Horizontal Gas-liquid Research Articles

Experimental characterization of gas–liquid intermittent subregimes by phase density function measurements

The transition between plug and slug flow patterns in a horizontal gas–liquid flow is studied experimentally by means of statistical analysis of the gas density function time series. Two parameters are introduced in order to characterize the transition: the ratio of the probability density function maxima and the power spectrum mean frequency. For a given liquid superficial velocity, the partial derivative of these parameters displays abrupt changes in value with respect to the gas superficial velocity where the flow pattern transition occurs. The experiments were carried out on an air–water flow in a horizontal pipe of i.d. 0.08 m; the water mass flow rates were 3, 4.5, 7, and 10 kg/s, while the gas fraction of volume flow ranged from 0.2 to 0.8; single-fiber optical probes measured the gas density function time series at a sampling rate of 2 kHz. The results agree with the Mandhane map and provide a basis for a theoretical description of the transition.

Slug velocity and liquid layer thickness before an abrupt contraction in horizontal gas–liquid flow

This work presents the results of an experimental investigation into the effects of an abrupt area contraction on the upstream gas–liquid intermittent flow; in particular, the slug velocity (the velocity of slugs as seen from a fixed observer) and the thickness of the liquid layer in the stratified region between two slugs were measured by means of optical probes just before the contraction, and compared with the values obtained in a straight pipe with the same inner diameter and under the same flow conditions. The superficial velocities of the liquid were from 0.6 and 0.9 m/s, while that of the gas ranged from 0.3 to 5 m/s. The results indicate that the abrupt contraction results in a substantial reduction of slug velocity, and in a growth of the liquid layer thickness.

Correlations of the Holdup and Frictional Pressure Drop in Air-Water Two-Phase Flow in a Flat Capillary Rectangular Channel (Comparison between Vertical Upward and Horizontal Flows)

Both vertical upward and horizontal gas-liquid two-phase flows in a flat capillary rectangular channel were studied to clarify the flow phenomena, i. e., the velocity and the passing frequency of liquid lumps, the holdup and the frictional pressure drop. The dimension of the channel used was 9.9mm×1.1mm. The orientations of the channel were the wide side vertical and the wide side horizontal in horizontal line. The differences between the flow characteristics in such orientations were investigated. New correlations of holdup and frictional pressure drop for flat capillary channels were proposed, in which the effect of aspect ratio has been taken into consideration.

Slug Frequency Measurement Techniques in Horizontal Gas-Liquid Flow

Slug Frequency Measurement Techniques in Horizontal Gas-Liquid Flow

Slug frequency measurement techniques in horizontal gas-liquid flow

Slug frequency measurement techniques in horizontal gas-liquid flow

Axial viewing studies of horizontal gas–liquid flows with low liquid loading

An axial viewing system is described which was used to investigate the behaviour of the liquid phase in two-phase gas–liquid flows in a 0.079 m diameter horizontal pipe. The region investigated was that of flows with relatively high gas velocities and low liquid loading. Such flows are typically experienced in gas-condensate lines. Axial images obtained using the new system, show that the bulk of the liquid flows within a layer at the bottom of the pipe; however, at higher gas or liquid velocities, a significant portion is entrained as droplets within the gas core and is seen to deposit around the pipe periphery. A process that appeared to contribute significantly to the entrainment was captured using a high speed video recording system. Waves on the liquid layer were observed to converge and breakup into rapidly moving spray droplets. This process appeared to be random, but more frequent with increase in gas or liquid velocities. Secondary droplet breakup was also observed; large droplets were seen to balloon out and burst. The video evidence obtained using the axial viewing method, clearly identified the process of entrainment and deposition as the dominant contributor to the liquid phase transfer to the upper part of the pipe wall.

Mass transfer enhancement due to surface wave formation at a horizontal gas–liquid interface

Absorption of CO 2 at the horizontal gas–liquid interface of flowing shallow water was enhanced by waves induced by a vibrating air column. Wave amplitude and mass transfer peaked at wave frequencies of 4.5– 6 Hz . At the peak frequency, mass transfer increased quadratically with wave amplitude and was independent of liquid flow rate. Peak mass transfer was approximately 45% greater than in the absence of waves. Increasing the viscosity of the liquid phase reduced mass transfer.

Prediction of the Circumferential Film Thickness Distribution in Horizontal Annular Gas-Liquid Flow

This paper develops a liquid film symmetry correlation and a liquid film thickness distribution model for horizontal annular gas-liquid pipe flows. The symmetry correlation builds on the work of Williams et al. (1996) (Droplet Flux Distributions and Entrainment in Horizontal Gas-Liquid Flows,” Int. J. Multiphase Flow, Vol. 22, pp. 1–18). A new correlating parameter is presented. The liquid film thickness model is based on the work of Laurinat et al. (1985) (Film Thickness Distribution for Gas-Liquid Annular Flow in a Horizontal Pipe,” PhysicoChem. Hydrodynam., Vol. 6, pp. 179–195). The circumferential momentum equation is simplified to a balance between the normal Reynolds stress in the film’s circumferential direction and the circumferential component of the weight of the film. A model for the normal Reynolds stress in the circumferential direction is proposed. The symmetry correlation is used to close the model equations. The model is valid for films with disturbance waves, and is shown to be applicable to air-water flows over a range of conditions from low velocity asymmetric to high velocity symmetric annular flows. [S0098-2202(00)02102-7]

Slug length prediction in near horizontal gas–liquid intermittent flow

The statistical distribution of slug lengths in a gas–liquid pipeline evolves along the pipe as short slugs collapse. This results from a lack of development of the liquid velocity profile at the rear of each slug that is shorter than a minimum stable length. Experiments have been conducted in a 50 mm diameter tube inclined at +5° to the horizontal to determine the rate of collapse of short slugs as a function of their length. The data covers both stagnant and turbulent flowing liquid, in both the elongated bubble and slug flow regimes. The data shows that all the turbulent flow results may be correlated by the same expression regardless of the value of the slug liquid Reynolds number. This correlation is employed within a model adapted for horizontal flow that assumes a random distribution of slug lengths at the pipe entrance and calculates the evolution of the slug length distribution along a pipe. The results of the model are compared against experimentally determined slug length distributions obtained in a 16 m long pipe. The comparison between the calculated and observed average slug length is shown to be quite good over a range of flow rates and the overall shape of the length distributions is predicted well by the model.

Influence of Froude number on physical processes determining frequency of slugging in horizontal gas–liquid flows

The effect of the liquid Froude number on the physical processes for forming slugs was investigated for cocurrent air–water flow through a 7.63 cm horizontal pipeline that was 20 m long. The transient characteristics are described by using simultaneous measurements of the liquid holdup at multiple locations. When the Froude number of the stratified flow at the onset of slugging is less than unity, the frequency is related to the propagation, upstream of gravity waves caused by the formation of slugs. A stochastic model is used to describe the formation for Froude numbers greater than unity. These results provide a physical basis for developing models for the frequency of slugging.

On the use of the stratified momentum balance for the deduction of shear stress in horizontal gas–liquid pipe flow

The use of the stratified flow momentum balance for the deduction of interfacial and liquid wall shear stresses from experimental measurements is examined. A systematic error analysis is applied to the governing equations using the principle of maximum uncertainty. A series of air–water experiments were conducted in 50 and 80 mm diameter pipes, in which gas pressure drop, liquid height and gas wall shear stress were measured. A framework for the correlation of the deduced shear stresses is proposed from the experimental measurements. The uncertainty analysis is used to show that the definition of mean liquid height does not significantly influence the overall results. The development of empirical equations based on such methods may lead to total uncertainties of up to 40%, irrespective of accuracy of the experimental data or the appropriateness of the correlating technique. Comparisons with state-of-the-art correlations for the liquid wall and interfacial friction factor data showed even larger discrepancies between measurement and prediction.

Local studies in horizontal gas–liquid slug flow

The axial liquid-phase velocity profile development in a horizontal air–water plug/slug flow-pattern was experimentally investigated by simultaneously using two hot-film anemometers. One of the probes which was kept in a fixed location was exclusively used as a phase identifier while the other probe was traversed vertically for local velocity measurements. The experimental observations were focused on the intermittent and transient characteristics of the slug flow-pattern. It was shown that the velocity rapidly develops into an asymmetric but nearly fully-developed profile within liquid slug with the maximum value occurring below the pipe center line. On the other hand it was documented that the velocity never develops into a quasi-fully-developed profiles within the liquid layer below a passing slug. At a given location the velocity gradually decelerates toward the gas slug tail, but rapidly accelerates towards the wake of the gas slug.

Droplet flux distributions and entrainment in horizontal gas-liquid flows

Measurements of entrainment and of droplet distributions are presented for air and water flowing in a horizontal 0.095 m pipe. A stratification of drops is observed because of the influence of gravity. Entrainment does not depend on pipe diameter and it increases strongly with increasing gas velocity. At small gas velocities, it is lower in horizontal pipes than in vertical pipes because gravitational settling enhances deposition.

Linear stability of stratified channel flow

Linear stability of horizontal gas-liquid stratified flow was solved using a tau spectral method that is valid for all wavenumbers. Pressures of 0.1–10 atm and liquid viscosities of 1–600 cP were examined. Comparison of these results with Kelvin-Helmholtz, integral momentum and rigorous long wave expansion approaches indicates that the approximate models do not correctly predict the point of neutral stability. The discrepancies in the models are due to more than differences in the calculation of interfacial perturbation stress components and differences in the base states. Stability predictions that include gas phase turbulence, as modeled with either a polynomial velocity profile or with imposed boundary conditions obtained from measured pressure and shear stress variations, are similar to laminar results if the interfacial stress and liquid depth are the same. The long wave stability boundary is found to correlate well for different channel height, density ratio and viscosity ratio, using a gas superficial Froude number corrected with a square root of density ratio and a liquid superficial Froude number. For gas-liquid channel flow waves that grow fastest typically have dimensionless wavenumbers of order unity. Their growth rate scales as a corrected gas Reynolds number to the first power. If the gas-liquid depth ratio is less than approximately one, long waves can be unstable before moderate wavelength waves. Under conditions where unstable moderate wavelength waves appear within a couple of meters, it can take 20–50 times this length for slowly growing long wavelength waves, which can destroy regime stability, to appear.

Effects of the liquid viscosity on the phase distributions in horizontal gas-liquid slug flow

In the present paper, experimental results of the investigations of the effect of liquid viscosity on the phase distribution in two-phase air-liquid slug flow in horizontal pipes are represented. As the liquid phases, oil with viscosities in the range from 14 to 37 mPas and water are used. The experimental investigations are conducted in a horizontal pipe with an inner diameter of 59 mm. The superficial liquid viscosities and the superficial air velocities in the test section are varied in a range from 0.15 to 1.5 m/s and from 0.1 to 13.5 m/s, respectively. The liquid hold-up within the liquid slug zone, the liquid holdup in the elongated bubble zone and the flow averaged liquid holdup are measured by means of a multibeam γ-ray densitometer. The results indicate that there are significant differences in the distribution of liquid and gas when oil and air or water and air are used in the experiments. By increasing the liquid viscosity, increasing values for the flow averaged liquid holdup and the volume averaged liquid holdup inside the liquid slug and the elongated bubble zone are obtained.

Prediction of liquid level distribution in horizontal gas-liquid stratified flows with interfacial level gradient

A one-dimensional momentum equation has been derived based on a two-fluid model and used to predict the axial distribution of liquid level or void fraction in steady, cocurrent, gas-liquid stratified flows in horizontal circular pipes and rectangular channels. The equation is carefully formulated to incorporate the effect of interfacial level gradient. Two different critical liquid levels are found from the momentum equation and are adopted as a boundary condition to calculate the liquid level or void fraction distribution upstream of the channel exit. The predicted void fraction distributions are compared with the experimental data obtained in a rectangular channel in this work and other data reported for large-diameter pipes. Good agreement is shown for air-kerosene, air-water and stream-water stratified flows with a smooth gas-liquid interface.

Pressure profiles for slugs in horizontal pipelines

Measurements are presented of the variation of pressure caused by the passage of a stable or unstable slug in a horizontal gas-liquid flow. Good agreement is obtained with a model which considers the pressure change as the sum of contributions from a hydraulic jump in the front of the slug, the wall drag in the body of the slug and a sudden change in the velocity of the liquid in the rear of the slug.

Horizontal gasliquid flow at close to atmospheric conditions

Air—liquid co-current flow was investigated using a 0.0935 m i.d. horizontal pipe under atmospheric conditions. The effect of liquid viscosity on flow pattern, holdup and pressure loss was investigated using water and glycerine solutions with dynamic viscosities up to 0.1 N s m −2. A dual technique combining visual observation allied to pressure loss fluctuation analysis was found to distinguish between flow patterns objectively. Theoretical and empirical flow pattern maps are compared against the data. The empirical maps of Lin and Hanratty, and Andritos are shown to give the best flow pattern predictions for air—water and air—glycerine solution data, respectively. Most theoretical transition criteria were unable to handle changes in either pipe diameter or liquid viscosity.

Effects of Pressure on Velocity of Liquid Lumps in Horizontal Gas-Liquid Two-Phase Flow.

phase flow under a wide range of pressures from near atmospheric pressure to a highly elevated pressure of about 20 MPa. The liquid lumps of interest are liquid slugs, huge waves, disturbance waves and ephemeral large waves. Velocities were determined using time-series signals of cross-sectional mean liquid holdup which were electrically detected at two axially separated locations of the test tube. It is demonstrated how the mean values of liquid lump velocity and its standard deviation depend on the gas and liquid flow rates and pressure. It is obtained that the liquid lump velocity and its standard deviation are obviously affected by the pressure and are related to flow patterns. The comparison of liquid lump velocities between vertical upward flow and horizontal flow is also discussed.

Time varying behavior of cocurrent gas-liquid flows in packed beds

The time-varying fluid mechanics of the pulse flow regime are studied by the use of continuous pressure measurements. It is found that pulses can occur either or intermittently and the time period between pulses scales inversely with a modified Reynolds number. A simple physical basis for this scaling is suggested by a one-dimentional map. The results emphasize the importance of knowing the time scale of pulses and suggest that it is worthwhile to look for similar scalings for other intermittent flow phenomena such as slugs in horizontal gas-liquid flows.