Showing papers on "Slug flow published in 1986"

Separated two‐phase flow and basaltic eruptions

Abstract: Fluid dynamical models of volcanic eruptions are usually made in the homogeneous approximation where gas and liquid are constrained to move at the same velocity. Basaltic eruptions exhibit the characteristics of separated flows, including transitions in their flow regime, from bubbly to slug flow in Strombolian eruptions and from bubbly to annular flow in Hawaiian ones. These regimes can be characterized by a parameter called the melt superficial velocity, or volume flux per unit cross section, which takes values between 10−3 and 10−2 m/s for bubbly and slug flow, and about 1 m/s for annular flow. We use two-phase flow equations to determine under which conditions the homogeneous approximation is not valid. In the bubbly regime, in which many bubbles rise through the moving liquid, there are large differences between the two-phase and homogeneous models, especially in the predictions of gas content and pressure. The homogeneous model is valid for viscous lavas such as dacites because viscosity impedes bubble motion. It is not valid for basaltic lavas if bubble sizes are greater than 1 cm, which is the case. Accordingly, basaltic eruptions should be characterized by lower gas contents and lower values of the exit pressure, and they rarely erupt in the mist and froth regimes, which are a feature of more viscous lavas. The two-phase flow framework allows for the treatment of different bubble populations, including vesicles due to exsolution by pressure release in the volcanic conduit and bubbles from the magma chamber. This yields information on poorly constrained parameters including the effective friction coefficient for the conduit, gas content, and bubble size in the chamber. We suggest that the observed flow transitions record changes in the amount and size of gas bubbles in the magma chamber at the conduit entry.

Method and apparatus for controlled chemical reactions

Abstract: A method and apparatus for effecting accelerated chemical reactions at elevated temperatures and pressures including wet oxidation of materials in waste streams. An influent waste stream or first reactant is directed through a long tubular coil within a containment vessel with a second reactant being added to the influent. The flow rates for the gas and liquid components of the waste stream are set to cause plug or slug flow of the waste stream through the tubular coil. The curvature of the coil tends to induce a secondary flow in the liquid such that the liquid tends to travel in a spiral thereby providing more intimate mixing of the reactants; and the containment vessel may be partitioned to form a series of successive compartments to permit different temperature gradients to be applied to successive sections of the tubing coil across its length. Alternatively, a tube in tube construction for the tubular coil permits counter current heat exchange betwen the waste stream within the inner tube and the returning stream. Multiple injection and extraction points are provided along the path of the tubular coil to permit gas, liquid or solid reactants to be added or extracted as desired. The tubular coil provides an excellent environmental container for the reaction stream and is extremely suitable for temperature control to permit continuous heat input, autogenic operation and continuous heat extraction.

A model for gas-liquid slug flow in a vertical tube

Abstract: Modelisation du comportement hydrodynamique d'un ecoulement piston turbulent ascendant gaz-liquide, basee sur une analyse detaillee de la structure de l'ecoulement. Mecanisme de l'ecoulement du liquide dans le bouchon contenant des bulles de gaz dispersees, en vue d'evaluer la fraction de vide du bouchon de liquide

A general design equation for air lift pumps operating in slug flow

Abstract: Air lift pumps are finding increasing use where pump reliability and low maintenance are required, and where corrosive, abrasive, or radioactive fluids must be handled. Although air lifts are used in nuclear fuel reprocessing plants, no general, theoretically sound equation has been proposed in the literature for tall air lift design. Such an equation is developed from two-phase flow theory to predict the height to which an air lift pump operating in the slug flow regime can lift a given volumetric flow rate of liquid, given the air flow rate and pressure at the point of gas introduction. The widely used drift-flux model for the prediction of holdup is combined with an approximate relationship to predict pressure loss, and is substituted into the total pressure differential. Integration of the resulting equation provides an explicit formula for the calculation of lift. Experimental work using a variety of liquids in a 38 mm dia. air lift test installation supports the new design equation and demonstrates its flexibility.

Heat transfer in horizontal plug and slug flow for gas-liquid and gas-slurry systems

Abstract: Heat transfer coefficients and flow characteristics were measured in a horizontal loop of 51.5 mm diam. pipe. The viscosities of liquid and slurry were changed in the range of 0.8-55 mPa •.s. The experiments were carried out in the regimes of plug and slug flow. The pressure drop in the straight sections was smaller than that predicted by the Lockhart-Martinelli correlation and was correlated by a newly proposed equation. The gas holdup increased with increasing gas velocity and decreasing liquid or slurry velocity. A correlation of the gas holdup is presented. The heat transfer coefficients, h, increased with increasing liquid velocity and decreased with increasing liquid viscosity. In the range of the superficial gas velocity Ug=0.2-1 m•s-1, h was nearly independent of the gas velocity. In the range of Ug=1-10m•s-1, however, h increased with increasing gas velocity. The values of h in the present experiments as well as those in the literature are well described by the equation proposed in this work.

Hydrodynamic Stability of Inverted Annular Flow in an Adiabatic Simulation

Abstract: Inverted annular flow was simulated adiabatically with turbulent water jets, issuing downward from large aspect ratio nozzles, enclosed in gas annuli. Velocities, diameters, and gas species were varied, and core jet length, shape, breakup mode, and dispersed core droplet sizes were recorded at approximately 750 data points. Inverted annular flow destabilization led to inverted slug flow at low relative velocities, and to dispersed droplet flow, core breakup length correlations were developed by extending work on free liquid jets to include this coaxial, jet disintegration phenomenon. The results show length dependence upon D/sub J/, Re/sub J/, We/sub J/, ..cap alpha.., and We/sub G/,rel. Correlations for core shape, breakup mechanisms, and dispersed core droplet size were also developed, by extending the results of free jet stability, roll wave entrainment, and churn turbulent droplet stability studies.

Transient Heat Transfer in Channel Flow with Step Change in Inlet Temperature

Abstract: A second-order accurate finite-difference scheme based on a modification of the upwind method for the classical wave equation is proposed for transient forced convection in laminar channel flow. Stability analysis is performed, and the modified equation is established. The problem involving a step change in inlet temperature is studied for both slug and parabolic flow inside a parallel-plate duct. The accuracy of the numerical scheme is examined by comparing the results to the exact solutions available for the case of slug flow. Numerical results are presented for the average flow temperature and local Nusselt number as a function of position along the channel at different times, and the thermal wave propagation phenomena are physically interpreted.

Method and apparatus for investigating two-phase flow in pipes

Abstract: First and second gamma ray attenuation measurements (10,17;10,16) are made with one-shot-collimators (22, 21) to measure repetitively the void fractions of said mixture in lower and upper portions (14L, 14U) of a transverse section through a pipe along which is flowing a gas and liquid mixture. Statistical features are derived from the measurements and are used in logical classification steps to determine the type of flow pattern (B, SW, AW, I, DB, SS), the profile of a large gas bubble in intermittent or slug flow, and the gas and liquid volume flow rates in slug flow.

Pressure Drop and Heat Transfer for Tree Phase Flow : 1st Report, Flow in Horizontal Pipes

Abstract: Three phase pressure drop in horizontal pipes was investigated for air-water-sand flow in 0.0416m I.D. pipe at atmospheric pressure and temperature and for hydrogen-solvent-pulverized coal flow in 0.0109m I.D. pipe at high pressure and temperature with use of a tube furnace in the coal liquefaction test plant. When the three phase flow was regarded as a two phase gas-slurry flow, the pressure drop in the three phase flow was flow was found to be reasonably well corre-coefficients for air-water-sand flow seem to be the same as those of simple slurry flow though some scatterings are observed.

Laminarization and Reversion to Turbulence of Low Reynolds Number Flow Through a Converging to Constant Area Duct

Abstract: Airflow in fully developed turbulent state between two parallel plates was accelerated through a linearly converging section, and then it flowed into a parallel-plate channel again. The Reynolds number 2hu m /ν was 10,000 and the acceleration parameter K in the accelerating section was 8 × 10−6 . Fluctuations of streamwise velocity as well as time-mean velocity profiles were measured at ten traversing stations located along the test channel by a hot-wire anemometer. It was found that the flow, partly laminarized in the accelerating section, continued to laminarize in the first part of the downstream parallel-plate section and then the reversion to turbulence occurred in the way similar to the case of natural transition in a pipe, where the transition proceeds through a regime of the so-called turbulent slug flow.

Turbulent slug and velocity field in the inlet region for a pulsatile pipe flow.

Abstract: This paper describes the characteristics of a turbulent slug and the velocity field in the inlet region for a pulsatile pipe flow. Experimental conditions of the present study were arranged systematically as to pulsation frequency and velocity amplitude ratio A1. The experimental results about the onset of a turbulent slug, the propagation behavior of the slug, and the distributions of velocity and turbulence intensity inside the slug were compared with those for a turbulent patch in a pulsatile boundary layer on a flat plate. A close analogy was shown between them. When A1 is smaller than 0.608, the velocity distribution for the laminar region outside a slug was approximated by an analytical solution previously presented for a pulsatile laminar flow with smaller A1.

Circulation and slugging in a fluid bed gasifier fitted with a draft tube

Abstract: We have developed a fluidized bed containing a draft tube for the purpose of gasifying low grade coal. The bed is effectively divided into two zones by the draft tube, and by conducting the gasification and combustion reactions separately in the two zones we have been able to produce synthesis quality gas using air and steam as the gasifying agents. In the draft tube where coal and steam react, the solids move upward in slug flow. It seems particularly advantageous to operate the draft tube in the square nosed slugging regime, because of the improved gas solid contacting which results from the high dense phase gas flow rate and low rise velocity of the slugs. The rate of solid transport induced by this slug flow up the draft tube and around the bed is experimentally and theoretically described.

Prediction of flow regime transitions in vertical upward three phase gas-liquid-solid flow

Abstract: The effect of solids concentration on the bubble to bubble-slug, bubble-slug to slug, and slug to churn flow transitions in gas-liquid-solid vertical flow has been studied using air, water, and glass beads in a 5 cm ID plexiglass column. Several simple models to predict the transitions were developed utilizing criteria proposed for gas-liquid vertical flow. These criteria are shown to apply equally well in the absence or presence of the solid phase With solids present in the flow, the bubble to bubble-slug and bubble-slug to slug flow transitions both occurred at lower superficial gas velocities because of increased bubble coalescence. However, the slug to churn flow transition was unaffected by solids in the range of flow conditions studied.

Time series analysis of gamma densitometry signals

Abstract: Single, narrow-beam densitometry has been developed as a method for determining the flow regime and void fraction for industrial liquid gas experiments at high pressures and temperatures in a vertical, thick-walled, steel vessel. To develop suitable techniques, the experimental conditions were simulated using a transparant air/water column. In the transition region from bubbly to slug flow, a time sequence of four regimes, viz. annular flow, partially developed annular flow, coalescing bubble flow and bubbly flow were visually identified in a given cross section. Gamma rays were used to interrogate a column diameter, and digital time series analysis methods were applied. Amplitude spectral densities were used to determine any periodicity in the gas phase flow. The average void fraction for periodic gas flows was obtained by analysis of probability density distributions (PDD). The time sequence of the flow regimes was obtained from the signal magnitude of the diametral void fractions and the time spent in each regime was measured by the associated probability. The results compared well with those obtained from other methods. In the bubbly flow region, the standard deviation of the PDD exceeded that expected for nuclear counting. This bubble noise was assessed with respect to bubble properties.

Investigation of two-phase flow processes in coal slurry/hydrogen heaters. Final report

Abstract: Experimental and analytical results are presented for two-phase slug flow in a horizontal, transparent pipe at large diameter (6.75 in.) at high gas density (20 times the density of air at atmospheric pressure) and at liquid viscosities ranging from 1 to 1000 centipoise. The test section replicates 1 1/2 rectangular coils (40 ft by 10 ft) of a fired heater in a coal liquefaction plant. Regime transtion, pressure drop, void fraction, and slug characteristic data have been obtained for liquid superficial velocities ranging from 0.2 to 6 ft/s and gas superficial velocities ranging from 0.2 to 12 ft/s. Regime transition results have been compared with the Taitel-Dukler analytical flow regime map. The transition from stratified to slug flow, which is underpredicted by the original analysis, has been studied in particular. Comparison with the dimensionless transition criterion (gas Froude number) shows that increased liquid viscosity increases the liquid level at which the transition occurs. Pressure drop data at the transition have been used to evaluate the interfacial shear and to show that it is greater than is assumed in the Taitel-Dukler analysis. Sensitivity studies for the transition criterion and interfacial shear illustrate exactly why the transition is underpredicted on the flowmore » regime map and how the predictions can be improved. Photos of the flow patterns illustrate the mechanism of slug formation at high viscosity compared with low viscosity. Pressure drop, void fraction, and slug characteristic results are compared with an analysis for pressure drop in slug flow, demonstrating better predictive capability of this model at large pipe size, high gas density, and high viscosity, compared with correlations from the literature. The pressure drop model is also shown to be in excellent agreement with coal liquefaction pilot plant data. 34 refs.« less

Geothermal Two-Phase Wellbore Flow: Pressure Drop Correlations and Flow Pattern Transitions

Abstract: In this paper we present some basic concepts of two-phase flow and review the Orkiszewski (1967) correlations which have been suggested by various investigators to perform well for geothermal wellbore flow situations. We also present a flow regime map based on the transition criteria used by Orkiszewski (1967) and show that most geothermal wells flow under slug flow regime. We have rearranged bubble- to slug-flow transition criterion used by Orkiszewski (1967) to show that the transition depends on the dimensionless pipe diameter number in addition to dimensionless liquid and gas velocity numbers. Our aim is also to identify what research may lead to improvements in two-phase pressure drop calculations for geothermal wellbore flow.

The propagation of turbulent slugs in a pulsatile pipe flow.

Abstract: Hot wire measurements were made on the propagation of turbulent slugs in a pulsatile pipe flow. The pulsatile pipe flow was produced by imposing a sinusoidal oscillation on a steady flow. Two types of slug behavior are observed according to whether the pulsation frequency f is low or high, and this paper presents two typical examples with f=0.300 and 2.082 Hz. In both examples the ap·pear·ance of the turbulent slug is periodic, in contrast to the randomness that is characteristic of transition in steady flows. Such periodicity is also known in transitional pulsatile boundary layers on a flat plate. At f=0.300 Hz a turbulent slug generates near the entrance (x/D≒3) and propagates downstream, where x is the axial coordinate and D is the pipe diameter. The velocities of leading and trailing edges of the slug can be predicted by steady flow data. Meanwhile at f=2.082 Hz numerous slugs are generated in the region downstream of x/D≒25 in the middle stage of the accelerating period. After a short time, a new slug appears at x/D≒3 and propagates downstream. Later its leading edge merges with the preceding slugs at x/D≒40 and then these slugs become a single slug. This slug disappears at almost the same time as in an earlier stage of the following accelerating period, due to the combined effect of viscous dissipation and acceleration. The velocity and tur·bu·lence intensity downstream of x/D≒50 were found not to change axially.

複合流路における気液二相流体の流動 : 第1報,副流路への気液流重配分

Abstract: Fully developed two-phase air-water flows through a multiple channel consisting of two interconnected subchannels were investigated. The purpose was to obtain more reliable data on the flow distributions when the cross-sectional area of the subchannels is different. Five test sections were used. The three of them were rectangular ones aligned vertically, and their area ratio of the subchannels were 1.5, 2 and 3.. The two others had the semi-circular subchannels of the area ratio 1.5 ; one was arranged horizontally while the other vertically. Void drift phenomena have been observed not only in vertical flows but also in horizontal flows. A more prominent void drift took place in a slug flow while less pronounced in a bubble flow and an annular flow. It was seen that the channel geometry has very little effects on the flow distributions and the flow pattern transitions. This supports the so-called one dimensional subchannel analysis in such a multiple channel.

急減圧下での水平管内における沸騰現象に関する研究 : 第1報, 流動様式の観察と圧力の時間変化

Abstract: Blowdown experiments with emphasis on an early response of depressurization were carried out on Freon-114 liquid in a horizontal tube. Visual investigations using a high-speed camera indicated that the initial flow pattern was bubbly around flashing inception points, but expansion and growth of the bubbles caused downstream liquid pushed out of the tube with a single phase. This pattern, like a slug flow, will give a higher discharge rate at an early stage of the blowdown than those calculated by current analytical models, since choking conditions will not be reached at the outlet. Measurements of local pressure and temperature showed a thermal non-equilibrium effect, which was indicated by a pressure holdup under the initial saturation pressure ; also, that the time lag of flashing inception after the passing of an initial expansion wave was less than 1ms, and the relaxation time from the non-equilibrium state was around 2 to 3 ms.

Hydroshock theory in a two-phase gas-liquid mixture in a slug flow regime

Abstract: A study is made of the intensity of a hydroshock in a two-phase gas-liquid mixture in a slug flow regime in the case when a pipeline is shut off by a liquid slug. The intensity is studied as a function of the length of the shut-off section of the liquid slug, the content of gas bubbles in the liquid slugs, and the pipeline shut-off law, and with allowance for the shock-wave character of the process [1, 2]. The calculated data using the shock-wave theory agree well with the experimental data of [3] and, unlike the results of the linear theory of [3], make it possible to determine the intensity of the hydroshock not only in the case of weak waves, but also in the case of waves of moderate intensity.

気泡の閉塞により誘起される不安定流動 : 第1報,発生機構に関する実験結果

Abstract: Thermo-hydraulic instability has been investigated experimentally in a boiling two-phase flow system composed of a vertical N-shaped boiling channel and an adiabatic bypass between an inlet and an outlet plenums, using Freon 113. The instability caused by the blockage of vapor to flow, which does not belong to Boure's classification of two-phase flow instabilities, is observed. The flow maps in each section, the static characteristics, the stable flow limits and the characteristics of this instability have been studied under various conditions to clarify the occurring mechanism. It makes the inlet velocity decrease for a vapor slug combined in the inverted U-shaped bend to obstruct the flow. The vapor slug flowing into the downcomer with the growth rises the void fraction in the downcomer and causes the gravitational loss to increase. Therefore, flow instability is brought about.