Micro-fabricated electrolytic micro-bubblers

Rectangular node electrodes were developed with micro-fabrication techniques in order to produce an electrolytic micro-bubbler which, while subjected to a convective flow, can generate bubbles of nearly uniform size with mean diameters of 50 μm or less. The devices were fabricated and placed in both a quiescent tap water chamber and a water channel operating at laminar flow rates. The effect of applied voltage and flow conditions on the bubbles generated was tested on single electrode pairs. Videos of the bubbles generated by the devices were taken from which the bubble sizes and generation rates were recorded.It was found that higher applied voltages coincided with smaller average bubble size, a narrower distribution of bubble sizes, higher bubble fluxes, and a higher current efficiency. The imposition of a hydrodynamic flow was shown to reduce average bubble size, narrow the range of bubble sizes, and reduce current efficiency. Smaller electrode nodes were seen to reduce the number of active nucleation sites and thus produced a narrower bubble size distribution. In the best case, the bubble distribution was nearly mono-disperse with a small average bubble diameter (ca. 40 μm) with repeatable mean bubble diameters and bubble fluxes. Intermediate values of the ratio of electrode node width and cathode/anode spacing provided the largest average bubble diameters.     

Some features of spray breakup in effervescent atomizers

The near orifice spray breakup at low GLR (gas to liquid ratio by mass) values in an effervescent atomizer is studied experimentally using water as a simulant and air as atomizing gas. From the visualizations, the near orifice spray structures are classified into three modes: discrete bubble explosions, continuous bubble explosions and annular conical spray. The breakup of the spray is quantified in terms of the mean bubble bursting distance from the orifice. The parametric study indicates that the mean bubble bursting distance mainly depends on airflow rate, jet diameter and mixture velocity. It is also observed that the jet diameter has a dominant effect on the bubble bursting distance when compared to mixture velocity at a given airflow rate. The mean bubble bursting distance is shown to be governed by a nondimensional two-phase flow number consisting of all the aforementioned parameters. The location of bubble bursting is found to be highly unsteady spatially, which is influenced by flow dynamics inside the injector. It is proposed that this unsteadiness in jet breakup length is a consequence of varying degree of bubble expansion caused due to the intermittent occurrence of single phase and two-phase flow inside the orifice.     

Insights in hydrodynamics of bubbling fluidized beds at elevated pressure by DEM–CFD approach

A numerical simulation was conducted to study the effect of pressure on bubble dynamics in a gas–solid fluidized bed. The gas flow was modeled using the continuum theory and the solid phase, by the discrete element method (DEM). To validate the simulation results, calculated local pressure fluctuations were compared with corresponding experimental data of 1-mm polyethylene particles. It was shown that the model successfully predicts the hydrodynamic features of the fluidized bed as observed in the experiments. Influence of pressure on bubble rise characteristics such as bubble rise path, bubble stability, average bubbles diameter and bubble velocity through the bed was investigated. The simulation results are in conformity with current hydrodynamic theories and concepts for fluidized beds at high pressures. The results show further that elevated pressure reduces bubble growth, velocity and stability and enhances bubble gyration through the bed, leading to change in bed flow structure.     

Formation of the finely dispersed gas phase in upward and downward fluid flows

The influence of the fluid and gas flow rate parameters on the diameter of bubbles separating from a single capillary immersed in a fluid flow in a channel of 1×1 cm cross-section is investigated. The dependences of the mean bubble diameter on the fluid velocity and the gas flow rate through the capillary are obtained. The frequency of the bubble detachment from the capillary is estimated. It is shown that the histogram of the bubble size distribution changes qualitatively with the excess of a certain critical bubble detachment frequency, which is associated with the transition from the discrete to jet mode of detachment.     

Characteristics of developing vertical bubbly flow under normal and microgravity conditions

The axial development of the void fraction profile, interfacial area concentration and Sauter mean bubble diameter of adiabatic nitrogen-water bubbly flows in a 9 mm-diameter pipe were measured using stereo image processing under normal and microgravity conditions. The flow measurements were performed at four axial locations (axial distance from the inlet, z normalized by the pipe diameter, D, z/D = 5, 20, 40 and 60) and with various flows: superficial gas velocity of 0.00840-0.0298 m/s, and superficial liquid velocity of 0.138-0.914 m/s. The effect of gravity on radial distribution of bubbles and the axial development of two-phase flow parameters is discussed in detail based on the obtained database and visual observation. Following Serizawa-Kataoka’s phase distribution pattern criteria under normal gravity conditions, the phase distribution pattern map was developed. Similar to normal gravity two-phase flows, wall, core and intermediate void peak patterns are observed under microgravity conditions but a transition void distribution pattern is not observed in the current experimental conditions. The data obtained in the current experiment are expected to contribute to the benchmarking of CFD simulation of phase distribution pattern and interfacial area concentration in forced convective pipe flow under microgravity conditions.     

Bubble cluster formation in shear-thinning inelastic bubbly columns

The mean rise velocity of bubble swarms ascending in shear-thinning fluids was experimentally measured in a rectangular bubble column. Great care was taken to produce nearly mono-dispersed bubble swarms and to use shear-thinning fluids with negligible elastic effects. In this manner, it was possible to isolate the effect of the hydrodynamic interaction between bubbles in the column caused by the thinning behavior of the liquid. It was found that the mean rise velocity of the bubbles was larger than that of an individual bubble, in accordance with previous studies. The magnitude of the swarm velocity was found to be greatly influenced by the appearance of bubble clusters. The bubble clusters, which appeared for certain values of the flow index and bubble diameter, were found to have a very different structure from those observed in Newtonian liquids. Furthermore, it was found that the appearance of clusters produced a dramatic increase of the agitation within the column. A set of conditions was identified for the appearance of bubble clusters in shear-thinning inelastic bubbly columns.     

Interfacial area concentration in gas–liquid bubbly to churn flow regimes in large diameter pipes

This study performed a survey on existing correlations for interfacial area concentration (IAC) prediction and collected an IAC experimental database of two-phase flows taken under various flow conditions in large diameter pipes. Although some of these existing correlations were developed by partly using the IAC databases taken in the low-void-fraction two-phase flows in large diameter pipes, no correlation can satisfactorily predict the IAC in the two-phase flows changing from bubbly, cap bubbly to churn flow in the collected database of large diameter pipes. So this study presented a systematic way to predict the IAC for the bubbly-to-churn flows in large diameter pipes by categorizing bubbles into two groups (group 1: spherical or distorted bubble, group 2: cap bubble). A correlation was developed to predict the group 1 void fraction by using the void fraction for all bubble. The group 1 bubble IAC and bubble diameter were modeled by using the key parameters such as group 1 void fraction and bubble Reynolds number based on the analysis of Hibiki and Ishii (2001, 2002) using one-dimensional bubble number density and interfacial area transport equations. The correlations of IAC and bubble diameter for group 2 cap bubbles were developed by taking into account the characteristics of the representative bubbles among the group 2 bubbles and the comparison between a newly-derived drift velocity correlation for large diameter pipes and the existing drift velocity correlation of Kataoka and Ishii (1987) for large diameter pipes. The predictions from the newly-developed two-group IAC correlation were compared with the collected experimental data in gas–liquid bubbly to churn flow regimes in large diameter pipes and their mean absolute relative deviations were obtained to be 28.1%, 54.4% and 29.6% for group 1, group 2 and all bubbles respectively.     

Pressure-oscillation defoaming for viscoelastic fluid

The enhancement of bubble rising velocity was experimentally investigated by mechanically applying an oscillating pressure to a single small air bubble (e.g., 1 mm³) in a viscoelastic fluid. For shear-thinning fluids, the cyclic change in bubble diameter induced by the oscillating pressure generates a continuous strong local flow near the bubble surface. Consequently, the apparent liquid viscosity is reduced and the bubble rising velocity increases by 400 times or more compared to the case without oscillating pressure. However, for a Newtonian fluid, almost no effect was observed with oscillating pressure. Time-series data of the longitudinal and horizontal bubble diameters were obtained experimentally using a stroboscope and a video system, and these data were used to estimate the local shear rate and the local shear viscosity. The increase in bubble rising velocity estimated from the shear viscosity behavior agreed well with the experimental data. Additionally, a periodic change in the bubble shape from a sphere at the maximum bubble size to a cusped shape at the minimum bubble size was observed under strong oscillatory pressure.     

Axial development of subcooled boiling flow in an internally heated annulus

For the main purpose of database construction in order to develop the interfacial area transport equation, axial developments of local void fraction, interfacial area concentration, bubble Sauter mean diameter, interfacial velocity, and bubble number density were measured in boiling water bubbly flows in a vertical-upward internally heated annulus using a double-sensor conductivity probe. The annulus channel consisted of an inner rod with a diameter of 19.1 mm and an outer round tube with an inner diameter of 38.1 mm, and the hydraulic equivalent diameter was 19.1 mm. A total of 11 data sets were acquired consisting of four inlet liquid velocities, 0.500, 0.664, 0.987 and 1.22 m/s, two heat fluxes, 100 and 150 kW/m², and two inlet liquid temperatures, 95.0 and 98.0°C. The axial developments of the flow parameters were discussed based on the measured data in detail. In addition to the database construction, the measured data validated recently proposed constitutive equations for the distribution parameter, drift velocity, and bubble Sauter mean diameter, which will improve the accuracy of the drift-flux model in subcooled bubbly flow.     

微通道人工泡状流的Lattice Boltzmann数值模拟

采用高频电控热激发汽泡的方式构造微通道人工泡状流,可以有效抑制微通道沸腾流动的不稳定性和强化传热。本文基于Lattice Boltzmann大密度比多相流复合模型,数值研究了通道内人工泡状流的流动和传热,通过比较分析不同发泡频率的泡状流,量化分析了汽泡运动和增长对微通道流动与传热的相互影响。一方面着重分析了汽泡运动对微通道运动边界层以及汽泡相变增长对热边界层的影响,另一方面也研究了边界层对汽泡动力行为的影响,所得结论对研究抑制微通道沸腾流动不稳定性和强化传热有参考意义。     

The minimum in-line coalescence height of bubbles in non-Newtonian fluid

The minimum in-line coalescence height of bubbles generated from a submerged nozzle was investigated experimentally in shear thinning non-Newtonian fluid at lower Reynolds number (2∼60). Carboxymethyl cellulose sodium (CMC) aqueous solution and carbon dioxide were used as the liquid phase and the gas phase, respectively. The process of the formation, movement and in-line coalescence of bubbles was visualized and recorded by a high-speed digital camera. The influences of bubble size, bubble generation frequency and liquid property on the minimum in-line coalescence height of bubbles were investigated by changing nozzle diameter, gas flow rate and the mass concentration of CMC aqueous solutions. For a given liquid, the generating frequency and size of bubbles increased but the minimum coalescence height of in-line bubbles decreased when the nozzle diameter and gas flow rate were increased. When the nozzle diameter and gas flow rate were fixed, the shear-thinning effect of CMC aqueous solution became stronger with increasing CMC mass concentration, which led to the increase in both the terminal rise velocity and average acceleration of the trailing bubble, consequently, the minimum in-line coalescence height of bubbles decreased. An empirical correlation for estimating the minimum in-line bubble coalescence height was proposed, the calculating values accords well with experimental data with a mean relative deviation only 7.6%.     

Evaluation of flow patterns and elongated bubble characteristics during the flow boiling of halocarbon refrigerants in a micro-scale channel

In the present study, quasi-diabatic two-phase flow pattern visualizations and measurements of elongated bubble velocity, frequency and length were performed. The tests were run for R134a and R245fa evaporating in a stainless steel tube with diameter of 2.32 mm, mass velocities ranging from 50 to 600 kg/m² s and saturation temperatures of 22 °C, 31 °C and 41 °C. The tube was heated by applying a direct DC current to its surface. Images from a high-speed video-camera (8000 frames/s) obtained through a transparent tube just downstream the heated sections were used to identify the following flow patterns: bubbly, elongated bubbles, churn and annular flows. The visualized flow patterns were compared against the predictions provided by Barnea et al. (1983) [1], Felcar et al. (2007) [10], Revellin and Thome (2007) [3] and Ong and Thome (2009) [11]. From this comparison, it was found that the methods proposed by Felcar et al. (2007) [10] and Ong and Thome (2009) [1] predicted relatively well the present database. Additionally, elongated bubble velocities, frequencies and lengths were determined based on the analysis of high-speed videos. Results suggested that the elongated bubble velocity depends on mass velocity, vapor quality and saturation temperature. The bubble velocity increases with increasing mass velocity and vapor quality and decreases with increasing saturation temperature. Additionally, bubble velocity was correlated as linear functions of the two-phase superficial velocity.     

Slug and bubble flows in a flat sheet ultrafiltration module: Experiments and numerical simulation

In this paper, the effect of gas bubbling including slug and bubble flows on enhancing shear force in an ultrafiltration (UF) process in a flat sheet module is investigated experimentally by image processing and numerically using the OpenFOAM software. In order to study characteristics of bubbles in the slug and bubble flows, the flat sheet module is analyzed by a video system facilitated with a high speed camera. The experimental results show that the average diameter of the slug flow is much larger than that of the bubble flow. The permeate flux for the slug and bubble flows is increased by 78% and 30%, respectively, compared to the case with no gas bubbling. The numerical results are shown to be in good agreement with those of the measurements both qualitatively and quantitatively. The results of simulations also demonstrate that although both flow patterns increase the shear stress by increasing the velocity gradient and/or vorticity, the shear stress induced by the slug flow is considerably larger. Therefore, the slug flow with a higher induced shear stress is more effective on the enhancement of the permeate flux in a UF process.     

Effect of gas expansion on the velocity of a Taylor bubble: PIV measurements

The effect of gas expansion on the velocity of a Taylor bubble was studied experimentally. The velocity field in the liquid ahead of a Taylor bubble was measured by particle image velocimetry (PIV), and the bubble velocity was measured with two pairs of laser diodes and photocells. The experiments were done in a 7.0 m long vertical tube with a 32 mm internal diameter. Solutions of carboxymethylcellulose (CMC) polymer with weight percentages between 0.01% and 0.1% were used. The expansion of slug gas induces an increase in the bubble velocity and a corresponding displacement of the liquid ahead of the bubble. The velocity of the bubble increases by an amount equal to the maximum velocity in the liquid displaced. For the solutions studied, the induced velocity profile was parabolic and the bubble velocity increase was equal to the liquid velocity at the tube axis, i.e., twice the mean velocity in the liquid displaced. The corrected velocity obtained by subtracting the velocity increase from the value of the bubble velocity is independent of the bubble length.     

Effects of bubble size on heat transfer enhancement by sub-millimeter bubbles for laminar natural convection along a vertical plate

Injection of sub-millimeter bubbles is considered a promising technique for enhancing natural convection heat transfer for liquids. So far, we have experimentally investigated heat transfer characteristics of laminar natural convection flows with sub-millimeter bubbles. However, the effects of the bubble size on the heat transfer have not yet been understood. The purpose of this study is to clarify the effects of the bubble size on the heat transfer enhancement for the laminar natural convection of water along a vertical heated plate with uniform heat flux. Temperature and velocity measurements, in which thermocouples and a particle tracking velocimetry technique are, respectively used, are conducted to investigate heat transfer and flow characteristics for different bubble sizes. Moreover, two-dimensional numerical simulations are performed to comprehensively understand the effects of bubble injection on the flow near the heated plate. The result shows that the ratio of the heat transfer coefficient with sub-millimeter-bubble injection to that without injection ranges from 1.3 to 2.2. The result also shows that for a constant bubble flow rate, the heat transfer coefficient ratio increases with a decrease in the mean bubble diameter. It is expected from our estimation based on both experimental data and simulation results that this increase results from an increase in the advection effect due to bubbles.     

Elongated bubbles in microchannels. Part I: Experimental study and modeling of elongated bubble velocity

The velocity of elongated vapor bubbles exiting two horizontal micro-evaporator channels with refrigerant R-134a was studied. Experiments with tube diameters of 509 and 790 μm, mass velocities from 200 to 1500 kg/m² s, vapor qualities from 2% to 19% and a nominal saturation temperature of 30 °C were analyzed with a fast, high-definition digital video camera. It was found from image processing of numerous videos that the elongated bubble velocity relative to that of homogeneous flow increased with increasing bubble length until a plateau was reached, and also increased with increasing channel diameter and increasing mass velocity. Furthermore an analytical model developed for a diabatic two-phase flow, has been proposed that is able to predict these trends. In addition, the model shows that the relative elongated bubble velocity should decrease with increasing pressure, which is consistent with the physics of two-phase flow.     

Modeling of bubble behaviors and size distribution in a slab continuous casting mold

Population balance equations combined with Eulerian–Eulerian two-phase model are employed to predict the polydispersed bubbly flow inside the slab continuous-casting mold. The class method, realized by the MUltiple-SIze- Group (MUSIG) model, alongside with suitable bubble breakage and coalescence kernels is adopted. A two-way momentum transfer mechanism model combines the bubble induced turbulence model and various interfacial forces including drag, lift, virtual mass, wall lubrication, and turbulent dispersion are incorporated in the model. A 1/4th scaled water model of the slab continuous-casting mold was built to measure and investigate the bubble behavior and size distribution. A high speed video system was used to visualize the bubble behavior, and a digital image processing technique was used to measure the mean bubble diameter along the width of the mold. Predictions by previous mono-size model and MUSIG model are compared and validated against experimental data obtained from the water model. Effects of the water flow rate and gas flow rate on the mean bubble size were also investigated. Close agreements by MUSIG model were achieved for the gas volume fraction, liquid flow pattern, bubble breakage and coalescence, and local bubble Sauter mean diameter against observations and measurements of water model experiments.     

Numerical simulation of an axisymmetric separated and reattached flow over a longitudinal blunt circular cylinder

This article presents a numerical investigation of turbulent flow in an axisymmetric separated and reattached flow over a longitudinal blunt circular cylinder. The governing equations were discretized by the finite-volume method and SIMPLER method was applied to solve the equations on a staggered grid. The turbulent flow was numerically simulated using the standard k–ε, Abe–Kondoh–Nagano (AKN) and Shear Stress Transport (SST) turbulence models. The comparisons made between numerical results and experimental measurements showed that the SST model is superior to other models in the present calculation.Computations were performed for three different Reynolds numbers of 6000, 10 000 and 20 000 based on the cylinder diameter. To our knowledge, this study represents the first numerical investigation of the present flow configuration. The computational results were validated with the available experimental data of reattachment length, mean velocity distribution and wall static pressure coefficient in the turbulent blunt circular cylinder flows. Further, other characteristics of the flow, such as turbulent kinetic energy, pressure, streamlines, and the velocity vectors are discussed.The results show that the main characteristics of the turbulence flow in the separation region, such as reattachment length or velocity profiles, are nearly independent of the Reynolds number. The obtained results showed that a secondary separation bubble may appear in the main separation bubble near the leading edge. Furthermore, it was found that the turbulent kinetic energy has a large effect on the formation of the secondary bubble.     

Interfacial structure in an air–water planar bubble jet

The objective of the current study is to better understand the interfacial structure and its development in an air–water planar bubble jet, as well as to provide a unique benchmark data set for a 3D thermal-hydraulic analysis code. Both flow visualization and local measurements were performed in three characteristic flow conditions at four elevations along a test section with a cross section of 200 mm in width and 10 mm in gap. A high-speed digital video camera was applied in the flow visualization study to capture the flow structures and bubble interaction phenomena, while a miniaturized four-sensor conductivity probe was used to acquire the time-averaged local void fraction, interfacial velocity, and bubble number frequency. Also, the interfacial area concentration and the averaged bubble Sauter mean diameter were obtained from the local measurements. The lateral bubble transport and bubble interaction mechanisms were clearly demonstrated in the acquired data.     

Two-phase flow patterns of a top heat mode closed loop oscillating heat pipe with check valves (THMCLOHP/CV)

This research is aimed at studying the two-phase flow pattern of a top heat mode closed loop oscillating heat pipe with check valves. The working fluids used are ethanol and R141b and R11 coolants with a filling ratio of 50% of the total volume. It is found that the maximum heat flux occurs for the R11 coolant used as the working fluid in the case with the inner diameter of 1.8 mm, inclination angle of ?90?, evaporator temperature of 125?C, and evaporator length of 50 mm. The internal flow patterns are found to be slug flow/disperse bubble flow/annular flow, slug flow/disperse bubble flow/churn flow, slug flow/bubble flow/annular flow, slug flow/disperse bubble flow, bubble flow/annular flow, and slug flow/annular flow.