Showing papers on "Slug flow published in 2006"

Liquid Water Removal from a Polymer Electrolyte Fuel Cell

Abstract: Liquid water transport and removal from the gas diffusion layer GDL and gas channel of a polymer electrolyte fuel cell PEFC are studied experimentally and theoretically. In situ observations of the liquid water distribution on the GDL surface and inside the gas channel were made in an operating transparent PEFC. Liquid droplet formation and emergence from the GDL surface are characterized and two modes of liquid water removal from the GDL surface identified: one through droplet detachment by the shear force of the core gas flow followed by a mist flow in the gas channel, and the other by capillary wicking onto the more hydrophilic channel walls followed by the annular film flow and/or liquid slug flow in the channel. In the former regime, typical of high gas flow rates, the droplet detachment diameter is correlated well with the mean gas velocity in the channel. In the latter regime characteristic of low gas flow rates, liquid spreading over hydrophilic channel surfaces and drainage via corner flow were observed and analyzed. A theory is developed to determine what operating parameters and channel surface contact angles lead to sufficient liquid drainage from the fuel cell via corner flow. Under these conditions, the fuel cell could operate stably under a low flow rate or stoichiometry with only a minimum pressure drop required to drive the oxidizer flow. However, when the corner flow is insufficient to remove liquid water from the gas channel, it was observed that the annular film flow occurs, often followed by film instability and channel clogging. Channel clogging shuts down an entire channel and hence reduces the cell’s active area and overall performance.

Controllable gas-liquid phase flow patterns and monodisperse microbubbles in a microfluidic T-junction device

Abstract: This letter describes the gas-liquid phase flow patterns and the mechanism of generation of monodisperse microbubbles in a T-junction microfluidic device using the crossflowing shear-rupturing technique. The bubble size is ranged from 100 to 500μm. The air phase states as isolate air slugs, “pearl necklaces,” periodic isolate bubbles, zig-zag bubble patterns, and multiple-bubble layer can be observed in the wider measured channel. The bubble size relates with the continuous phase flow velocity and viscosity as Vb∝1∕(μcuc), while being almost independent of surface tension γ and air phase flow rate Qg, for the conditions used in this work. The bubble formation mechanism by using the crossflowing shear-rupturing technique is different from the hydrodynamic flow focusing and both geometry-dominated breakup techniques. Our system provides independent control of both the size and volume fraction of dispersed bubbles.

Gas slug ascent through changes in conduit diameter: Laboratory insights into a volcano‐seismic source process in low‐viscosity magmas

Abstract: Seismic signals generated during the flow and degassing of low-viscosity magmas include long-period (LP) and very-long-period (VLP) events, whose sources are often attributed to dynamic fluid processes within the conduit. We present the results of laboratory experiments designed to investigate whether the passage of a gas slug through regions of changing conduit diameter could act as a suitable source mechanism. A vertical, liquid-filled glass tube featuring a concentric diameter change was used to provide canonical insights into potentially deep or shallow seismic sources. As gas slugs ascend the tube, we observe systematic pressure changes varying with slug size, liquid depth, tube diameter, and liquid viscosity. Gas slugs undergoing an abrupt flow pattern change upon entering a section of significantly increased tube diameter induce a transient pressure decrease in and above the flare and an associated pressure increase below it, which stimulates acoustic and inertial resonant oscillations. When the liquid flow is not dominantly controlled by viscosity, net vertical forces on the apparatus are also detected. The net force is a function of the magnitude of the pressure transients generated and the tube geometry, which dictates where, and hence when, the traveling pressure pulses can couple into the tube. In contrast to interpretations of related volcano-seismic data, where a single downward force is assumed to result from an upward acceleration of the center of mass in the conduit, our experiments suggest that significant downward forces can result from the rapid deceleration of relatively small volumes of downward-moving liquid.

Discrete element simulation for pneumatic conveying of granular material

Abstract: The pneumatic transport of granular material is a common operation frequently employed to transport solid particles from one location to another. It is well established in the literature that different flow regimes can arise in such transportation processes depending on the system geometry and operating conditions used. In this study, the pneumatic transports of solid particles in both vertical and horizontal conveying lines were studied numerically using the discrete element method coupled with computational fluid dynamics. The simulation outputs corresponded well with reported experimental observations in terms of the different flow regimes obtained at different operating conditions. In the vertical pneumatic conveying simulations, two different flow patterns corresponding to the experimentally observed dispersed flow and plug flow regimes were obtained at different gas velocities and solid concentrations. Similarly, the homogeneous flow, stratified flow, moving dunes, and slug flow regimes previously reported to occur in horizontal pneumatic conveying were also reproduced computationally in this study. Solid concentration profiles obtained by spatial averaging along the length of the pipe showed a symmetrical but non-uniform distribution for dispersed flow and an almost flat distribution for plug flow in vertical pneumatic conveying. The profile for stratified flow in horizontal pneumatic conveying showed higher solid concentration near the bottom wall due to the effects of gravitational settling, while that for slug flow was flat. Hysteresis in solid flow rates was observed in vertical pneumatic conveying near the point where transition between the dispersed and plug flow regimes was expected to occur. Solid flow rates were also found to be more sensitive towards the coefficient of friction than the coefficient of restitution of particles and the pipe walls in a sensitivity analysis study of these parameters. © 2005 American Institute of Chemical Engineers AIChE J, 2006

An Online Flow Pattern Identification System for Gas–Oil Two-Phase Flow Using Electrical Capacitance Tomography

Abstract: An online and noninvasive gas-oil two-phase flow pattern identification system is proposed based on the electrical capacitance tomography (ECT) technique. A fuzzy pattern recognition technique is introduced to solve the flow pattern identification problem because the flow pattern and its transition are fuzzy in nature. Images captured by the ECT system are analyzed through image processing and a pattern recognition technique. The flow pattern identification results are achieved by fuzzy sets theory. The online flow pattern identification system is established, and typical flow patterns, such as the homogeneous flow, the stratified flow, the annular flow, and the slug flow, in horizontal pipeline can be identified. The experimental results show that the online flow pattern identification system is effective

Subsea multiphase pumping systems

Abstract: In subsea multiphase pumping systems, the use of a gas-liquid cylindrical cyclone (GLCC) as a separator to recirculate liquid from pump discharge to pump suction, especially during high gas inlet conditions from a multiphase petroleum stream. Further contemplated is protection of the pump from momentary high gas inlet conditions due to an incoming slug flow profile from a petroleum stream, via transforming a naturally varying multiphase petroleum stream into separated phases for measured distribution to the pump suction and ensuring a minimum liquid flow.

Carbon dioxide evaporation in a single microchannel

Abstract: Despite its importance for designing evaporators and condensers, a review of the literature shows that heat transfer data during phase change of carbon dioxide is very limited, mainly for microchannel flows In order to give a contribution on this subject, an experimental study of CO2 evaporation inside a 08 mm-hydraulic diameter microchannel was performed in this work The average heat transfer coefficient along the microchannel was measured and visualization of the flow patterns was conducted A total of 67 tests were performed at saturation temperature of 233°C for a heat flux of 1800 W/(m2°C) Vapor qualities ranged from 0005 to 088 and mass flux ranged from 58 to 235 kg/(m2s) An average heat transfer coefficient of 9700 W/(m2°C) with a standard deviation of 35% was obtained Nucleate boiling was found to characterize the flow regime for the test conditions The dryout of the flow, characterized by the sudden reduction in the heat transfer coefficient, was identified at vapor qualities around 085 Flow visualization results showed three flow patterns For low vapor qualities (up to about 025), plug flow was predominant, while slug flow occurred at moderated vapor qualities (from about 025 to 050) Annular flow was the flow pattern for higher vapor qualities

Surfactant Use for Slug Flow Pattern Suppression and New Flow Pattern Types in a Horizontal Pipe

Abstract: A set of experiments was performed to study flow pattern suppression in horizontal air-water pipe flow by means of surfactant additive Results suggest that addition of the surfactant to the gas-liquid flow significantly reduces the occurrence of slug flow In addition, previously unreported flow patterns were observed to exist between slug and dispersed bubble flows It is concluded that new mechanisms for slug flow transition need to be considered

Foam flow of oil-refrigerant R12 mixture in a small diameter tube

Abstract: This work presents an experimental investigation of the ester oil ISO VG10-refrigerant R134a mixture flow with foam formation through a straight horizontal 3.22 mm ID diameter, 6.0 m length tube. An experimental apparatus was designed to permit the measurement of both pressure and temperature profiles along the tube as well as the visualization of the flow patterns of the two-phase flow. Tests were performed at different mass flow rates, several refrigerant mass fractions at the inlet of the flow, and inlet mixture temperatures around 28 and 39 °C. A liquid mixture flow with constant temperature and pressure gradient could be seen at the inlet of the tube. As the flow proceeded towards the exit of the tube the pressure drop produced a reduction of the refrigerant solubility in the oil yielding to formation of the first bubbles. Initially, small and few bubbles could be noticed and the flow behaved as a conventional two-phase flow. Eventually, the bubble population increased and foam flow was observed at the exit of the flow. Due to the great formation of bubbles, both the temperature and pressure gradient of the mixture were greatly reduced in this region of the flow.

Turbid two-phase slug flow in a microtube: Simultaneous visualization of structure and velocity field

Abstract: Frequency domain optical coherence tomography with phase-resolved algorithm is presented to perform high-resolution, cross-sectional imaging of turbid two-phase slug flow in a microtube. Imaging and quantifying the interfacial structure and velocity field in a turbid two-phase flow are important to understand the mechanism of bioreactions and flow dynamics in biological systems. The authors demonstrate here that optical coherence tomography can measure detailed interfacial structures, bubble velocity, and velocity field inside liquid slugs. The radial liquid velocity was quantified without refraction correction. Two toroidal vortices per liquid slug were visualized which is the essential mechanism for radial heat and mass transfers.