Showing papers on "Slug flow published in 1999"

Direct numerical simulation of a puff and a slug in transitional cylindrical pipe flow

Abstract: A direct numerical simulation of transitional pipe flow is carried out with the help of a spectral element method and used to investigate the localized regions of ‘turbulent’ flow that are observed in experiments. Two types of such regions can be distinguished: the puff and the slug. The puff, which is generally found at low values of the Reynolds numbers, is simulated for Re = 2200 where the Reynolds number Re is based on the mean velocity UB and pipe diameter D. The slug occurs at a higher Reynolds number and it is simulated for Re = 5000. The computations start with a laminar pipe flow to which is added a prescribed velocity disturbance at a given axial position and for a finite time. The disturbance then evolves further into a puff or slug structure.The simulations confirm the experimentally observed fact that for a puff the velocity near the leading edge changes more gradually than for a slug where an almost discontinuous change is observed. The positions of the leading and trailing edges of the puff and slug are computed from the simulations as a function of time. The propagation velocity of the leading edge is found to be constant and equal to 1.56UB and 1.69UB for the puff and slug, respectively. For the trailing edge the velocity is found to be 0.73UB and 0.52UB, respectively. By rescaling the simulation results obtained at various times to a fixed length, we define an ensemble average. This method is used to compute the average characteristics of the puff and slug such as the spatial distribution of the mean velocity, the turbulent velocity fluctuations and also the wall shear stress. By computing particle trajectories we have investigated the entrainment and detrainment of fluid by a puff and slug. We find that the puff detrains through its trailing edge and entrains through its leading edge. The slug entrains fluid through its leading and through most of its trailing edge. As a consequence the fluid inside the puff is constantly exchanged with fluid outside whereas the fluid inside a slug remains there. These entrainment/detrainment properties which are in agreement with the measurements of Wygnanski & Champagne (1973) imply that the puff has the characteristics of a wave phenomenon while the slug can be characterized more as a material property which travels with the flow.Finally, we have investigated in more detail the velocity field within the puff. In a coordinate system that travels with the mean velocity we find recirculation regions both near the trailing and leading edges which agrees at least qualitatively with experimental data. We also find streamwise vortices, predominantly in the trailing-edge region which have been also observed in experiments and which are believed to play an important role in the dynamics of the transition process.

Flow Pattern Transition During Gas Liquid Upflow Through Vertical Concentric Annuli—Part I: Experimental Investigations

Abstract: In the present work, extensive experiments have been carried out on air-water upflow through concentric annuli to identify the distribution of the two phases in the bubbly, slug, and churn flow regimes. A parallel plate type conductivity probe has been indigenously designed and constructed from a unique material for this purpose. The probability density function analysis of the probe signals has been performed for a better appraisal of the flow situation. The unique design of the probe and its extensive use at different axial and azimuthal positions have enabled us to note the asymmetric phase distribution in the slug flow regime. Based on the experimental results, an insight has been obtained into the physical mechanism underlying the transitions between different flow regimes. This has enabled the development of mechanistic models for the transition boundaries between the bubbly-slug and the slug-churn flow regimes. They have been reported in a subsequent paper.

Calculation of Two-Phase Pressure Drop for Vertical Upflow in Narrow Passages by Means of a Flow Pattern Specific Model

Abstract: Allow pattern specific model is presented, which is used to estimate the overall two-phase pressure drop for vertical upflow. The model first predicts the flow pattern that is present. For bubbly flow a drift flux model is employed, slug flow is dealt with by a mechanistic description invoking separate section around and between the Taylor bubbles. For annular flow, a phenomenological approach is used, utilizing published descriptions for the rates of entrainment and deposition and film thickness, but employing a new, improved correlation for interfacial friction factor. It was found that the same model gave the best description of churn flow. The model has been tested against data from small diameter circular and noncircular channels obtained from experiments described in the paper, as well as published experimental data. The agreement is good over all flow patterns.

Countercurrent Gas−Pseudoplastic Liquid Two-Phase Flow

Abstract: Countercurrent flow limitation (flooding), two-phase flow patterns, and void fraction in countercurrent flow of gas−pseudoplastic liquids in a vertical and an inclined channel (with a 68° inclinati...

Flow Pattern Transition During Gas Liquid Upflow Through Vertical Concentric Annuli—Part II: Mechanistic Models

Abstract: In this paper the transition boundaries between different flow regimes during cocurrent upflow of gas liquid two-phase mixture through concentric annuli has been predicted theoretically. On the basis of the experimental observations (Das et al., 1999), mechanistic models have been proposed to formulate mathematical equations of the regime boundaries as functions of the annulus dimensions, the physical properties, and velocities of the two phases. The analysis has yielded the bubbly-slug transition to occur at a limiting void fraction of 0.2, and the slug-churn transition to occur due to flooding of the liquid films by the Taylor bubbles. A comparison of the model predictions with experimental data corroborate the suitability of the proposed mechanisms.

Shannon entropy characteristics of two-phase flow systems

Abstract: A method to calculate Shannon entropy of two-phase flow systems from the power spectral density of the output signal is introduced in this article. Two series of experiments are made successively. One is used to investigate the density wave instability in a test facility to study the Chinese 200 MW nuclear heating reactor. After studying the Shannon entropy of the inlet pressure drop in all 534 experimental tests under different heating power, inlet subcooling, operating pressure, and inlet orifice resistance, we find that tests with higher negative Shannon entropy (negentropy) are unstable while those with lower negentropy are stable, like energy used in many fields. The other experiment was made to research two-phase flow regimes in vertical pipes. We calculate the time-dependent negentropy of the air–water flow through the power spectral density of the pressure drop. From the results, we find that the negentropy of the bubble flow is the smallest, the slug flow’s negentropy is the largest, and the negentropy of the annular flow is between the bubble flow and annular flow.

Effect of Drag-Reducing Agent on Slug Characteristics in Multiphase Flow in Inclined Pipes

Abstract: The effect of drag-reducing agent (DRA) on multiphase flow in upward and downward inclined pipes has been studied. The effect of DRA on pressure drop and slug characteristics such as slug translational velocity, the height of the liquid film, slug frequency, and Froude number have been determined. Experiments were performed in 10-cm i.d., 18-m long plexiglass pipes at inclinations of 2 and 15 deg for 50 percent oil-50 percent water-gas. The DRA effect was examined for concentrations ranging from 0 to 50 ppm. Studies were done for superficial liquid velocities between 0.5 and 3 m/s and superficial gas velocities between 2 and 10 m/s. The results indicate that the DRA was effective in reducing the pressure drop for both upflow and downflow in inclined pipes. Pressure gradient reduction of up to 92 percent for stratified flow with a concentration of 50 ppm DRA was achieved in 2 deg downward inclined flow. The effectiveness of DRA for slug flow was 67 percent at a superficial liquid velocity of 0.5 m/s and superficial gas velocity of 2 m/s in 15 deg upward inclined pipes. Slug translational velocity does not change with DRA concentrations. The slug frequency decreases from 68 to 54 slugs/min at superficial liquid velocity of 1 m/s and superficial gas velocity of 4 m/s in 15 deg upward inclined pipes as the concentration of 50 ppm was added. The height of the liquid film decreased with the addition of DRA, which leads to an increase in Froude number.

Measuring device for the gas-liquid flow rate of multiphase fluids

Abstract: The invention is a kind of measuring device for the gas-liquid flow rate of multiphase fluids. The device includes a means for generating static slug flow and a means for measuring the flow rate by cross-correlation. These means are mounted on the pipe through which the multiphase fluids such as gas liquid flow. The device includes a multiphase fluid regulating means and a multiphase fluid fraction measuring means. These means are mounted on the pipe through which the multiphase fluids such as gas liquid flow by cross-correlation orderly. Said multiphase fluid regulating means lies in the downstream of said measuring means or said multiphase fluid fractions measuring means lies in the upstream of said generating means for static slug flow. The device also includes a means for data acquisition and data procession. The means is used for acquiring and processing the measuring data. The flow rate of each fraction is calculated from the multiphase fluid fractions measuring means and the measuring means for the flow rate by cross-correlation.

Continuous Synthesis of TiO2 fine Particles and Increase of Particle Size Using a Two-Stage Slug Flow Tubular Reactor

Abstract: A novel continuous process has been developed, which consists of a two-stage slug flow tubular reactor (SFTR) to increase the size of titanium dioxide fine particles. In a single-stage process, particle growth stops at a certain conversion level with unreacted titanium ethoxide remaining. In this process, however, water is added to the particle suspension containing unreacted titanium ethoxide to allow the particle growth to proceed again. The particles prepared using the two-stage SFTR process have a larger mean particle size and smaller geometric standard deviation (GSD) than those from the single-stage process. The two-stage process also provides a higher conversion to particles than the single-stage process does. A smaller amount of water fed into the first stage and a larger amount of water fed into the second stage results in a larger mean particle size, higher conversion to particles, and smaller GSD. The width of the particle size distribution of the two-stage process is almost the same as that of the single-stage process. The calculated results using a mathematical model are in agreement with the experimental data.

A mechanistic model for predicting pressure drop in vertical upward two-phase flow

Abstract: A comprehensive mechanistic model is formulated to predict flow patterns, pressure drop, and liquid holdup in vertical upward two-phase flow. The model identifies five flow patterns: bubble, dispersed bubble, slug, churn, and annular. The flow pattern prediction models are the Ansari et al. (1994) model for dispersed bubble and annular flows, the Chokshi (1994) model for bubbly flow, and a new model for churn flow. Separate hydrodynamic models for each flow pattern are proposed. A new hydrodynamic model for churn flow has been developed, while Chokshi's slug flow model has been modified. The Chokshi and Ansari et al. models have been adopted for bubbly and annular flows, respectively. The model is evaluated using the expanded Tulsa University Fluid Flow Projects (TUFFP) well data bank of 2052 well cases covering a wide range of field data. The model is also compared with the Ansari et al., (1994), Chokshi (1994), Hasan and Kabir (1994), Aziz et al. (1972), and Hagedorn and Brown (1964) methods. The comparison results show that the proposed model performs the best and agrees well with the data.

Application of image processing techniques to air/water two-phase flow

Abstract: Image processing techniques have been used extensively in many different applications today. In particular, in fluid mechanics, image processing has become a powerful technique to study the flow phenomena, the flow pattern and the flow characteristics of two-phase flow. This paper presents a new application of image processing techniques to two-phase bubble/slug flow in a vertical pipe. Based on image processing techniques (image filtering for noise reduction, edge detection and thresholding for image enhancement, etc.), the results obtained are showed that this technique has many advantages. It is able to study together, in very short time, one image contains a large number of bubbles and the large amount of images, while the other methods such as point by point measurements technique or by using a digitizing table for digitization cannot be applicable. Moreover this technique also enable to identify automatically, to measure fast and relatively accurate the parameters such as size, shape of the bubble. These studies promise a great progress for an application of image processing techniques to study the complicated flow phenomena, the flow pattern and the flow characteristics of multiphase flow.

Wettability Effect on the Flow Pattern of Air-water Two-phase Flows in a Vertical Circular Pipe

Abstract: 機械工学や化学工学などの分野でみ られる気液二相流の 大部分は濡れ性のよい管路内を流れており,古 くから行わ れてきた膨大な研究によって,流 動様式はもとよりの こと, 各流動様式における気体と液体の流動特性 についても豊富 な情報が蓄積 されている図)。 一方 ,本 研究で対象とする鉄鋼精錬の分野で も,気 液二 相流は多くのプロセスでみ られるが,壁 から融体への汚染 をできるだけ少なくするために,反 応容器や管路には融体 との濡れ性の悪い材料が用いられている5)。ところが,管 路は一般に不透明であり,融 体の温度 も高 く,し かも実験 に危険が伴うことから濡れ性 と上記諸特性との関係につい て系統的知見は得 られていない。わずかにレイノルズ数が 小さい場合の水平円管内における流動形態の一端が水モデ ル実験に基づいて報告 されているに過 ぎない6)。周知の よ うに,濡 れ性は接触角θcで定量化される。ここではθc<90° のとき濡れがよい,θc>90°の とき濡れが悪いと定義する。 既存のプロセスの効率向上はもとより,新 しいプロセス を開発するに際 しては,反 応容器や管路内の融体の流動特 性や気泡の分散状況を正確に把握する必要がある。 このよ うな認識に基づき,最 近では流動と濡れ性 との関係につい ても注目が集 まり,活 発な研究が開始 されつつある7,8)。 著者らもRH脱 ガス装置やガス吹込み浸漬 ノズル内の流 動特性を把握す るための基礎的研究 として,空 気一水系の 垂直管内気液二相流の流動様式および流動特性 と濡れ性 と の間の関係に着目し,実 験を始めている9鋤。本研究では, 実際の連続鋳 造設備における浸漬 ノズル内の溶鋼とアルゴ ンガスの流れの向 きとは異なるが,ま ず水と空気がともに 上向きに流れている場合の流動様式に着目し,管 路と水 と の接触角 θcを36°, 77°, 104°と広範囲に変化 させて流動様 式に関する実験 を行った。