An experimental study of two-phase flow in simulated reduced-gravity condition: dispersed droplet to slug flow transition and slug flow

The results from an experimental study of reduced-gravity two-phase flows are reported in this paper. The experiments were conducted in simulated reduced-gravity conditions in a ground-based test facility with a circular test section of 25 mm inner diameter. The flow conditions for which data were acquired lie in the dispersed droplet to slug flow transition and slug flow regime. Local data were acquired for 17 different flow conditions at three axial locations. The acquired data complement and extend those discussed in an earlier paper by the authors (Vasavada et al. in, Exp Fluids 43: 53–75, 2007). The radial profiles and axial changes in the local data are analyzed and discussed in this paper. The area-averaged data, in conjunction with the local data, are discussed to highlight important interaction mechanisms occurring between fluid particles, i.e., drops. The data clearly show the effect of progressive coalescence leading to formation of slug drops. Furthermore, the shape of slug drops in reduced-gravity conditions was observed to be different from that in normal-gravity case. The analyses presented here show the presence of drop coalescence mechanisms that lead to the formation of slug drops and transition from dispersed droplet flow to the slug flow regime. The most likely causes of the coalescence mechanism are random collision of drops driven by turbulence eddies in the continuous phase and wake entrainment of smaller drops that follow preceding larger drops in the wake region. Data from flow conditions in which the breakup mechanism due to impact of turbulent eddies on drops illustrate the disintegration mechanism.     

Experimental study of gas-liquid slug flow in a small-diameter vertical pipe

Experimental investigation of upward gas-liquid slug flow in a vertical pipe in 15 mm ID has been carried out. The electrochemical method which permits the determination of the value and direction of instantaneous wall shear stress as well as the mean and fluctuating components of the liquid velocity is used for measurements. It is shown that the change of the sign of the velocity near the wall usually occurs at the moment of slug passage; the time-averaged wall shear stress at low liquid velocities is significantly lower than the value obtained by means of common prediction methods. The results of measuring of the local void fraction. liquid velocity and components of liquid velocity fluctuations are presented. The time-dependent behavior of the instantaneous hydrodynamic characteristics is described.     

Correlation of the liquid volume fraction in the slug for horizontal gas-liquid slug flow

Experimental data for gas holdup in liquid slugs are reported for two different pipe sizes (2.58 cm and 5.12 cm I.D.). A simple empirical correlation is developed and is shown to be a significant improvement over the only other published correlation proposed by Hubbard (1965). The results of this investigation are important for the development of a mechanistic model for the prediction of pressure drop and holdup for slug flow in pipes.     

Heat transfer during gas hydrate formation in gas-liquid slug flow

A model of heat transfer during gas hydrate formation at a gas-liquid interface in gas-liquid slug flow is suggested. Under the assumption of perfect mixing in liquid plugs, the recurrent relations for temperature in then-th liquid plug and heat and mass fluxes from then-th gas slug are derived. Total mass and heat fluxes in gas-liquid slug flow during gas hydrate formation are determined.     

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

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.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 188–190, September–October, 1985.     

On the transition between slug flow and two-layer flow of a gas-liquid mixture in a horizontal tube

The conditions necessary for the existence of slug flow are considered and the boundary of the region of its existence is constructed. Comparison with the author's own data and published experimental data reveals agreement of the results.     

A model for slug frequency during gas-liquid flow in horizontal and near horizontal pipes

Slug formation is an entry region phenomena. Waves form on a growing stratified film eventually blocking the gas to form a slug. The liquid level drops when the slug is swept away and the waves disappear. Then the film rebuilds its level in a time equal to the inverse frequency. The process is modelled and the results shown to be in agreement with experiment.     

An experimental study of diabatic spiral vortex flow

In spiral vortex flow, between concentric cylinders with the inner cylinder rotating and the outer stationary, the addition of a thermal gradient across the gap is a known complicating factor. The present diabatic study for narrow and wide gaps (radius ratios N=0.955 and N=0.8), with a heated outer and adiabatic inner cylinder, was undertaken to investigate this problem. The heat transfer characteristics and the modes of transition have been investigated together with the relationship between them. Using standard on-line digital computer techniques, the onset of vortex flow and its higher transitions have been shown to cause a sharp increase in Nusselt number. At higher Taylor numbers, of the order of 10⁶, a marked change in the Nusselt number occurs with the onset of the transition to periodic turbulent vortex flow. Outer wall heating is seen to affect the modes of transition. Diabatic critical Taylor numbers are much higher than those for adiabatic conditions and are found to depend on the close approach of the vortices to the outer wall     

Mass transfer during gas absorption in a vertical gas-liquid slug flow with small bubbles in liquid plugs

A model is developed for the analysis of mass transfer during isothermal absorption in a vertical gas-liquid slug flow at large Reynolds numbers with liquid plugs containing small bubbles. Simple formulas for mass flux from the N-th unit cell of gas-liquid slug flow and for total mass flux from N unit cells are derived. In the limiting case the derived formulas for mass transfer during gas absorption in a slug flow with liquid plugs containing small bubbles recover the derived expressions for mass transfer in slug flow without small bubbles in the liquid plugs. Using the developed model recommendations concerning the design of the absorber operating in a slug flow regime are suggested. Received on 28 July 1997     

An analysis of holdup in horizontal two-phase gas-liquid flow

The Butterworth form of correlation for holdup in two-phase gas-liquid flow is justified theoretically for certain conditions. In addition, a wide range of experimental data were used to show that holdup data may be broadly classified into three major groups based on the flow pattern, and different relationships were found to represent the data in each group. Thus for slug and plug flow, the holdup is given by the Armand type of equation; for stratified flow the holdup is given by the theoretical equations which are derived while annular flow data are satisfactorily represented by a semi-empirical correlation.     

An experimental study of planar entry flow of rubber compound

An automated rheometer based on an injection molding machine is developed for the evaluation of entry flow problems. Several entry flow geometries having different contraction and expansion angles and different channel lengths are tested. Two pressure transducers are flushmounted along the die length and a displacement transducer is installed to measure the screw motion. Signals generated by the pressure transducers and displacement transducer are supplied to an A/D converter and an IBM PC/AT computer. The pressure losses for a rubber compound are measured between two cross-sections along the flow direction. The time evolution of pressure with overshoot during flow before and after the entry region is observed. At low flow rates the pressure drops of the expansion flow are larger than those of the contraction flow. At high flow rates the pressure drops of the contraction flow become higher than those of the expansion flow. A ratio of the pressure drop to absolute pressure before the entry is found to be almost independent of flow rate.     

An experimental study of the flow emerging from porous walls

The fluid mechanics of air flow exiting from porous materials is investigated. Porous walls with filter ratings differing by about three orders of magnitude are studied. The flow behavior is investigated for its spatial and temporal stability. The measurements show that coalescence effects cause jet development which gives rise to a stable, spatial velocity nonuniformity termed pseudo-turbulence. Beyond a critical mean velocity, the pseudo-turbulence shows a leveling trend and the flow exhibits jets whose dimensions are independent of wall porosity and provide a unique length scale for this type of flow field (1 mm). A correlation is developed to predict the onset of fully developed jets. The impact of using porous wall materials in cold flow solid rocket motor testing is addressed.This study was supported by the Experimentation Branch, Aerophysics Division, NASA Marshall Space Flight Center. The useful discussions with Mr. Werner Dahm are deeply appreciated. The authors thank the useful comments of one of the reviewers and acknowledge the help of the associated technicians and NASA/ED-31 personnel during the conduct of this experiment.     

An experimental study on the flow behavior of micellar solutions

At higher concentration levels, the inner structure of micellar solutions cannot be detected directly by optical means. Nevertheless, the flow behavior of the micellar solutions reflects their micellar structures. Hence, in this study the material behavior of micellar surfactant solutions was investigated by rheometrical means in steady and oscillatory shear flows. The flow behavior of the solutions was found to be strongly dependent on the concentration of the surfactants. At very low concentrations, the surfactant solution shows Newtonian behavior. With increasing concentration, a transition to shear thinning behavior and increasing viscoelasticity was found. The complex material structure is modeled according to the flow behavior by discrete and continuous relaxation time spectra, depending on the concentration. Received: 3 May 2000/Accepted: 18 September 2000     

Experimental study on hydrodynamic characteristics of upward gas–liquid slug flow

To clarify the impacts of the hydrodynamic boundary layer and the diffusion boundary layer in the near wall zone on gas–liquid two-phase flow induced corrosion in pipelines, the hydrodynamic characteristics of fully developed gas–liquid slug flow in an upward tube are investigated with limiting diffusion current probes, conductivity probes and digital high-speed video system. The Taylor bubble and the falling liquid film characteristics are studied, the effects of various factors are examined, and the experimental results are compared with the data and models available in literature. The length of Taylor bubble, the local void fraction of the slug unit and the liquid slug, the shear stress and mass transfer coefficient in the near wall zone, are all increased with the increase of superficial gas velocity and decreased with the increase of superficial liquid velocity, whereas the length of liquid slug and the liquid slug frequency are changed contrarily. The alternate wall shear stress due to upward gas–liquid slug flow is considered to be one of the major causes for the corrosion production film fatigue cracking. A normalized formula for mass transfer coefficient is obtained based on the experimental data.     

An updated three-zone heat transfer model for slug flow boiling in microchannels

This work proposes a novel physics-based model for the fluid mechanics and heat transfer associated with slug flow boiling in horizontal circular microchannels to update the widely used three-zone model of Thome et al. (2004). The heat transfer model has a convective boiling nature and predicts the time-dependent variation of the local heat transfer coefficient during the cyclic passage of a liquid slug, an evaporating elongated bubble and a vapor plug. The capillary flow theory, extended to incorporate evaporation effects, is applied to estimate the bubble velocity along the channel. A liquid film thickness prediction method also considering bubble proximity effects, which may limit the radial extension of the film, is included. The minimum liquid film thickness at dryout is set to the channel wall roughness. Theoretical heat transfer models accounting for the thermal inertia of the liquid film and for the recirculating flow within the liquid slug are utilized. The heat transfer model is compared to experimental data taken from three independent studies. The 833 slug flow boiling data points cover the fluids R134a, R245fa and R236fa, and channel diameters below 1 mm. The proposed evaporation model predicts more than 80% of the database to within ±30%. It demonstrates a stronger contribution to heat transfer by the liquid slugs and correspondingly less by the thin film evaporation process compared to the original three-zone model. This model represents a new step towards a complete physics-based modelling of the bubble dynamics and heat transfer within microchannels under evaporating flow conditions.     

The presence of slug flow in horizontal two-phase flow

One of the flow regimes occurring in horizontal two-phase flows is characterized by periodic large waves “surging” along the tube. This flow, called “slug” flow, has been frequently observed in low and high pressure gas liquid systems, but it has been noticed that slugging is absent in certain liquid-liquid two-phase systems. A method is developed giving the necessary conditions for the presence of slug flow. This method quantitatively explains the observed absence of slugging in certain liquid-liquid flows.