Liquid Slug Research Articles

Double condenser pulsating heat pipe cooler

A novel pulsating heat pipe cooler (PHP) based on automotive technology is presented. This technology uses numerous aluminum Multiport Extruded (MPE) tubes disposed in parallel to achieve the desired compactness. The sub-channels of the MPEs are connected in a serpentine manner by means of fluid distribution elements integrated in the two condenser manifolds. This configuration enables the oscillation of liquid slugs and elongated bubbles between the evaporator and the condenser areas. This power electronics cooling system with a double condenser area was experimentally characterized and the thermal performances measured with R245fa are presented in this paper. The influence of different parameters such as the heat load, fluid filling ratio and orientation are investigated. The results show that the device is orientation-free: the performance was identical in all orientations, vertical, horizontal and anti-vertical.

Characteristics of oscillating flow in a micro pulsating heat pipe: Fundamental-mode oscillation

Theoretical and experimental analyses are performed for oscillating flow in a micro pulsating heat pipe (MPHP). A meandering rectangular micro-channel with a hydraulic diameter of 667μm is engraved on a silicon wafer to form a five-turn closed-loop. Flow visualization through a glass top using a high-speed camera is conducted together with temperature measurement for thermal characterization of the MPHP. The MPHP is observed to have a harmonic oscillating motion: each liquid slug in the MPHP is observed to oscillate at frequency ranging from 40 to 50Hz with a phase difference of 2π/5 between adjacent slugs. A closed-form expression for the oscillating motion of the slugs is suggested from a vapor spring-liquid mass model. To quantitatively explain the oscillating mechanism by the vapor spring, a link between the spring motion of the vapor plug and heat transfer to the vapor plug is found: Expansion and contraction of the vapor plug are shown to result from continuous evaporation and condensation at the liquid film enveloping the vapor plug. The evaporation heat transfer and the condensation heat transfer are shown to be out of phase and in turn result in a nonzero net heat transfer rate. To mathematically express the relationship between the net heat transfer rate and the spring motion of the vapor plug, a semi-analytic model is proposed. A semi-analytic expression for the vapor spring constant is developed and validated with experimental results. This study on the fundamental-mode oscillation in MPHP is expected to be used as a building block for investigating more complex oscillating motion in PHPs.

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.

Transition of plug to slug flow and associated fluid dynamics

Transition of plug to slug flow is associated with bubble detachment from elongated bubble tail or bubble entrainment inside the liquid slug. The mechanism responsible for this transition was earlier identified by Ruder and Hanratty (1990) and Fagundes Netto et al. (1999) based on the shape of the hydraulic jump observed at elongated bubble tail region. The transition mechanism reported by Ruder and Hanratty (1990) and Fagundes Netto et al. (1999) was only based on their flow visualization study. Plug to slug transition and associated dynamics of bubble detachment from the elongated bubble is analysed in the present paper using flow visualization and local velocity measurements. Experiments are reported for 13 different inlet flow conditions of air and water phases. Images of plug/slug flow structures are captured at a rate of 4000 FPS using FASTCAM Photron camera and the local values of axial liquid velocity are measured using LDV system synchronised with a 3D automated traverse system. LDV measurement of local liquid velocity in the liquid slug and liquid film establishes the reason for detachment of bubbles from the slug bubble tail.

Theoretical analysis of start-up power in helium pulsating heat pipe

An analytical model for one-turn helium pulsating heat pipes (PHPs) with single liquid slug and vapor plug is established in present study. When an additional heat power takes place in the evaporating section, temperature and pressure will increase. The pressure wave travels through vapor and liquid phases at different speed, producing a pressure difference in the system, which acts as an exciting force to start up the oscillating motion. Results show that the start-up power of helium PHP is related to the filling ratio. The start-up power increases with the filling ration. However, there exist an upper limit. Furthermore, the start-up power also depends on the inclination angle of PHP. When the inclination angle increases, the heat input needed to start up the oscillating motion decreases. But for one-turn helium PHP, it can not be started up when the inclination angle is up to 90°, equalling to horizontal position,. While the inclination angle ranges between 0° (vertical position) and 75°, it can operate successfully.

Numerical and experimental investigations of thermally induced oscillating flow inside a capillary tube

A numerical model has been developed for a system consisting of a liquid slug and a vapour plug oscillating in a tube closed at one end, and connected to a reservoir at a constant pressure at the other end, which represents the most basic configuration of a Pulsating Heat Pipe (PHP). The thermally driven self-sustained oscillations of the system result from evaporation and condensation phenomena occurring at two zones of the tube (separated by an adiabatic section), one being cooled and the other being heated, simultaneously. The modelling principles of this system had been posed in previous works. In this work, the equation describing the liquid film evaporation has been substantially improved in the light of recent experimental results: both, the thickening of the film and the shortening of its length due to the evaporation at the triple line are taken into account. Furthermore, the transient heat conduction equation is solved in both the tube and the liquid film in order to calculate the temperature of the evaporator, which is a key parameter of the model. Moreover, an experimental bench is presented to measure the pressure variations inside an oscillating liquid slug. The results show that the classical correlations of fluid mechanics are relevant to model the oscillation of this system. Finally, a parametric study is carried out to understand the influence of the thermal properties of both the liquid and the tube on the start-up of the system. The thermal effusivity of both these materials is found to be an important criterion to indicate the conditions under which oscillations can commence and relain self-sustained.

CFD Analysis of Multi turn Pulsating Heat pipe

Pulsating Heat Pipe (PHP) is a heat exchanger device which absorbs heat from evaporator region and transfers it to the condenser region. The flow in pipe is Multi-Phase flow. Vapour plugs and Liquid slugs are formed in PHP due to the capillary action.CFD modeling is done in ANSYS CFX with two turns of PHP and Methanol is used as the working fluid and a fill ratio of 60%. At evaporator boundary, heat flux that is equivalent to 10 W to 70 W is supplied and the condenser boundary is set as heat flux of range 4423 W/m^2 and in the adiabatic section heat flux is zero. The obtained CFD results are compared with the experimental paper [1].The CFD analysis is performed and the outputs of the simulations are plotted in graphs and contours. Decrease in the methanol temperature at the evaporator suggests that heat is carried away to the condenser part. Change in volume fractions of methanol and air in three regions viz. evaporator, adiabatic region and condenser reflects to the flow pattern of the fluid inside the PHP.

Three-Dimensional Bubble Shape Estimation in Two-phase Gas-liquid Slug Flow

Wire-mesh sensors produce 3-D data of void fraction distribution at high resolution thus being an appropriate tool to investigate two-phase gas-liquid flows. Slug flow is typically found in petroleum production lines. This type of flow is characterized by the intermittent occurrence of gas bubbles and liquid slugs along the pipe. An important issue of these flows is the existence of a variety of regimes, depending on the flow rates of gas and liquid. The quantitative and qualitative information about shapes of the bubble nose and tail allows to study and to model the flow characteristics in order to increase safety and efficiency of oil production operations. In this paper, we describe two methods to estimate typical bubble shape from wire-mesh sensor data, which are based on ensemble mean and median approaches. Results from a set of two-phase flow experiments in inclined tubes, show that both approaches produce similar estimations, however since median is a type of robust estimator, contours of bubbles are better defined. 3-D images of typical bubbles, for eight different operational conditions, were generated and reveal some details about bubble shape. In addition, the bubble identification was validated by measuring the gas flow rate in comparison with reference instrumentation.

Green enhanced oil recovery (GEOR)

Green enhanced oil recovery (GEOR) is a chemical enhanced oil recovery (EOR) method involving the injection of specific green chemicals (surfactants/alcohols/polymers) that effectively displace oil because of their phase-behaviour properties, which decrease the interfacial tension (IFT) between the displacing liquid and the oil. In this process, the primary displacing liquid slug is a complex chemical system called a micellar solution, containing green surfactants, co-surfactants, oil, electrolytes and water. The surfactant slug is relatively small, typically 10% pore volume (PV). It may be followed by a mobility buffer such as polymer. The total volume of the polymer solution is typically ~1 PV. This study was conducted to examine the effectiveness of the combination of microbial by-products Bacillus subtilise strain JF-2 bio-surfactant and alcohol in recovering residual oil. It also considered whether bio-surfactant capability could be improved by blending it with non-ionic green surfactant. The study consisted of a phase behaviour study, IFT measurement and core-flooding experiments. In the phase behaviour study, it was found that 0.5% alkyl polyglycosides (APG) and 0.5–1.00% of butanol at 2% NaCl gave stable middle phase micro-emulsion. Non-ionic (APG 264) and anionic (bio-surfactant) mixtures are able to form stable middle phase micro-emulsion. Based on IFT reduction, two low concentrations (40 and 60 mg/l) of JF-2 bio-surfactant were identified where IFT values were low. The bio-surfactant and butanol formulation produced a total ~39.3% of oil initially in place (OIIP).

Modeling mass transfer in a Taylor flow regime through microchannels using a three-layer model

The process of convection and diffusion in a Taylor regime of gas–liquid flow through microchannels is modeled based on a three-layer mass transfer model in the axisymmetric formulation. The circulation circuit of Taylor vortices is described in the form of outer and inner layers surrounded by a thin film that does not participate in circulation. Due to the assumption about the predominance of convection over radial diffusion in the inner layer (the Peclet number order of magnitude is nearly 105) and a uniform concentration distribution over the cross sections of each layer, the problem is confined to a quasi-one-dimensional problem with boundary conditions of special type. The obtained numerical solutions allow us to determine the kinetics of mass transfer from a liquid to a channel wall in detail, calculate the average mass-transfer coefficient, and reveal optima in the dependences of the mass-transfer coefficient on the two-phase flow velocity and the capillary diameter; furthermore, there is also an optimum for the length of a liquid slug. The obtained results allow us to understand the pattern of Taylor convection and diffusion fluxes and to reveal the reasons for a decrease in mass fluxes under nonoptimal conditions. This enables the correction of selected geometric and process parameters during the design of microreactor equipment.

Experimental and theoretical research on a ammonia pulsating heat pipe: New full visualization of flow pattern and operating mechanism study

This paper presents a novel study on performance of pulsating heat pipe using ammonia as working fluid. Firstly, a new full visualization experiment with high speed camera is conducted, to study the relationship between the flow patterns and thermo-hydrodynamics of the PHP. The tested PHP, consisting of 6 turns, is fully made of quartz glass tubes with 6mm outer diameter and 2mm inner diameter. The filling ratio for ammonia fluid is 70%. Wall temperature fluctuations of several key positions are recorded under a series charge of heat transfer rates, which are from 25W to 520W. In visualization results, the motion identities, flow pattern variations, breakup and coalescence between the vapor plugs and liquid slugs are illustrated and discussed. In addition, the actual velocity of the vapor is computed from the experimental measurement in this paper, which is never found in other literature but is very important for the mechanism study in theoretical model.Furthermore, a theoretical model including the dynamical characteristics and heat transfer is investigated coupled with the thermal driving force, friction force and capillary force variations as the flow patterns changed at different transport powers, which are derived from the full visualization experiment. The theoretical results are compared with the experimental results analytically, and the operating mechanism of PHP will be discussed in detail finally.

Flow models and numerical schemes for single/two-phase transient flow in one dimension

In the two-phase flow field, a traditional mathematical model for simulating the transition of severe slugging flow presents a challenge when liquid slugs completely block pipelines. Accordingly, an advanced and practical slug model that is derived from a mixture model associated with a slip closure is essential to solving the problem in cooperation with the two-fluid model. The model can offset numerical instability that arises from the discontinuous function of the friction factor across the transition from one flow pattern to the other. Two numerical schemes, the non-iterative and the iterative, are developed, and the proposed schemes can stably predict the transient problems under the Courant–Friedrichs–Lewy (CFL) condition for semi-implicit/implicit schemes. In the present work, pressure transients produced by a complex phenomenon, named water hammer effect, are captured using the single-phase flow model in one-dimension to verify the applicability of the numerical schemes and the friction factor model. At last, the analysis of the two-phase transient flow in a pipeline-riser system indicates that the significant advantage of the present schemes is the robustness that the numerical prediction of the severe slugging behaviour is accurate and stable.

An experimental method for controlled generation and characterization of microchannel slug flow boiling

This study uses high-speed imaging to characterize microchannel slug flow boiling using a novel experimental test facility that generates an archetypal flow regime suitable for high-fidelity characterization of key hydrodynamic and heat transfer parameters. Vapor and liquid phases of the fluorinated dielectric fluid HFE-7100 are independently injected into a T-junction to create a saturated two-phase slug flow, thereby eliminating the flow instabilities and flow-regime transitions that would otherwise result from stochastic generation of vapor bubbles by nucleation from a superheated channel wall. Slug flow boiling is characterized in a heated, 500μm-diameter borosilicate glass microchannel. A thin layer of optically transparent and electrically conductive indium tin oxide coated on the outside surface of the microchannel provides a uniform heat flux via Joule heating. High-speed flow visualization images are analyzed to quantify the uniformity of the vapor bubbles and liquid slugs generated, as well as the growth of vapor bubbles under heat fluxes ranging from 30W/m2 to 5160W/m2. A method is demonstrated for measuring liquid film thickness from the visualizations using a ray-tracing procedure to correct for optical distortions. Characterization of the slug flow boiling regime that is generated demonstrates the unique ability of the facility to precisely control and quantify hydrodynamic and heat transfer characteristics. The experimental approach demonstrated in this study provides a unique platform for the investigation of microchannel slug flow boiling transport under controlled, stable conditions suitable for model validation.

Experimental study of the hydrodynamic behaviour of slug flow in a horizontal pipe

This paper investigates the unsteady hydrodynamic behaviour of slug flow occurring within an air–silicone oil mixture, within a horizontal 67mm internal diameter pipe. A series of slug flow regime experiments were performed for a range of injected air superficial velocities (0.29–1.4ms−1) and for liquid flows with superficial velocities of between 0.05–0.47ms−1. A pair of Electrical Capacitance Tomography (ECT) probes was used to determine: the slug translational velocities of the elongated bubbles and liquid slugs, the slug frequencies, the lengths of elongated bubbles and the liquid slugs, the void fractions within the elongated bubbles and liquid slugs. The pressure drop experienced along the pipe was measured using a differential pressure transducer cell (DP cell). A comparative analysis of the current experimental data and that previously published experimental confirms good agreement.

Characterisation of gas-liquid two-phase flow in minichannels with co-flowing fluid injection inside the channel, part II: gas bubble and liquid slug lengths, film thickness, and void fraction within Taylor flow

Current research proofs the potential of apparatuses containing minichannel flow structures to intensify gas-liquid-solid contacting processes. The excellent heat and mass transfer in these devices as well as a sharp RTD mainly result from the Taylor flow regime. A proper design of corresponding contactors requires precise information on the provided interfacial areas. However, the characterisation of gas-liquid Taylor flow with industrially relevant fluids at elevated pressure and created by capillary injection devices gained little attention so far.This work analyses adiabatic gas-liquid Taylor flow in a square minichannel of 1.0mm hydraulic diameter using water, water-glycerol, or water-ethanol mixtures as liquid phase and hydrogen or nitrogen as gas phase to cover a broad range of material parameters. In the mixing zone located within the flow channel, gas was injected into the co-flowing liquid by so-called capillary injectors with variable inner diameter (0.184, 0.317, 0.490mm).Two different bubble forming mechanisms were identified leading to a complex interaction between physical properties of the fluids, geometrical parameters and the observed gas bubble and liquid slug lengths. According to the Pi-theorem, these lengths were affected by 6 dimensionless groups, namely (uG,s/ uL,s), ReL, WeL, (dIn,CI/ dh), (dOu,CI / dh), and Θ*. Based on more than 370 experimental data, novel correlations to predict gas bubble and liquid slug lengths were developed.

Expanding Taylor bubble under constant heat flux

Modelization of non-isothermal bubbles expanding in a capillary, as a contribution to the understanding of the physical phenomena taking place in Pulsating Heat Pipes (PHPs), is the scope of this paper. The liquid film problem is simplified and solved, while the thermal problem takes into account a constant heat flux density applied at the capillary tube wall, exchanging with the liquid film surrounding the bubble and also with the capillary tube outside medium. The liquid slug dynamics is solved using the Lucas-Washburn equation. Mass and energy balance on the vapor phase allow governing equations of bubble expansion to be written. The liquid and vapor phases are coupled only through the saturation temperature associated with the vapor pressure, assumed to be uniform throughout the bubble. Results show an over-heating of the vapor phase, although the particular thermal boundary condition used here always ensures an evaporative mass flux at the liquid-vapor interface. Global heat exchange is also investigated, showing a strong decreasing of the PHP performance to convey heat by phase change means for large meniscus velocities.

Experimental study of the contribution of liquid film for liquid-liquid Taylor flow mass transfer in a microchannel

Liquid-liquid two-phase Taylor flow mass transfer in microchannels, which has many advantages for heat and mass transfer operations, should be studied to develop practical applications in various chemical processes. To understand the effects of various parameters on mass transfer and develop an estimation method of mass transfer performance, mass transfer in microchannels was examined by an evaluation of phenol extraction from dodecane to water. Under stable Taylor flow conditions in a 1mm inner diameter glass channel, the organic phase (dodecane) formed liquid droplets and the aqueous phase (water) formed liquid slugs with a liquid film enclosing the organic phase droplets. The effect of the droplet length on mass transfer was investigated with a microchannel device that consists of a circular channel with a 1mm inner diameter and union tees with inner diameters of 2.0mm and 2.4mm. Although the droplets lengths were varied in the range of 7.4mm to 30.5mm, approximately the same volumetric mass transfer coefficients (Ka) were obtained. This result suggested that the side interface of the droplets that contact the thin liquid film becomes a mass transfer interface as well as an end interface. This occurs because the side interface area increases with increasing droplet length and droplet volume while the specific interfacial area remains approximately constant. The volumetric mass transfer coefficients were obtained for various cross-sectional shapes and hydraulic equivalent channel diameters at various flow velocities. The mass transfer coefficients were derived by dividing the volumetric mass transfer coefficients by the specific interfacial area. It was found that the Sherwood number (Sh), which is a dimensionless expression of the mass transfer coefficient, has excellent correlation with the Reynolds number (Re); thus, the estimation of the mass transfer performance for a given channel and given operational conditions can be determined using the correlation between Sh and Re.

Maximum Heat Transfer and Operating Temperature of Oscillating Heat Pipe

It is reported that the operating temperature of an oscillating heat pipe (OHP) at an operating limit is not dependent on the ambient temperature but that the maximum heat transfer is dependent on this. In this study, using different ambient temperature conditions, a 15-turn OHP filled with HFC-134a as a working fluid was operated until it dries out. The maximum heat transfer was found to vary with changes in the ambient temperature, but the operating temperature at an operating limit, which depends on the filling ratio (FR) of the working fluid, was found to be constant. At the operating limit, the operating temperature decreased with an increase in the FR when the ratio was greater than 50 wt.%. Visualization experiments and calculations were used to confirm that there is an increase in the liquid volume in the OHP in accordance with an increase in the heat input and that ultimately the OHP fills with the liquid, resulting in the failure of OHP operation. In contrast, at the operating limit, when the FR was less than 50%, the operating temperature increased in line with an increase in the FR. In this case, it is assumed that the volume of liquid slugs decreases as the heat input increases, thus causing the OHP to dry out. This theory is explained using a P–V diagram of the working fluid in the OHP. The OHP thermodynamic cycle reaches a saturated liquid or vapor line before it reaches a critical point if a specified volume is shifted from the specified volume at the critical point. The optimum FR for maximum heat transfer is therefore decided by the void ratio at the critical point of the working fluid.

Evaporation-induced natural convection of a liquid slug of binary mixture inside a microchannel: effect of confinement

Both experimental and simulation studies have been carried out on internal convection of an evaporating liquid slug of aqueous NaCl solution inside a microcapillary. Effect of confinement due to the extended channel length beyond the interface of the liquid slug has been investigated by placing the liquid slug inside the microcapillary of different lengths. Micro-PIV technique has been used for measurement of velocity field inside the liquid slug. Simulation studies have been carried out using COMSOL Multiphysics software for reporting the evaporative flux distribution on the meniscus and the concentration field distribution inside the liquid slug. The combined experimental and simulation studies successfully explain the underlying flow physics. Evaporation from the liquid–air interface of the slug induces buoyancy-driven Rayleigh convection. Evaporative flux of the interface depends on the extended length of the microcapillary beyond the liquid slug. The presence of extended channel region beyond the meniscus suppresses the evaporation from the meniscus due to the absence of evaporation flux normal to the channel wall. Evaporation occurs primarily from only one meniscus when the slug is located at one end of a long channel. Evaporation occurs from both the menisci when both the menisci are directly exposed to the atmosphere. Evaporation from only one meniscus of a slug leads to one recirculation bubble inside the liquid slug, whereas evaporation from both the menisci leads to two recirculation bubbles inside the liquid slug. Liquid slug with asymmetric extended channel length beyond the liquid slug interface leads to asymmetric evaporative flux, concentration field distribution and recirculation bubble size. The extended channel length beyond an evaporating liquid slug can influence/control the performance of a digital microfluidic system/device.

Investigation of Entropy Generation Through the Operation of an Unlooped Pulsating Heat Pipe

In the present study, an unlooped pulating heat pipe has been considered with two liquid slugs and three neighboirng vapor plugs.The governing equations such as momentum, energy and mass equations are solved explicitly except the energy equation of liquid slugs.The aim of the present study is to calculate the entropy generation through the performance of a pulsating heat pipe. Additionally, the effects of different pipe diameters and evaporator temperatures have been investigated. Besides, Bejan number has been derived for each case study to investigate the share of heat transfer in entropy generation. According to the results, by increasing the pipe diamtere the sensible and latent heat transferred into the pulsating heat pipe enhance and the liquid slugs oscillate in high amplitudes. On the other hand, the entropy generation value increases as the pipe diameter increases. The evaluated Bejan numbers show that the viscous effects share in entropy generation decreases as the pipe diameter increase. Previous studies reports that there is a threshold of pulsating heat pipe diameter to operate. However, the results of the present paper demonstrate that the using pulsating heat pipes are not reasonable in small diameters. Moreover, the results show that the heat removing performance of pulsating heat pipe improves as the temperature difference of the evaporator and condenser increases. The calculated results demonstrate increment in total entropy generation in high evaporator temperatures. Moreover, the Bejan number will increase by any increment in the evaporator temperature and this phenomenon reveals the insignificant role of viscous effects in high evaporator temperatures. To validate the calculations, the results have been compared to the previous ones. This comparison shows very good agreement between them.