Falling Liquid Film Research Articles

Temperature Field Measurements In The Cross-Section Of Falling Liquid Films By Laser-Induced Fluorescence

Thin wavy liquid films a reused in many applications as heat a nd mass transfer promoters thanks to their great performances for a small liquid consumption. To further understand the exact mechanisms of coupling between the wavy fluid dynamics and the heat transfer, a laser-induced fluorescence technique has been developed. The technique enables a measurement of the temperature field inside of a wavy film flowing down an inclined plane with a high spatial resolution. This paper reports on the development of the technique. Water liquid films were investigated under two-dimensional waves conditions. The measurements reveal the regions where mixing is enhanced by convective circulation zones and enables the evaluation of the intensification of the heat transfer due to the waves.

Experiments in free falling and downward cocurrent annular flows – Characterization of liquid films and interfacial waves

Falling liquid films and downward cocurrent flows in rounded shape pipes have been experimentally studied during the last decades, estimating the evolution of its major characteristics. The most important variables during the formation and growth of surface waves in falling downward flows have been measured using conductance probes.The main objective of the current research paper is to study the dependency of the characteristics of the thin liquid layer for downward cocurrent annular flows. The GEPELON experimental facility consists of a vertical pipe with 3.8 m of useful test length. Two pipe diameters have been analysed in this experimental study, 42 and 30 mm, in which the range covered by the liquid Reynolds number varies between 570 and 8500 and 800–7900 respectively, while the gas Reynolds numbers vary from 0 to 7.9·104 and from 0 to 1.1·105 respectively for the mentioned pipe diameters. Up to five conductance probes have been placed along the pipes test sections to capture the liquid film thickness fluctuations along time at different distances of the pipe entrance for both developing and fully developed regions.After the study and analysis of the experimental data, the central point of this paper has been the development of new correlations for the liquid film thicknesses and the two major properties of the interfacial waves. Their adjustment procedure has been carried out in terms of dimensionless numbers, aiming to provide more general relationships. In particular, the magnitudes that characterise the interface behavior have been measured, particularly film thicknesses, average disturbance wave amplitudes, and disturbance wave frequencies for each boundary condition.An additional part of the document contains an extensive comparison between the results obtained in this study and the data and expressions of other authors. It has been confirmed the significant dispersion existing among different researchers, especially when analysing variables related to the interfacial waves. This highlights the lack of knowledge in some aspects even today. The different correlations proposed have been calculated based on the best fit of the data from all the series of experiments carried out in this study. Comparisons of the behaviour of these correlations with data from other researchers have also been included.

A numerical investigation of three-dimensional falling liquid films

In this article, we present a full three-dimensional numerical study of thin liquid films falling on a vertical surface, by solving the full three-dimensional Navier–Stokes equations with a hybrid front-tracking/level-set method for tracking the interface. General falling film flow applications span across many types of process industries but also occur in a multitude of natural and environmental applications such as ice sheets, glaciology and even volcanic lava flows. In this study, we propose three configurations of falling films. Two of them, with small and moderate Reynolds number, are set to mimic pulsed and forced falling film types inside a minimum periodic domain, able to cover entirely the temporal evolution of a single wave. The latest example, corresponding to a high Reynolds number, is initialised with a flat interface without any specific perturbations. For the first time, this study highlights the natural transition from a non-deformed interface to its first streamwise disturbance (two-dimensional wavy flow), and then a second spanwise wave disturbance (three-dimensional wavy flow).

Heat transfer and wetting behaviour of falling liquid films in inclined tubes with structured surfaces

This study examines wetting behaviour and evaporation heat transfer of a falling liquid film inside a circular tube carrying grooves at its internal surface. The experiments include five different structures (four helical, one longitudinal) and one smooth tube (Cu, d i ≈ 27 mm ) with counter-current flow of liquid (downwards) and vapour (upwards). Results are obtained for liquid propane films ( P r = 2 . 79 , R e smooth tube = 160 − 645 ) at different twisting angles and height of the grooves, tube inclination angles ( β = 0° − 20° from the vertical), mass flow rates (from 5 kg/h up to 20 kg/h) and driving temperature differences (2 K, 3 K and 4 K) mainly at a film inlet temperature of 293.15 K. For wetting behaviour investigations the liquid film is visually observed at the lower end of the tube. The experiments reveal that the internal structures used lead to a strong improvement in both wetting behaviour and heat transfer, even up to inclination angles of 20° . The amount of capillary transported liquid within the grooves coupled with groove geometry itself are the main impacts on wetting behaviour and will be discussed within this paper. • Helically twisted structures resulting better wetting and heat transfer. • Greater groove mass flow rate supportive for greater heat transfer. • High aspect ratios supportive for high heat transfer. • Driving temperature difference influences wetting and heat transfer. • Small inclination with small influence on heat transfer.

Design and experimental demonstration of enabling components for simultaneous water purification and space conditioning using thermally driven absorption heat pumps

• Absorber/desorber for water-purifying absorption heat pump designed and tested. • 90% removal of organic contaminants from graywater achieved. • Up to 99% removal of dissolved solids from NaCl solution demonstrated. • Good agreement (AAD = 8.6%) between desorber model and experiments. A horizontal-tube, falling-film heat exchanger capable of acting as an absorber or desorber in a small-scale (300-W cooling, 10.5 kg day −1 water purification) water-purifying absorption heat pump cycle was designed with LiBr as the absorbent. A design model for the heat exchanger that accounted for both heat transfer and mass transfer effects in the falling liquid film during desorption and absorption was formulated. Parametric studies were conducted for tube lengths ranging from 0.12 to 0.30 m and for absorber equilibrium solution outlet temperatures ranging from 35 to 45 °C. Based on the results of these simulations, a tube length of 0.24 m and an absorber equilibrium solution outlet temperature of 37.5 °C were selected. Batchwise experiments were conducted to validate the component model and determine the water purification potential of the cycle. Three feed solutions were tested: DI water, synthetic graywater, and 3.5 wt% NaCl. Measurements showed 90% removal of contaminants from synthetic graywater and 99% removal of NaCl from the 3.5 wt% NaCl solution. The measured heat transfer, after correcting for the presence of air in the chamber, demonstrated an average absolute deviation (AAD) of 18.6% for operation as an absorber and an AAD of 8.6% for the desorber.

Influence of Thermocapillary Forces on the Characteristics of Falling Liquid Films

Influence of Thermocapillary Forces on the Characteristics of Falling Liquid Films

Spatial distribution of heat transfer coefficient in the vicinity of wetting front during falling liquid film cooling of a vertical hot wall

Spatial distribution of the heat transfer coefficient (HTC) in the vicinity of the wetting front is of crucial importance in predicting the wetting velocity during liquid film cooling of a high-temperature solid surface. However, mainly due to the lack of experimental data, no sufficiently reliable model has been developed so far for the HTC distribution in this fundamental thermal-hydraulic situation. In the present work, experiments using a high-speed infra-red camera were carried out to measure the transient of wall temperature distribution during wetting of a high-temperature vertical wall with a falling liquid film. Based on the HTC distributions calculated from the measured temperature data, dimensionless correlations were developed for the HTC distribution near the wetting front. It was confirmed that the propagation velocities of the wetting front (wetting velocity) calculated using the proposed correlations agree with the experimental data accumulated in this work well.

Peripheral heat transfer prediction of the subcooled falling liquid film on a horizontal smooth tube

The peripheral heat transfer correlation of the subcooled falling liquid film outside the horizontal tube was established using the present numerical data under different liquid flow rates, heat fluxes, tube sizes, distributor heights, and liquid temperatures. The results indicated that the present correlation can predicate 93% of 2654 data within ±25% in θ = 2°–15°, 92% of 2254 data within ±15% in θ = 15°–165°, and 80% of 1442 data within ±30% in θ = 165°–178°. The peripheral heat transfer correlation can predict 74% of 903 data and 80% of 700 data, respectively, within 0°–180° and 15°–165° with the deviations of ±30%. The sensitivity analysis indicated that the parameter angle and the distributor height have the most and least significant impact on the peripheral heat transfer correlation, respectively.

Spanwise structuring and rivulet formation in suspended falling liquid films

In thin films flowing down the underside of an inclined planar substrate, surface topology is known to evolve towards a spanwise structuring of rivulets. While experiments imply a connection between long-term spanwise structuring and primary instabilities (PI) of the film surface at low Reynolds number (Re), this connection is not conclusive if a larger parameter space of Re and Kapitza number (Ka) is considered. Using an integral boundary layer model for falling liquid films and varying imposed initial conditions, Re, Ka, and wall inclination, we numerically investigate long-term rivulet evolution and its connection to PI, obtaining excellent concordance with previous experimental data.

Engineered mucoadhesive microparticles of formoterol/budesonide for pulmonary administration

In the present study, a multi-component system comprised of dipalmitylphospatidylcholine (DPPC), Chitosan, Lactose, and L-Leucine was developed for pulmonary delivery. Microparticles were engineered by the spray drying process and the selection of the critical parameters was performed by applying experimental design. The microcarriers with the appropriate size and yield were co-formulated with two active pharmaceutical ingredients (APIs), namely, Formoterol fumarate and Budesonide, and they were further investigated. All formulations exhibited spherical shape, appropriate aerodynamic performance, satisfying entrapment efficiency, and drug load. Their physicochemical properties were evaluated using Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR), and Differential Scanning Calorimetry (DSC). The aerodynamic particle size characterization was determined using an eight-stage Andersen cascade impactor, whereas the release of the actives was monitored in vitro in simulated lung fluid. Additional evaluation of the microparticles’ mucoadhesive properties was performed by ζ-potential measurements and ex vivo mucoadhesion study applying a falling liquid film method using porcine lung tissue. Cytotoxicity and cellular uptake studies in Calu-3 lung epithelial cell line were conducted to further investigate the safety and efficacy of the developed formulations.

Characteristics of HFC-134a absorption rate with [Bmim][Tf2N] in falling film type absorber

To develop an absorption heat pump with a hydrofluorocarbon refrigerant, this study evaluated its absorption rate with a falling liquid film type absorber. The HFC-134a refrigerant and [Bmim][Tf2N] ionic liquid were selected as the absorbent pair. The experimental apparatus was designed using the volumetric method, and the effects of the absorbent mass flow, temperature, and pressure conditions on the absorption rate were evaluated. Additionally, we calculated the absorption rate coefficient using the HFC-134a absorption transport model. The experimental results revealed that the absorption rate affected the desired amount of absorption equilibrium under the given temperature and pressure conditions. The absorption rate increased, while the amount of absorption at the set temperature and pressure decreased. Additionally, the outlet amount of absorption in the absorber increased as the absorbent mass flow down onto the heat exchanger tube increased. However, the amount of absorption was saturated as the absorbent mass flow increased. We monitored the absorbent flow behavior in the heat exchanger tube. The gas–liquid surface area was not changed by the absorbent mass flow, and the thickness of the absorbent liquid film increased. The absorption rate coefficient was determined using the HFC-134a absorption transport model, and the absorber height was calculated for up-scaling. The calculation results revealed that, to achieve 80% of absorption equilibrium, the absorber height must be 4.3 times greater than the experimental value. Thus, the heat exchanger with high wettability must be designed with a high absorption rate.

Temperature distribution in the cross section of wavy and falling thin liquid films

Wave instabilities of falling liquid films are crucial in many applications to enhance heat and mass transfer. Despite the importance of this issue, the interplay between the heat transfer and the wavy dynamics of falling films is still not completely understood. To get more insight, a planar laser-induced fluorescence technique has been developed for imaging the temperature distribution in the cross section of thin liquid films (approximately 0.5–1 mm thick), which are falling down an inclined heated surface. This study reports on the implementation of this imaging technique. It also discusses its advantages and limitations for the investigation of the heat transfer in the falling liquid films. Two-dimensional flow conditions and regular waves are considered for the reconstruction of a complete temperature field in the waves. Measurements provide new understanding of the wave ability to generate mixing within the film. Temperature maps reveal preferential regions where mixing occurs first, before eventually spreading to the rest of the film if the wave amplitude and the travel distance are large enough. The increase in the heat transfer coefficient is directly related to the internal mixing observed in the temperature images.

Experimental Validation of Falling Liquid Film Models: Velocity Assumption and Velocity Field Comparison.

This publication focuses on the experimental validation of film models by comparing constructed and experimental velocity fields based on model and elementary experimental data. The film experiment covers Kapitza numbers Ka = 278.8 and Ka = 4538.6, a Reynolds number range of 1.6–52, and disturbance frequencies of 0, 2, 5, and 7 Hz. Compared to previous publications, the applied methodology has boundary identification procedures that are more refined and provide additional adaptive particle image velocimetry (PIV) method access to synthetic particle images. The experimental method was validated with a comparison with experimental particle image velocimetry and planar laser induced fluorescence (PIV/PLIF) results, Nusselt’s theoretical prediction, and experimental particle tracking velocimetry (PTV) results of flat steady cases, and a good continuity equation reproduction of transient cases proves the method’s fidelity. The velocity fields are reconstructed based on different film flow model velocity profile assumptions such as experimental film thickness, flow rates, and their derivatives, providing a validation method of film model by comparison between reconstructed velocity experimental data and experimental velocity data. The comparison results show that the first-order weighted residual model (WRM) and regularized model (RM) are very similar, although they may fail to predict the velocity field in rapidly changing zones such as the front of the main hump and the first capillary wave troughs.

Experimental study of the effect of surfactants on the characteristics of waves in three-dimensional wave regimes

Effect of a surfactant Triton X-100 on three-dimensional wave regimes of the falling liquid film flow was experimentally studied. The shadow method and laser induced fluorescence measurements revealed that for surfactant concentrations at which complete suppression of wave motion is not observed, the flow characteristics at the three-dimensional wave regime differ from the characteristics of a pure water flow. Most likely, these differences are associated with differences in the structure of flows under the three-dimensional wave, which were revealed by simultaneous measurements of the thickness and volumetric measurements of the velocity in the wave at the initial section of the flow. For films with the surfactant concentration of 2000 mg/l, the capillary ripples in front of the three-dimensional wave are weakly expressed, there are no backflows, and transverse flows in the area of the capillary precursors are much weaker than in the case of pure water.

Experimental study of the three-dimensional interfacial wave structure of freely falling liquid film in a vertical large pipe diameter

Most of the experimental and theoretical studies on the film thickness of falling liquid films are focussed on either small diameter pipes or flat plates. Almost, all these studies provide time series data of the film thickness measured at single positions around the pipe wall or along the pipe section, which might not reflect the real flow structure in both axial and circumferential directions. This paper reports the interfacial structure of freely falling liquid films (liquid Reynolds numbers, ReL=618–1670) in a vertical large diameter pipe (127mm) using advanced Multi-Probes Film Sensor (MPFS). Unlike smaller diameter pipes where the waves are characterised as coherent rings, the waves found in this paper with larger diameter pipe were much localised around the pipe circumference and evolved with time in the axial direction. Skewness and Kurtosis numbers showed two distinct linear trends with two different slopes intersecting at ReL=960. One of the significant contributions of the current paper is that the new experimental reconstructed three-dimensional wave structure data for the falling liquid film in a vertical large pipe diameter will be available in the literature which would be of interest to many modellers who need such data to validate their CFD models.

Transient, Two-Dimensional Heat Conduction Model for Rewetting a Hot Vertical Plate Surface by a Falling Liquid Film.(Dept.M)

A transient, two-dimensional heat conduction model for rewetting a hot plate surface by a falling liquid film is developed. It is assumed that the plate thermal properties are Independent on temperature, and the heat transfer coefficient is a constant value in the wet region and zero in the dry region. The mathematical model equations are transformed to a dimensionless form solved numerically . The results indicate that the heat conduction problem in the rewetting process is substantially two-dimensional one, not only for thick plates, I.e., a large Bolt number, but also for a low initial wall temperature. An explicit simple formula for predicting the rewetting velocity is obtained by correlating the numerical results, Comparisons with other studies are made.

INSTABILITY OF A GRAVITY-DRIVEN LIQUID FILM CONFINED IN AN INCLINED RECTANGULAR DUCT

This study deals with the confinement effects due to top and side walls on the stability of a falling liquid film flowing down an incline. To this aim, we consider a liquid film flowing between two infinite plates and in rectangular ducts in the presence of an adjacent gas phase, focusing on the case of zero net gas flow rate. A rigorous linear stability analysis is conducted for three gas-liquid systems of particular interest in applications. This enables us to demonstrate the effect of the gas-liquid density ratio and viscosity ratio in small channels/ducts and shallow downward inclinations. Although the analysis considers all wave numbers, the neutral stability boundary is found to be defined mainly by long-wave perturbations in the two-plate (TP) geometry, as well as in the rectangular ducts. The decrease of the channel height for the considered liquid-gas systems always stabilizes the liquid film. However, the Kapitza criterion for the onset of Kapitza waves on the surface of a falling liquid film can either overpredict or underpredict the stability limits of the channel/duct flows, depending on the channel height, its aspect ratio, and the viscosity and density ratios of the phases. The falling liquid film, bounded by the side walls, becomes unstable at larger flow rates with the decrease of the duct aspect ratio. The presence of recirculating gas in the channel/duct is found to affect the stability through the dynamic shear stress components at the gas-liquid interface. Therefore, its presence must be taken into account.

CFD modeling of mass transfer in Gas-Liquid-Solid catalytic reactors

This work investigates Gas-Liquid-Solid mass transfer coupled to heterogeneous catalytic reaction using Computational Fluid Dynamics (CFD). The numerical model is based on the Volume-of-Fluid (VOF) approach, coupled with a convection–diffusion equation for mass transfer resolution. First, the numerical method is validated on a falling liquid film over a semi-infinite planar surface. Then, a micro-structured reactor with α-methylstyrene hydrogenation is studied. A good agreement is found between experimental data of Tourvieille et al. (2013) and simulation results. Afterwards, a vertical spherical beads string is investigated. Convective transport by transversal velocities is identified as an important contributor to the overall Gas-Liquid-Solid mass transfer. While the film model is applicable in pure diffusion regimes, the resistances-in-series model is not relevant and over-estimates the real mass transfer by nearly 30% when mass transfer occurs in liquid film flow without bulk.The present work shows how CFD can be an effective tool for predicting hydrodynamic and catalyst geometry effect on mass transfer in Gas-Liquid-Solid reactors.

Three-dimensional dynamics of falling films in the presence of insoluble surfactants

Abstract

Improvement of CO2 absorption by Fe3O4/water nanofluid falling liquid film in presence of the magnetic field

AbstractCarbon dioxide is considered as the main greenhouse gas responsible for climate change. Absorption of CO2 by liquid phase has gained attention as a means of mitigating environmental challenges. In this study, the effect of Fe3O4 nanoparticles on the CO2‐water mass transfer coefficient was experimentally evaluated in the presence and absence of a magnetic field. The absorption experiments were carried out in a falling liquid film absorber system in laminar and turbulent flow regimes. Fe3O4/water nanofluid was used in 0.001‐0.05 vol% concentrations. The results show that adding Fe3O4 nanoparticles to water increases the mass transfer coefficient (MTC), and that it increases with an increase in nanofluid concentration. For a concentration of 0.05 vol% nanofluid, the MTC was improved 111% and 13.7% in the turbulent and laminar flow regimes, respectively. The mass transfer coefficient of CO2 in water and effective mass transfer coefficient in nanofluid were increased up to 10% and 29% in the presence of a parallel alternative magnetic field, respectively.