Thickness Of Thin Liquid Film Research Articles

Pixel-dependent laser-induced fluorescence method for determining thin liquid film thickness distribution

Thin liquid films on free surfaces or inside restricted spaces are widely encountered and can have significant effects in heat transfer, coating, biofilm growth, and lubrication. The thickness and distribution of these thin liquid films are important determinants of performance, but cannot easily be measured accurately. This paper presents a methodological study on determining the thickness of thin liquid films by means of laser-induced fluorescence. A model of fluorescence imaging and measurement is established based on the fluorescence energy transmission. A novel pixel-dependent laser-induced fluorescence method is then proposed and compared with commonly used methods using experimental test results. The accuracy and precision of the measurements are discussed in detail. It is found that the measurement performance is greatly influenced by the nonlinearity, heterogeneity, and random noise of the optical system and imaging technique. The proposed pixel-dependent laser-induced fluorescence method effectively eliminates or ameliorates the effects of these factors and enhances the measuring accuracy.

Thin liquid film thickness measurement using stepped-index-type optical fiber sensors

Thin liquid film thickness measurement using stepped-index-type optical fiber sensors

Numerical simulation of flow characteristics of falling-film evaporation of R32/R134a non-azeotropic refrigerant outside a horizontal tube

Horizontal-tube falling-film evaporator is regarded as an effective alternative to full-liquid evaporator because of its higher heat transfer coefficient and lower refrigerant charge. In the process of falling-film evaporation, the thin liquid film thickness (δ) is an important parameter for characterizing the hydrodynamics, and accurate prediction of δ can effectively prevent the reduction in heat transfer efficiency caused by local wall dryout. In this study, non-azeotropic refrigerant R32/R134a was taken as the research object. By incorporating the multi-component phase change model and contact angle model into the governing equations, a two-dimensional numerical model of falling-film evaporation outside a horizontal circular tube was established. The effects of the spray height (H), tube diameter (d), Reynolds number (Re), inlet temperature (Tinlet), and mass fraction of R32 in the liquid-phase mixture (MR32_liquid) on δ were analyzed under sheet flow. The results show that an increase in Tinlet, H, and d is favorable for increasing the average δ (δave), whereas an increase in MR32_liquid and Re leads to a reduction in δave. Besides, the decreasing rate of the δave decreases gradually as H and d increase. When H is 4 mm, the local δ (δlocal) first decreases and then increases as the circumferential angle (Φ) increases. When H ranges from 6 mm to 13 mm, as Φ increases, δlocal first increases slightly, then decreases, and finally increases significantly near the wake region. The δlocal of a larger tube diameter is thicker than that of a smaller tube diameter near the impact region, whereas in the subsequent regions, the δlocal of a larger tube diameter is thinner than that of a smaller tube diameter. The minimum δlocal occurs at Φ between 130° and 140°, and with an increase in H and d, Φ corresponding to the minimum δlocal decreases.

Effects of tube cross-sectional shapes on flow pattern, liquid film and heat transfer of n-pentane across tube bundles

The heat transfer of hydrocarbon refrigerant across tube bundles have been widely used in refrigeration. Three-dimensional simulation model using volume of fluid (VOF) was presented to study the effects of tube shapes on flow pattern, film thickness and heat transfer of n-pentane across tube bundles, including circle, ellipse-shaped, egg-shaped and cam-shaped tube bundles. Simulation results agree well with experimental data in the literature. The liquid film thickness of sheet flow and heat transfer for different tube shapes were obtained numerically. The flow pattern transition occurs lower vapor quality for ellipse-shaped tube than other tube shapes. For sheet flow, the liquid film on circle tube and ellipse-shaped tube is symmetrically distributed along the circumferential direction. However, the liquid film on egg-shaped tube at circumferential angles (θ) = 15°–60° is thicker than θ = 135°–165°. The liquid film on cam tube at θ = 15°–60° is slightly thinner than θ = 135°–165°. The liquid film thickness varies from thinner to thicker for ellipse-shaped, cam-shaped, egg-shape and circle within θ = 15°–60°. The effect of tube shape is insignificant on thin liquid film thickness. Ellipse-shaped tube has largest heat transfer coefficient for sheet flow. In practical engineering, the tube shape could be designed as ellipse to promote heat transfer.

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.

Measurement of thin liquid film thickness in pipes based on optical interferometry

Measurement of thin liquid film thickness in pipes based on optical interferometry

Dynamics of liquid film rupture under local heating

Rupture dynamics of a horizontal liquid film non-uniformly heated from below was experimentally studied in a wide range of liquid viscosity. To visualize the liquid surface deformations and disruption of the film, an optical schlieren system was used. The instantaneous local film thickness was measured using a chromatic confocal sensor. In order to measure the thickness of thin liquid films, the substrate was covered with a black coating eliminating reflection of the incident light. Angular and frequency characteristics of the confocal sensor were studied in detail. The process of film rupture can be divided into three stages: 1) film thinning down to a residual film on the heater; 2) existence of a stable residual film for some time; 3) rupture and dry out of the residual film. The thickness of the residual film was found to strongly depend on the liquid viscosity: for water it is about 10 µm, whereas for silicone oil PMS-200 it is about 275 µm. The data obtained for various working liquids were generalized by a dimensionless correlation.

Non-contact measurement of the thickness of a surface film using a superimposed ultrasonic standing wave

Most methods used to measure the thickness of thin liquid or solid surface films and coatings need access to the coated surface. In this work reflected ultrasonic pulses were used to measure a coating thickness from a solid back face. Piezoelectric transducers on the solid back face emitted ultrasound waves and received the waves that bounced off the front face. The magnitude of the reflected wave was dependent on the film thickness at the front face. Most pulse-echo ultrasonic approaches use the time-of-flight through the surface layer to determine its thickness. However, as the film becomes thinner, the reflected echoes overlap and there is often an acoustic mismatch between the solid and the surface film that reduces the signal strength. In this work, we propose the use of an ultrasonic continuously repeated chirp longitudinal wave to amplify the effect of the surface film. Multiple reflections interfere within the solid to form a superimposed standing wave whose amplitude spectrum is highly dependent on the surface film thickness thus overcoming the acoustic mismatch problem. Two bare 10 MHz piezoelectric elements were bonded to a 10 mm thick aluminium solid in a pitch-catch arrangement such that one continuously sends repeating chirp ultrasound waves and the other acts as the receiver. The transmitter was set to send a repeating chirp wave of 4 ms duration corresponding to the bandwidth of the transducer in order to maximise signal amplitude. The incident and reflected waves constructively and destructively interfere to form a superimposed standing wave within the solid. The solid/surface film to solid/air boundary condition frequency spectra ratio showed the film resonant frequency modes as minima. Using this technique epoxy coatings ranging from 70 μm to 350 μm were measured and showed a good correlation with independent measurements using a surface profilometer.

Review and Prospect of the Measurement Technology of the Thickness of the Liquid Film on the Wall of the Corrugated Plate Dryer

The corrugated plate dryer is an important steam-water separation device in the steam generator in the secondary loop of a light water reactor (LWR). It is essential to understand its steam-water separation mechanism. Most studies have shown that the separation efficiency of the corrugated plate is related to the thickness of the liquid film on the wall of the corrugated plate. The relationship between the thickness of the liquid film and the critical airflow velocity of the corrugated plate has been studied by us. At present, scholars mostly use image method, ultrasonic method and capacitance method to measure the thickness of thin liquid film. We have analyzed the application of these three methods in liquid film thickness measurement. Some important examples are listed. The results show that the capacitance method is recommended for liquid film thickness measurement experiments whereas its accuracy is not particularly high. To pursue the higher accuracy, the image method is recommended. Planar laser-induced fluorescence (PLIF) technology is the first choice among image methods. Ultrasonic method is applicable in the case that the measurement data is large.

Hyperspectral imaging for dynamic thin film interferometry

Dynamic thin film interferometry is a technique used to non-invasively characterize the thickness of thin liquid films that are evolving in both space and time. Recovering the underlying thickness from the captured interferograms, unconditionally and automatically is still an open problem. Here we report a compact setup employing a snapshot hyperspectral camera and the related algorithms for the automated determination of thickness profiles of dynamic thin liquid films. The proposed technique is shown to recover film thickness profiles to within 100 nm of accuracy as compared to those profiles reconstructed through the manual color matching process. Subsequently, we discuss the characteristics and advantages of hyperspectral interferometry including the increased robustness against imaging noise as well as the ability to perform thickness reconstruction without considering the absolute light intensity information.

A technical review of research progress on thin liquid film thickness measurement

The thickness of the thin liquid film and its effects have always been a research hotspot in nuclear power applications and operating nuclear power plants, because the flow phenomenon of the liquid film is extremely common in multiphase flow research. Based on papers published in recent years, novel research progress on thin film thickness is reviewed from the following two perspectives: the experimental measurement and the theoretical model of liquid film thickness. For the experimental measurement, methods mainly include the PLIF method, the capacitance method, and the ultrasonic method. The following contents in this part are mainly reviewed from the PLIF method, the capacitance method, the ultrasonic method, and other methods for the measurement of the liquid film thickness on the wall, the liquid film thickness on the tube wall, and other aspects of the liquid film thickness. For the model, the theoretical model of liquid film thickness on the wall is mainly reviewed from two aspects: vertical flow and horizontal flow. Other models are mainly reviewed from the following aspects. They are annular film thickness, liquid film thickness for gas–liquid annular flow, and film thickness in pipes.

Thin film thickness measurements in two phase annular flows using ultrasonic pulse echo techniques

The electric power generation and oil/gas production industries have a strong interest in the physical characterization of conducting and non-conducting liquid films that are formed during the flow of liquids in pipes. Conducting and non-conducting liquid films do not lend themselves to the same characterization techniques due to the different requirements originating from their electrical properties. Techniques based on the use of ultrasound are extremely attractive for that purpose as they do not depend on the electrical properties of the liquid and are also non-invasive. This paper presents the application of ultrasonic techniques for measuring the thickness of wavy thin liquid films (<6 mm) in vertical pipes. Initial benchtop experiments were performed, and different signal processing methods were implemented in order to identify the most suitable depending on the film thickness. For a film thickness >0.5 mm a time of flight method was utilized whereas for a film thicknesses <0.5 mm a frequency method and time domain method were utilized. These methods were validated using a theoretical volume measurement on a static system. The studied methods were then tested on downward and upward vertical flow experimental rigs with pipe diameters of 127 mm and 34.5 mm respectively. The results of the experiments using ultrasonic methods showed good agreement with the measurements obtained using a multi pin film sensor and a concentric conductance probe, highlighting the potential that ultrasound offers in thin film measurements.

Fast and Accurate Thickness Mapping of Thin Liquid Films

The thickness of thin liquid films is of great interest to industrial processes and life science. However, there are not appropriate quantitative experimental tools for an adequate study of film evolution in case of not-ideal conditions. Here, we show the application of a holographic system for the evaluation of the 3D topography and thickness of evolving protein films. We use a custom holographic microscope that combines quantitative phase imaging with materials engineering. This technique offers an unprecedented level of details and we anticipate that it will promote a deeper understanding of the underlying physics of thin film dynamics.

Conductimetry technique for the measurement of thin liquid film thickness between two solid surfaces in relative motion: hydrodynamic lubrication

This paper presents an approach for the measurement of a liquid film thickness in contact with a rough surface, based on electrical conductance sensor. This type of sensor consists of two electrodes mounted flush on a wall and using an electrolyte solution representing the liquid film. Assessing of the generated current between a pair of electrode is used as a measure of the film thickness. The liquid film is contained between two parallel surfaces that one of which is coated with a certain roughness, while the other is smooth. The electrical probes are placed on the smooth surface and the facing rough surface is removable allowing a free move of the wall. In this way, a rotational or a sliding motion is imposed on the rough wall allowing a browse of an entire surface relative to the electrode, in order to determine the influence of roughness on the film thickness measurements. In addition, series of signal acquisition were carried out with imposed pressures on the upper plate for the characterization of the effect of the film compression on the measured thicknesses. The principle of this electrochemical technique is briefly explained, as well as how the lateral distance between the electrodes impacts the measuring range limit. The experimental setup is described and used to study the liquid film flow with several configurations of the wall surface. The obtained results demonstrated the feasibility of this type of non-intrusive estimation, as it was possible to estimate the variable film thickness which is depending on the peaks and valleys of the rough surface. This is promising, because measuring this parameter remains difficult. Finally, the results analysis allows to synthesize the advantages and limitations of this method.

Rupture of thin liquid film based premature critical heat flux prediction in microchannel

Accurate prediction of critical heat flux (CHF) is vital for the application and safety of compact heat exchanger with flow boiling in microchannel. In view of this, the current paper presents a criterion model to predict the premature CHF caused by upstream compressible volume instability (or flow reversal) in microchannel, i.e. the premature CHF is affirmed when the period of flow reversal is in excess of the maximum duration of the existence of the thin liquid film. Partial differential equation (PDE) for the thin film thickness is developed to analyze the transient characteristic of thin liquid film thickness, and film breakup occurs when the thin liquid film is evaporated to a critically low thickness. Mass-spring model is employed to predict the period of reversed flow with the upstream compressibility volume acting as the spring and the liquid column constituting the mass. Both of the maximum duration of liquid film existence and periodic flow reversal decrease with an increase of CHF, and increase with the increase of heat-to-mass flux ratio (or Boiling number). Premature CHF can be eliminated by increasing flow rate and pressure drop multiplier parameter, and the CHF increases with the increase of mass flux and the pressure drop multiplier parameter respectively. The periodic flow reversal model can satisfactorily predict the experimental oscillation periods with the maximum relative error of ±25%, and the CHF predicted by the current criterion is in possession of accuracies within the relative error of ±22.5%.

Evaporation thermal anslysis of Swallow-tailed Axial-grooved Heat Pipe

A detailed mathematical model that describes evaporating characteristics through thin liquid film at the evaporator section of swallow-tailed axial-grooved heat pipe was developed. The numerical simulation results about thin film profile, liquid-vapour interface temperature, evaporating rate and heat flux at the evaporating thin film region were given by the current investigation and the effect of superheat on the liquid-vapour interface temperature, evaporating mass rate and heat flux was discussed. Meanwhile, thermal model of the meniscus region at the evaporating section was developed to calculate the rate of heat transfer. The ratio of the heat conduction in the evaporating thin liquid film region and total heat rate were also discussed. It is indicated that the thickness of thin liquid film rises in a nearly linear fashion. The disjoining pressure can be neglected with increasing the liquid film thickness, tends to be negligibly small. The heat transfer rate at the intrinsic meniscus cannot be compared with that of the evaporating liquid film region.

Research of Interficial Shear effects on Heat Transfer Characteristics of Complete Condensation in Vertical Tube

The effects of interficial shear between liquid film and flowing-steam cannot be neglected during the steam condensation process in vertical tubes, and the Nusselt condensation model should be modified. In this paper, the heat transfer characteristics of steam condensation in vertical tube were investigated by experiments. Both the Nusselt theoretical model and the modified model considering interficial shear effects were adopted and compared with the experimental measurements. The results show that interfacial shear stress have significant influence on the liquid film flow state and thin liquid film thickness, which further results in higher condensation heat transfer coefficient. As the modified model considers interficial shear correctly, and its calculation results agree well with the experimental results.

Transient measurement of thin liquid films using a Shack–Hartmann sensor

A Shack–Hartmann (SH) wavefront sensor was used to measure the thickness of thin liquid films of n–octane on silicon substrates. A SH device consists of an array of microlenses spaced from an image sensor by their effective focal length. A planar wavefront imposed on a SH device would produce uniformly spaced foci on the sensor for each lenslet. A distorted wavefront would then shift the foci by a calculable amount. Typically used for optical system alignment and calibration, the SH device was coupled with magnifying optics to determine the distortion of a planar wavefront source upon refraction through a film and reflection by a polished substrate. Geometrical optics, in the form of ray transfer matrices, were used to determine the location and orientation of rays emanating from the film surface given their positions and angles recorded by the SH device. The SH foci movements were translated into sample coordinates and film slopes, which were subsequently integrated to produce the film profile.

Experimental study and modeling of disturbance wave height of vertical annular flow

Disturbance waves play an important role in interfacial transfer of mass, momentum, energy and modeling of liquid film behavior in annular two-phase flow. In view of this, air–water annular flow visualization experiments have been conducted at different gas and liquid velocity in a tubular test section having an inside diameter of 2.54cm, and high-speed videos of vertical upflow have been analyzed to extract the liquid film and disturbance waves data by the Matlab code. The average thickness of thin liquid film has been studied as functions of both gas and liquid phase velocities, i.e. the thickness of thin liquid film increases with increasing liquid velocity and decreases with increasing gas flow, which is in the form of δD=2.03Relf0.15Reg-0.6 according to the current experimental data fitting. On the basis of the thin liquid film thickness and taking the Kelvin–Helmholtz instability into account, i.e. from the perspective of the physical mechanism of disturbance wave formed, a semi-empirical model, i.e. ΔδD=1400ug″uf-1/3(ρf-ρg″)gδ2σ5/8, of disturbance wave height has been proposed. Comparing the results of this model with the existing correlations available in the prediction of disturbance wave height, the model can satisfactorily predict the experimental data available in literature.

The influence of material hardness on liquid droplet impingement erosion

This paper describes the experimental study on the liquid droplet impingement erosion of metal materials to understand the influence of material hardness on the erosion rate. The experiment is carried out using a water spray jet apparatus with a condition of relatively thin liquid film thickness. The metal materials tested are pure aluminum, aluminum alloy, brass, mild steel, carbon steel and stainless steel. The liquid droplets considered are 30±5μm in volume average diameter of water, which is the same order of droplet diameter in the actual pipeline in nuclear/fossil power plants. In order to understand the influence of material hardness on the liquid droplet impingement erosion, the scanning electron microscope (SEM) observation on the eroded surface and the measurement of erosion rate are carried out in the terminal stage of erosion. The experimental results indicate that the erosion rates are expressed by the droplet velocity, volume flux, Vickers hardness and the liquid film thickness, which are fundamentals of the liquid droplet impingement erosion. The empirical formula shows that the power index for droplet velocity dependency is found to be 7 with a scattering from 5 to 9 depending on the materials, while the power index for Vickers hardness dependency is found as −4.5.