Liquid Film Thickness Research Articles

Development and application of an optimal three-wavelength combination for liquid film measurement with absorption spectroscopy

In the simultaneous measurement of liquid film temperature and thickness based on multi-wavelength absorption spectroscopy, selecting optimal wavelength combinations can significantly improve the measurement accuracy. In the work, the absorption spectra of e-liquid at different temperatures were measured firstly. And ten sets of two-wavelength and three-wavelength combinations were then established based on five specific wavelengths from the absorption spectra, respectively. And the measurement accuracy of all the combinations were validated with a home-made calibration system. Among them, an optimal three-wavelength combination were selected. Finally, the evaporation processes of e-liquid films at three initial thicknesses (921/780/629 μm) on a horizontal quartz plate were then investigated with the optimal combination. The variation trends of film temperature and thickness measured by the combination were consistent with imaging method and thermocouple. It was found that the optimal three-wavelength combination could achieve high accuracy in simultaneous measurement of liquid film temperature and thickness.

Spreading Behavior of Single Oil Droplet Impacting Surface with/without a Thin Liquid Film

When oil droplets impact a solid surface for oil-air lubrication, they may spread out to produce a thin oil layer on the surface, which serves as a significant source of lubrication. A test rig was constructed in this research to observe the spreading behavior of oil droplets impacting surfaces from both frontal and lateral views. From the frontal view, laser-induced fluorescence techniques are used to measure the thickness of the oil layer quantitatively during the spreading of oil droplets. While the lateral view can observe the shape evolution of the droplets. Oil droplet spreading patterns on the sheet with dry surfaces and with different thin liquid film thicknesses were studied, and the effect of viscosity and the thickness of the thin liquid film on spreading radius and spreading thickness is considered. The experimental findings demonstrate that the maximum spreading factor, the spreading central layer thickness, and the apparentness of retraction all increase as viscosity increases. The retraction is obviously impacted by thin liquid films, and the retraction weakens as the thin liquid film thickness increases.

Hydrodynamics of liquid-liquid two-phase flow accounting for the coupling of dean vortices and Poiseuille flow

Hydrodynamics of liquid–liquid (L-L) two-phase flow in a confined helical microchannel are investigated via a VOF-CSF model, and further the influences of continuous phase flowrate, interfacial tension, viscosity ratio, helical screw pitch, helical radius as well as the diameter ratio of droplet to channel were systematically ascertained. To achieve the precise control over the interface deformation in a confined helical microchannel, the correlation equations with several key dimensionless numbers are established to predict the droplet aspect ratio, eccentricity and the minimum liquid film thickness. Especially, the minimum film thickness can be used to distinguish the flow field structures, and when the dimensionless outer minimum liquid film thickness (δ*out) is greater than 0.16, there is only one complete vortex system structure inside the droplet. Otherwise, two vortices are observed. Besides, the Q-criterion is used to quantitatively confirm the disappearance of vortices inside the droplet. Finally, the force field characteristics are also discussed to reveal the corresponding droplet interface deformation mechanisms, and it is found that, the droplet is driven forward by the differential pressure force, and deformed by the shear force along its flow direction. Both of the Dean force and Coriolis force usually control the interface deformation of droplet in the radial direction. The results in our study offer valuable guidance for designing the L-L two-phase flow microsystems to realize the goal of process intensification.

Agglomeration behavior of the dense medium in a gas–solid separation fluidized bed

In this study, the atomic force microscopy (AFM) was used to measure the micro-forces between different particles and study the influence of different operating parameters on the formation of particle clusters in the gas–solid fluidized bed separation process. In this way, the mechanism of liquid bridge formation and fracture can be elucidated, thereby providing a theoretical basis for inhibiting the formation of particle clusters. The results indicated that for relative humidity (RH) < 40%, the extrusion liquid bridge is the dominant factor for the formation of the particle liquid bridge, and the liquid film thickness, h, and liquid film volume, Vsq, vary linearly with RH; however, once the RH is > 40%, these relationships become nonlinear. The adhesion force between magnetite powder particles was found to be the largest and that between pulverized coal particles was found to be the smallest, with the mixture of the two particle types being in between. Further, the adhesion between particles increased monotonically with RH, while showing a maximum value with increasing particle separation. A theoretical analysis of the critical fracture distance, Drupture*, of the particle liquid bridge showed that, as the liquid bridge volume, V*, and contact angle, θ, increase, Drupture* gradually increases, but its rate of increase rate. The theoretical analysis was found to be largely consistent with the experimental data, only deviating at low RH.

A numerical study of liquid film dynamics in multi-nozzle spray cooling of downward-facing surface

In a consideration of spray cooling as the potential cooling mechanism for the in-vessel melt retention (IVR) strategy of nuclear reactors because of its superior heat removal efficiency, the SPAYCOR experiment has been conducted at KTH to investigate the spray cooling capacity of multiple nozzles applied to a downward-facing heated surface. In the present study, the dynamics of liquid film on the downward-facing surface resulting from the multi-nozzle spray are numerically simulated by using a coupled Eulerian-Lagrangian method implemented in the OpenFOAM platform. Prior to simulation of the SPAYCOR experiment, the numerical approach is used to calculate two theoretical setups which have known analytical solutions, with the objective to validate the models in predicting liquid film dynamics either in spray or on an inclined surface. In the simulation of the SPAYCOR experiment, the predicted film morphology shows a good agreement with the experimental observation. What's more, the influential factors, including the inclination of the downward-facing heater surface, the nozzle-to-surface distance as well as the nozzle-array layout, are also investigated numerically in the present study. The simulation results show that a decreasing nozzle-to-surface distance does not only lead to a thicker liquid film and a lower velocity in the vicinity of each spray coverage, but also increases non-uniformity of the liquid film. The nozzles-array layout has little influence on the average liquid film thickness and velocity, but significantly affects the film morphology.

Measurement of the Thin Liquid Film at the Wet-Gas Outlet of the Supersonic Separator by FPC Conductance Sensor

Gas-liquid two-phase flow widely exists in the natural gas transportation industry. Accurate measurement of the liquid film is conducive to enhancing the efficiency and economy of supersonic separators. Therefore, a flexible printed circuit (FPC) liquid film conductance sensor based on array arrangement is designed. Firstly, the geometric parameters of sensors are optimized by the finite element method. The modules of electrical signal excitation and collection system based on FPGA are designed. A liquid film calibration device is built to calibrate the FPC sensor, the liquid film thickness is fitted through calibration experiments, and the fitting curve error is within ±5.0 %. Secondly, to analyze the liquid film thickness and velocity at the wet-gas outlet section of the supersonic separator, serval experiments are carried out in different working conditions, and the signals of liquid film thickness are processed by time and frequency domain analysis. The results reveal that the liquid film thickness has a bimodal characteristic, where the thickness of the thin base layer is about 70 μm, and fluctuating layer thickness is related to working conditions. The normalized energy ratio and fluctuation characteristics significantly changed under different frequency bands. At the same time, the existence of shock waves seriously influences the generation of liquid film, resulting in a decrease in the liquid film thickness seriously. Last, the cross-correlation algorithm is used to analyze the liquid film velocity. The different signal transmitter and receiver electrode orders can realize the velocity measurement in different directions. The experimental results show that the liquid film velocity at the outlet of the supersonic separator maintains at 54.8 mm/s ~ 339.0 mm/s when the back pressure ranges from 0.350-0.717, the maximum <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RMSE</i> of flow direction velocity is 4.51 mm/s and the maximum <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RMSE</i> of circumferential direction velocity is 3.38 mm/s.

GAS-LIQUID FILM DISTRIBUTION AND SEPARATION OF CONDENSATION FROM A DOWNWARD-FLOWING STEAM-NONCONDENSABLE GAS MIXTURE ONTO A HORIZONTAL TUBE AT SUBATMOSPHERIC PRESSURE

Based on the boundary layer, a mathematical model was established to investigate the thicknesses of liquid and gas films and the steam condensate distribution in the presence of noncondensable gas outside a horizontal tube at pressures lower than atmospheric pressure. The model accurately predicts the steam condensation heat transfer with noncondensable gas. The influences of the noncondensable gas mass fraction, pressure, velocity, and surface subcooling upon the condensation heat transfer, the thicknesses of liquid and gas films, and condensate distribution around the horizontal tube have been studied. The liquid film thickness rises as the surface subcooling raises but reduces with raising pressure and mass fraction of noncondensable gas. The gas film thickness rises with rising surface subcooling and noncondensable gas mass fraction, reducing as the pressure rises. The thicknesses of gas and liquid films and condensate distribution significantly affect the local heat transfer. The liquid film separation velocity reduces with raising pressure, surface subcooling, and mass fraction of noncondensable gas, which is most influenced by pressure.

Analysis of the breaking process and the components of the crown structure after a single droplet hits thin liquid film

• Three types of crown breakup: splash breaking, hole breaking, and mixed breaking. • Fluid film viscosity and droplet Weber number affect the crown breakup. • Empirical correlations to classify three types of crown breakup was proposed. The breaking process and the components of the crown formed when the single droplet impact on different liquid films were investigated experimentally and simulatly. Laser-induced fluorescence (LIF) methods were used to study the breakup process of the crown structure. The composition of the incident droplets in this test is ethanol, and the glycerol aqueous solution was used as the liquid film. Results show that at different droplet Weber numbers, liquid film thickness, and liquid film viscosity, it is reasonable to classify the breakup of crown structure into the following three regimes: splash breaking, hole breaking, and mixed breaking. The characteristics of each breaking regime, such as the breaking mechanism, diameter of crown structure and spatial distribution of the secondary droplets formed after breakup, have been qualitatively analyzed. The viscosity of the liquid film and the Weber number of incident droplets have significant influence on the breakup of the crown structure. With decreasing viscosity and increasing Weber number, the crown structure breaking process transits from unbroken to splash breaking, hole breaking, and finally mixed breaking. According to the experimental results, a fitting formula for describing critical parameters of the different breaking regimes of crown structure is established. To further quantify the material components of the crown structure and the mixing of the incident droplets with the liquid film, Volume of Fluid (VOF) model and species transport model were used to establish a model for simulation calculations, and the effects of droplet Weber number, liquid film thickness and liquid film viscosity were clarified.

RECENT PROGRESS ON HEAT TRANSFER PERFORMANCE AND INFLUENCING FACTORS OF DIFFERENT MICROCHANNEL HEAT SINKS

The microchannel heat sink (MCHS) is an efficient thermal management technology widely used in various fields, including electronic equipment, automobiles, and aerospace. In this paper, the recent advances in cross-sectional shape, coolant type, flow channel shape, flow pattern, and application scenarios of the MCHS are systematically reviewed. The liquid film thickness in circular microchannels is the smallest, followed by rectangle, trapezoid, and triangle sections. Conversely, the pressure drop experienced exhibits an inverse relationship with the liquid film thickness. Comparatively, the heat transfer performance of the liquid phase surpasses that of the gas phase, and the two-phase coolant consistently outperforms the single-phase coolant. The study also investigates the impact of flow direction and shape on heat transfer performance. It is found that the implementation of wavy, fractal, and cavity structures enhances heat transfer performance at the expense of increased fluid motion variability, resulting in a loss of pressure drop. Additionally, this paper discusses the occurrence of laminar and turbulent flow phenomena within MCHSs and summarizes their respective influences on heat dissipation performance. On the basis of the aforementioned findings, four key applications of MCHSs are emphasized, accompanied by recommendations for their present utilization and future development. Future research endeavors will concentrate on striking a balance between altering the shape and material characteristics of MCHSs to optimize heat transfer performance while developing novel theoretical models continuously.

Flow and heat transfer model for turbulent-laminar/turbulent gas-liquid annular flows

• Physical process of heat transfer for annular flow was mathematically formulated. • Effect of flow orientations and parameters on heat transfer process was analyzed. • Heat transfer was significantly enhanced by gas in flow regime of annular flow. • Dependence of heat transfer multiplier on void fraction and pressure drop multiplier was identified. • A heat transfer correlation was developed based on Chilton & Colburn analogy. Two-phase gas-liquid annular flows are characterized by the liquid flowing in the annular-shaped channels as liquid film and gas flowing in the center as gas core. Adequate understanding of the flow and heat transfer mechanism is important. This paper aims at developing a model for flow and heat transfer in two-phase annular flows. First, the physical process of flow and heat transfer for two-phase annular flow is mathematically formulated and a new heat transfer model is developed based on two-fluid concept. Then, the new model is well validated using the experimental void fraction, liquid film thickness, and heat transfer coefficient collected from various sources. Third, the effect of the orientations and flow parameters on the heat transfer of two-phase annular flows are comprehensively investigated. Finally, the dependence of the two-phase heat transfer multipliers on the void fractions and pressure multipliers is identified quantitatively and a simple heat transfer correlation is developed based on Chilton & Colburn analogy.

ELUCIDATION OF INTERNAL FLOW AND ITS EFFECT ON THE JET OF A TWIN-FLUID ATOMIZER FOR GAS TURBINE CROSSFLOW COMBUSTORS

Conventional liquid fuel jet injection into a crossflow is widely used for large-scale power generation in gas turbine combustors. The authors proposed a new twin-fluid injection technique as a potential alternative and conducted a series of studies on the internal flow of the atomizer and liquid jet/spray characteristics. In this study, experimental and numerical studies were first performed on internal flow behaviors and liquid and atomizing air interactions. The important behaviors of the internal flow of the atomizer, such as the dynamic change in the liquid film thickness, the rapidly changing velocity of the wave front, and the liquid impingement distance on the inner wall of the atomizer, were measured by both the experiment and numerical analysis. Atomizing air generates a high-speed oscillating annular liquid film flow that flows at the exit of a twin-fluid atomizer. This dynamic of the annular liquid film is considered fairly advantageous for achieving good atomization, which is investigated by analyzing only the liquid's behaviors in jet/spray and twin-fluid injection in a crossflow. High-speed photography and image processing were applied to observe the liquid jet/spray phenomena in the crossflow and reveal the atomization characteristics such as jet/spray trajectories and breakup length. Furthermore, the atomizing air can dominate the trajectory of the liquid jet/spray in the crossflow. The twin-fluid injection provides improved atomization properties compared to conventional liquid-only injection.

R245fa flow boiling heat transfer in a sintering and electroplating modulated tube

• R245fa flow boiling heat transfer is investigated in a modulated tube coated by using sintering and electroplating technology. • Flow patterns and dynamic liquid height analyses are proposed to explore the heat transfer enhancement mechanisms. • The micro-nano scale coating on the tube inner wall could modulate flow pattern to enhance heat transfer. • The maximum EF and PEC can reach 2.87 and 2.79. The flow boiling performance of R245fa was investigated in a bare stainless-steel tube (BT) and a sintering and electroplating modulated stainless-steel tube (SET). The test tubes had the same inner diameters of 10.02 mm and effective heat transfer lengths of 800 mm. The mass fluxes, inlet vapor mass qualities and heat fluxes were in ranges of 192.61-705.35 kg/(m 2 ·s), 0.01–0.9 and 4.99-74.97 kW/m 2 , respectively. Three flow patterns were identified in both BT and SET. The annular flow pattern conversion line of SET was earlier than that of the BT. With the increase in vapor quality, the boiling heat transfer coefficient increased and then decreased at q≤ 45 kW/m 2 , whereas it decreased at q ≥60 kW/m 2 . For the BT, the heat transfer coefficient was weakly affected by the mass flux, but it increased with the increase in mass flux for the SET. For the BT and SET, the frictional pressure drop increased and then decreased with the increase in mean vapor quality at a certain heat flux. Owing to the strong wettability of SET, the thick liquid film at the tube bottom in stratified flow was forced to the tube top and the liquid droplets in annular flow were captured by the surface coating. Thus, flow patterns were modulated to enhance heat transfer. The maximum of the heat transfer enhancement ratio and performance evaluation parameter can reach 2.87 and 2.79, respectively.

Dielectric response of confined water films from a classical density functional theory perspective.

We re-examine the problem of the dielectric response of highly polar liquids such as water in confinement between two walls using simple two-variable density functional theory involving number and polarisation densities. In the longitudinal polarisation case where a perturbing field is applied perpendicularly to the walls, we show that the notion of the local dielectric constant, although ill-defined at a microscopic level, makes sense when coarse-graining over the typical size of a particle is introduced. The approach makes it possible to study the effective dielectric response of thin liquid films of various thicknesses in connection with the recent experiments of Fumagalli et al., [Science, 2018, 360, 1339-1342], and to discuss the notion of the interfacial dielectric constant. We argue that the observed properties as a function of slab dimensions, in particular the very low dielectric constants of the order of 2-3 measured for thin slabs of ∼1 nm thickness do not highlight any special properties of water but can be recovered for a generic polar solvent having similar particle size and the same high dielectric constant. Regarding the transverse polarisation case where the perturbing field is parallel to the walls, the associated effective dielectric constant as a function of slab dimensions reaches bulk-like values at much shorter widths than in the longitudinal case. In both cases, we find an oscillatory behaviour for slab thicknesses in the one nanometer range due to packing effects.

Numerical study of inlet eccentricity on liquid film spreading and splitting in centrifugal granulation assisted thermal energy recovery

Centrifugal granulation assisted thermal energy recovery(CGATER) is a method of recovering waste heat from molten slag by centrifugal granulation on a rotating disc. Disturbance and positioning errors can cause the incoming liquid column to deviate from the center of the disc. In this study, numerical simulation was conducted to investigate the effect of inlet eccentricity on liquid film flow spreading, liquid filament formation and fragmentation, and droplet size distribution during CGATER. The study shows that the eccentric granulation results in a new splitting pattern in which the liquid filament grows to a certain length with the rotation of the disc and then breaks from the bottom; the thickness of the liquid film on the eccentric granulation disc is consistent with the radial velocity distribution of the liquid film. Finally, the general dimensionless prediction correlation of particle median diameter is established for the eccentric situation. • A simulation of centrifugal granulation under eccentric entry was carried out. • Eccentric entry creates a new mode of liquid filament splitting. • Consistent liquid film thickness and radial velocity distribution on the disc. • A dimensionless particle size prediction equation with eccentricity was proposed.

Modeling of droplet dynamics with soluble surfactant by multi-relaxation-time phase-field lattice Boltzmann method

The multiphase fluid system in the presence of surfactant is frequently encountered in numerous scientific and engineering applications. Developing a model for accurately simulating such a complex system is of great significance. In this work, we propose a multi-relaxation-time phase-field lattice Boltzmann model for simulating droplet dynamics with soluble surfactants. The accuracy and validity of the model are verified by benchmark cases including static droplet and Rayleigh–Taylor instability tests. The effects of surfactant, capillary number, and density ratio on single-droplet deformation and two-droplet interaction under shear flow are investigated. Simulation results indicate that the Marangoni stress generated by the inhomogeneous distribution of surfactant at the interface plays the role of promoting droplet deformation and hindering droplet coalescence. Within the studied range, it tends to be much easier for droplets to deform with the decrease in density ratio. The increase in the capillary number and surfactant concentration is conducive to promoting the deformation and breakup of droplets. In addition, a higher surfactant concentration is found to result in greater liquid film thickness between droplets, which would hinder the coalescence of the droplets.

Dryout quality prediction in helical coils based on non-uniform liquid film thickness distribution: A model study

Dryout occurrence has an adverse effect on the thermal efficiency of helically coiled steam generators used in thermal energy storage systems. In this study, a mechanistic model based on the non-uniform annular liquid film thickness distribution was developed to evaluate the quality of dryout occurrence in helical coils. The inner-side and outer-side liquid films were investigated, including the local effects of entrainment, deposition, and dryout, while the film interface profile in the tube cross-section was simplified as a half-ellipse. The deposition correlations reported in the literature were modified by considering the centrifugal effect. The entrainment correlations were compared and optimized to improve the dryout prediction. The proposed model achieved satisfactory agreement on a boiling crisis dataset of 994 test points with a mean absolute error of 35.2%, which is better than existing models. The current work can be used in thermohydraulic design and performance prediction for thermal energy storage systems.

Numerical simulation of two-phase flow in a microchannel for evaluating liquid film thickness and its Reynolds number dependency

Numerical simulation of two-phase flow in a microchannel for evaluating liquid film thickness and its Reynolds number dependency

PENGARUH TEGANGAN PERMUKAAN PADA TEBAL FILM CAIRAN DAN KARAKTERISTIK GELOMBANG ANTARMUKA ALIRAN DUA FASE STRATIFIED HORIZONTAL

This experimental study aims to examine the effect of the surface tension on the characteristics of unidirectional air-liquid interface waves on a horizontal pipe. The pipe used is an acrylic pipe with a diameter of 26 mm. To vary the surface tension, the working fluid consisted of 100% water and water mixed with 2% and 5% butanol. with surface tension (σ) values ​​of 72, 58, and 42.5 mN/m, respectively. The wave characteristics studied in this research include the interface wave structure which is analyzed from visual observations and parameters by image processing. The superficial velocities of gas and liquid were varied in the range of 4-16 m/s and 0.02-0.1 m/s, respectively. Wave velocity and frequency were determined by using cross-correlation and power spectra density (PSD) from time-series data of liquid film thickness. Generally, velocity increases as the gas superficial velocity and surface tension decrease, the high gas superficial velocity the dominant frequency is high.

Time-dependent force in high-speed liquid droplet impacting on a wet wall

Recent research (Fujisawa, 2022a) has shown that the time-dependent force exerted on a droplet during high-speed [O (100 m/s)] impact on a planar rigid wall is quite different from that of low-speed [O (1 m/s)] impact case owing to the compressibility of the droplet. Here, we consider a more general case of a high-speed liquid droplet impacting a wall where a liquid film is present on the wall surface. Three-component Euler simulations were performed to determine the characteristics of the time-dependent force exerted on the droplet during high-speed liquid droplet impact on a planar rigid wet wall. Several initial liquid film thicknesses were considered. The strength of the force exerted on the droplet decreases in magnitude as the liquid film thickness increases, whereas the total force increases with increasing liquid film thickness owing to the presence of a water hammer shock that is reflected inside the liquid film.

Observing the impact force and low-speed droplet behavior of wet surfaces with very-thin liquid films

The impact of a low-speed droplet on a very-thin liquid film on a solid surface was analyzed experimentally by observing the impact force and droplet behavior. A high impact force and delayed peak impact time were observed with very-thin liquid films (with h/d < 0.05; h is the liquid-film thickness and d is the droplet diameter). On increasing the film thickness, the impact force gradually increased. The increased impact force could be attributed to splash formation on the wet surface arising from lateral-jet formation at the instant of droplet impact, and interactions with the liquid film. Furthermore, this result was in agreement with numerical simulation results of the surface impact-pressure variation which indicated impact-pressure damping immediately after droplet impact, with a subsequent spread of the wet-surface impact-pressure area, thereby causing an increase in the impact force at a later stage. Thus, impact-pressure damping and force increments occurred at different time ranges after droplet impact.