Measurement Of Liquid Film Thickness Research Articles

Measurement of liquid water film thickness on opaque surface with diode laser absorption spectroscopy

Liquid film is usually formed on opaque surfaces in various industrial processes, and the investigation of liquid film is very important for understanding and optimization of the relevant processes. Here, a novel on-line measurement method based on diode laser absorption spectroscopy (DLAS) was developed to determine the film thickness on opaque surfaces. The evaporation process of static liquid films both on a silver-coated mirror (smooth opaque surface) and an aluminum alloy plate (rough opaque surface) were investigated with the proposed method. Ultrasonic pulse-echo method (UPEM) was simultaneously applied to validate the measurement accuracy of DLAS method, it revealed that the average thickness deviation between these two methods during film evaporation process on silver-coated mirror were 4.9% and on aluminum alloy plate were 7.1%, respectively. A flow liquid film on aluminum alloy surface was then studied, it was found that the average thickness deviation between DLAS and UPEM at low flow speed were 6.5% and at higher flow speed were 9.5%, respectively. And the fluctuation frequency of the liquid film at both flow speeds were also in good agreement with these two methods.

Transient film thickness and microscale heat transfer during flow boiling in microchannels

The underlying mechanism of heat transfer for several sub-processes that control a bubble growth cycle during flow boiling in microchannels is studied in this article by means of high frequency measurements of liquid film thickness and temperature accompanying synchronous visualization. The test section is made up of glass tube having an internal diameter of 0.94mm with the Indium Tin Oxide (ITO) electrically conductive layer as heaters. The initial liquid film thickness formed by a boiling bubble slug agreed well with Taylor’s law. For most cases of the liquid film thickness evolution, the thinning of the liquid film was found to be much quicker than the prediction when considering evaporation effect alone. A new model for predicting the liquid film thickness evolution under flow boiling condition was developed by inclusion of the combination effect of evaporation and shear stress. It was found that the sub-processes of a bubble growth cycle were dependent on heat flux. The cyclical fluctuation of the temperature was due to the different heat transfer capability of the sub-processes. The time ratio and relative importance of the different heat transfer mechanisms were quantitatively estimated by linking the transient temperature fluctuations with the liquid film thickness variations.

Liquid film thickness measurement underneath a gas slug with miniaturized sensor matrix in a microchannel

Measurement of liquid film thickness is essential for understanding the dynamics of two-phase flow in microchannels. In this work, a miniaturized sensor matrix with impedance measurement and MEMS technology to measure the thin liquid film underneath a bubble in the air–water flow in a horizontal microchannel has been developed. This miniaturized sensor matrix consists of 5 × 5 sensors where each sensor is comprised of a transmitter and a receiver electrode concentrically. The dimension and performance of the sensor electrodes were optimized with simulation results. The maximum diameter of the sensor ring is 310 µm, allowing a measurable range of liquid film thickness up to 83 µm. These sensors were distributed on the surface of a wafer with photolithography technology, covering a total length of 8 mm and a width of 2 mm. A spatial resolution of 0.5 × 2.0 mm2 and a temporal resolution of 5 kHz were achieved for this sensor matrix with a measurement accuracy of 0.5 µm. A series of microchannels with different heights were used in the calibration in order to achieve the signal-to-thickness characteristics of each sensor. This delicate sensor matrix can provide detailed information on the variation of film thickness underneath gas–water slug directly, accurately and dynamically.

Effect of IR Transceiver orientation on gas/liquid two-phase flow regimes

Multiphase flows play a vital role in many industrial and naturally occurring processes. Recent trend of miniaturization in mini/micro fluid reactors, compact heat exchangers and micro thrusters requires a thorough knowledge on multiphase flow phenomena in mini/micro channels. The present work is focused on the effect irradiation behavior of infrared rays (IR) during gas liquid two phase flow consisting of thin liquid films inside a mini channel. The influence of size and shape of the slug regime and liquid film thickness on IR rays is analyzed with COMSOL Multi physics package. Experiments are carried out in a 2.5mm diameter borosilicate glass tube with wall thickness of 0.3mm. The refraction and transmittance behavior of IR rays on slug and bubbly flow is studied by analyzing the Current-time output of an IR photodiode kept at different angles with the test section. The results are found to be in good agreement with experimental image processing technique and COMSOL results. The results obtained will be useful for designing of IR sensor arrays sensitive to multiphase flows. It can also be used for measurement of liquid film thickness with proper calibration.

Design of instantaneous liquid film thickness measurement system for conductive or non-conductive fluid with high viscosity

In the paper, a new capacitive sensor with a dielectric film coating was designed to measure the thickness of the liquid film on a flat surface. The measured medium can be conductive or non-conductive fluid with high viscosity such as silicone oil, syrup, CMC solution and melt. With the dielectric film coating, the defects caused by the humidity in a capacitor can be avoided completely. With a excitation frequency 0-20kHz, the static permittivity of capacitive sensor is obtained and stable when small thicknesses are monitored within the frequency of 0-3kHz. Based on the measurement principle, an experimental system was designed and verified including calibration and actual measurement for different liquid film thickness. Experimental results showed that the sensitivity, the resolution, repeatability and linear range of the capacitive sensor are satisfied to the liquid film thickness measurement. Finally, the capacitive measuring system was successfully applied to the water, silicone oil and syrup film thickness measurement.

Flow regimes during condensation in superheated zone

Heat transfer experiments with flow visualization were performed with R134a in a smooth horizontal tube in order to verify that condensation occurs outside the two-phase region. The visualization performed for this work is unusual in that a diabatic visualization section was used, more closely approximating the behavior of the fluid in an actual condenser than typical adiabatic visualization experiments. In addition to the flow visualization, liquid film thickness measurements were taken and used to determine the cumulative latent heat transferred from the refrigerant. The measured film thicknesses and recorded videos confirm the presence of liquid in the bulk superheated region. Condensate appeared first as droplets and rivulets on the tube wall and then formed a thin film. The flow later passed through misty annular and wavy flow regimes as the refrigerant transitioned to the two-phase region.

Feasibility Test of a Liquid Film Thickness Sensor on a Flexible Printed Circuit Board Using a Three-Electrode Conductance Method.

Liquid film thickness measurements under temperature-varying conditions in a two-phase flow are of great importance to refining our understanding of two-phase flows. In order to overcome the limitations of the conventional electrical means of measuring the thickness of a liquid film, this study proposes a three-electrode conductance method, with the device fabricated on a flexible printed circuit board (FPCB). The three-electrode conductance method offers the advantage of applicability under conditions with varying temperatures in principle, while the FPCB has the advantage of usability on curved surfaces and in relatively high-temperature conditions in comparison with sensors based on a printed circuit board (PCB). Two types of prototype sensors were fabricated on an FPCB and the feasibility of both was confirmed in a calibration test conducted at different temperatures. With the calibrated sensor, liquid film thickness measurements were conducted via a falling liquid film flow experiment, and the working performance was tested.

A new flow regime map and void fraction model based on the flow characterization of condensation

The paper presents flow characterization of condensation based on experiments with R134a in 6.1mm inner diameter horizontal round tube. The paper presents flow visualizations and liquid film thickness measurements with mass fluxes from 50 to 200kgm−2s−1and heat fluxes from 5 to 15kWm−2, showing the effect of mass flux and heat flux on the onset of condensation, flow regime, film distribution and void fraction. All the measurements are taken at constant pressure of 1.319MPa, which corresponds to a saturation temperature of 50°C. The result of flow visualization reveals that the condensation always starts in the bulk superheated region, which is generally defined as the beginning of condensing superheated region, as annular flow and the flow regime is strongly affected by the mass flux, instead of the heat flux. Based on the flow visualization results and underlying physics in the condensing superheated region, a new diabatic flow regime map is proposed to better represent the physics and predict flow regimes in both the two-phase and the condensing superheated region. The range of prediction of the new flow regime map extends over quality one up to the onset of condensation and the early stages of condensation are strictly in annular flow regime. A film thickness measurement technique for round tube is described as well as results of calibration. The film thickness measurement demonstrates that void fraction drops below one in superheated region, which is not taken into account for most conventional void fraction models. In addition, both the mass flux and heat flux affect the void fraction by altering the onset of condensation, a new void fraction model is proposed to include this mechanism. Moreover, having information of film distribution provides an opportunity to more realistically model the film geometry inside of the tube.

Measurements of interfacial dynamics of gas–liquid displacement in a capillary

Measurement of liquid film thickness in gas–liquid plug/slug flows is a challenging task. A novel laser interference method for measuring the interfacial film thickness of gas–liquid displacement in a plug flow has been developed. This novel technique utilizes light scattering from different liquid/gas interfaces in forming interference fringes. The interference fringes are used for calculating the film thickness. A set of simultaneous equations is derived based on geometrical optics. The experiment set up is not complex and is easy to install. The fringes are recorded by a charge-coupled device high speed camera and the image data are calculated using fast Fourier transform (FFT) and a non-linear least squares Levenberg–Marquardt algorithm. The uncertainty of this measurement technique is quite small (0.3 μm) and the entire film thickness profile can be measured at the same time.

Novel method for the measurement of liquid film thickness during fuel spray impingement on surfaces.

This paper describes the development and application of a novel optical technique for the measurement of liquid film thickness formed on surfaces during the impingement of automotive fuel sprays. The technique makes use of the change of the light scattering characteristics of a metal surface with known roughness, when liquid is deposited. Important advantages of the technique over previously established methods are the ability to measure the time-dependent spatial distribution of the liquid film without a need to add a fluorescent tracer to the liquid, while the measurement principle is not influenced by changes of the pressure and temperature of the liquid or the surrounding gas phase. Also, there is no need for non-fluorescing surrogate fuels. However, an in situ calibration of the dependence of signal intensity on liquid film thickness is required. The developed method can be applied to measure the time-dependent and two-dimensional distribution of the liquid fuel film thickness on the piston or the liner of gasoline direct injection (GDI) engines. The applicability of this technique was evaluated with impinging sprays of several linear alkanes and alcohols with different thermo-physical properties. The surface temperature of the impingement plate was controlled to simulate the range of piston surface temperatures inside a GDI engine. Two sets of liquid film thickness measurements were obtained. During the first set, the surface temperature of the plate was kept constant, while the spray of different fuels interacted with the surface. In the second set, the plate temperature was adjusted to match the boiling temperature of each fuel. In this way, the influence of the surface temperature on the liquid film created by the spray of different fuels and their evaporation characteristics could be demonstrated.

Measurement of Liquid Film Thickness via Light Absorption and Laser Tomography

This paper presents film thickness measurement over a vertical falling liquid film, carried out with Light Absorption (LAbs) and a 2D laser tomography techniques known as Level Detection and Recording (LeDaR). The presented test cases are in the low Reynolds regime (Re = 1 - 8) and include undisturbed conditions, i.e. constant flow rate, and inlet-forced conditions, i.e. flow rate pulsing at frequencies in the range fp = 12-18Hz. The liquid used is Dipropilene Glycol (DPG), with Kapitza number Ka = 4.6. The results include average film thickness in the steady case, wave celerity and wave profile in the forced conditions. All the measurements are scaled using the Skhadov scaling, to facilitate the comparison with analytical low-dimensional models. Both LAbs and LeDaR techniques are presented in detail, including calibration, uncertainties and data processing aspects.

Measurements of liquid film thickness, concentration, and temperature of aqueous urea solution by NIR absorption spectroscopy

Measurements of liquid film thickness, concentration, and temperature of aqueous urea solution by NIR absorption spectroscopy

Error analysis and liquid film thickness measurement in gas–liquid annular flow

The parameters measurement of liquid film in gas–liquid two-phase annular flow is of great significance for the study of heat and mass transfer and flow characteristics. The paper focuses on the thickness measurement and flow evolution analysis of the liquid film in downward gas–liquid two-phase annular flow. Compared with the traditional measurement methods, the method combined the laser-induced fluorescence (LIF) technique with the high-speed photography is developed to measure the liquid film thickness and analyze the flow distribution. Firstly, the geometric optical path model of the organic glass pipeline was established based on the Gauss optics to recognize the real and virtual film images and quantitatively estimate the measurement linearity. And then the digital image processing technology was developed to achieve non-intrusive measurement of liquid film thickness. Finally, the film thickness distribution in downward gas–liquid two-phase annular flow was further analyzed. The method can avoid the interference with the flow field. Meanwhile, the measurement results by experiment show that the film thickness manifests stochastic distribution, which corresponds to the features of randomness and small flow in the process of liquid film flow.

Measurements of liquid film thickness, concentration and temperature of aqueous NaCl solution by NIR absorption spectroscopy

A multi-wavelength near-infrared (NIR) diode laser absorption sensor has been developed and demonstrated for real-time monitoring of the thickness, solute concentration, and temperature of thin films of urea–water solutions. The sensor monitors the transmittance of three near-infrared diode lasers through the thin liquid film. Film thickness, urea mass fraction, and liquid temperature were determined from measured transmittance ratios of suitable combinations of lasers. Available laser wavelengths were selected depending on the variation of the NIR absorption spectrum of the solution with temperature and solute concentration. The spectral database was measured by a Fourier transform infrared spectrometer in the range 5500–8000 cm−1 for urea solutions between 5 and 40 wt% and temperatures between 298 and 338 K. A prototype sensor was constructed, and the sensor concept was first validated with measurements using a calibration cell providing liquid layers of variable thickness (200–1500 µm), urea mass fraction (5–40 wt%) and temperature (298–318 K). Temporal variations of film thickness and urea concentration were captured during the constant-temperature evaporation of a liquid film deposited on an optically polished heated quartz flat.

Heat transfer and flow characteristics of rising Taylor bubbles

The heat transfer enhancement due to slug flow has been widely studied both experimentally and numerically, yet identification of the main heat transfer mechanisms has been debated and little experimental evidence is available. In this work, an infrared thermography technique, high-speed visualization, and a film thickness sensor were used to characterize vapor Taylor bubbles rising in a vertical, co-current, liquid flow. Measurements of the local heat transfer around each bubble indicated that the largest enhancement resulted from turbulent mixing in the bubble wake due to vortex shedding at the tail. The effect of the vortices on heat transfer was found to decline with increasing liquid velocity as a result of shortened residence time of the bubbles over a particular tube location. Liquid film thickness measurements showed excellent agreement with correlations when the film was fully developed. Bubble velocities and vortex shedding frequencies were experimentally determined for several liquid velocities.

Measurement of liquid film thickness in micro tube annular flow

Heat transfer in micro scale two-phase flow attracts large attention since it can achieve large heat transfer area per unit volume. At large flow rate and high quality, annular flow becomes one of the major flow regimes in micro two-phase flow. Heat is transferred by evaporation or condensation of the liquid film, which are the dominant mechanisms of micro scale heat transfer. Therefore, liquid film thickness is one of the most important parameters in modeling the heat transfer phenomena. In the present study, time averaged annular liquid film thickness is measured by laser confocal displacement meter (LCDM), and the gas–liquid interface profile is observed by a high-speed camera. Glass tubes with inner diameters of D=0.3, 0.5 and 1.0mm are used. Degassed water and air are used for working fluids, and the total mass flux is varied from G=100 to 500kg/m2s. Flow patterns are observed and flow pattern map based on Reynolds numbers of gas and liquid flows is suggested. Pressure drop is measured and compared with the prediction using Lockhart and Martinelli parameter. Pressure drop is well predicted with Lockhart–Martinelli correlation. Dimensionless mean film thickness is then plotted against quality, and compared with the annular film model assuming flat gas–liquid interface. Flat interface model overestimated the experimental data. It is considered that the shear stress on the gas–liquid interface in the real annular flow is larger than that estimated in the ideal flat interface model. Prediction using new empirical correlation considering the effect of the non-flat gas–liquid interface showed good agreement with the experiment data.

Experimental and computational investigation of vertical downflow condensation

This explores downflow condensation in a circular tube both experimentally and computationally using FC-72 as a working fluid. A highly instrumented condensation module is used to map detailed axial variations of both wall heat flux and wall temperature, which are used to determine axial variations of the condensation heat transfer coefficient. The experimental results are compared to predictions of a two-dimensional axisymmetric computational model using FLUENT. The study provides detailed construction of the model, including choice of interfacial phase change sub-model, numerical methods, and convergence criteria. The model is shown to yield good prediction of the heat transfer coefficient. The computed temperature profiles exhibit unusual shape, with steep gradient near the annular liquid film interface as well as near the wall, and a mild gradient in between. This shape is shown to be closely related to the shape of the eddy diffusivity profile. These findings point to the need for future, more sophisticated measurements of liquid film thickness, and both velocity and temperature profiles, to both validate and refine two-phase computational models.

Measurements of liquid film thickness of a wide horizontal co-current stratified air-water flow

A horizontal co-current stratified two-phase flow was studied in a duct with a rectangular section. Its dimensions were such that the top and side walls did not influence the interfacial waves. The film thickness was almost of the millimeter. The Reynolds number based on the mean height and velocity in each phase varied from 0 to 1.16•105 in air and from 69 to 271 in water. These experimental conditions allowed the observation of the smooth, 2D wavy and 3D wavy stratified flow patterns. The interfacial waves were instantaneously measured on a 100 mm long line segment in the longitudinal midspan plane of the channel with the Level Detection and Recording technique. These measurements yielded statistical characteristics and highlighted the spectral differences between the 2D and 3D wavy stratified flow patterns.

Measurement of liquid film thickness in air — water two phase flows in conventional and mini channels using image processing

The measurement and study of liquid films in the case of two phase flows is significant in many heat transfer and mass transfer applications, such as chemical process industries, micro reactors, coating processes and in boilers. The focus of the present study was to measure and characterize the thickness of the liquid films for various two phase flow regimes in conventional and in mini channels using a non-intrusive technique. Experiments were performed on tubes of diameters 0.6, 1.5, 2.6 and 3.4mm. The superficial velocities of gas and liquid are in the range of 0.01–50 and 0.01–3m/s, respectively. The flow patterns were recorded with a high speed camera. A method to determine the two phase flow velocity using image registration has been discussed. Morphological processing and gray scale analysis were used to determine the liquid film thickness and characterize the flow regimes. The flow patterns identified are bubbly, dispersed bubbly, slug, slug-annular, wavy-annular, stratified, and annular. The flow regimes were validated with flow maps available in the literature. The liquid film thickness was identified by distance transform technique in image processing. The magnitude of film thickness varied with liquid and gas flow velocities. The film thickness was represented in terms of capillary number. The variation in film thickness along the length of the flow regime has been discussed. A relation between the liquid film thicknesses measured using the non-intrusive image processing technique and capillary number for the conventional and mini tubes is proposed based on the analysis. $$\begin{array}{*{20}c} {\frac{h} {d} = 2.03Ca^{0.13} We^{0.52} for Bo > 1} \\ {\frac{h} {d} = 1.08Ca^{0.4} We^{0.35} for Bo < 1} \\ \end{array} $$

Measurement of Liquid Film Thickness around Horizontal Tube Bundle by Optical Technique for Optimizing Evaporator Design and Manufacturing

This paper proposes the improvement of design and manufacturing of Falling Film Horizontal Tube Evaporators (FFHTE) through optimizing different parameters such as tubes pitch, tubes diameter and material and liquid film flow rate. These design and operational parameters have a significant influence on the hydrodynamic of the liquid film (eg: wetability of the tubes, scale deposition, heat transfer coefficient, etc.). Due to the complexity of the liquid film flow around the horizontal tube bundle, the experimental approach is preferred than modeling because it gives a better understanding of the phenomena occurring in the heat exchanger. In this paper one experiment was carried out to investigate liquid film flow around a single horizontal tube. A particular attention was taken for the measurement of liquid film thickness around the tube using a novel optical technique based on light reflection. The influence of the tubes pitch, tube diameter, height of the liquid distribution system and the liquid mass flow on the transitions between falling-film modes and film thickness is investigated and the results are compared to other data obtained from the literature. It was found that tubes wetability and heat transfer increased with increasing the vertical tube pitch. To account for fouling and heat transfer performance, a tube spacing value of 1.3 was recommended.