Liquid Slug Research Articles

Startup and transport characteristics of oscillating heat pipe using ionic liquids

This paper attempted to investigate the effects of ionic liquids (ILs) addition on the startup, operation and thermal performance of a closed loop copper oscillating heat pipe (OHP, four turns). The liquid slug oscillation characteristics and flow pattern of the OHP with various heat input were also experimentally examined utilizing high-speed camera. The experimental results showed that for higher ILs mass fractions, the OHP behaved a lower startup power (decrease of 29 W) and startup temperature (decrease of 11 °C), which are favorable of not only facilitating a faster startup of the OHP but also effectively avoiding the “dry-out” problem. When the mass fraction of ILs was 0.67%, the ILs-water based OHP substantially had the same amplitude and the velocity as water-based OHP. When the ILs addition was respectively 19.1% and 44.4% (mass fraction), the oscillation liquid slug displayed a high frequency and small amplitude characteristics. In case of a low mass fraction of ILs, ILs-water based OHP nearly perform the same heat transfer performance as the water based OHP presented. However, the heat transfer performance of the ILs-water based OHP showed a slight decrease of 10% at higher heat input (>220 W), which in general did not affect the overall thermal performance and operational stability of the OHP. A modified correlation of Kutateladze number (Ku) was established and could basically predict the thermal performance within approximately ±10% deviation in the vertical closed loop OHP at a filling ratio around 65% considering the effects of ILs addition.

Intermittent flow initiation in a horizontal tube: quantitative visualization and CFD analysis

© 2018, The Brazilian Society of Mechanical Sciences and Engineering. The onset of slugging in stratified gas–liquid flow in a 26.4-mm ID horizontal pipe was investigated experimentally using a high-speed digital video processing technique. Spatio-temporal displacement maps were produced to show the evolution of two-phase flow structures (waves, slug precursors and liquid slugs) in the flow entrance region of the air–water mixture flow. The two-phase distribution derived from the visualization technique was in good agreement with measurements performed using non-intrusive capacitance sensors. Numerical results from a hybrid CFD approach that combined the two-fluid model and volume-of-fluid method to track the position of the large-scale interface in time and space also agreed well with the quantitative flow visualization data for phase distribution, slug length and slug frequency. The relative errors between the flow structure velocities calculated by cross-correlating the numerical and experimental (image analysis) phase distribution signals were 6.8 and − 7.4% for the slug and plug flow cases, respectively.

Multiple Bounces and Oscillatory Movement of a Microdroplet in Superhydrophobic Minichannels

A droplet jumping intensification device of droplets surrounded by gas has effectively integrated small droplets (d = 0.30–0.60 mm) generated by an electrohydrodynamics system (EHD) and superhydrophobic minichannel to enhance gas absorption or chemical reaction due to the multiple bounces of the droplets. Droplet internal circulation was formed after impact on the superhydrophobic surface in the process of droplet bounce back and forth, with deformation in the superhydrophobic minichannel, which was like a liquid slug moving in the meandering channels. It was shown that the bouncing frequency went up with the decreasing height of the minichannel, meaning that the bouncing movement became stronger in the confined space. The droplet trajectory could be better controlled by the height of the minichannel, droplet size, and impact Weber number, which was investigated experimentally and analytically.

Air–Liquid Segmented Continuous Crystallization Process Optimization of the Flow Field, Growth Rate, and Size Distribution of Crystals

The continuous crystallization behavior of an air–liquid segmented tubular crystallization process was investigated using glycine as a model compound. Process parameters such as aspect ratio of the liquid slug, Reynolds number, seed loading, tubing material, slug shape, and tube crystallizer arrangement were optimized experimentally for their effect on the growth rate, size distribution, and morphology of the produced crystals. An Eulerian liquid–solid two-phase model coupled with a realizable k−e model in a computational fluid dynamics platform was also used to simulate the suspension state and the flow trajectory of crystals in a liquid slug. The results of our study indicated that a uniform crystal size benefiting from a heightened flow dynamics of crystals can be achieved under the following conditions: a slug aspect ratio of 1:1, 2 wt % seed loading, a Reynolds number higher than 22.87, a tubing material with highly hydrophobic properties and a resulted highly spherical slug shape. The growth rate of...

Mechanistic studies of enhanced oil recovery by imidazolium-based ionic liquids as novel surfactants

Ionic liquids are recently considered as an alternation to surfactants for their application in enhanced oil recovery (EOR) because of their promising surface-active properties. In the present study, the ability of a series of synthesized ionic liquids (C8mimBF4, C10mimBF4 and C12mimBF4) to reduce interfacial tension (IFT) and change the wettability of oil-wet rock have been investigated. Results demonstrated that all three synthesized ionic liquids enhanced interfacial properties and rock-wetting characteristics. The ionic liquids were able to significantly reduce IFT and remained stable at high temperature and saline conditions. Addition of organic alkali also showed a synergistic effect on IFT reduction between crude oil and ionic liquid solution. FTIR and zeta potential measurements were conducted to establish the mechanism of wettability alteration of oil-wet rock. Emulsification tests confirmed the capability of the ionic liquids to emulsify the trapped oil, which is an important mechanism of chemical EOR. The loss of ionic liquids by adsorption on rock surface was studied, and the adsorption data were analyzed by Langmuir and Freundlich models. Microemulsion study showed Winsor type III behavior with ultra-low IFT, which is beneficial for EOR application. Sand pack flooding experiments were conducted to study the EOR efficiency using the synthesized ionic liquids and around 32.28% additional recovery was observed after the conventional water flooding with injection of ionic liquid, polymer and alkali slugs.

Dynamic changes in gas-liquid mass transfer during Taylor flow in long serpentine square microchannels

The present work focuses on the hydrodynamics variation and mass transfer characteristics of Taylor flow along long serpentine microchannels with a square cross-section. The volumetric mass transfer coefficient (kLa) is regarded as the transient change value to characterize the gas-liquid mass transfer process of CO2 in water. All experimental data of Taylor bubble are obtained from 1000 continuously captured images. An online high-speed imaging method and the unit cell model are adopted in this study. The effects of gas and liquid flow rates, together with microchannel geometry are investigated on Taylor bubble characteristics in terms of length, velocity and the mass transfer performance.Taylor bubble length shrinks and subsequently plateaus out along the flow direction from the T-junction, resulting in the decrease in Taylor bubble velocity. kLa in a unit cell gradually decreases along the serpentine microchannel, and increases as the channel cross-sectional area decreases. As the gas flow rate increases under a given liquid flow rate, a critical point is found for the evolution of kLa and kL (that is the liquid phase mass transfer coefficient). The results indicate that the contribution of the circulation in the liquid slug to kL is dominant before the critical point compared to the leakage flow in the liquid film. All these findings in this work give important information to understand the dynamic change in gas-liquid Taylor flow mass transfer within microchannels. They will serve as basis for designing and optimizing gas-liquid multiphase microreactors in the future.

Millimeter-scale liquid metal droplet thermal switch

Devices capable of actively controlling heat flow have been desired by the thermal management community for decades. The need for thermal control has become particularly urgent with power densification resulting in devices with localized heat fluxes as high as 1 kW/cm2. Thermal switches, capable of modulating between high and low thermal conductances, enable the partitioning and active control of heat flow pathways. This paper reports a millimeter-scale thermal switch with a switching ratio >70, at heat fluxes near 10 W/cm2. The device consists of a silicone channel filled with a reducing liquid or vapor and an immersed liquid metal Galinstan slug. Galinstan has a relatively high thermal conductivity (≈16.5 W/mK at room temperature), and its position can be manipulated within the fluid channel, using either hydrostatic pressure or electric fields. When Galinstan bridges the hot and cold reservoirs (the “ON” state), heat flows across the channel. When the hot and cold reservoirs are instead filled with the encapsulating liquid or vapor (the “OFF” state), the cross-channel heat flow significantly reduces due to the lower thermal conductivity of the solution (≈0.03–0.6 W/mK). We demonstrate switching ratios as high as 15.6 for liquid filled channels and 71.3 for vapor filled channels. This work provides a framework for the development of millimeter-scale thermal switches and diodes capable of spatial and temporal control of heat flows.

Flow disturbances induced by an orifice plate in a horizontal air-water flow in the slug regime

An experimental study of the flow disturbances induced by an orifice plate in an air-water slug flow is carried on a horizontal test section with 0.026 m and 1009 pipe diameters long. Three orifices with distinct area contraction ratios are tested at three axial positions along the test section. The instantaneous orifice pressure difference as well as the absolute pressure and void fraction along the test section are measured. The data is further processed to get the bubble nose velocities and the slug frequency. The pressure difference at the orifice and the absolute pressure along the test section reveal a pulsatile pressure behavior due the intermittent passage liquid slugs and elongated bubble through the orifice. The spectral analyses disclose a match between the frequency of passage of slugs through the orifice and the frequency of the orifice pressure fluctuations. The orifice disturbances on the void fraction, bubble nose velocity and frequency of passage exhibits a dependency on the orifice size and to the distance from the mixer.

Experimental and numerical investigation on slug initiation and initial development behavior in hilly-terrain pipeline at a low superficial liquid velocity

The initiation and initial development behavior of gas–liquid slug flow is experimentally and numerically investigated in a hilly–terrain pipeline at a low superficial liquid velocity. The test section consists of three parts: three sections of horizontal pipe, one ascending pipe, and one descending pipe. The inclined pipes are at an angle of 10°, all sections are connected by elbow pipes, and the inner diameter is 40 mm. Air and water, with a superficial liquid velocity ranging from 0.002 m/s to 0.1 m/s, are used as working fluids, which are operated at ambient temperature and pressure. Differential pressure transmitters and ultrasound Doppler velocimetry are used to measure the pressure drop and liquid film height, and velocity, respectively. Numerical simulation is simultaneously carried out using the coupled level-set and volume of fluid method, and is verified through comparison with experimental data. Slug initiation mechanisms are discussed based on experimental observation and numerical prediction. The characteristics of liquid slug initiation and initial development are also explained using pressure drop fluctuation, liquid holdup, and phase distribution features. The overall qualitative and quantitative analysis can contribute to the improvement of recent theoretical and numerical models.

NUMERICAL INVESTIGATION OF THERMAL PERFORMANCE OF A CRYOGENIC OSCILLATING HEAT PIPE

The oscillating heat pipe (OHP) with simple structure and high heat transfer effi ciency has been proposed recently for cryogenic use. In this paper, a model of fluid flow and heat transfer on multiple liquid slugs and vapor plugs in an OHP has been developed. The cryogenic OHP with working fluid of nitrogen is studied using the model. Oscillatory flow and circulatory flow are observed. The mass flow rate of liquid slugs and the mean wall temperature of the evaporator section are calculated. Furthermore, the effective thermal conductivity of the cryogenic OHP is determined, which shows much higher thermal performance than that of high purity metal. The effects of the heat input and the filling ratio on the effective thermal conductivity are discussed. Finally, the calculated effective thermal conductivity is compared with experimental results, and it is seen that this model correctly predicts the thermal performance of a cryogenic OHP in the case of a medium and low heat input.

Wettability control of droplet durotaxis.

Durotaxis refers to cell motion directed by stiffness gradients of an underlying substrate. Recent work has shown that droplets also move spontaneously along stiffness gradients through a process reminiscent of durotaxis. Wetting droplets, however, move toward softer substrates, an observation seemingly at odds with cell motion. Here, we extend our understanding of this phenomenon, and show that wettability of the substrate plays a critical role: while wetting droplets move in the direction of lower stiffness, nonwetting liquids reverse droplet durotaxis. Our numerical experiments also reveal that Laplace pressure can be used to determine the direction of motion of liquid slugs in confined environments. Our results suggest new ways of controlling droplet dynamics at small scales, which can open the door to enhanced bubble and droplet logic in microfluidic platforms.

Numerical investigation on pulsating heat pipes with nitrogen or hydrogen

With flexible structure and excellent performance, pulsating heat pipes (PHP) are regarded as a great solution to distribute cooling power for cryocoolers. The experiments on PHPs with cryogenic fluids have been carried out, indicating their efficient performances in cryogenics. There are large differences in physical properties between the fluids at room and cryogenic temperature, resulting in their different heat transfer and oscillation characteristics. Up to now, the numerical investigations on cryogenic fluids have rarely been carried out. In this paper, the model of the closed-loop PHP with multiple liquid slugs and vapor plugs is performed with nitrogen and hydrogen as working fluids, respectively. The effects of heating wall temperature on the performance of close-looped PHPs are investigated and compared with that of water PHP.

Experimental study of Large-scale cryogenic Pulsating Heat Pipe

Pulsating Heat Pipes (PHP) are passive two-phase heat transfer devices consisting of a long capillary tube bent into many U-turns connecting the condenser part to the evaporator part. They are thermally driven by an oscillatory flow of liquid slugs and vapor plugs coming from phase changes and pressure differences along the tube. The coupling of hydrodynamic and thermodynamic effects allows high heat transfer performances. Three closed-loop pulsating heat pipes have been developed by the DACM (Department of Accelerators, Cryogenics and Magnetism) of CEA Paris-Saclay, France. Each PHP measures 3.7 meters long (0.35 m for the condenser and the evaporator and 3 m for the adiabatic part), being almost 20 times longer than the longest cryogenic PHP tested. These PHPs have 36, 22 and 12 parallel channels. Numerous tests have been performed in horizontal position (the closest configuration to non-gravity) using nitrogen as working fluid, operating between 75 and 90 K. The inner and outer diameters of the stainless steel capillary tubes are 1.5 and 2 mm respectively. The PHPs were operated at different filling ratios (20 to 90 %), heat input powers (3 to 20 W) and evaporator and condenser temperatures (75 to 90 K). As a result, the PHP with 36 parallel channels achieves a certain level of stability during more than thirty minutes with an effective thermal conductivity up to 200 kW/m.K at 10 W heat load and during forty minutes with an effective thermal conductivity close to 300 kW/m.K at 5 W heat load.

Measurement of gas phase characteristics in vertical oil-gas-water slug and churn flows

In the present study, gas phase characteristics of oil-gas-water slug and churn flows in a vertical upward pipe with 20 mm inner diameter (ID) are experimentally investigated. We firstly measure the fluctuating signals of a traversable bi-optical fiber probe at different radial positions. The gas phase flow parameter distributions (local gas velocity, local gas holdup) are obtained and the flow structure is uncovered by calculating the series of gas bubble chord lengths at different radial positions. Additionally, in order to describe the flow structure of slug flow, the relative length of the liquid slug, the profile distributions of gas holdup and bubble size in liquid slug are investigated. To understand the nonlinear dynamic characteristics of slug and churn flows, multi-scale cross entropy (MSCE) algorithm is applied to analyze the signals of a high-resolution half-ring conductance sensor and bi-optical fiber probe. The result indicates that multi-scale cross entropy can be an effective tool for validating the measurement of gas phase characteristics by using the traversable bi-optical fiber probe.

Numerical investigation of effect of film dynamics on fluid motion and thermal performance in pulsating heat pipes

In the present study, a one-dimensional numerical model for pulsating heat pipes (PHPs) is presented. The balance equations of mass, momentum, and energy were solved for liquid slugs, vapor plugs and also for liquid films, along with the heat conduction equation for the tube wall. The spatial and temporal variations of the liquid-film thickness were directly simulated, and the film was allowed to dry out when the local thickness decreased to the roughness height of the tube wall. The numerical results showed good agreement with the experimental data, not only for a vertical PHP but also for horizontal and inclined PHPs with various different parameters, including number of turns, working fluid, filling ratio, and operating temperature. Furthermore, the effect of the dynamics of the liquid film on flow behavior and heat transfer was examined numerically. It was confirmed that the oscillating motion of the fluid inside a horizontal PHP cannot be completely predicted when the film dynamics are ignored. For a vertical PHP, on the other hand, circulating motion was predicted regardless of the film dynamics, and the role of the film dynamics on the prediction of thermal performance was not significant. The present model is considered the first to predict the thermal performance of a horizontal PHP without introducing any fitting parameters.

Effect of Gas Channel Wall Contact Angle on a Liquid Water Slug Dynamics in a Straight Gas Channel of a Trapezoidal PEMFC

The volume of fluid method is used to investigate the behavior of a liquid water slug in a PEMFC trapezoidal gas channel(GC) with a open angle of 60 degrees. To evaluate the effect of the contact angle of the top and side walls, the gas diffusion layer water coverage ratio(GWCR) and water volume fraction(WVF) in a inspection control volume are analyzed. As the contact angle increases, GWCR increases and WVF decreases. The cases with the GC contact angle of 60 and 80 degrees show the more favorable water removal characteristics compared to the other cases in a GC flooding condition.

Modeling Liquid Slugs Accelerating in Inclined Conduits

In this article, we simulate traveling liquid slugs in conduits, as they may occur in systems carrying high-pressure steam. We consider both horizontal and inclined pipes in which the slug is accelerated by a suddenly applied pressure gradient, while at the same time, gravity and friction work in the opposite direction. This causes a steep slug front and an extended slug tail. The shapes of front and tail are of interest since they determine the forces exerted on bends and other obstacles in the piping system. The study also aims at improving existing one-dimensional (1D) models. A hybrid model is proposed that enables us to leave out the larger inner part of the slug. It was found that the hybrid model speeds up the two-dimensional (2D) computations significantly, while having no adverse effects on the shapes of the slug's front and tail.

Continuous Reactive Precipitation in a Coiled Flow Inverter: Inert Particle Tracking, Modular Design, and Production of Uniform CaCO3 Particles

A multiphase flow profile inside a helically coiled tubular device (HCTD) was observed by using a high-speed camera. Gas–liquid slug flow observations revealed that the Taylor vortices are influenced by secondary flow due to the centrifugal force acting perpendicular to the flow direction. Hence, mixing inside the liquid slug is enhanced by the combination of Dean and Taylor vortices in HCTD. The modular design of a specific type of HCTD, that is, the coiled flow inverter (CFI) is elucidated by the representation of a new design space diagram. Continuous precipitation of calcium carbonate (CaCO3) was investigated for modular CFI made of polyvinyl chloride (PVC) tubes (di = 3.2 mm) with slug flow patterns. CaCO3 was continuously precipitated along CFI with a conversion of ca. 90%. CFI provided a narrower particle size distribution with median particle diameters around 28 μm and more uniform morphology in comparison to a batch reactor.

Suspension stability of slurry Taylor flow: A theoretical analysis

A theoretical model for predicting the suspension stability of slurry Taylor flow is presented. Force balance analysis is conducted on a particle that moves downward along the leading meniscus of a Taylor bubble towards the thin liquid film between the bubble and the channel wall. The concept of maximum resistance force is proposed for calculating critical values of capillary number, particle size and density, and contact angle that determine whether solid particles stay suspended in the same liquid slug or fall through the thin liquid film and travel among different liquid slugs. It was found that the deformation of bubble surface enforced by the particle motion appeared independent of the particle material that has rough surface. The maximum capillary force decreased as the capillary number increased, but increased faster than the downward drag force as the particle size increased. A particle with relatively more hydrophobic surface was more prone to fall into the liquid film. It was implied that devices of Taylor flow might be applied to achieve particle selection based on particle size, density and surface hydrophobicity. The model proves capable of accurately predicting the suspension stability of slurry Taylor flow for the design and operation of industrial chemical reaction processes.

Study of high viscous multiphase phase flow in a horizontal pipe

Heavy oil accounts for a major portion of the world’s total oil reserves. Its production and transportation through pipelines is beset with great challenges due to its highly viscous nature. This paper studies the effects of high viscosity on heavy oil two-phase flow characteristics such as pressure gradient, liquid holdup, slug liquid holdup, slug frequency and slug liquid holdup using an advanced instrumentation (i.e. Electrical Capacitance Tomography). Experiments were conducted in a horizontal flow loop with a pipe internal diameter (ID) of 0.0762 m; larger than most reported in the open literature for heavy oil flow. Mineral oil of 1.0–5.0 Pa.s viscosity range and compressed air were used as the liquid and gas phases respectively. Pressure gradient (measured by means differential pressure transducers) and mean liquid holdup was observed to increase as viscosity of oil is increased. Obtained results also revealed that increase in liquid viscosity has significant effects on flow pattern and slug flow features.