気液二相流

Methods of measurement and data obtained for drop sizes and velocity in annular gas/liquid flow have been reviewed. In particular, possible sources of bias of bias in the results have been examined. The interrelation of data from different sources is considered and regions where different behaviour predominates are identified.

From the passage geometry, noncircular channel can be divided into two categories-a single channel and a multiple channel consisting of subchannels. This exposition covers some of the principal properties of both single-phase and two-phase flows in these channels. Frictional pressure drop, bubble rising velocity, void fraction and flow distributions to each subchannel are considered.

A review is made of the various models and basic equations of gas-liquid two-phase flows which are indispensable for accurate analyses of the phenomena. As the most detailed model, a local instantaneous representation of mass, momentum and energy conservation is shown. Then, averaging procedures for the various physical quantities and their derivatives of two-phase flow are described with a particular attention to the existence of the interface. Based on the local instantaneous formulation and averaging methods, the averaged formulations of basic equations of two-phase flow are stated. Two-types of formulations based on mixture models and two-fluid models are shown. The mixture model is further divided into homogeneous model, a slip model and a drift flux model depending upon the treatment of velocity differences between phases. Approximations, assumptions and limitations of applications of the basic equations based on each model are described.

Recently, many studies on annular mist flows have been reported. Most of them focus on a flow which has been fully developed or has reached its equilibrium state. On the flow in the non-equiliblium state, however, there are only a very few systematic studies. But two-phase flow phenemena in actual industrial devices are often in non-equilibrium. Therefore it is necessary to accumulate and analyze the fundamental data on the flow in non-equilibrium, in order to improve the efficiency of instruments connected with a two-phase flow, and to assure the generality and validity for the experimental data on two-phase flow. In the light of the above-mentioned facts, the author scrutinized some gas-liquid mixing methods in a two-component, annular mist flow closely related to the non-equilibrium length. The gas-liquid mixing devices involved can be classified into the following six classes: porous sinter, annular slot, center single jet, multi-jet, multi-hole and air-blast atomizer. Next, a variety of items relevant to the study of mixing methods, such as mixing devices, experimental conditions and measuremetns and details of some of those involved in this field of research, are set out in a table. Finally, some examples of flow behavior in the non-equilibrium region of an annular mist flow are looked at briefly.

今回は流動様相という語句の周辺の用語について述べる。この語句は気液二相流特有の学術用語というより、一般的な意味あいが強いし、この語句のもとに定義されている気液二相流の様相自体もまた、使用している研究者によって必ずしも同じではないように思える。すなわち流動現象の複雑さと、それを観察する研究者のphilosophyによって、同じ語句のもとに違った流動様相を頭に描いている可能性がある。このことは、以下の説明にみられるように定量的に定義されているのではなく、観察者の感性に依っている面が多いことにも示される。したがって本解説にはあいまいな点が多く、かつ著者の独断も多く含まれているので、賢明なる読者諸氏の意見をいただければ幸いである。なお流動様式の名称としてここで述べなかった語句は数多いが、それらについては文献 (5) に詳述されているので、それらを参照していただきたい。

数式・数量やあいまいさの入りようのない明りょうな表現による言葉、で定義されていない場合は、同じ語句であっても使用者によって若干異なった意味あるいは現象を表わしていることがある。前回に述べた流動様式の定義にもその一例が見られた。本稿では同じような現象を対象にしながらも、観点や立場によって表現すなわち語句 (用語) が変わってくることもあり得ることを「沸騰二相流」という語句を介して考えてみたい。

前回「沸騰二相流」その英語句として考えられるboiling two-phase flowなる語が、歴史的に数奇な意味あいを持っていること、また実際この語句が一部のみの調査ではあるが、英文誌にみられないことなどについて述べた。今回はその続きとして沸騰流の分類に関連する基本的語句についてふれる。沸騰の分類として伝熱工学資料 [1] にはA: 液体温度によりサブクール沸騰と飽和沸騰、B: 液体の流動状況によりプール沸騰と強制対流沸騰の二つの面から分類してある。このほか、C: 沸騰の機構により核沸騰と蒸発、およびD: 沸騰が生じている場の幾何形状により内部沸騰と外部沸騰とに分類することもできる。以下にはこれらに基づいて記述する。なお沸騰に関する用語は数多いが、ここでは「沸騰二相流」という語句の周辺の用語にとどめる。

Since the existence of many bubbles is frequently encountered in the flow of liquid in hydraulic machineries such as water turbines, pumps, etc., research on the behavior of multiple bubbles has been recently paid much attention to problems related closely with the cavitation phenomena, or cavitation damage. In this paper, I will explain briefly the studies of multiple bubbles in our laboratory at the Institute of Fluid Science, Tohoku University.

Since a thin radial liquid film flow on a stationary and rotating disk has the velocity profile of a boundary layer, a remarkable transition from laminar to turbulent flow occurs at a sufficiently large Reynolds number, being attributable to the amplification of disturbance inside the liquid film. The instability of the flow changes from a viscous-to an inflectional-type with increasing effect of the disk rotation on the flow. The former instability is essentially the same as that of the boundary layer on a flat plate in a stream, whereas the latter is similar to that of the three-dimensional boundary layer on a rotating disk in still fluid. In the present article, the feature of liquid surface in the transition region and characteristic properties of the disturbance detected through wall pressure fluctuation are reviewed in comparison with the linear stability theory.

An interdisciplinary research activity, Science on Form, started in 1980. The society ‘Science on Form, Japan’ was established in 1984. In addition to three regular domestic symposium a year, it organized two International Symposiums of Science on Form, the 1st in 1985 and the 2nd in 1988 both in Tsukuba. It publishes three issues of the Journal Forma in English every year. It covers all fields of Science. The actual activities and some topics will be introduced in this report. In these years, some complimentaly societies are organized and the related activities grow more and more wide to include art. An International Symposium KATACHIU SYMMETRY was held in November 21-25, also in University of Tsukuba. It played a role of an international and cultural contribution from Japan with some view based on the background of our Japanese culture and was an enjoyable symposium.

The rapid gas dynamic expansion in a free jet is used as a tool to investigate the creation of the condensed phase out of the gas phase by nucleation and cluster growth. Starting from experimental results that yield information about the existence of clusters and the heat transferred to the gas phase, the modeling of a gas-cluster system is presented using a molecular kinetic approach which considers the mass and energy transfer between gas and clusters in the free-molecular flow regime. Some numerical results are presented which show fair agreement with the experimental results. However, it is stressed that considerably more detailed information, which could possibly obtained now from numerical experiments, is needed to be able to further refine models for the fluxes at phase interfaces, especially for small systems.

液体中の圧力が, その温度における飽和蒸気圧程度まで低下すると、気泡が発生する。この現象およびその後の諸過程をキャビテーションと呼ぶ。キャビテーションは、古くから水力機械の性能を考える上で、重要な研究課題となっているが、昨今、キャビテーションは、こうした工学的問題に限らず、物理・化学・医学の分野でも様々な話題を提供している。本報では、キャビテーションに関連したいくつかの用語の説明を行うとともに、気泡力学で用いられる代表的な単一運動方程式を紹介する。

カオス (決定論的カオス) とは何かについては、専門家の間でも必ずしも統一された見解があるわけではない。通俗的には、カオス的な系は「決定論的な関係式によって記述される運動であるにも拘わらず、一見不規則に見える複雑な非周期的運動を行う力学系」であり、「初期値の僅かな違いが、長時間後の系の挙動に大きな差異をもたらす」とされている。しかし、実験データから、対象系がカオス的であるかどうかを判定しなければならない場合、カオスに関する上記の説明はあまり役立たない。本稿では、カオスとそれに関連する諸概念を、数学的に厳密な観点からではなく、実験を通じてカオスと接する工学者の立場から解説する。

The recent development on studies of the phase transition from a vapor to liquid phase at the vapor-liquid interface is reviewed. An experimental method by a shock tube makes it possible to determine the condensation coefficient through the measurement of the growth rate of a liquid film formed on the shock tube endwall behind a reflected shock wave. Theoretical basis for the method is given on the basis of the analysis of gas dynamics equations with kinetic boundary conditions. The condensation coefficients of methanol, acetic acid, and carbontetrachloride vapor are determined with good accuracies. Computer simulations of dynamic processes of evaporation and condensation at the interface are also introduced.

Basic equations and constitutive equations of gas-liquid two-phase flow were explained based on the various modeling. The local instantaneous formulation and averaged formulation of basic equations were described based on typical models such as homogeneous model, drift flux model and two-fluid model. The constitutive equations for these averaged basic equations were discussed. The modeling of turbulence in gas-liquid two-phase flow were also explained and basic equations and constitutive equations of turbulence were described.

Gas-liquid two-phase flows exist in a wide range of applications and enabling technologies. If, however, “understanding” we mean that the phenomenon in two-phase flow system can be predicted in terms of governing parameter, it should be concluded that two-phase flow is poorly understood item, and many questions are still open. A thorough theoretical approach with such microscopic experimental results as three-dimensional and high spatiotemporal measurements is needed to assess the performance of gas-liquid two-phase flow. My study of gas-liquid two-phase flows at microgravity conditions and in small diameter pipe is outlined in this report, especially for which regarding to interfacial area transport and drift flux. Research on two-phase at micro- or reduced-gravity conditions and in small pipe is expected to be conducted more widely in the future, shedding light on two-phase flow phenomena in piping system.

Research topics I have been working for the last 27 years related to rising bubbles are reviewed with the emphasis of surfactant effect on the multiscale structure of bubbly flows. Small amounts of surfactant can drastically change single bubble behaviors. For example, a bubble in aqueous surfactant solution rises much slower than one in purified water. This phenomenon is explained by the so-called Marangoni effect caused by a non-uniform concentration distribution of surfactant along the bubble surface. More interestingly, this Marangoni effect influences not only the rising velocity, but also the lateral migration in the presence of mean shear. Furthermore, these phenomena influence the multiscale nature of bubbly flows and cause a drastic change in the bubbly flow structure. In this article, these interesting phenomena are reviewed for the celebration of 30th anniversary of the Japan Society of Multiphase Flow

In this article, our experimental studies on the characteristics of adiabatic two-phase flows in microchannels have been reviewed. In these experiments, horizontal circular capillary tubes of 75-250 μm I.D. and a square channel with hydraulic diameter of 250 μm were used as the test channel to study the effects of the tube diameter and the channel shape on the characteristics. In order to clarify the effects of liquid properties, Newtonian liquids as well as non-Newtonian ones were used as the test liquid, while nitrogen gas as the test gas. Furthermore, the effects of gas-liquid inlet and mixing conditions have been studied by adopting several mixing methods. The significant effects have been seen on two-phase flow characteristics, i.e., two-phase flow pattern, void fraction, bubble velocity, bubble length and pressure drop, in microchannels. Also, characteristics of the two-phase flow through singularities such as bend, contraction and expansion in microchannel are addressed.

Dynamic wetting / dewetting, which is characterized by the motion of the contact line (the triple phase line of the liquid-fluid interface and the solid surface), is ubiquitously-observed phenomenon in two- or three- phase flow field confined by solid walls. The behavior of the contact line and the contact angle is one of the major boundary conditions for the description of the behavior of the bulk interface, understanding of the behaviors are hence highly required. In this article, recent topics and future prospect on the interfacial flow and wetting are briefly introduced, with some personal histories of the authors’ studies on that field.

Gas-liquid two-phase flow is seen in various engineering disciplines and understanding of its interfacial structure is a great importance for proper model development. Flow regime maps have been developed by various researchers in the past. However, identification of flow regime involves subjectivity and technical difficulty as the nature of flow regime transition mechanism and criteria are still yet unknown. In the present article, various two-phase flow regime identification methods utilizing machine learning (ML) approach will be reviewed. Two-phase flow features such as two-phase mixture impedance, dynamic force signal, and high-resolution images from high-speed camera were selected for the ML model training and testing. For the machine learning models, artificial neural network (ANN), convolutional neural network (CNN), and convolutional long short-term memory (ConvLSTM) were tested, depending on the feature types. As a result, the ML approach can identify two-phase flow regime with high accuracy. There still remains issues related to ML usage, including hyperparameter selection, and ability to explain its decisions. However, adaptation of ML tools for multiphase flow research fields may provide utmost benefits for efficient signal and image classification approach.

The aim of the present study is to create new technologies for solving problems such as ship fouling control and ballast water treatment in the field of maritime sciences. We report the investigation on underwater shock waves generated by the collapse of fine bubbles in cavitating flow fields. The cavitating jets are generated by four types of orifices and a triple-plunger pump. The schlieren method is used to clearly capturing the underwater shock waves. The schlieren images reveal that the local high-brightness region appeared in the shadow of the bubble flow moves downstream accompanied with shock wave generation. The relationship between the movement of the high-brightness region and the shock wave generation is correlated to the frequency of the chained collapse of cavitation bubbles. It is suggested that the movement of the high-brightness region depends on the jet flow velocity, the number density of bubbles, and the bubble contraction rate determined from the bubble diameter, the internal pressure of bubbles, and the strength of external shock wave pressures.

Contactless sample manipulation is attracting great attention in a wide range of fields, including analytical chemistry, biological science, and food industry. A better understanding on the droplet dynamics in acoustic levitation is vital to realize the lab-in-a-drop technology. Here, this paper reviews the recent research progress on the contactless droplet manipulation driven by acoustic levitation, focusing on droplet levitation, transport, coalescence, mixing, and evaporation dynamics are presented and discussed.