11Cr19 stainless steels. First, helium cluster diffusivities are calculated at a variety of helium cluster sizes and temperatures for systems with and without dislocations. Second, the process of diffusion of helium atoms to join existing helium bubbles is not deterministic and is hence studied using ensemble simulations for systems with and without vacancies, interstitials, and dislocations. We find that bubble nucleation depends on diffusion of not only single helium atoms, but also small helium clusters. Defects such as vacancies and dislocations can significantly impact the diffusion kinetics due to the trapping effects. Vacancies always increase the time for helium atoms to join existing bubbles due to the short-range trapping effect. This promotes bubble nucleation as opposed to bubble growth. Interestingly, dislocations can create a long-range trapping effect that reduces the time for helium atoms to join existing bubbles. This can promote bubble growth within a certain region near dislocations.

This paper reports a study of bubble nucleation by fission fragments in superheated water. The experimental work was conducted using a small bubble chamber especially built for the program. The minimum superheat necessary for nucleation of visible bubbles by fission fragments (the threshold) was measured at temperatures between 380 and 440°F.Predictions of the threshold are based on comparison of the energy and linear energy transfer (LET) of fission fragments with the values required for bubble nucleation. Because of the variation in fission-fragment energy, the comparison is made on the basis of the median, 80’th percentile, and maximum energy and LET of the fragments present in the experiment.The data indicate that the LET comparison is the appropriate basis for prediction of the threshold. Using an empirically adjusted value of the LET required for nucleation, the calculated threshold agrees reasonably well with the data but becomes increasingly discrepant with increasing temperature. Reasons ...

<p>Deep Long Period (DLP) earthquakes have been observed in many volcanic regions and are often considered as one of the important precursors to volcanic eruptions. At the same time, the physics of the source of these earthquakes remains unclear. We focus our study on Klyuchevskoy group of volcanoes in Kamchatka, Russia, one of the World’s most active volcanic system. The DLP earthquakes in this region occur at the limit between the lower crust and the upper mantle at depths of 30-35 km where ductile flow is expected to dominate rock deformation. Their occurrence also appears to correlate with the eruptive activity. Therefore, this is natural to consider that their generating mechanism is not related to brittle mechanism but rather to pressure fluctuations in the magmatic system as often suggest for the LP seismicity in general. We suggest a possible generating mechanism related to the rapid pressure changes caused by nucleation and growth of gas bubbles in response to the slow decompression of over-saturated magma. The pressure variation is simulated using the mathematical model of bubble nucleation and growth accounting for multiple dissolved volatiles (H<sub>2</sub>O-CO<sub>2</sub>) and diffusive gas transfer from magma into growing bubbles. Results of simulations show that fast pressure increase followed by its relaxation almost to its initial level is not very sensitive to the assumptions on the values of governing parameters. Typical pressure changes of a few tens of MPa in a volume of 3500 m<sup>3</sup> occurring on time scales of fractions of a second to a second following bubble nucleation and growth can generate seismic waves with amplitudes similar to those recorded by seismographs in the vicinity of the Klyuchevskoy volcano.</p>