Bubble Growth Behavior Research Articles

풀 핵비등시 단일 기포 성장에 대한 벽면 과열도의 영향에 관한 연구

Nucleate pool boiling experiments for R11 under a constant wall temperature condition were carried out. A microscale heater array was used for the heating and the measurement of high temporal and spatial resolution by the Wheatstone bridge circuit. Very sensitive heat flow rate data were obtained by the control for the surface condition with high time resolution. The measured heat flow rate shows a discernable peak at the initial growth stage and reaches an almost constant value. In the thermal growth region, bubble shows a growth proportional to . The bubble growth behavior is analyzed with a dimensionless parameter to compare with the previous results in the same scale. As the wall superheat increases, the departure diameter and the departure time increase, and the waiting time decreases. But the asymptotic growth rate is not affected by the wall superheat change. The effect of the wall superheat is resolved into the suggested growth equation. Dimensionless parameters of time and bubble radius characterize the thermal growth behavior well, irrespective of wall condition. The comparison between the result of this study and the previous results shows a good agreement at the thermal growth region. The quantitative analysis for the heat transfer mechanism is conducted with the measured heat flow rate behavior and the bubble growth behavior. The required heat flow rate for the volume change of the observed bubble is about twice as much as the instantaneous heat flow rate supplied from the wall.

냉매 이성분 혼합물의 포화 풀핵비등 특성에 관한 연구

Saturated nucleate pool boiling experiments for binary mixtures, which are consisted of refrigerant R11 and R113, were performed with constant wall temperature condition. Results for binary mixtures were also compared with pure fluids. A microscale heater array and Wheatstone bridge circuits were used to maintain the constant temperature of the heating surface and to obtain heat flow rate measurements with high temporal and spatial resolutions. Bubble growth images were captured using a high speed CCD camera synchronized with the heat flow rate measurements. The departure time for binary mixtures was longer than that for pure fluids, and binary mixtures had a higher onset of nucleate boiling (ONB) temperature than pure fluids. In the asymptotic growth region, the bubble growth rate was proportional to a value between and . The bubble growth behavior was analyzed to permit comparisons with binary mixtures and pure fluids at the same scale using dimensionless parameters. There was no discernable difference in the bubble growth behavior between binary mixtures and pure fluids for a given ONB temperature. And the departure radius and time were well predicted within a error. The minimum heat transfer coefficient of binary mixtures occurred near the maximum value, and the average required heat flux during bubble growth did not depend on the mass fraction of R11 as more volatile component in binary mixtures. Finally, the results showed that for binary mixtures, a higher ONB temperature had the greatest effect on reducing the heat transfer coefficient.

포화상태 풀비등시 단일기포의 성장에 관한 연구

Nucleate boiling experiments on heating surface of constant wall temperature were performed using R113 for almost saturated pool boiling conditions. A microscale heater army and Wheatstone bridge circuits were used to maintain a constant wall temperature condition of heating surface and to measure the heat flow rate With high temporal and spatial resolutions Bubble images during the bubble growth were taken as 5000 frames per second using a high-speed CCD camera synchronized With the heat flow rate measurements The bubble growth behavior was analyzed using the new dimensionless parameters for each growth regions to permit comparisons With previous experimental results at the same scale. We found that the new dimensionless parameters can describe the whole growth region as initial and later (thermal) respectively The comparisons showed good agreement in the initial and thermal growth regions. In the initial growth region<br/> including surface tension controlled, transition and inertia controlled regions as divided by Robinson and Judd,<br/> the bubble growth rate showed that the bubble radius was proportional to t2/3 regardless of working fluids and heating conditions. And ill the thermal growth region as also called asymptotic region, the bubble showed a growth rate that was proportional to t1/5, also. Those growth rates were slower than the growth rates proposed ill previous analytical analyses The requited heat flow rate for the volume change of the observed bubble was estimated to be larger than the heat flow rate measured at the wall. Heat, which is different from the Instantaneous heat supplied through the heating wall, can be estimated as being transferred through the interface between bubble and liquid even With saturated pool condition. Tins phenomenon under a saturated pool condition needs to be analyzed and the data from this study can supply the good experimental data With the precise boundary condition (constant wall temperature).<br/>

Single bubble growth in saturated pool boiling of binary mixtures

Nucleate pool boiling experiments for binary mixtures, which are consisted of R11 and R113, were performed with constant wall temperature condition. Results for binary mixtures were also compared with pure fluids. A microscale heater array and Wheatstone bridge circuits were used to maintain the constant temperature of the heating surface and to obtain heat flow rate measurements with high temporal and spatial resolutions. Bubble growth images were captured using a high-speed CCD camera synchronized with the heat flow rate measurements. The departure time for binary mixtures was longer than that for pure fluids, and binary mixtures had a higher onset of nucleate boiling (ONB) temperature than pure fluids. In the asymptotic growth region, the bubble growth rate was proportional to a value between t 1/6 and t 1/4. The bubble growth behavior was analyzed to permit comparisons with binary mixtures and pure fluids at the same scale using dimensionless parameters. There was no discernible difference in the bubble growth behavior between binary mixtures and pure fluids for a given ONB temperature. And the departure radius and time were well predicted within a ±30% error. The minimum heat transfer coefficient of binary mixtures occurred near the maximum | y− x| value, and the average required heat flux during bubble growth did not depend on the mass fraction of R11 as more volatile component in binary mixtures. Finally, the results showed that for binary mixtures, a higher ONB temperature had the greatest effect on reducing the heat transfer coefficient.

Visual observation of CO2 foaming of polypropylene‐clay nanocomposites

AbstractUsing a newly developed high‐pressure autoclave, which has two sapphire windows on the walls, we visually observed the batch physical foaming of polymer‐clay nanocomposites to understand the effect of nano‐sized clay on the initial stage of foaming. With CO2 as a physical foaming agent, polypropylene‐montmorillonite clay nanocomposites were foamed at 150°C. A high‐speed digital camera with a microscope could observe the bubble nucleation and bubble growth behavior of the early stage of foaming in situ. The series of micrographs was analyzed in order to investigate the effect of clay content on bubble nucleation and growth. The experiments, together with CO2‐solubility and diffusivity data, show that the clay enhances bubble nucleation as a nucleation agent and retards the growth of bubbles at the early stage of foaming. Polym. Eng. Sci. 44:1004–1011, 2004. © 2004 Society of Plastics Engineers.

Single bubble growth in saturated pool boiling on a constant wall temperature surface

Nucleate pool boiling experiments with constant wall temperatures were performed using R11 and R113 for saturated pool boiling conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain a constant wall temperature condition and to obtain measurements with high temporal and spatial resolution. Accurate heat flow rate data were obtained from microscale heater array by controlling the surface conditions at a high temporal resolution. Images of the bubble growth were captured using a high-speed CCD camera synchronized with the heat flow rate measurements. The geometry of the bubble was obtained from the images. In the asymptotic growth region, the bubble showed a growth rate that was proportional to t 1/5, which was slower than the growth rate proposed in previous analytical analyses. The bubble growth behavior was analyzed using a new dimensionless parameter to permit comparisons with previous results at the same scale. The comparisons showed good agreement in the asymptotic growth region. A non-dimensional correlation for the bubble radius that can predict the bubble growth and the heat flow rate simultaneously, was suggested. The required heat flow rate for the volume change of the observed bubble was estimated to be larger than the instantaneous heat flow rate measured from the wall. Heat, other than the instantaneous heat supplied from the wall, is estimated to be transferred through the interface between bubble and liquid, even with saturated pool conditions. This phenomenon under a saturated pool condition needs to be analyzed and the data from this study can supply the good experimental data with the precise boundary condition (constant wall temperature).

Partial Nucleate Boiling on the Microscale Heater Maintaining Constant Wall Temperature

Nucleate pool boiling experiments for pure R11 with a constant wall temperature condition were carried out. A microscale heater array and Wheatstone bridge circuit were used to maintain a constant wall temperature condition and to obtain measurements with high temporal and spatial resolution. The images of bubble during the growth period were captured by the high speed CCD camera synchronized with heat flow rate measurement. The bubble shape immediately after inception was an oblate spheroid with a small contact angle. The geometrical shape permitted a large amount of heat to be supplied from the wall. In the asymptotic growth region, the bubble shape is almost truncated sphere. The measured heat flow rate showed a discernable peak in the initial growth region, and reached almost constant value. A quantitative analysis for the heat transfer mechanism was performed using the measured heat flow rate and bubble growth behavior for saturated pool boiling conditions. The required heat flow rate for the volume change of the observed bubble was larger than the instantaneous heat flow rate supplied from the wall.

An experimental study on resonance of oscillating air/vapor bubbles in water using a two-frequency acoustic apparatus

A two-frequency acoustic apparatus is employed to study the growth behavior of vapor-saturated bubbles driven in a volumetric mode. A unique feature of the apparatus is its capability of trapping a bubble by an ultrasonic standing wave while independently driving it into oscillations by a second lower-frequency acoustic wave. It is observed that the growing vapor bubbles exhibit a periodic shape transition between the volumetric and shape modes due to resonant coupling. In order to explain this observation, we performed an experimental investigation on resonant coupling of air bubbles and obtained the following results: First, the induced shape oscillations are actually a mixed mode that contains the volume component, thus, vapor bubbles can grow while they exhibit shape oscillations. Second, the acoustically levitated bubbles are deformed and therefore, degeneracy in resonant frequency is partially removed. As a result, the vapor bubbles exhibit the shape oscillations in both the axisymmetric mode and asymmetric (three-dimensional) modes. Nonlinear effects in addition to the frequency shift and split due to deformation creates overlapping of the coupling ranges for different modes, which leads to the continuous shape oscillations above a certain bubble radius as the bubble grows.

Visual Observations of Batch and Continuous Foaming Processes

The visual observations of batch and continuous foaming processes were conducted to understand the bubble nucleation and bubble growth behaviors in polymers. The batch foaming was performed using a newly developed high-pressure cell, where two sapphire windows were equipped on the walls so as to observe the early stage of bubble nucleation and growth behaviors with the help of a high-speed digital camera and microscope. In the batch process, homo polypropylene was foamed at different pressure release rates using CO2 as a physical blowing agent to see the effects of operating condition on the bubble nucleation and growth rates. The in situ observation could identify that (1) the bubble nucleation and growth occur simultaneously, (2) the influence region, where the nucleation was suppressed, exists around bubbles, and (3) the nucleation rate and the growth rate increase as the pressure release rate increases. The continuous foaming was performed using a different visualization unit, which consists of an autoclave and an extrusion slit-die with quartz windows. It was found that the nucleation mechanism in the continuous foaming, i.e., extrusion foaming, was different from that of batch foaming. In the continuous extrusion foaming, the nucleation could be induced by flow and/or shear stress, and by cavitations brought by the surface roughness of the wall.

Modeling Diffusion-Induced Bubble Growth in Polymer Liquids

Accurate modeling of diffusion-induced bubble growth is essential for the development of efficient polymer foaming processes. Consequently, a large number of transport models of this complex phenomena have been formulated. In most previous studies, one or more simplifying approximations have been invoked to reduce mathematical complexity. In this paper, we present and compare several models of bubble growth in liquids and examine the effects blowing agent concentration, liquid viscosity and elasticity on bubble growth dynamics. In addition, we compare predicted and measured bubble growth behavior in two polymer foaming systems.

Low-temperature irradiation behavior of uranium–molybdenum alloy dispersion fuel

Irradiation tests have been conducted to evaluate the performance of a series of high-density uranium–molybdenum (U–Mo) alloy, aluminum matrix dispersion fuels. Fuel plates incorporating alloys with molybdenum content in the range of 4–10 wt% were tested. Two irradiation test vehicles were used to irradiate low-enrichment fuels to approximately 40 and 70 at.% 235U burnup in the advanced test reactor at fuel temperatures of approximately 65 °C. The fuel particles used to fabricate dispersion specimens for most of the test were produced by generating filings from a cast rod. In general, fuels with molybdenum contents of 6 wt% or more showed stable in-reactor fission gas behavior, exhibiting a distribution of small, stable gas bubbles. Fuel particle swelling was moderate and decreased with increasing alloy content. Fuel particles with a molybdenum content of 4 wt% performed poorly, exhibiting extensive fuel–matrix interaction and the growth of relatively large fission gas bubbles. Fuel particles with 4 or 6 wt% molybdenum reacted more rapidly with the aluminum matrix than those with higher-alloy content. Fuel particles produced by an atomization process were also included in the test to determine the effect of fuel particle morphology and microstructure on fuel performance for the U–10Mo composition. Both of the U–10Mo fuel particle types exhibited good irradiation performance, but showed visible differences in fission gas bubble nucleation and growth behavior.

VISUALIZATION OF BOILING PHENOMENA IN A BEAD-PACKED STRUCTURE

A visual study was conducted on the boiling behavior, especially the dry-out and rewetting process, in a three-dimensional porous structure made of staggered glass beads. The bead-packed structure was filled with water, and heated from below. Using a high-speed video imaging system, pore-scale bubble growth behavior in the bead-packed structure was observed. In the pore-scale view, it was found that the narrow-gap corner between a glass bead and the heated wall is a dual-effect area that is a very active wetting area as well as an active bubble-generation area. Replenishment that plays a very important role on the dry-out and rewetting process is a unique characteristic in the bead-packed structure, and also the liquid supply for wetting effect of the narrow-gap corner zone.

Mechanical response of sediments to bubble growth

Modeling the process of bubble growth in sediments requires an understanding of the physics that controls bubble shape and the interaction of the growing bubble with the sediment. To acquire this understanding we have conducted experiments in which we have injected gas through a fine capillary into natural and surrogate sediment samples and have monitored pressure during bubble growth to provide information about stress and strain. In gas injection studies with natural sediment samples, we have observed two modes of bubble growth behavior. One of these modes, characterized by a saw-tooth record of pressure as the bubble grows, is consistent with fracture of the medium. Observations indicate that bubble growth by fracture should correspond to bubbles that are coin- or disk-shaped. This shape is confirmed in observations of bubbles in natural sediments and in our studies of bubble injection into gelatin, a surrogate sediment material. Interpretation of the stress–strain results for bubble growth also required that we measure Young’s modulus, E. The measurements show E to be near 0.14 MN m 2, which differs by more than 4 orders of magnitude from values that have been reported in the literature. Our measurements of E give substantially better estimates of bubble shape than are predicted using the literature values. Our data are interpreted with linear elastic fracture mechanics (LEFM) which predicts that the critical pressure for bubble growth will depend on the bubble volume, V raised to the −1/5 power. While evidence of substantial heterogeneity in sediment properties is apparent in our results, this V −1/5 dependence is confirmed. Through application of LEFM theory, we have determined the critical stress intensity factor, K 1c, a material property and the principal determinant of bubble shape and growth by fracture. Our values of K 1c range from ∼2.8×10 −4 MN m −3/2 to ∼4.9×10 −4 MN m −3/2 for our natural sediment samples from Cole Harbor, Nova Scotia. We have also estimated the critical stress intensity factor for Eckernförde Bay samples by analyzing published images of natural bubbles. The K 1c obtained in this way is similar to our Cole Harbor results and is ∼5.5×10 −4 MN m −3/2.

Polymeric Foaming Simulation for Extrusion Processes

A numerical simulation for polymeric foaming extrusion processes was conducted. Combining classical nucleation rate and bubble growth models with a non-Newtonian fluid model of a flow, a simultaneous bubble nucleation and growth behavior in a flow field was simulated. Simulation results were compared with the experimental data obtained by visual observations at a foaming extruder, where a polypropylene resin was physically foamed. The effects of physical parameters in foaming model on bubble size and number density calculation were intensively examined by sensitivity analysis.

907 減圧下での加熱面上で発生する気泡の挙動とそのまわりの流れの可視化

907 減圧下での加熱面上で発生する気泡の挙動とそのまわりの流れの可視化

Growth of gas bubbles in the foam extrusion process

In comparison to other kinds of crosslinked thermoset foam, recyclable polyolefin foam is an environmentally friendly material, but it increases the degree of foaming in the foam extrusion process. In order to overcome this shortcoming, the effect of material and process variables on the degree of foaming and the growth behavior of the gas bubbles was studied by using polyolefins, such as polypropylene (PP), low density polyethylene (LDPE), and high density polyethylene (HDPE). These materials were foamed in extrusion process with a blowing agent consisting of sodium bicarbonate and citric acid. A higher degree of foaming was obtained with a low temperature of cooling water and high melt temperature. The take-up speed did not influence the degree of foaming. It was also observed from cell morphologies that the size of the gas bubbles was reduced with increased take-up speed and decreased melt temperature, but the size of the gas bubbles was nearly invariant as the temperature of the cooling water increased. In addition, the effects of the molecular weight, the molecular structure, the melt viscosity, and the melt tension on the degree of foaming and the growth of the gas bubbles were investigated. It was revealed that the degree of foaming and the size of the gas bubbles were directly related to the melt tension of the polymer melt, regardless of the structure and the molecular weight of the polymer resin and the temperature of the polymer melt. This implies that the ability to sustain the cell in the melt state is a critical foaming parameter in the extrusion process. © 2000 John Wiley & Sons, Inc. Adv Polym Techn 19: 97–112, 2000

Numerical investigation of bubble coalescence characteristics under nucleate boiling condition by a lattice-Boltzmann model

A numerical study was performed to investigate the characteristics of bubble growth, detachment and coalescence on vertical, horizontal, and inclined downward-facing surfaces. The FlowLab code, which is based on a lattice-Boltzmann model of two-phase flows, was employed. Macroscopic properties, such as surface tension ( σ) and contact angle ( β), were implemented through the fluid–fluid ( G σ ) and fluid–solid ( G t ) interaction potentials. The model predicted a linear relationship between the macroscopic properties ( σ, β) and microscopic parameters ( G σ , G t ). The simulation results on bubble departure diameter appear to have the same parametric dependence as the empirical correlation. Hydrodynamic aspects of bubble coalescence are investigated by simulating the growth and detachment behavior of multiple bubbles generated on horizontal, vertical, and inclined downward-facing surfaces. For the case of horizontal surface, three distinct regimes of bubble coalescence were represented in the lattice-Boltzmann simulation: lateral coalescence of bubbles situated on the surface; vertical coalescence of bubbles detached in a sequence from a site; and lateral coalescence of bubbles, detached from the surface. Multiple coalescence was predicted on the vertical surface as the bubble detached from a lower elevation merges with the bubble forming on a higher site. The bubble behavior on the inclined downward-facing surface was represented quite similar to that in the nucleate boiling regime on a downward-facing surface.

212 局所的に加熱された面から離脱する気泡の挙動と流れの可視化

212 局所的に加熱された面から離脱する気泡の挙動と流れの可視化

45 単一気泡及び気泡合体挙動の可視化

In order to clarify the phenomena of bubble growth and coalescence, we studied the growth and release behavior of bubbles formed at a nozzle tip, the coalascence of two bubbles formed at a dual-nozzle tip, the coalescene of many bubbles in a liquid, and the flow behavior of bubble groups. We also studied the conditions at the onset of bubble coalescence in terms of a hypothesis, derived from the earlier observation results, that bubble coalescence is strongly affected by the degree of swing in the paths of the bubbles.

Rates of shrinkage and growth of air bubbles entrained in wheat flour dough.

The rates of shrinkage at constant temperature, and growth under a temperature rise below 100°C, of bubbles entrained in wheat flour dough were analyzed and compared with those of a bubble in water. The rate of shrinkage of bubbles in flour dough was controlled by the diffusion of dissolved air from the surface of bubbles to the bulk of flour dough. The apparent diffusion coefficient of the dissolved air in wheat flour dough with the water fraction of 0.49 calculated from the shrinkage of bubbles, was (3.2 ± 1.5) × 101−1 m2/sec (19°C), and (6.4 ± 2.0) × 10−11 m2/sec (42°C). However, the growth behavior of bubbles in flour dough under a temperature rise was very different from that predicted from the diffusion theory. The critical radius of bubbles to grow was larger than that estimated from the diffusion theory. The mechanism of growth of bubbles in wheat flour dough, which was different from that of a bubble in water, is a subject that needs to be clarified.