Liquid Film Flow Research Articles

Effect of irregular heat source/sink on the radiative thin film flow of MHD hybrid ferrofluid

Ferrofluids are the colloidal suspensions of magnetic nanoparticles and base fluids. It has several medical applications like drug targeting, cell separation, magnetic resonance imaging, etc. The dispersion of more than one magnetic nanoparticle into the convectional fluid is called hybrid nanofluid. It has various technological applications like damping, dynamic sealing, heat dissipation, etc. Due to the immense applications of the ferrofluids, in this analysis, we examined the liquid film flow and heat transfer of hybrid ferrofluid in the attendance of radiation and irregular heat source/sink. We considered the magnetite (Fe3O4) and cobalt ferrite (CoFe2O4) nanoparticles and suspended them into water–ethylene glycol (EG) mixture (50–50%). A mathematical model is developed for the present investigation and solved numerically after applying the suitable similarity transformations. The impact of various governing non-dimensional parameters on the momentum, energy fields, and local Nusselt number of ferro- and hybrid ferrofluids is studied with the aid of graphical illustrations. It is perceived that the rate of heat transfer is higher in hybrid ferrofluid than that of ferrofluid.

Concentration of cattle blood by moisture freezing

Prospects for the use and processing of cattle blood for food and pharmaceutical purposes were described in the article. The process of blood cyclic concentration by the method of moisture freezing was investigated. A crystallizer with a flowing liquid film was used in the experiments. Research showed that increasing of the refrigerant boiling point in the crystallizer evaporator causes a decrease in the specific amount of frozen from the blood ice. Increase in the amount of frozen ice contributes to an increase in the liquid flow rate that irrigates the crystallizer evaporator. Increase of the initial solids content causes a decrease in the frozen ice value. A decrease in the refrigerant boiling point in the crystallizer evaporator causes an increase in the solids content of the solution obtained by frozen ice melting. An increase in the blood flow irrigating the crystallizer evaporator causes a decrease in the solids content in the solution obtained by frozen ice melting. It characterizes the amount of loss of target substances that are removed with frozen ice. An increase in the initial content of dry substances in the blood causes an increase in the content of soluble substances in the frozen ice solution. Spectral analysis of the chemical composition of the concentrated blood showed its high quality, while preserving almost all proteins, bilirubin and lecithin.

Driving mechanisms of ratchet flow in thin liquid films under tangential two-frequency forcing

In a recent paper, we demonstrated the emergence of ratchet flows in thin liquid films subjected to tangential two-frequency vibrations [E. Sterman-Cohen, M. Bestehorn, and A. Oron, “Ratchet flow of thin liquid films induced by a two-frequency tangential forcing,” Phys. Fluids 30, 022101 (2018)], and asymmetric forcing was found to be a sole driving mechanism for these ratchet flows. In this paper, we consider other two-frequency excitations and reveal an additional driving mechanism of an emerging ratchet flow when the acceleration imparted by forcing is symmetric with respect to a certain moment of time within the forcing period (this type of forcing referred to as “symmetric forcing”). This driving mechanism exhibits an intricate interaction between forcing, capillarity, and gravity. We find that in contradistinction with the case of asymmetric forcing where the flow intensity reaches a constant value in the large-time limit, in the case of symmetric forcing the flow intensity exhibits oscillatory variation in time. We also discuss the flow intensity variation of the emerging ratchet flows with the fundamental wavenumber of the disturbance.

Mathematical Model of Mass Transfer in Randomly Packed Columns with Phase Maldistribution

The paper considers the processes of mass transfer between phases in countercurrents of gas and liquid in stationary packed beds of industrial randomly packed column apparatus. Such beds are widely used in heat and mass transfer, separation, and reaction processes in the oil and gas processing, petrochemistry, and other industries. An approximate approach of mathematical modeling of mass transfer in a randomly packed bed at turbulent gas motion and countercurrent laminar wave flow of liquid film is analyzed. The basic concepts of the multispeed continuum model, in which the transfer equations are written for each phase separately, are used. The phases fill one space, the volume of the continuous phase (gas) exceeding by far that of the liquid one. The interaction of the phases is reflected in averaged transfer terms, which take into account the interfacial transfer phenomena. Those are the mass transfer coefficients and the driving forces of the processes. The concentration profiles of the components are found from the solution to differential mass transfer equations written for a cylindrical channel with a volume source of mass. This approach can be used in the absence of experimental data on the structure of gas and liquid flows in a packed bed, for example, when designing new contact elements.

Local liquid film behavior of annular two-phase flow on rod-bundle geometry-I. Experimental phenomenon and analysis

Local liquid film behavior of annular flow on rod bundle geometry is vital for accurate prediction of dryout. This paper is the first part of a two-part study concerning measurement and analysis of local liquid film behavior of annular flow on rod bundle geometry. In this paper, air-water annular flow experiments have been performed at different gas and liquid superficial velocity conditions in a 3 × 3 simulating PWR fuel rod-bundle test-section, and a conductivity-based technique is developed to measure the local dynamical liquid film thickness. Large amplitude disturbance waves appear at all low liquid phase superficial velocity conditions through down-burst mechanism caused by the highly turbulent gas flow and strong inter-subchannel interaction. Wave shapes do not change drastically by the variation of superficial liquid velocity since liquid flow tends to fill out the rod bundle corner due to the nature of annular flow. For disturbance wave dominating the film flow case, secondary flow in subchannel produces a circumferential force to shear off the liquid film, and the liquid droplets are redistributed, resulting in the liquid film thickness for the central region being thicker than that of gap region. Cross-correlation analysis indicates that gap and central regions are in possession of circumferential coherent wave structure with the same interfacial velocity, and the wave position for the central region is a little ahead of that for gap region since a higher wave has a longer leeward and upwind faces. The current study on local liquid film behaviors for rod bundle and the possible mechanisms that causing the discrepancies in local liquid film structures can be further used for the mechanism model development for the liquid film flow on the rod bundle. Part II of this study will introduce a mechanism model as an alternative approach to local liquid film thickness prediction for annular flow in rod bundle geometry.

Numerical Analysis of Laminar Convective Condensation with the Presence of Noncondensable Gas Flowing Downward in a Vertical Channel

The purpose of this paper is to study and perform a numerical analysis of the simultaneous processes of mass and heat transfer during the condensation process of a steam in the existence of noncondensable gas (NCG) inside a descending vertical channel. In this study, the flow of the vapor-air mixture is laminar and the saturation conditions are prevailing at the inlet of the channel. The coupled control equations for liquid film, interfacial conditions, and mixture flow are solved together using the approach of finite volume. Detailed and valuable results are presented both in the liquid condensate film and in the mixing regions. These detailed results contain the dimensionless velocity and dimensionless temperature profiles in both phases, the dimensionless mass fraction of vapor, the axial variation of the dimensionless thickness of the film liquid δ⁎, and the accumulated condensate rate Mr as well the local Nusselt number Nuy. The relative humidity at the inlet varies from 60% to 100% and the inlet temperature from 40°C to 80°C. The results confirm that a decrease in the mass concentration of NCG by the increasing the inlet relative humidity has a direct influence on the liquid film layer, the local number of Nusselt, and the variation of condensation rate accumulated through the channel. The results also designate that an increase of the inlet relative humidity and the inlet temperature ameliorates the condensation process. The comparison made for the coefficient of heat transfer due to condensation process and the condensate liquid film thickness with the literature results is in good concordance which gives more credibility to our calculation model.

The unsteady liquid film flow of the carbon nanotubes engine oil nanofluid over a non-linear radially extending surface

The enhancement of heat transfer through carbon material is the objective of this study. The renowned class of carbon identified as single walled carbon nanotubes and multi walled carbon nanotubes, nanofluid-flow over a non-linear and unstable surface has been explored. The thermophysical properties of the two sorts of carbon nanotube have been implemented from the experimental outputs in the existent literature using engine oil as a base fluid. The viscous dissipation term has also been included in the energy equation improve the heat transfer rate. The thickness of the nanofluid thin layer is kept variable under the influence of the unstable and non-linear stretching of the disk. The elementary governing equations have been transformed into coupled non-linear differential equations. The problem solution is achieved through BVP 2.0 package of the optimal homotopy analysis method. The square residual error for the momentum and thermal boundary-layers up to the 20th order approximations have been obtained. The numerical ND-solve method has been used to validate the he optimal homotopy analysis method results. The impact of the model parameters vs. velocity field and temperature distribution have been shown through graphs and tables. The impact of the physical parameters on the temperature profile and velocity, pitch for both multi wall carbon nanotubes and single walled carbon nanotubes is gained in the range of 0 ? ? ? 4%. From the obtained results it is observed that the single walled carbon nanotubes nanofluids are more efficient to improve the heat transfer phenomena as compared to the multi wall carbon nanotubes.

CFD modeling of liquid film reversal of two-phase flow in vertical pipes

The phenomenon of two-phase flows is characterized by its wide range of presence in nature and industrial applications. In gas and oil production, the simultaneous two-phase flow of gases and liquids often occurs in gas wells. The produced gases flow rate decreases over the years until reaching a critical flow rate when they lose their ability to lift the associated liquids upward and the liquid film reverses direction, which triggers liquid loading. Liquid loading causes the produced liquids to accumulate in the bottom of the wellbore, which causes a high back pressure that reduces the well production rate till production is ceased eventually. The critical gas velocity exists in the churn flow regime which is mainly characterized by the oscillatory behavior of the liquids flow field. This study employs CFD techniques to model the churn flow in a 3-inch-diameter vertical pipe near the critical gas flow rate for different liquid flow rates. This work utilizes the two-fluid Eulerian model along with the RNG (Re-Normalization Group) k-ε turbulence model to investigate the behavior of the flow field in a two-dimensional axisymmetric computational domain. Simulations were carried out using the commercial software ANSYS Fluent 18.2 with air and water as the two working fluids. The model results showed a good agreement with the experimental data and proved the mesh and time independence of the model. Oscillatory behaviors of the liquid film flow rate, shear stress, and pressure were observed along with the formation of interfacial waves. Detailed information about the velocity, shear stress, and pressure behaviors is presented. Accordingly, recommendations are suggested for future considerations.

Liquid Flow Morphology of Viscous Systems in Structured Packings: Investigations by X-ray Tomography

In the modeling of gas-liquid separation processes in structured packings, fluid dynamics is often reduced to a uniform liquid film flow over the packing surface. However, previous experiments with X-ray tomography indicate that this assumption is not valid when the liquid-phase viscosity is significantly higher than 1 mPa s. In order to improve existing modeling approaches for viscous systems, a better understanding of the influence of the liquid viscosity on the liquid flow inside the structured packing is necessary. In this work, X-ray tomography is used to investigate the flow morphology of liquid systems with low surface tension and a viscosity up to 50 mPa s. An empirical correlation that describes the hold-up fraction of the existing flow patterns is given and a modelling approach is proposed that allows to consider the influence of the different flow patterns on mass transfer.

Investigation of Two-Phase Flow in a Hydrophobic Fuel-Cell Micro-Channel

This paper presents a quantitative visualization study and a theoretical analysis of two-phase flow relevant to polymer electrolyte membrane fuel cells (PEMFCs) in which liquid water management is critical to performance. Experiments were conducted in an air-flow microchannel with a hydrophobic surface and a side pore through which water was injected to mimic the cathode of a PEMFC. Four distinct flow patterns were identified: liquid bridge (plug), slug/plug, film flow, and water droplet flow under small Weber number conditions. Liquid bridges first evolve with quasi-static properties while remaining pinned; after reaching a critical volume, bridges depart from axisymmetry, block the flow channel, and exhibit lateral oscillations. A model that accounts for capillarity at low Bond number is proposed and shown to successfully predict the morphology, critical liquid volume and evolution of the liquid bridge, including deformation and complete blockage under specific conditions. The generality of the model is also illustrated for flow conditions encountered in the manipulation of polymeric materials and formation of liquid bridges between patterned surfaces. The experiments provide a database for validation of theoretical and computational methods.

Experimental investigations of the fluid dynamics in liquid falling films over structured packing geometry

The objective of this study was to adapt an optical non-invasive measurement methodology for the investigation of the fluid dynamics in thin liquid falling films. The technique should allow to directly observe effects of the surface topology of structured packings on the flow field and film thickness distribution during flow over real packing surfaces. Three-component planar velocity vector fields were determined by means of stereoscopic particle image velocimetry. Optical distortion could be avoided by measuring from the back through specially-prepared transparent moldings of packing surfaces which match the refractive index of the liquid. The adapted stereoscopic particle image velocimetry was validated for laminar liquid film flow over a smooth inclined plate. The results for laminar flow over an inclined plate with a tetrahedral micro-structured packing surface indicate an increased exchange of fluid elements over the liquid film height and in the liquid film between micro-structures. Furthermore, the technique allowed spatially resolved complex observations of liquid flow behavior in real packing channel geometries. The methodology may be used to attain basic data for validation of numerical studies to improve understanding the geometry effects of complex surface topologies on liquid film flow dynamics and for numerical studies on packing structure optimization.

Evolution of surface droplets and flow patterns on C/SiC during thermal ablation

Degradation of high temperature ceramics caused by thermal ablation is a great challenge for maintaining structural integrity and stability under service conditions. In order to advance the understanding of the thermal ablation mechanisms of C/SiC composites, we adopt here the optical observation system to visualize the surface evolution of C/SiC in wind-tunnel test, and demonstrate experimentally the various flow characteristics of liquid SiO2 generated on the sample surface at different temperatures. Three different flow patterns (droplet flow, stream flow, and liquid film flow) corresponding to 1600 ℃, 1750 ℃, and 1950 ℃ were observed on the sample surface. Numerical simulation shows that the radius of the droplets plays a critical role in determining the flow patterns, as evidenced by the experiments. Droplets with large radius (≥0.5 mm) deform severely and cause “tailing”; droplets with small radius deform little under shear. We also observe that the large SiO2 droplet will continuously break into small droplets and eventually the droplets will oscillate with a certain radius. With the ablation rate increasing, a large number of droplets merge with each other to form a liquid film, and periodic K-H (Kelvin-Helmholtz) instability occurs at the gas-liquid interface.

The mechanism of drainage gas recovery and structure optimization of an internal vortex tool in a horizontal well

Effective drainage gas recovery technology is the key to solving the problem of the liquid loading of gas well and of enhanced gas recovery. An internal vortex tool is designed for solving the problem of liquid loading in horizontal wells. This study conducted a computational fluid dynamics analysis the efficiency of the drainage gas recovery of the tool in the horizontal well, to optimize the structure of the tool. The results show that most of the liquid film flows along the pipe wall in the internal vortex tool, and the gas flows through the central area of the pipe at a high speed with fog. The internal vortex tool can increase the gas phase velocity and separate the gas and liquid. The structural parameters of the internal vortex tool for minimum pressure loss and the maximum drainage efficiency is the same diameter as the tubing and casing for the runner diameter, 42.8° for the swirl angle, 8 mm for the helix height, 6 mm for helix width and 4 for the number of lead. The study on the flowing of a gas liquid swirling flow in the internal vortex tool proves the feasibility of its application in horizontal wells. It can provide theoretical guidance and a practical basis for efficient liquid loading using the internal vortex tool in the future.

Heat transfer enhancement in MHD free surface flow by controlling the electromagnetic force with fin structure

A partly insulated fin (PIF) structure has been proposed to mitigate the temperature stratification of flowing-type liquid metal divertors. This study conducted a three-dimensional magnetohydrodynamics (MHD) flow and heat transfer numerical simulation to investigate the influence of the PIF on the velocity and the temperature field inside the film flow. An analytical model simulated a liquid metal film flow of 25 mm thickness under a transverse magnetic field up to 1.0 T. The simulation was conducted with a PIF, a metal fin, and without a fin. The results showed the generation of a high velocity region near the surface and the velocity was 2.68 times higher than the case without a fin due to the MHD effect. In addition, an upward flow was observed in front of the PIF. The contribution of the flow field generated by the fin to the temperature field was confirmed through numerical analysis, and the highest temperature of the liquid metal surface decreased by 0.96% at its maximum. A deformation of the surface due to the fin was estimated using the pressure distribution obtained from the simulation. The result indicated expected deformation is 21.8 μm at the magnetic field of 1.0 T.

Modeling and simulation of the gas-liquid separation process in an axial flow cyclone based on the Eulerian-Lagrangian approach and surface film model

Axial flow cyclones are a widely used gas-liquid separation equipment in the chemical industry, petroleum industry, nuclear industry, etc. The gas-liquid separation process inside an axial flow cyclone involves complex phenomena, including the deposition of droplets on the wall (forming a liquid film), the flow of this liquid film, and the entrainment of droplets in this film. Therefore, CFD simulations of the gas-liquid separation process in an axial flow cyclone are very challenging to perform. In the present study, the gas-liquid separation process inside an axial flow cyclone is successfully modeled using the Eulerian-Lagrangian approach and a surface film model. The gas core is described by the Eulerian-Lagrangian approach, and the liquid film flow is modeled based on a two-dimensional thin-film assumption. Then, the proposed model is implemented by using OpenFOAM and validated against various experimental data sources. Subsequently, the proposed model is employed to simulate the flow field in the cyclone, which directly affects the separation performance. Furthermore, the gas-liquid two-phase flow inside the cyclone with high operating pressure is investigated using the proposed model, and the relevant characteristics are analyzed. The research results are of great significance to the design, operation, and optimization of axial flow cyclones.

Investigation on emergency core coolant bypass with local measurement of liquid film thickness using electrical conductance sensor fabricated on flexible printed circuit board

An experimental work was conducted in a reduced-scale downcomer annulus of a nuclear reactor pressure vessel in order to investigate the behavior of liquid films under emergency core coolant (ECC) bypass conditions and to obtain high definition data for validating two-phase flow prediction capability of computational fluid dynamics (CFD) codes. Air and water at ambient pressure and temperature were used as a working fluid, creating a multidimensional two-phase flow in the test section. The main instrumentation was an electrical conductance sensor that was developed to measure the local thickness of the liquid film. The sensor has an array of 24 × 24 measuring points, each of which has 12 mm × 6 mm of the sensing area. The fabrication of the electrodes on a flexible printed circuit board enabled the sensor to be installed on the curved surface. The time-averaged liquid film thickness, which was measured by the developed sensor, shows the influence of the lateral air flow on the liquid film flow, and it was compared with visual observations. As the air velocity increased, a droplet appeared that was created in the thick part of the liquid film, and the thinned liquid film after entrainment could be confirmed from the measured thickness distribution. In this study, qualitative and quantitative analyses of the measurement results show the reliability of the developed sensor and help to understand the liquid film behavior in the ECC bypass phenomenon. Furthermore, the measured thickness distribution can contribute to validating the CFD codes, which have not been validated sufficiently because of a lack of local measurement data for the ECC bypass condition.

Application of different advanced oxidation processes for the removal of chloroacetic acids using a planar falling film reactor

Advanced oxidation processes (AOPs) are considered as an effective and promising method for the degradation and mineralization of aqueous recalcitrant organic pollutants. In this study, application of ozonation and various types of AOPs including photocatalysis, Fenton alone and their combinations were investigated and compared for the degradation and mineralization of chloroacetic acids (CAAs) in aqueous solutions, using a planar falling film reactor. CAAs are widely available in water treated by chlorination processes and are resistance against ozonation in the darkness. The results of the present work showed that the plain ozonation was inefficient method for the destruction of the CAAs as only about 2% degradation was observed after 90 min treatment. However, the best results were achieved by ozone in combinations with other oxidation processes. Furthermore, a synergistic effect on the removal rate was observed when these processes were exposed to the UVA light. Among the examined processes, combination of photo-Fenton with ozonation was found to be the fastest one for CAAs degradation. The effects of different parameters such as initial concentration of Fe2⁺, H₂O₂ and CAAs in photo-Fenton combined with ozonation were investigated. The optimum ratio of 0.12 of Fe2⁺/H₂O₂ concentration was found to give the best result for CAAs degradation. The degree of CAAs mineralization, measured by the total organic carbon removal, as well as the effect of falling liquid film flow rate on the removal of CAAs were also studied and discussed.

Influence of a single microstructure on local mass transfer in liquid film flows

The influence of a single microstructure on liquid-side controlled mass transfer is studied for liquid film flow over an inclined plate with a film height of less than 1 mm. Planar Laser Induced Fluorescence (PLIF) is adapted and used to measure oxygen absorption at three gas flow rates. The liquid is a mixture of water, ethanol and glycerol. A ruthenium complex is used as a fluorescence indicator. By combining PLIF with dynamic fluorescence quenching, local concentration fields are visualized and determined through image processing. The results show that the local mass transfer coefficient decreases in the flow direction on a smooth inclined plate. Compared to a completely smooth surface, the local mass transfer tends to be enhanced in the direct vicinity of a single microstructure. The impact of the microstructure on local mass transfer depends on the gas flow conditions and the measurement position. The highest local mass transfer coefficients are measured, where the local film thickness is lowered due to the structure and with lower gas flow rates.

Dryout prediction with CFD model of annular two-phase flow

Two-phase flow and heat transfer are of interest to industrial applications due to its high efficiency. In a diabatic annular two-phase flow, the liquid film is depleted by both entrainment of liquid droplets and by evaporation. When the liquid film experiences almost complete depletion and cannot cover the wall, the heat transfer between the fluid and the channel wall significantly deteriorates, leading to the onset of boiling transition called dryout. While the dryout is milder than the departure from nucleate boiling (DNB) occurring in low quality two-phase flows, it could still challenge and damage the channel wall. As a result, the dryout occurrence needs to accurately predicted and avoided in practice, such as in boiling water reactors (BWRs). Research interests haven been recently focused on dryout prediction with annular flow modeling, with three fields of gas, droplets and liquid film accounted for. In the current study, one unified computational fluid dynamics (CFD) model for annular flow was developed for dryout applications. The model is employing a separate solver of two-dimensional conservation equations to predict propagation of a thin boiling liquid film on solid walls. The film model is coupled to a solver of three-dimensional conservation equations describing the gas core, which is assumed to contain a saturated mixture of vapor and liquid droplets. All the major interaction phenomena between the liquid film and the gas core flow have been accounted for, including the liquid film evaporation as well as the droplet deposition and entrainment. The resultant unified framework for annular flow has been applied to the steam-water flow with conditions typical for a BWR. The simulation results for the liquid film flow and dryout occurrence show favorable agreements with the available experimental data.

Numerical study of the falling film thickness around the tube bundle with different spacings between spray holes and tubes under tilt and sloshing conditions

The shell-side flow patterns and liquid film thickness (δ) distribution of the falling film are extremely important for the comprehensive performance of the spiral-wound heat exchangers. In this paper, 3-D simulations calculated with volume of fluid (VOF) method are performed to study the inter-tube flow modes and the distribution characteristics of δ around the tube. At first, the effects of different spray densities, longitudinal inter-tube spacings (St) and spray holes arrangements (Sh) on the δ distribution are studied to determine the appropriate values. After that, the influence of tilt and sloshing conditions are examined based on the selected geometric parameters. The results show that although the value of St has a significant influence on the fluctuation range of δ in stable zone, the average δ in stable zone could remain in the same level. For the spray holes arrangement, the decrease of Sh does not thicken the δ in stable zone but promotes a 46% axial length expansion for the crest zone with a thicker δ. The tilt conditions significantly deteriorate the inter-tube flow modes transitions, which makes the column flow much easier convert to sheet flow. Under roll motion, a secondary crest zone emerges nearby the original main crest zone and its δ is 30%–40% thinner than that of the original main crest zone.