Slug Motion Research Articles

Isolated slug traveling in a voided line and impacting at an end orifice

Driven by upstream high-pressure steam, liquid slugs in nuclear power plant pipelines impact the end orifice at high speed, leading to bursting pipelines and threatening the plant's safety. This research aimed to accurately and efficiently assess the dynamic behavior of an isolated slug driven by pressurized air in a voided line with an end orifice. An improved one-dimensional (1D) model for the slug motion and impact was established. The dynamic variation of the pressure at both the slug's tail and front, the variation of the slug length, and the frictional resistance coefficient in the model was obtained by three-dimensional (3D) computational fluid dynamics (CFD). Based on 27 cases with different pipeline diameters and tank pressures, it was observed that the driving air pressure had a quadratic relationship and that the slug length had a constant rate of decrease vs the slug tail displacement. Finally, the decrease in the driving air pressure behind the slug, the increase in the air pressure ahead of the slug, the holdup coefficient, and the friction factor obtained from the 3D CFD results were interpreted in the 1D model, and the velocity histories of the liquid slug were found to be in excellent agreement with the 3D CFD solutions.

Capillary flow of two immiscible liquids initially separated by a Taylor gas bubble

We investigate the capillary displacement of silicone oil columns by water in tandem with a Taylor gas bubble in horizontally aligned tubes that initially separate the two immiscible liquids. The experiments show that the gas slug's motion forces the drainage of the prefilling liquid through a thin film deposited on the tube's wall. The displaced oil is then recovered at the advancing water meniscus. The dynamics of the capillary displacement and drainage are studied as functions of the silicone oil and gas bubble volumes. We find that the Taylor bubble's length has a small influence on the capillary displacement and drainage of the prefilling liquid. The study presents an adapted model for the capillary drainage and oil recovery using Taylor's law for the deposited film thickness on the tube's wall, which collapses all experimental data onto a single curve.

Motion of a viscous slug on heterogeneous surfaces: crossover from stick–slip to steady sliding

We present a theoretical study of viscous slug motion inside a microscopically rough capillary tube, where pronounced stick–slip motion can emerge at slow displacement rates. The mathematical description of this intermittent motion can be reduced to a system of ordinary differential equations, which also describe the motion of a pendulum inside a fluid-filled rotating drum. We use this analogy to show that the stick–slip motion transitions to steady sliding at high displacement rates. We characterize this crossover with a simple scaling relation and show that the crossover is accompanied by a shift in the dominant energy dissipation mechanisms within the system.

Strain gauge measurements of an oscillating heat pipe from startup to stable operation

Oscillating heat pipe (OHP) devices offer high heat flow (Q) and low thermal resistance (Rth) capabilities with convenient form factors for applications in spacecraft and terrestrial thermal management. Though prior measurements have used intrusive strain measurements in a single OHP turn to study the complex thermal-fluid mechanisms of OHP operation, minimally-intrusive external strain measurements on the casing of a multi-turn OHP have not been previously reported. Here, we affix strain gauges to the exterior casing of an ammonia filled flat plate OHP and use strain measurements to quantify the frequency response of ammonia vapor pressure fluctuations over a broad Q range. For all Q, the Fourier transformed strain signals referenced to the Q=0 W background are well-described by a low-pass filter fitting function with a low-frequency amplitude a and characteristic “knee” frequency c. In all cases, a increases with increasing Q due to the enhanced vapor pressure fluctuations at larger thermal loads. In the startup regime of Q<10W, we observe large temporal deviations in the strain response, large Rth>0.1K/W, and c values that increase with increasing Q. In the stable operating regime for Q near 100W, we observe a stable frequency response, a minimum value of Rth=0.02K/W, and Q-independent c values that saturate near 10Hz. Our strain gauge findings are consistent with a mechanism of intermittent boiling near startup followed by stable vapor plug/liquid slug motion at higher Q. Thus, our measurements show that externally placed and minimally invasive strain gauges canprovide diagnostic capabilities and additional insight into operation in both the startup and stable regimes for multi-turn, lightweight OHP devices used in aerospace and terrestrial thermal management applications.

An experimental study on real–time analysis of two–phase peristaltic slug flows in dialysis machines

Two–phase flows appear in many industrial and biomedical applications. One of the most vital biomedical applications of two–phase flows is in hemodialysis machines due to air embolism and heparin injection. Since these flows have a very complex and intermittent nature, studying their dynamics is a very challenging and fundamental problem. The purpose of this article is to present an experimental study on the dynamics of two–phase peristaltic slug flows. The measurement strategy is based on the image processing technology. The characteristic parameters of the two–phase pulsatile slug flows, including the slug length, as well as the translational velocity and frequency of the slug motion, are measured, and the effect of the liquid flow rate and liquid superficial velocity is investigated. The results show that the average and maximum slug velocities, and also the dominant amplitude of the slug velocity increase with the flow rate and liquid superficial velocity, while it is not possible to clearly predict a correlation between the liquid superficial velocity and the slug length. The measurement strategy presented in this article can be used in the control and alarm systems of smart dialysis machines.

Wall shear stress during slug motion in an annular channel

Wall shear stress during slug motion in an annular channel

Inertial measurement unit as a tool within dense phase pneumatic conveying. Investigation into velocity measurement accuracy, pressure and velocity relationships in slug flow

This paper presents the results of using an inertial measurement unit (IMU) to study various dynamic relationships in horizontal slug flow pneumatic conveying. The accuracy of the IMU was assessed and compared to particle image velocimetry (PIV) and once good agreement was confirmed it was used to investigate various aspects of slug flow. Relative movement between core particles and slugs tails and heads was assessed using relative pressures and quantified times spent in a slug. It was found that the propagation of particles backwards through a slug is relatively constant. Pressure-velocity relationship was observed that was theorised to be related to variations in stationary layer ahead of the slugs. Observations of further nuanced features of slug motion are also included to demonstrate the capabilities of IMUs in capturing the many dynamic aspects of the flow.

Dynamics of gas slugs in a vertical annular channel

Experimental studies of gas slug motion in an annular channel were performed. The channel consisted of two concentric pipes with the diameters of 32 and 10 mm. Experiments were performed for different values of liquid velocity. Slug velocity was measured using optical sensors. Wall shear stress measurements were performed using an electrodiffusional technique. It was shown that the shape of the slug in the annular channel differs significantly from that in a pipe. The distribution of wall shear stress along the slug was presented for different positions of the slug.

Kinetics of gravity‐driven slug flow in partially wettable capillaries of varying cross section

AbstractA mathematical model for slug (finite liquid volume) motion in not‐fully‐wettable capillary tubes with sinusoidally varying cross‐sectional areas was developed. The model, based on the Navier‐Stokes equation, accounts for the full viscous terms due to nonuniform geometry, the inertial term, the slug's front and rear meniscus hysteresis effect, and dependence of contact angle on flow velocity (dynamic contact angle). The model includes a velocity‐dependent film that is left behind the advancing slug, reducing its mass. The model was successfully verified experimentally by recording slug movement in uniform and sinusoidal capillary tubes with a gray‐scale high‐speed camera. Simulation showed that tube nonuniformity has a substantial effect on slug flow pattern: in a uniform tube it is monotonic and depends mainly on the slug's momentary mass/length; an undulating tube radius results in nonmonotonic flow characteristics. The static nonzero contact angle varies locally in nonuniform tubes owing to the additional effect of wall slope. Moreover, the nonuniform cross‐sectional area induces slug acceleration, deceleration, blockage, and metastable‐equilibrium locations. Increasing contact angle further amplifies the geometry effect on slug propagation. The developed model provides a modified means of emulating slug flow in differently wettable porous media for intermittent inlet water supply (e.g., raindrops on the soil surface).

Transformation of a Water Slug in Free Fall Under the Conditions of Exposure to an Air Flow Orthogonal to the Direction of the Slug Motion

An experimental study has been made of the influence of an orthogonal (side) air flow propagating with a velocity to 5 m/s on the phases of transformation of a water slug with an initial volume of 0.05–0.5 liter in free fall from a height of 3 m. Use was made of Phantom V411 and Phantom Miro M310 high-speed video cameras and a Tema Automotive software system with the function of continuous tracking. The laws of retardation of the phases of transformation of the water slug from the instant of formation to that of formation of a droplet cloud under the action of the air flow orthogonal to the direction of the slug motion, and also of the deceleration, removal, and destruction of the droplets and fragments of water separating from the slug surface, have been established.

Gravity-driven unsteady-state slug fall in capillaries – modeling and experimental verification

ABSTRACTInfiltration of liquid droplets into dry porous media often occurs in industrial and natural settings, which has been widely modeled as liquid slug flow in capillaries. This work focuses on gravity-driven slug motion in vertically oriented capillary tubes. To model the propagation and evolution of the slug, a mathematical model was set on the basis of Newton’s second law and the law of conservation of mass. The model includes terms like slug’s inertia, deposited film, dynamic contact angle, slug’s advancing and receding menisci hysteresis, and it particularly highlights the direct effect of the trailing film on the change of slug mass. In order to verify this model, experiments on water slug with different lengths of initial slugs were conducted in two Pyrex glass capillaries that are partially wettable. It was found that both the length and the velocity of the slug vary during the slug motion in every case. Then the experimental results were simulated with the established model by carefully presetting two fitting parameters, αa and αh, that are related to the dynamic contact angle at the advancing meniscus and the thickness of the trailing film, respectively. The good agreement between the experimental and theoretical results demonstrates that the present model is capable of describing the unsteady-state dynamics of slugs fall in porous media.

The effect of gas diffusion layer compression on gas bypass and water slug motion in parallel gas flow channels

Water slugs form in the gas flow channels of polymer electrolyte membrane fuel cells (PEMFCs) which hinder reactant transport to the catalyst layer. We report a study correlating video images of slug formation and motion with pressure/flow measurements in parallel gas flow channels. Slugs move when the differential gas pressure exceeds the force to advance the contact lines of the slug with the channel walls. Water slugs can divert the gas flow through the gas diffusion layer (GDL) beneath the ribs to adjacent channels. The flow diversion can cause slugs to stop moving. Slug size and motion has been correlated with in situ GDL permeabilities as functions of GDL compression. Compression reduces the GDL permeability under the ribs much more than the GDL permeability under the channel. A model is presented to describe the spatio‐temporal location of slugs in a PEMFC flow field. © 2014 American Institute of Chemical Engineers AIChE J, 61: 355–367, 2015

Effects of Superhydrophobic and Superhydrophilic Surfaces on Heat Transfer and Oscillating Motion of an Oscillating Heat Pipe

The effects of superhydrophobic surface and superhydrophobic and superhydrophilic hybrid surface on the fluid flow and heat transfer of oscillating heat pipes (OHPs) were investigated in the paper. The inner surfaces of the OHPs were hydrophilic surface (copper), hybrid surface (superhydrophilic evaporation and superhydrophobic condensation section), and uniform superhydrophobic surface, respectively. Deionized water was used as the working fluid. Experimental results showed that superhydrophobic surface influenced the slug motion and thermal performance of OHPs. Visualization results showed that the liquid-vapor interface was concave in the OHP with copper surface. A thin liquid film existed between the vapor plug and the wall of the OHP. On the contrary, the liquid-vapor interface took a convex profile in the OHP with superhydrophobic surface and the liquid-vapor interface contact line length in the hybrid surface OHP was longer than that in the uniform superhydrophobic surface OHP. The liquid slug movements became stronger in the hybrid surface OHPs as opposed to the copper OHP, while the global heat transfer performance of the hybrid surface OHPs increased by 5–20%. Comparing with the copper OHPs, the maximum amplitude and velocity of the liquid slug movements in the hybrid surface OHPs increased by 0–127% and 0–185%, respectively. However, the maximum amplitude and velocity of the liquid slug movements in the uniform superhydrophobic OHPs was reduced by 0–100% and 0–100%, respectively. The partial dryout phenomenon took place in OHPs with uniform superhydrophobic surface. The liquid slug movements became weaker and the thermal resistance was increased by 10–35% in the superhydrophobic surface OHPs.

Dynamic Force on an Elbow Caused by a Traveling Liquid Slug

The impact force on an elbow induced by traveling isolated liquid slugs in a horizontal pipeline is studied. A literature review reveals that the force on the elbow is mainly due to momentum transfer in changing the fluid flow direction around the elbow. Therefore, to accurately calculate the magnitude and duration of the impact force, the slug arrival velocity at the elbow needs to be well predicted. The hydrodynamic behavior of the slug passing through the elbow needs to be properly modeled too. A combination of 1D and 2D models is used in this paper to analyze this problem. The 1D model is used to predict the slug motion in the horizontal pipeline. With the obtained slug arrival velocity, slug length, and driving air pressure as initial conditions, the 2D Euler equations are solved by the smoothed particle hydrodynamics (SPH) method to analyze the slug dynamics at the elbow. The 2D SPH solution matches experimental data and clearly demonstrates the occurrence of flow separation at the elbow, which is a typical effect of high Reynolds flows. Using the obtained flow contraction coefficient, an improved 1D model with nonlinear elbow resistance is proposed and solved by SPH. The 1D SPH results show the best fit with experimental data obtained so far.

Effects of hydrophilic surface on heat transfer performance and oscillating motion for an oscillating heat pipe

In this paper, a series of experiments were performed to investigate the effects of superhydrophilic and hydrophilic surfaces on the heat transfer performance and slug motion for oscillating heat pipes (OHPs). Deionized water was used as the working fluid. The surfaces of the OHPs were copper, superhydrophilic, hydrophilic and hydrophobic surfaces with contact angles of 73.4°, 0°, 12.9° and 141.5°, respectively. The heat transfer performance in six-turn OHPs was higher than in four-turn OHPs. Experimental results showed that the surface wettability remarkably influenced the slug motion and thermal performance of OHPs. The liquid slug movements became stronger both in superhydrophilic and hydrophilic OHPs as opposed to the copper OHP, while the global heat transfer performance of the superhydrophilic and hydrophilic OHPs increased in the six-turn OHP. For six-turn OHPs, the maximum displacement of the liquid slug in the hydrophilic OHPs and superhydrophilic OHPs increased by 5–60% and 25–60%, respectively, in comparison with that of copper OHPs. The heat transfer performance of superhydrophilic OHPs and hydrophilic OHPs increased by 5–15% and 15–25%, respectively, in comparison with that of copper OHPs. The hydrophilic surface improved the heat transport capability of OHPs, and the maximum displacement and velocity of the liquid slug increased. The startup temperature of the four-turn and six-turn OHPs varied from 45 to 55°C and 35 to 45°C, respectively. The startup temperature of the four-turn and six-turn superhydrophilic OHPs varied from 40 to 55°C and from 35 to 50°C, respectively. The startup temperature of the four-turn and six-turn hydrophilic OHPs varied from 40 to 50°C and from 30 to 40°C, respectively. The hydrophilic OHP also showed a better startup performance than the copper OHP. At heat input of 150W, the thermal resistance for the four-turn hydrophobic OHPs was about two times greater than that of copper OHPs. The global heat transfer performance of the superhydrophilic OHP for the four-turn OHP was lower than that of the four-turn OHPs with hydrophilic and pure copper OHPs.

Numerical Study on Mobilization of Oil Slugs in Capillary Model with Level set Approach

Numerical investigation on mobilizing oil slugs trapped in an axisymmetric capillary tube saturated with water was conducted. The governing equations for the incompressible two-phase core-annular flow in a capillary model were derived. Numerical solutions for capturing the evolution of interface between oil and water were developed using a level set approach. A projection method was applied to obtain the velocity field of the two phase flow. The existence of a water film between the oil slug and solid tube wall was evident in mobilizing the oil slug, and the water film clearly reduced the external pressure required to mobilize the oil slug. This result matches with the experimental phenomena reported in the literature. New phenomena in oil slug mobilization and motion were also found in the numerical investigations. The results obtained showed the high efficiency of the presented approach and the significance of water film in improving the mobilization of oil slugs.

The role of the gas diffusion layer on slug formation in gas flow channels of fuel cells

Water drops emerge from large pores of the hydrophobic Gas Diffusion Layers (GDL) into the cathode gas flow channel of Polymer Electrolyte Membrane (PEM) Fuel Cells. The drops grow into slugs that span the cross-section of the flow channels. The slugs detach and are forced out the gas flow channel by the air flow. An acrylic micro-fluidic flow cell with a 1.6 mm gas flow channel and a 100 μm liquid pore through a carbon paper GDL has been used to quantitatively determine slug volumes, velocity of slug motion, and the force required to move slugs as functions of the gas and liquid flow rates. In a channel with 4 acrylic walls, slugs detach immediately upon formation. A porous GDL wall allows gas flow to bypass the slugs, thus allowing slugs to continue to grow after spanning the open area of the channel. The differential pressure to detach and move slugs is equal to the dynamic interfacial force on a slug normalized by the cross-sectional area of the channel. The dynamic interfacial force is equal to the difference between the downstream (advancing) and upstream (receding) contact lines of the water with the channel walls. Slugs will stop moving if the differential pressure drop for gas flow to bypass the slug and flow through the GDL under the rib separating the channels is less than the differential pressure required to move the slug. The results improve our physical insight into the state of water hold up in PEM fuel cells.

Experimental studies of the pressures generated by a liquid oxygen slug in a magnetic field

The strong paramagnetic susceptibility of liquid oxygen (LOX) has established it as a good candidate for a cryogenic magnetic fluid system. While its properties have been known for several decades, a fundamental understanding of the behavior of LOX in a magnetically controlled fluid system is needed for the development of a suitable space application that can operate reliably and efficiently. This study conducted quantitative experiments on the dynamics of a LOX slug in a tube when subjected to electrically-induced magnetic fields within a solenoid. The experiments used a quartz tube with an inner diameter of 1.9 mm and LOX slugs of 0.6, 1.3, 1.9, 2.5, and 3.2 mm length at various initial positions relative to the solenoid. The pressures generated by the motion of the LOX slug under the magnetic force were recorded to characterize the pressure differential generated and the breakdown of the slug. The highest attainable pressure differential was found to be 1.45 kPa, which correlated well to theoretical predictions once the analysis accounted for the resistance heating of the solenoid. The noted differences between experimental results and theory could also be attributed to impeded slug motion from shear and mass forces. Within the workable pressure range, however, an optimal slug length was found which appropriately balances the pressure, shear, and magnetic forces in the system. This paper presents the experimental data on the dynamics and the maximum pressure differential generated by a LOX slug in a magnetic field and discusses the viability of LOX in a magnetic fluid management system intended for space applications.

Gravity-driven slug motion in capillary tubes

The velocity of a liquid slug falling in a capillary tube is lower than predicted for Poiseuille flow due to presence of menisci, whose shapes are determined by the complex interplay of capillary, viscous, and gravitational forces. Due to the presence of menisci, a capillary pressure proportional to surface curvature acts on the slug and streamlines are bent close to the interface, resulting in enhanced viscous dissipation at the wedges. To determine the origin of drag-force increase relative to Poiseuille flow, we compute the force resultant acting on the slug by integrating Navier–Stokes equations over the liquid volume. Invoking relationships from differential geometry we demonstrate that the additional drag is due to viscous forces only and that no capillary drag of hydrodynamic origin exists (i.e., due to hydrodynamic deformation of the interface). Requiring that the force resultant is zero, we derive scaling laws for the steady velocity in the limit of small capillary numbers by estimating the leading order viscous dissipation in the different regions of the slug (i.e., the unperturbed Poiseuille-like bulk, the static menisci close to the tube axis and the dynamic regions close to the contact lines). Considering both partial and complete wetting, we find that the relationship between dimensionless velocity and weight is, in general, nonlinear. Whereas the relationship obtained for complete-wetting conditions is found in agreement with the experimental data of Bico and Quéré [J. Bico and D. Quéré, J. Colloid Interface Sci. 243, 262 (2001)], the scaling law under partial-wetting conditions is validated by numerical simulations performed with the Volume of Fluid method. The simulated steady velocities agree with the behavior predicted by the theoretical scaling laws in presence and in absence of static contact angle hysteresis. The numerical simulations suggest that wedge-flow dissipation alone cannot account for the entire additional drag and that the non-Poiseuille dissipation in the static menisci (not considered in previous studies) has to be considered for large contact angles.

On Thermocapillary Propulsion of Microliquid Slug

A liquid slug in a microchannel can be propelled by temperature difference between the ends of the slug. This study presents an analytical model to predict the velocity of such slug motion. The models thus far developed only considers very long slugs where the viscous dissipation near the contact line is negligible compared with the bulk dissipation. Therefore, here we develop a novel model to account for the shear stress near the contact line so that it can be applied to slugs of general configuration. It is shown that the bulk dissipation dominates when the slug length is much greater than the channel's hydraulic diameter. However, the wedge dissipation dominates when the slug length is much smaller than the hydraulic diameter. A simple experiment using a mineral oil microslug shows that our theory considering both the wedge and bulk dissipation agrees with the measurement results.