Periodic bubble emission and appearance of an ordered bubble sequence (train) during condensation in a single microchannel

Condensation of steam in a single microchannel, silicon test section was investigated visually at low flow rates. The microchannel was rectangular in cross-section with a depth of 30 μm, a width of 800 μm and a length of 5.0 mm, covered with a Pyrex glass to allow for visualization of the bubble formation process. By varying the cooling rate during condensation of the saturated water vapor, it was possible to control the shape, size and frequency of the bubbles formed. At low cooling rates using only natural air convection from the ambient environment, the flow pattern in the microchannel consisted of a nearly stable elongated bubble attached upstream (near the inlet) that pinched off into a train of elliptical bubbles downstream of the elongated bubble. It was observed that these elliptical bubbles were emitted periodically from the tip of the elongated bubble at a high frequency, with smaller size than the channel width. The shape of the emitted bubbles underwent modifications shortly after their generation until finally becoming a stable vertical ellipse, maintaining its shape and size as it flowed downstream at a constant speed. These periodically emitted elliptical bubbles thus formed an ordered bubble sequence (train). At higher cooling rates using chilled water in a copper heat sink attached to the test section, the bubble formation frequency increased significantly while the bubble size decreased, all the while forming a perfect bubble train flowing downstream of the microchannel. The emitted bubbles in this case immediately formed into a circular shape without any further modification after their separation from the elongated bubble upstream. The present study suggests that a method for controlling the size and generation frequency of microbubbles could be so developed, which may be of interest for microfluidic applications. The breakup of the elongated bubble is caused by the large Weber number at the tip of the elongated bubble induced by the maximum vapor velocity at the centerline of the microchannel inside the elongated bubble and the smaller surface tension force of water at the tip of the elongated bubble.     

Numerical study on the formation of Taylor bubbles in capillary tubes

Numerical simulations have been carried out for the transient formation of Taylor bubbles in a nozzle/tube co-flow arrangement by solving the unsteady, incompressible Navier–Stokes equations. A level set method was used to track the two-phase interface. The calculated bubble size, shape, liquid film thickness, bubble length, drift velocity, pressure drop and flow fields of Taylor flow agree well with the literature data. For a given nozzle/tube configuration, the formation of Taylor bubbles is found to be mainly dependent on the relative magnitude of gas and liquid superficial velocity. However, even under the same liquid and gas superficial velocities, the change of nozzle geometry alone can drastically change the size of Taylor bubbles and the pressure drop behavior inside a given capillary. This indicates that the widely used flow pattern map presented in terms of liquid and gas superficial velocities is not unique.     

低排放燃烧室空气雾化喷嘴射流破碎特性研究

针对当前广泛应用于低排放燃气轮机燃烧室中的空气雾化喷嘴,采用大涡模拟（Large Eddy Simulation,LES）和流体体积法（Volume of Fluid,VOF）研究了其在流动模糊（Flow Blurring,FB）和流动聚焦（Flow Focusing,FF）模式下射流一次破碎过程的差异。结果表明：两种模式的射流一次破碎过程均可分为3个阶段,气液交界面波动阶段、射流发展阶段和射流破碎阶段;喷嘴内部回流区的演变决定了气液交界面的波动程度,流动模糊模式下射流在后两个阶段的径向速度和形态变化程度均远高于流动聚焦模式,气泡回流过程在其射流破碎阶段占据主导地位,液体管道内气泡分布位置与涡的强度呈正相关。     

环状出口气泡雾化喷嘴液膜破碎过程与喷雾特性

主要研究了环状出口气泡雾化喷嘴出口下游液膜破碎过程与喷雾特性．当气液质量流量比为零时，出口下游形成空心封闭膜壳，表面波明显存在于液膜表面．随着气液质量流量比的增加，膜壳逐渐膨胀并在最薄弱部位被撕裂．利用DualPDA测量得到液雾颗粒的速度分布、直径分布与流通量分布特性；在主流区域存在负向运动的粒子同时颗粒平均速度明显降低．出口下游的速度分布曲线呈现双峰趋势，实验数据显示中心回流区域结束于距离出口30mm左右．索特平均直径的有关数据显示气泡的“爆炸”发生于出口下游5～15mm区域．流通量分布曲线也是双峰的，径向逐渐扩张，轴向逐渐降低，并且液雾主流区域流通量明显高于边缘区域的流通量．     

Resident time of a compound drop impinging on a hot surface

The resident time of a water-in-diesel compound drop impinging on a hot surface at a temperature higher than the Leidenfrost temperature was investigated experimentally. Past experimental evidence suggested that the resident time of a pure liquid drop was independent of the impact velocity. And this independency could also be seen for compound drops. For both pure drops and compound drops, the resident time became longer with increasing outer diameter of the drop. For water-in-diesel compound drops of a given outer diameter, the resident time decreased as the volume of the core water drop increased. By using a modified Weber number which took into account of the two interfaces of the compound drop, a correlation of the non-dimensional resident time was obtained and was in good agreement with the experimental data.     

Bubble generation in quiescent and co-flowing liquids

A numerical simulation has been accomplished to analyze the problem of dynamic bubble formation from a submerged orifice in an immiscible Newtonian liquid under the condition of constant gas inflow. We have considered two cases for the surrounding liquid, namely the liquid in a quiescent condition and the liquid as a co-flowing stream with the gas. The full cycle, from formation to detachment of the bubbles and the corresponding bubble dynamics, was simulated numerically by using a coupled level-set and volume-of-fluid (CLSVOF) method. The role of the liquid to gas mean velocity ratio, the Bond number and the Weber number in the bubble formation process was studied and the order of magnitude of forces involved in bubble dynamics are presented. Our simulation results show that the minimum radius of the neck decreases with a power law behavior and the power law exponent in a co-flowing liquid is less than 1/2 as predicted by the Rayleigh–Plesset theory for quiescent inviscid liquids. Single periodic and double periodic bubbling (with pairing and coalescence) regimes are observed in the present investigation. It is identified that a moderate co-flowing liquid may inhibit the bubble coalescence. The volume of the bubble and the bubble formation time decrease with increasing liquid to gas mean velocity ratios. For small Bond numbers, significant differences pertaining to bubble dynamics are observed between the co-flowing liquid and the quiescent liquid. Furthermore, the generation and breakup of the Worthington jet after bubble pinch-off and formation of tiny drops inside the detached bubbles are observed.     

Characteristics of interfacial profiles in upward and downward gas-liquid two-phase plug flow

Gas-liquid interfacial profiles in plug flow for both upward and downward flows were obtained using semi-supermultiple point-electrode probes, comprising 67 sensing tips arranged on a tube diameter. Typical interfacial profiles are demonstrated for both flows. Close inspection of the profiles reveals that four zones exist in a pair of gas and liquid slugs for upward plug flow and a high slip velocity region in downward plug flow. The lengths of the swelling liquid front zone and the wake zone were determined. The length of the wake zone strongly depends on the relative velocity between the liquid film around the gas slug and the liquid phase in the liquid slug. Characteristic distributions of bubbles within liquid slugs were found, i.e., three types of radial distributions of void fraction, namely saddle-shaped, trapezoidal and bullet-shaped distributions, exist for upward flow. The two types for downward flow exclude the saddle-shaped distribution. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25 (8): 568–579, 1996     

Computational Fluid Dynamics–Population Balance Modeling of Gas–Liquid Two-Phase Flow in Bubble Column Reactors With an Improved Breakup Kernel Accounting for Bubble Shape Variations

Abstract

Traditionally, bubble shapes have been assumed to be spherical in breakup models such as the one developed by Luo and Svendsen in 1996. This particular breakup model has been widely accepted and implemented into computational fluid dynamics (CFD) modeling of gas–liquid two-phase flows. However, simulation results from this model usually provide unreliable predictions about the breakage of very small bubbles. The incorporation of bubble shape variation into breakup models has rarely been documented in literature but the bubble shape plays an important role in the interactions with the surrounding eddies, especially when the effects of bubble deformation, distortion, and bubble internal pressure change are considered during the events of eddy-bubble collision. Thus, the assumption of spherical bubbles seems to be no longer appropriate in reflecting this phenomenon. This study proposes and implements a modified bubble breakup model, which accounts for the variation of bubble shapes when solving the population balance equations for CFD simulation of gas–liquid two-phase flows in bubble columns. The key parameters predicted by the modified breakup model have been compared with the ones predicted by the original model. The simulation results of interfacial area and mass transfer coefficient for larger bubbles have been greatly enhanced by the modified breakup model.     

Temperature fluctuation under bubbles of pool boiling in decompression

Experiments on pool boiling of water were performed under decompression. Temperature fluctuation was measured with three thermocouples which were equipped on the copper block surface by plating. Rapid temperature drops in this fluctuation arise at the same times. This shows the existence of evaporation of the microlayer under bubbles. Evaluation of this evaporation of the microlayer and rapid temperature drops revealed the existence of a new evaporation factor in obedience to the liquid surface overheating. Temperature drops are of two types at decompression. One is rapid drop due to cavity existence and the other is slow drop due to generation in the superheated bulk liquid on the mirror surface. This rapid temperature drop is gradual at the first stage. For verification of gradual drops, we introduced an equation concerning micro film thickness under the bubble on the basis of the viscosity and the preservation of momentum. This equation has an exponential factor for microfilms. Numerical calculations showed good agreement with the experimental data. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(7): 567–581, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10102     

A study of the influence of initial liquid volume on the capillary flow in an interior corner under microgravity

In this work the influence of initial liquid volume on the capillary flow in an interior corner is studied systematically by microgravity experiments using the drop tower, under three different conditions: the Concus–Finn condition is satisfied, close to and dissatisfied. The capillary flow is studied by discussing the movement of tip of the meniscus in the corner. Experimental results show that with the increase of initial liquid volume the tip location increases for a given microgravity time, the achievable maximum tip velocity increases and the flow reaches its maximum tip velocity earlier. However, the results for the three different conditions show some difference.     

Structure and temperature distribution of a stagnation-point Diesel spray premixed flame

We experimentally examine the flow and flame characteristics of a stagnation point premixed flame influenced by Diesel sprays. In the experiment, distributions of drop size, drop axial velocity and its fluctuation as well as the gas phase temperature are measured by using the phase-doppler particle analyzer and a thin thermocouple. As might be expected, similar to the gasoline spray flame, the partially prevaporized Diesel spray flame is composed of a weak blue flame zone, indicating the burning of methane fuel, and a strongly luminous zone containing many bright yellow lines showing the passages of burning Diesel drops. It is found that the axial temperature profiles at various radial positions consist of an upstream preheat region, a maximum temperature downstream of the blue flame and a downstream region with a declined temperature curve because of the heat loss to the quartz plate. The SMD of the drops increases from the upstream preheat region to a maximum near the blue flame and then decreases in the downstream burning zone. Along the axial position, the drops are decelerated in front of the flame but accelerated when passing through the blue flame. It is also interesting to note that the radial distributions of SMD and number density of drops in the upstream region are mainly influenced by small drops flowing outward, since the upstream vaporization of Diesel drops is very limited; while those in the downstream region should be influenced by both small drops flowing outward and Diesel drops burning. From the experimental observations, there are impinging and bouncing of Diesel drops downstream of the spray flame near the quartz plate, resulting in a small amount of soot and carbon deposits on the wall. These interesting phenomena will be reported in the near future.     

Microscale Contacting of Two Immiscible Liquid Droplets to Measure Interfacial Tension

In various microfluidic devices, surface tension and interfacial tension values are necessary to analyze the fluid behavior in microchannels, and evaluating the values of interfacial tension is especially important for gas–liquid and liquid–liquid flows. A pendant drop method is commonly used to measure the interfacial tension value; however, the pendant drop method requires strict accuracy in measuring the droplet size when the droplet has a nonspherical shape, as well as an accurate value of the density difference between the two liquids. In this work, a new measurement method called the “liquid bridge-inducing microscale contact method” has been developed in which the interfacial tension can be obtained from the bridging of two liquid droplets extruded from opposing ends of glass capillary tubes or formed on the ends of round metal rods. By measuring the radii of curvature of each liquid surface and interface, we calculate the Laplace pressure on the surface and interface, and derive the interfacial tension value using the Laplace equation. The results show that the values of interfacial tension obtained from the two methods are approximately the same and that the liquid bridge-inducing microscale contact method is capable of accurate interfacial tension measurements.     

高剪切力下气泡聚并与破碎特性的数值研究

了解气泡在剪切力场中的聚并与破碎机理及生长与运动特性,对于气液搅拌槽中桨叶的优化设计具有重要意义。将欧拉-欧拉模型与群体平衡模型(population balance model, PBM)进行耦合,对不同剪切力下的气液两相流场进行求解,研究了剪切力、高剪切力下进气速度、气泡塔高度对气泡聚并与破碎的影响,并结合气泡的聚并与破碎模型对高剪切力下气泡的聚并与破碎机理进行了探究。研究表明,剪切力主要影响气泡的破碎,当剪切力较小时其对气泡破碎的影响较小,随着剪切力的增大其对气泡破碎的影响逐渐显著,使小气泡的含量大幅增多;高剪切力下进气速度、气泡塔高度对气泡的聚并与破碎的影响不明显。     

The role of the gas diffusion layer on slug formation in gas flow channels of fuel cells

Water drops emerge from large pores of the hydrophobic Gas Diffusion Layers (GDL) into the cathode gas flow channel of Polymer Electrolyte Membrane (PEM) Fuel Cells. The drops grow into slugs that span the cross-section of the flow channels. The slugs detach and are forced out the gas flow channel by the air flow. An acrylic micro-fluidic flow cell with a 1.6 mm gas flow channel and a 100 μm liquid pore through a carbon paper GDL has been used to quantitatively determine slug volumes, velocity of slug motion, and the force required to move slugs as functions of the gas and liquid flow rates. In a channel with 4 acrylic walls, slugs detach immediately upon formation. A porous GDL wall allows gas flow to bypass the slugs, thus allowing slugs to continue to grow after spanning the open area of the channel. The differential pressure to detach and move slugs is equal to the dynamic interfacial force on a slug normalized by the cross-sectional area of the channel. The dynamic interfacial force is equal to the difference between the downstream (advancing) and upstream (receding) contact lines of the water with the channel walls. Slugs will stop moving if the differential pressure drop for gas flow to bypass the slug and flow through the GDL under the rib separating the channels is less than the differential pressure required to move the slug. The results improve our physical insight into the state of water hold up in PEM fuel cells.     

Mixing, transport and combustion in sprays

Recent advances concerning analysis of sprays and drop/turbulence interactions are reviewed. Consideration is given to dilute sprays and related dilute dispersed flows, which contain well-defined dispersed-phase elements (e.g. spherical drops) and have dispersed-phase volume fractions less than 1%; and to the near-injector, dense spray region, having irregularly-shaped liquid elements and relatively-high liquid fractions.

Early analysis of dilute sprays and other dispersed flows assumed either locally-homogeneous flow (LHF), implying infinitely-fast interphase transport rates, or deterministic separated flow (DSF) where finite interphase transport rates are considered, but interactions between dispersed-phase elements and turbulence are ignored. These limits are useful in some instances; however, recent evidence shows that both methods are deficient for quantitative estimates of the structure of most practical dispersed flows, including sprays. As a result, stochastic separated flow (SSF) methods have been developed, which treat both finite interphase transport rates and dispersed phase (drop)/turbulence interactions using random-walk computations for the dispersed phase. Evaluation of SSF methods for particle-laden jets; nonevaporating, evaporating and combusting sprays; and noncondensing and condensing bubbly jets has been encouraging, suggesting capabilities of current SSF methods to treat a variety of interphase processes. However, current methods are relatively ad hoc and many fundamental problems must still be resolved for dilute flows, e.g. effects of anisotropic turbulence, modification of continuous-phase turbulence properties by the dispersed phase (turbulence modulation), effects of turbulence on interphase transport rates, and drop shattering, among others.

Dense sprays have received less attention and are poorly understood due to substantial theoretical and experimental difficulties, e.g. the idealization of spherical drops is not realistic, effects of liquid breakup and collisions are difficult to describe, spatial resolution is limited and the flow is opaque to optical diagnostics which have been helpful for studies of dilute sprays. Limited progress thus far, however, suggests that LHF analysis may provide a useful first-approximation of the structure and mixing properties of dense sprays near pressure-atomizing injectors. Since dense-spray processes fix initial conditions needed to rationally analyze dilute sprays, more research is this area is clearly warranted.     

An investigation of two-phase flow pressure drop in helical rectangular channel

Numerical simulations have been carried out to evaluate the two-phase frictional pressure drop for air-water two-phase flow in horizontal helical rectangular channels by varying configurations, inlet velocity and inlet sectional liquid holdup. The investigations performed using eight coils, five different inlet velocity and four different inlet sectional liquid holdups. The effects of curvature, torsion, fluid velocity and inlet sectional liquid holdup on two-phase frictional pressure drop have been illustrated. It is found that the two-phase frictional pressure drop relates strongly to the superficial velocities of air or water, and that the curvature and torsion have some effect on the pressure drop for higher Reynolds number flows in large-scale helical rectangular channel; the inlet sectional liquid holdup only increases the magnitude of pressure drop in helical channel and has no effect on the development of pressure drop. The correlation developed predicts the two-phase frictional pressure drop in helical rectangular channel with acceptable statistical accuracy.     

Visualization of a principle mechanism of critical heat flux in pool boiling

A new experimental work was made to discover a principle mechanism of the burnout in pool boiling. Here, we directly observed a liquid layer structure under a massive vapor clot and the liquid layer-related burnout phenomenon. Based on the present observations, we have made a visual model for the formation and dryout of a liquid film under its vapor environment. At the formation process, liquid is trapped in interleaved space between growing bubbles and surface and the liquid trapping continues between coalesced bubbles and surface. In the dryout process, we especially observed vapor “holes” made by spontaneous breakup of discrete nucleating bubbles inside a vapor clot. The burnout can be triggered by the evaporation of the liquid film region expanded from rims of the holes.     

Influence of interfacial mass transfer and chemical reaction on breakup of low-solubility fluid jets in water

In this study, we report our experimental investigation on the influence of interfacial mass transfer and chemical reaction on breakup patterns of a liquid CO₂ jet in water. It was found in our experiments that when liquid CO₂ was injected into high-pressure and low-temperature water, the jet breakup resulted in a train of irregular cylinders. This cylinder-like breakup pattern is distinct from those of usual liquid-jet breakup, i.e. jet-breakup produces either a train of single uniform-sized drops or multiple non-uniform-sized drops. This unusual jet-breakup pattern was found to result from mass transfer and chemical reaction at the interface of the jet and the ambient water. The patterns of jet breakup found in this study also can occur in many other low-solubility fluid jets in water if molecular sizes of the jet fluids are in the range between 3.8 and 6.6 angstrom.     

用激光全息术研究轴针式喷油嘴的早期喷雾特性

作者利用同轴全息照相术研究了ZS4S1轴针式喷油嘴的早期喷雾机理和油滴空间分布。作者在常温、常压、无涡流条件下,拍摄了不同喷油压力下油雾场的全息照片。通过再现全息片并对已微粒化油滴的直径大小和数目进行判读后发现,在早期喷雾中,油注的头部区域就发生破碎;中心液注则一直持续到外围完全微粒化。本研究对加深喷雾过程的理解和建立轴针式喷油嘴精确的喷雾模型具有一定的价值。     

Liquid jet in a subsonic gaseous crossflow: Recent progress and remaining challenges

This article reviews published literature on the characteristics of a liquid jet injected transversally into a subsonic gaseous crossflow. The review covers the following aspects: (і) liquid jet primary breakup regimes, (іі) liquid jet trajectory and penetration, (ііі) liquid jet breakup length, and (іv) droplets features and formation mechanisms. The focus is on analyzing the role of different prominent parameters which include gaseous and liquid properties, and liquid injector geometry. The review revealed that gas Weber number plays a crucial role in defining non-turbulent primary breakup regimes, while liquid jet Weber number is of great importance for the transition to turbulent primary breakup. Jet-to-crossflow momentum flux ratio is the most important parameter for predicting the trajectory, penetration, and breakup length of a liquid jet in a crossflow. The characteristics of droplets disintegrated during the primary breakup are mostly influenced by the nozzle exit conditions, whereas the characteristics of droplets produced via the secondary breakup are strongly dependent on the velocity of cross airflow. Although the review revealed that substantial progress has been made in understanding this complex two-phase flow phenomenon, there still remain several shortcomings which require further research.