Bubble Eruption Research Articles

Fluctuations of concentration overpotential generated at gas-evolving electrodes

The electrode overpotential is usually expressed in terms of partial components related to ohmic, activation and concentration effects. For a gas-evolving electrode, the overpotential fluctuates because of bubble evolution but only the global overpotential fluctuations and, more recently, their ohmic part are easily accessible by routine experiments. In the present paper, high-amplitude fluctuations of concentration overpotential have been experimentally revealed by an anomalous behaviour of the current fluctuations induced by hydrogen evolution on platinum electrodes under potential control in alkaline media with or without the presence of a second mass-transport-controlled reaction. Inverted current jumps, reproducibly observed owing to an artificial bubble-nucleation site created on the edge of the rotating electrode, which provoked periodic bubble eruption, were ascribed to local concentration changes drastically affecting the current-lines distribution. These experimental results validate the theoretical models that predicted those anomalies but remained unverified up to now. A quantitative estimation of each overpotential component at bubble eruption is addressed by means of a simplified analytical approach supported by the simultaneous measurement of the electrochemical current noise and electrolyte resistance fluctuations.

Initial particle velocity spatial distribution from 2-D erupting bubbles in fluidized beds

Some results on particle image velocimetry (PIV) in 2-D freely bubbling fluidized beds are presented. The PIV applications were used in order to determine the initial particle velocity of bubble eruptions. A two-dimensional non-reacting fluidized bed was constructed to measure the origin of the ejected particles and the initial particle velocity distribution, using coarse sand particles. The bubble ejection mechanism was observed taking into account the origin of particles ejected, the initial particle velocity distributions as well as the effect of other neighbor exploding bubbles. Our results show that the assumption of linear dependence of initial velocity with the angle predicts the velocity faithfully only for purely vertical-ascent bubbles. Measurements of ejection velocities show that initial velocities in the combined layer are higher than those of the particles in the nose of the leading bubble. Avoiding coalescence of bubbles at the bed surface can lead to less particle entrainment out of the bed and consequently to shorter fluidized beds.

Hydrodynamics of gas–solid flow around immersed tubes in bubbling fluidized beds

A numerical study was conducted based on the gas–solid two-fluid model using the body-fitted coordinate system to analyze the behavior of particles and bubbles flow in bubbling fluidized beds without and with immersed tubes. The kinetic theory of granular flow was implemented in the model. The images of simulated instantaneous particle concentration and velocity gave the process of the formation, coalescence and eruption of bubbles. The effects of the tube pitch and superficial gas velocity on the fluidization in a bubbling fluidized bed were investigated. Calculated bubble frequencies without and with immersed tubes were in agreement with previous experimental and simulation findings. The wavelet multi-resolution analysis was used to analyze the simulated data of instantaneous particle concentration. From the random-like particle concentration fluctuations, the fluctuating components due to particle flow and bubble motion can be extracted based on the wavelet multi-resolution analysis over a time-frequency plane.

Pressure fluctuations in gas-solids fluidized beds

Pressure fluctuation data measured in a series of fluidized beds with diameters of 0.05, 0.1, 0.29, 0.60 and 1.56 m showed that the maximum amplitude or standard deviation increased with increasing the superficial gas velocity and static bed height for relatively shallow beds and became insensitive to the increase in static bed height in relatively deep beds. The amplitude appeared to be less dependent on the measurement location in the dense bed. Predictions based on bubble passage, bubble eruption at the upper bed surface and bed oscillation all failed to explain all observed trends and underestimated the amplitude of pressure fluctuations, suggesting that the global pressure fluctuations in gas-solids bubbling fluidized beds are the superposition of local pressure variations, bed oscillations and pressure waves generated from the bubble formation in the distributor region, bubble coalescence during their rise and bubble eruption at the upper bed surface.

Scanning electron microscopic observations of dough and bread supplemented with Gastrodia elata Blume powder

Dough and bread prepared from wheat flour containing varying amounts of added Gastrodia elata Blume (GEB) rhizome powder [0, 0.5, 1.0, 1.5, and 2.0% (w/w)] were examined by scanning electron microscopy (SEM) during fermentation and baking. The structure of the doughs containing added GEB was found to be related to the protein matrix. Further, it was found that large starch granules and strings of small starch granules play an important role in dough structure. The control dough (no added GEB) had a membrane-like structure, and doughs with 0.5–1.0% added GEB had membrane-like structures that were more developed than those of the control, resulting in increased bread volumes. At 1.5–2.0% GEB levels, however, the doughs tended to have mesh-like structures and result in decreased bread volumes. The dough samples with 0.5 and 1.0% added GEB powder had well-developed gluten matrices with evenly dispersed starch granules. These samples resulted in breads with numerous gas bubble eruptions on their surfaces and consequently in larger loaf volumes than were obtained at other levels of GEB. After the second fermentation, many expanded starch granules were observed and these starch granules were dispersed more evenly than after the first fermentation. In 0.5–1.0% GEB bread, many of the large starch granules had expanded after fermentation, but small starch granules had not. The data obtained in this study suggest that bread baked with 0.5–1.0% GEB exhibits a better loaf volume due to the more complete development of a gluten matrix.

Cold model characterisation of a fluidised bed catalytic reactor by means of instantaneous pressure measurements

The analysis of instantaneous pressure fluctuations is utilised in a cold modelling study of a pilot scale fluidised bed catalytic reactor for ethane oxychlorination. It is demonstrated that the fluid dynamic behaviour of a cold model of this fine particle (Geldart group A) system is well characterised by the frequency spectrum, the standard deviation and the cross-correlation of pressure signals detected at different locations within the bed and in the freeboard. The influence of operating parameters, such as gas flow rate, initial bed height, reactor configuration (internals) and average value of the local pressure, is investigated, and a comparison is made with the features exhibited by Geldart group B systems (fluidised bed combustors) for which the same diagnostic techniques have been applied. Finally, the applicability to fine particle suspensions of literature correlations for dominant frequency and propagation velocity of pressure waves is discussed with reference to the combined influence of the eruption of many small bubbles at the bed surface, and the compressibility of the gas contained within the particle-bed interstices.

Particle pressures generated around bubbles in gas-fluidized beds

The particle pressure is the surface force in a particle/fluid mixture that is exerted solely by the particle phase. This paper presents measurements of the particle pressure on the faces of a two-dimensional gas-fluidized bed and gives insight into the mechanisms by which bubbles generate particle pressure. The particle pressure is measured by a specially designed ‘particle pressure transducer’. The results show that, around single bubbles, the most significant particle pressures are generated below and to the sides of the bubble and that these particle pressures steadily increase and reach a maximum value at bubble eruption. The dominant mechanism appears to be defluidization of material in the particle phase that results from the bubble attracting fluidizing gas away from the surrounding material; the surrounding material is no longer supported by the gas flow and can only be supported across interparticle contacts which results in the observed particle pressures. The contribution of particle motion to particle pressure generation is insignificant.The magnitude of the particle pressure below a single bubble in a gas-fluidized bed depends on the bubble size and the density of the solid particles, as might be expected as the amount of gas attracted by the bubble should increase with bubble size and because the weight of defluidized material depends on the density of the solid material. A simple scaling of these quantities is suggested that is otherwise independent of the bed material.In freely bubbling gas-fluidized beds the particle pressures generated behave differently. Overall they are smaller in magnitude and reach their maximum value soon after the bubble passes instead of at eruption. In this situation, it appears that the bubbles interact with one another in such a way that the de uidization effect below a leading bubble is largely counteracted by refluidizing gas exiting the roof of trailing bubbles.

Characteristics of gas mixing in a circulating fluidised bed

Horizontal gas dispersion is studied by tracer-gas measurements and smoke visualisation in the riser of a 1/9-scale model of the Chalmers' circulating fluidised-bed (CFB) boiler, with complementary measurements in the boiler. The model was designed and operated according to fluid dynamic scaling-laws. The results are based on the time-average horizontal dispersion of gas, dynamics of the measured concentration of tracer gas and smoke visualisation of the gas flow. The fluidisation velocity in the boiler ranged from 1.2 to 4.3 m/s and the bed material was silica sand with an average particle diameter of 0.3 mm. When the cold scale-model was operated to resemble the conditions of the tracer-gas measurements in the full-scale boiler, the results from the two units were similar (scale model operated with a fluidisation velocity of 0.4 to 1.4 m/s and with 0.06-mm bronze particles). The scale model also covers conditions that cannot be reached in a full-scale boiler. For instance, changes in the characteristics of the gas mixing during a gradual increase of particle concentration in the riser, starting from an empty riser, could be studied. The gas flow is characterised by a combination of small-scale motion (widening of a gas plume) and large-scale motion (a meandering motion of the whole plume). Both types of motion contribute to the mixing of gas. The large-scale motion in the transport zone is linked to the bubble eruptions in the bottom bed and the horizontal dispersion is higher in the bottom- and splash zones than in the transport zone. The dispersion rate is higher at low gas velocities than at high velocities because of a great contribution from the large-scale motion generated by bubbles at low particle circulation rates.

Insights into the instantaneous freeboard flow above a bubbling fluidised bed

The freeboard of a fluidised bed reactor, that is, the space above the bed material where the fluidising gas disengages from solids before leaving the fluidisation vessel, can be a place where unexpected additional reaction can take place. In this study particle image velocimetry (PIV), implemented by image shifting, has been used to provide instantaneous gas velocity vector maps of the central vertical plane of a bubbling fluidised bed freeboard in order to study the flow pattern in a cold model of a fluidised bed used for polymer recycling. The velocity vector maps obtained show that the freeboard gas flow is characterised by the presence of eddies and vortices generated by bubble eruption at the bed surface.

Circulating particle flow and air bubble behavior at various superficial air velocities in two-dimensional gas–solid fluidized beds

Circulating particle flow and behavior of air bubbles in a two-dimensional gas-solid fluidized bed of various superficial air velocities are investigated by recording videos of movement of a plastic pellet put into the fluidized bed and rising air bubbles using a video camera. The movement velocity of the plastic pellet and properties of the air bubbles such as the bubble rising velocity and the bubble distribution coefficient, which shows the proportion of the bubbles erupting at the center of the bed surface, are measured by analyzing the videos. It is found that the plastic pellet moves following the circulating particle flow; the particles rise up at the center of a column and fall down near the side walls, and that the movement velocity increases with the superficial air velocity. The bubble rising velocity, the apparent erupting bubble size and the bubble distribution coefficient increase, and the bubble eruption frequency slightly decreases, with the superficial air velocity. These results indicate that the circulating particle flow is generated by the rising air bubbles. In particular, the fact that the air bubbles rise at the center of the column and coalesce with other bubbles is closely related to the generation of the circulating particle flow.

A study of bubbling and slugging fluidised beds using the two-fluid granular temperature model

This paper reports finite element numerical simulations of gas–solid fluidised beds using the two-fluid granular temperature model. The aim of the study has been to investigate the various phenomena that have been observed in fluidised beds but have not been subject to numerical investigation. Two fluidised beds, operating in the slugging and bubbling regimes, were modelled, and the formation, elongation, coalescence and eruption of bubbles described. The effect of an obstruction on the fluidisation efficiency in a fluidised bed was investigated. Granular temperature distribution inside the fluidised bed provided an indication of the regions on the obstruction, which could be susceptible to erosion by particle impact.

Study of the flow pattern above an erupting bubble in an incipiently fluidised bed using image shifting

The gas flow in the freeboard of a fluidised bed is strongly dependent on bubble eruption at the bed surface. Here, the gas flow above an erupting bubble has been studied by the injection of single bubbles in an incipiently fluidised bed. Gas velocity vector maps of the vertical central plane above the bed surface, i.e. a plane parallel to the direction of the overall flow, have been determined using image shifting (an implementation of particle image velocimetry). These vector maps are presented and discussed. A mechanism has been proposed to describe the process of single-bubble eruption.

Investigation of the bubble behaviour at the free surface of a large three-dimensional gas fluidized bed

Gas fluidization is generally associated with the formation of bubbles that critically influence the performance of fluidized bed processes (FBPs). Therefore, in the design, simulation and operation of FBPs, it is very essential to know the behaviour of the bubbles at the free surface. The size and growth of bubbles play an important role for determining properties such as bed expansion, solids entraiment, in-bed heat transfer and solid mixing. This paper presents a study on the behaviour of bubbles at the free surface of a large three dimensional gas-fluidized bed with square section of 61×61 cm2. Measurements were carried out to determine the effects of bed height and excess air velocity on the bubble eruption diameter, frequency and bubble fraction. All experiments were performed at freely bubbling mode and the flow characteristics of bubbles were recorded by a video camera. Bed materials used were 593 μm raw perlite and 1233 μm sand falling within the categories of Geldarts Groups B and D, respectively. The fixed bed height ranged from about 8–18 cm for raw perlite and 9–26 cm for sand. The excess air velocity was varied between 0·5 and 1·75 cm s−1 for raw perlite and 13 and 25 cm s−1 for sand. Equations related to the bubble count, frequency, flow area shape factor and through-flow coefficient were given using a modified form of two-phase theory of fluidisation. Observations were made to validate the two-phase theory for two different particles. The flow area shape factor was in the range of 0·47–0·81 for raw perlite and 0·20 to 0·57 for sand, with mean values of 0·6 and 0·4, respectively. The through-flow coefficient was found to be between −0·68 and 2·82 for raw perlite and between 3·27 to 15·87 for sand, and was larger than predicted values of classical bubble models. © 1998 John Wiley & Sons, Ltd.

Origin, propagation and attenuation of pressure waves in gas—solid fluidized beds

Recently reported results on the origin, propagation and attenuation of pressure waves in bubbling gas—solid fluidized beds are re-evaluated and the results are compared with additional experiments reported here. It is found that the measured pressure fluctuations are a result of slow and fast propagating pressure waves. Pressure waves with high propagation velocities (> 10 m/s) are unambiguously identified as compression waves, which move upwards and downwards. Upward moving compression waves originate from gas bubble formation and gas bubble coalescence. The amplitude of upward moving pressure waves is linearly dependent on the distance to the bed surface. Downward moving compression waves are caused by gas bubble eruptions at the fluidized bed surface, bubble coalescence and by changes in bed voidage. In this case the pressure wave amplitude is independent of the distance to the bed surface. Pressure waves with propagation velocities of less than 2 m/s are caused by rising gas bubbles. These pressure waves move upwards only, with an amplitude proportional to the bubble size. The average wave propagation velocity measured in a freely bubbling bed is lower than that predicted from the pseudo-homogeneous compressible wave theory owing to the presence of slowly rising gas bubbles. The average propagation velocity of pressure waves in a gas—solid circulating fluidized bed is adequately described as a function of local voidage by pseudo-homogeneous compressible wave theory. At low voidages in the bottom of the riser, the propagation velocity is lowered by the presence of gas bubbles or large gas voids.

Prediction of the particle flow conditions in the freeboard of a freely bubbling fluidized bed

The density and flux of particles in the freeboard of a freely bubbling fluidized bed is a function of the size, aspect ratio and frequency of bubbles erupting at the surface of the dense bed. To predict the velocity and amount of material thrown into the freeboard, a previous model for the eruption of a single bubble (Yule and Glicksman, 1989, Proceedings Engineering Foundation International Fluidization Conference, Canada) was reformulated to deal with the eruption of a multiple bubbles undergoing coalescence at the bed surface. The solution includes the explicit mechanics of particle elements in the nose of the leading bubble as well as the particle elements in the wakes of bubbles trapped between coalescing bubbles. The motive force for the particle acceleration is the excess gas flow through the cavity at the bed surface. The gas flow, in turn is found from a numerical solution spanning the bed from the upper surface to the distributor. The solutions for a single erupting bubble, two vertically aligned bubbles, and three vertically aligned bubbles were obtained numerically. The predicted vertical mass flux of particles decreases exponentially with height. The smooth fall off of the predicted flux with height above the bed is due to the consideration of separate particle elements which have different initial conditions at different points around the nose of the bubble. The solution for the erupting single sphere agrees with the measurements of George and Grace (1978, A.I.Ch.E.Symp. Ser. 74). For a freely bubbling bed, the frequency of bubble eruptions and the probability of coalescence at the bed surface can be obtained from a statistical model of the bubbling process which has been used before to predict accurately the expansion of bubbling beds (Glicksman et al., 1991, Chem. Engng Sci. 46, 1561–1571). Combining the statistical model with the predictions for single eruptions results in a prediction of the flux and density distributions in the freeboard. Measured density distributions above a freely bubbling bed, containing horizontal tubes, are in general agreement with the model predictions. The distributions are found to be primarily controlled by the bubble size at the bed surface, the volume fraction of bubbles in the bed and the excess gas velocity U U mf . The predicted exponential coefficient for density and mass flux vs height is consistent with the correlation of experimental results given by Kunii and Levenspiel (1990, Powder Technol. 61, 193).

Investigation on the vertical division of flow regions in a gas-solidfluidized bed by means of coherence function

A new method has been developed for better discrimination between flow regions in the vertical direction in gas-solid fluidized bed using the coherence functions of the pressure fluctuation signals. The experimental results show that the three different regions in the dense phase, i.e. inlet region, region of relatively stable bubble size and bubble eruption region can be distinguished clearly by this method. The effects of operating conditions, particle properties and static bed height on the division between these regions are also investigated.

On Hydrogen Transport and Edge Plasma Modeling of Liquid-Metal Divertors

The steady-state operational conditions for large tokamaks impose high performance requirements that cast suspicion on the employment of conventional solid surface divertors. Flowing liquid-metal divertors are thus being considered as an alternative. A preliminary evaluation is made of some aspects of this concept. To understand the hydrogen (i.e., deuterons and tritons) recycling behavior in liquid metals, the transport and chemistry aspects of the hydrogen/liquid-metal interaction are investigated, including hydrogen gettering, tritium inventory, and blistering caused by hydrogen bubble eruptions. It is shown that when operating in the high-recycling model (i.e., as the liquid metal is filled with deuterons and tritons, one implanting ion will immediately cause the emission of one neutral particle), lithium would have a large tritium inventory. Gallium, on the other hand, does not have the same problem because of its negligible hydrogen solubility and the decomposition of its hydrides in the temperature range of interest. However, to avoid blistering, the flow speed of a gallium neutralizer has to be high. An edge plasma simulation model is briefly introduced, and its outcome for the high-recycling liquid-metal (lithium and gallium) divertors is presented. This model gives more realistic predictions of plasma temperature than some of the existing simulation models.more » Results of this model show that denser and cooler edge plasmas can be achieved by liquid-metal divertors than by conventional stationary surface divertors. Evaporation and sputtering of liquid-metal divertors are shown not to be as serious a problem as might be suspected at first glance. 22 refs., 2 figs., 1 tab.« less

A gas and particle flow model in the freeboard of a fluidized bed based on bubble coalescence

A new model was developed to predict gas and particle velocity profiles in the freeboard of a bubbling fluidized bed under the consideration of bubble eruption mechanism at the dense bed surface. The bubble eruption mechanisms, namely single- and multiple-bubble eruptions, were considered to be the major factors which influenced the initial gas and particle velocities and consequently affected the flow field in the freeboard. A model based on the bubble through-flow velocity due to bubble eruption and consequent gas puffs was obtained to describe the gas velocity. A Lagrangian approach based on the interaction between the gas puffs or ‘ghost bubbles’ and ejected particles had been established to describe the particle motion in the freeboard. The results showed that the particle velocity profile in the freeboard had a two-level decay trend due to different bubble eruption mechanisms with a very good agreement for the gas puff velocity between the model and published data.

Frequency analysis of pressure fluctuation in fluidized bed plenum.

Pressure fluctuations in the plenum chamber of a solid-gas fluidized bed were measured and analyzed in order to develop a method for the diagnosis of fluidization conditions.Power spectral density functions obtained by FFT analysis of the pressure fluctuation data had three principal frequencies. These frequencies were compared with the bubble eruption frequency observed at the bed surface and the bubble generation frequency, which was obtained by an optic fiber probe installed above the distributor opening, and two of the three frequencies were found to agree well respectively with the obtained frequencies of bubble generation and of eruption. The third principal frequency was judged to be a natural frequency of the fluidized bed, since the frequency depended on the particle content in the fluidized bed and on the plenum volume. The frequency corresponding to bubble generation was observed to be influenced significantly by the natural frequency of the fluidized bed when the natural frequency had a low value.

Measurement of pressure fluctuations in two-dimensional gas-solid fluidized beds at elevated temperatures.

Pressure fluctuations were measured in a two-dimensional gas-solid fluidized bed at temperatures ranging from 13 to 270°C. The random pressure fluctuation signals were studied on-line by the statistical method. The results indicate that the frequency spectrum, major frequency, and mean pressure amplitude depend considerably on the gas excess velocity and temperature. A probability density function of pressure fluctuations with two peaks was observed in the upper portion of the bed, indicating bubble eruption behavior at room temperature. The two peaks in the p.d.f. are transformed into a single peak at higher temperature, and the location of maximum amplitude also changes at higher temperature. Both measurements of pressure drop fluctuations and differential pressure fluctuations were used in this study, and the latter were found to be superior in the study of bubble behavior.