Effects Of Slug Flow Research Articles

Slug flow-enhanced vacuum membrane distillation for seawater desalination: Flux improvement and anti-fouling effect

Vacuum membrane distillation (VMD) is characterized by high permeate flux and low conductive heat loss among the membrane distillation technologies. However, the inescapable polarization effect severely deteriorates the distillation performance and service life. Thus, this study aimed at establishing a novel slug flow-enhanced VMD system to effectively address the polarization problem and investigating the influence of flow behaviors on MD process. First, a novel slug flow-enhanced VMD system was developed. The transmembrane permeate flux was intensified by 28.1 %. The performance enhancement was mainly ascribed to the increased turbulence and intensified shear stress. What’s more, further anti-fouling test demonstrated that the effect of slug flow on membrane fouling was divided into two stages. In Stage I, the permeate flux just dropped from 18.1 to 17.7 kg/(m2·h) and only 2.2 % reduction was observed in slug flow-enhanced VMD system, while it dropped rapidly from 15.2 to 12.9 kg/(m2·h) and a significant reduction of 15.2 % was observed in conventional VMD system. This research would be useful to membrane distillation for seawater desalination.

Investigation on slug flow phenomenon in Alborz morning glory spillway and preventing its destructive effects

Slug flow is a flow pattern which occurs in conveyance systems containing a two phase-fluid flow. Large air bubbles entrapped along these systems interrupt the flow and conduct to undesirable pressures and their fluctuations. Most of the previous concerning studies on slug flow phenomenon were performed in micro-channels with small scales in which the expansion of the air bubbles was negligible. In contrast, we investigated the systems with large pressure drops which conduct to abrupt increases on the volume of air phase. In this research, the verification tests applying CFD techniques were performed in OpenFOAM software by interFoam solver. The performance of morning glory spillways is investigated under steady states. While during the occurrence of flood, by increasing the depth over the spillway crest, the discharge is augmented which conducts to entrap the air pockets with pressure and velocity fluctuations. These fluctuations influence on the life of structures and their performances. This study aimed on unsteady flow in a glory morning spillway and its consequences and proposing the measures for reducing the destructive effects of slug flow. The pressure and velocity fluctuation were considered as the indices for the performance of a hydraulic structure. Spatial and temporal variations of pressure and velocity along the spillway are evaluated. Also, the influences of spillway geometry on slug flow and the measures to attenuate its destructive effects are analyzed. The results showed that the better performances of morning glory spillway are coincided by the diameter of tunnel superior than the height of water above the crest.

Computational investigation on the effects of rib on the slug flow phenomenon; using OpenFOAM

Slug flow is a two-phase fluid flow pattern, characterized by a series of liquid slugs dispersed by relatively large air bubbles. Air bubbles produced and trapped during the slug flow phenomenon conduct to interruption in flow which induces pressure and velocity fluctuations. These effects have destructive circumstances in conduits and conveyance structures. This paper deals with studying numerically two-phase flows using computational fluid dynamic (CFD) techniques performed in OpenFOAM (an open source software) by interFoam solver. Most of the previous concerning studies on slug flow were performed in micro-channels with small scales in which the expansion of the air bubbles was negligible. By contrast, we investigated the systems with large pressure drops which conduct to abrupt increases on the volume of air phase. The slug flow phenomenon could be created by introducing air and water with different velocities at inlet of a culvert with air to water velocity ratio varied from 1.1 to 34. First, this study focused on temporal and spatial variations of pressure and velocity along the culvert. After that, by dimensional analysis, the nondimensional parameters influencing the slug flow phenomenon are extracted and analyzed. Finally, different strategies for reducing the destructive effects of slug flow, including the shape and location of ribs, are evaluated and the best strategy is proposed.

Pulse shear stress for anaerobic membrane bioreactor fouling control

Increase of shear stress at membrane surfaces is a generally applied strategy to minimize membrane fouling. It has been reported that a two-phase flow, better known as slug flow, is an effective way to increase shear stress. Hence, slug flow was introduced into an anaerobic membrane bioreactor for membrane fouling control. Anaerobic suspended sludge was cultured in an anaerobic membrane bioreactor (AMBR) operated with a side stream inside-out tubular membrane unit applying sustainable flux flow regimes. The averaged particle diameter decreased from 20 to 5 microm during operation of the AMBR. However, the COD removal efficiency did not show any significant deterioration, whereas the specific methanogenic activity (SMA) increased from 0.16 to 0.41 gCOD/g VSS/day. Nevertheless, the imposed gas slug appeared to be insufficient for adequate fouling control, resulting in rapidly increasing trans membrane pressures (TMP) operating at a flux exceeding 16 L/m2/h. Addition of powdered activated carbon (PAC) enhanced the effect of slug flow on membrane fouling. However, the combined effect was still considered as not being significant. The tubular membrane was subsequently equipped with inert inserts for creating a locally increased shear stress for enhanced fouling control. Results show an increase in the membrane flux from 16 L/m2/h to 34 L/m2/h after the inserts were mounted in the membrane tube.

A Conceptual Study of Finger-Type Slug Catcher for Heavy-Oil Fields

Summary The gas/liquid-separation processes in heavy- and extra-heavy-oil fields are performed mainly with gravity conventional separators. However, the separation efficiency of this equipment depends on the operating conditions, an appropriate design, and the properties of the fluids. Therefore, the separation efficiency is affected mainly by the high liquid viscosity, low pressures, and the low gas-flow rate that are present in heavy- and extra-heavy-oil fields. Additionally, these conditions increase the probability of slug-flow formation through pipelines, which causes operational problems, mainly in the separation process. This situation raises the need to design a gas/liquid separator able to handle viscous liquid, reduce the effects of slug flow, and perform an efficient separation. There are different technologies that can help improve gas/liquid separation, among them the finger-type slug catcher. This technology is usually used in gas fields or light- and medium-oil fields as a flow conditioner and slug-flow mitigator. However, this paper has changed that focus toward considering a heavy-oil design. This paper presents an improvement in the methodology of Sarica et al. (1990), predicting the dimensions of a finger-type slug catcher for heavy-oil fields. It is derived from the effect from the transition of stratified flow to nonstratified flow when the liquid phase is viscous and only considers slug-flow characteristics under normal flow; on the basis of the improvement, the required diameter and length of the finger are determined. This improvement is used to design a finger-type slug catcher for heavy-oil fields in the Orinoco belt. An economic comparison against conventional separators is presented, demonstrating the finger-type slug catcher to be more economical than the conventional separator.

Flux enhancement by a continuous tangential gas flow in ultrafiltration hollow fibres for drinking water production: Effects of slug flow on cake structure

A new process is proposed to reduce particulate membrane fouling by injecting air into the feed stream, creating a gas/liquid two-phase flow on the membrane surface. The injected air is supposed to form air slugs inside the hollow fibres; these slugs create high wall shear stresses and flow instabilities. These phenomena may prevent filtered particles from settling on the membrane surface, and so enhance the ultrafiltration flux. Experiments were carried out with clay suspensions in hollow fibre membranes. A range of air velocities and particle concentrations was examined. The air injection process led to an increase in the permeate flux, depending on the liquid velocity and transmembrane pressure, for all the various concentrations studied. For specific conditions, the flux can be increased by 155% using a critical gas velocity. Above this critical value, the flux is no longer enhanced. The air injection clearly modifies the cake structure, and seems to expand the cake. Evolutions of cake thickness, porosity and specific resistance with gas velocity are given and analysed.

Flux enhancement by a continuous tangetial gas flow in ultrafiltration hollow fibres for drinking water production: Effects of slug flow on cake structure

Flux enhancement by a continuous tangetial gas flow in ultrafiltration hollow fibres for drinking water production: Effects of slug flow on cake structure