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The objective of this work is to develop a film thickness and velocity measurement technique using laser intensity measurement in liquid film flow. This technique was developed for annular flow studies, but it has been scarcely used due to the equipment's complexity, as compared with other techniques. The laser technique uses the reflection of the...
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The objective of this work is to develop a film thickness and velocity measurement technique using laser intensity measurement in liquid film flow. This technique was developed for annular flow studies, but it has been scarcely used due to the equipment's complexity, if compared to other techniques. The laser technique uses the reflection of the la...
Citations
... Ohba et al. (1984Ohba et al. ( , 1985Ohba et al. ( , 1989 demonstrated that the relationship between the reflection intensity of optical fiber liquid film sensors and film thickness can be represented using a theoretical model based on Gaussian distribution, enabling the creation of calibration curves. Moreover, expanding the fiber diameter has made it possible to measure relatively thicker liquid films and their wave velocities (Oliveira et al., 2006). However, Yamaguchi et al. (2020) reported that this method leads to nearly constant reflection intensity for liquid film thicknesses below the fiber core diameter, resulting in a loss of sensor sensitivity through simulations. ...
... According to Ohba et al.(1984), the spatial intensity distribution of the emitted light can be described by a Gaussian distribution. Ohba et al. (1984), and Oliveira et al. (2006) proposed Equation (1) describing this spatial intensity distribution. ...
Optical fiber liquid film sensors utilizing the reflection intensity at the gas-liquid interface exhibit excellent environmental robustness. However, this technique needs to improve its sensitivity when measuring liquid film thicknesses approaching the fiber core diameter. In this study, we propose a method for accurately measuring liquid film thickness around the fiber core diameter by changing the spatial intensity distribution of laser light emitted from the fiber. We employ stepped-index-type optical fibers (SI-type) instead of the commonly used graded-index-type fibers (GI-type) to alter the spatial distribution and investigate the relationship between reflection intensity and liquid film thickness. The reflection intensity of SI-type optical fibers exhibits a linear response over a range approximately eight times the core radius, significantly enhancing sensitivity for thin films. In addition, we use a theoretical model for the spatial intensity distribution of light emitted from SI-type optical fibers, demonstrating a close match between the predictions and experimental results, and discussing the linear response. Finally, we demonstrate the liquid film thickness measurement of the liquid jet impinging surface.
... (vi) Attenuation techniques-When electromagnetic radiation, such as light, gamma-ray, X-ray are passed through a liquid-thin film, their intensity is attenuated, which is used to measure the film thickness [37][38][39] From the above discussion, and table 1, it is seen that the optical interferometry method has numerous advantages over the other techniques. First, it is non-intrusive. ...
Thin films of liquids formed on solid surfaces are of both fundamental and industrial importance. Therefore, the detection and analysis of thin-film profiles have been at the attention of the scientific community for decades. However, due to the small-scale nature, there exists a significant challenge to characterize thin films experimentally. The goal of the present review is to shed light on the recent developments in optical interferometry techniques for the characterization of thin-films. The review includes the efforts devoted to looking into thin-films of several applications, such as falling thin-films, and liquid crystal thin-films; by virtue of optical interferometry. Thereafter, how the technique has been extended to tribology has been reviewed. At last, the efforts devoted to combining reflectometry and interferometry for the characterization of thin liquid films with enhanced accuracy have been outlined.
... A common approach to experimentally determine the film thickness is laser induced fluorescence (LIF), where the fluid is marked with a fluorescent dye and the film geometry and thickness is deduced from a camera image [5,6,7]. Other methods employed so far are chromatic-confocal sensing that uses a colour fringe of a highly dispersive lens and the evaluation of a specific wavelength reflected from the film surface [8], white-light interferometry [9], or the evaluation of the film height-dependent light intensity that is back-reflected from a divergent laser beam [10]. All these approaches have in common that they can determine the film thickness and its temporal behaviour, but not the velocity or velocity profile inside the film. ...
Impinging circular free-surface water jets are used in challenging cooling and cleaning tasks. In order to develop simulation models for process optimization, validation data are required, which are currently not available. Therefore, the flow field of these jets is studied for the first time with the novel laser Doppler velocity profile sensor. The mean velocity field and fluctuations are measured within the stagnation and adjacent redirection region for radial coordinates up to 3 times the nozzle diameter. In the examined parameter range with jet velocities up to 17 m/s and nozzle diameters up to 5.2 mm, i.e. Reynolds numbers up to 69500, thin films of a few hundred micrometers are formed, which hinder the measurement with common optical measuring systems. Based on the measurement results, a comparatively low-cost Volume of Fluid simulation model is developed and validated that presumes a relaminarized film flow. The profiles measured and the simulated flow show very good agreement. In the future, the simulation model provides a basis for process optimization and the innovative measurement technology used will prospectively provide further detailed insights into other flows with high velocity gradients.
... The temporal resolution for measuring the liquid film is determined by the bandwidth of the photoelectric sensor of sampling rate of voltage recorder that records the light intensity. Further improvements to this approach have enabled measurement of relatively thick liquid films (up to several millimeters thick) and their wave velocities via increasing the number of fibers or expanding the fiber diameter (Oliveira, 2006). However, the measurement resolution decreases as the film thickness reduces because the light emitted from the fiber has a Gaussian distribution as discussed in the present paper; this distribution is characteristic of the widely used optical fiber (the graded index (GI)-type fiber), designed mainly for long-distance communications. ...
The accurate measurement of thin liquid films in various environments is essential for several industrial applications. In this regard, measurement procedures involving the use of optical fibers are preferred because such fibers are resistant to heat and pressure. Herein, we propose a high-accuracy method to measure liquid films with thicknesses of <100 μm (about fiber diameter) on the basis of the variation in the intensity distribution of the laser light emitted from the optical fiber; the thickness is measured by using the light reflected from the air-liquid interface (called the glare light in this study). First, instead of using light with a Gaussian distribution (characteristic of conventional graded-index fibers), we consider a stepped-index-type optical fiber with a step distribution. We model the distribution of the light emitted from the optical fiber and analyze the reflected light, i.e., glare light, via the ray-tracing method. We model three distributions: the Gaussian, point-like, and step distributions, and then we found that the step-distribution-based approach facilitates high-resolution measurements of liquid films with thicknesses less than optical fiber diameter. Moreover, the reflected light intensities for different film thicknesses closely agreed with the experimental results obtained using a stepped-index fiber. Remarkably, the intensity of the reflected light linearly decreases with the increase in the film thickness when using the step distribution. The numerical results quantitatively agreed with experiments; therefore, these results indicate the possibility of numerical calibration for liquid-film measurements with the use of the proposed step distribution model.
... Optical method has been widely used in liquid film thickness measurement due to its advantages of non-invasion and high precision. According to different measuring principles, optical methods can be divided into the following categories: laser intensity method [13][14]; laser scattering method [15]; laser velocimetry method [16]; total internal reflection method [17]; laser focus displacement meters [18]; and Particle Image Velocimetry (PIV) [19][20][21][22][23]. Nevertheless, optical methods are usually expensive and not suitable for engineering applications. ...
A distributed ultrasonic method for measuring the liquid film thickness of vertical multiphase slug flow and churn flow widely existed in industrial production is proposed in this paper. Firstly, According to the proportional relationship between transit time of ultrasonic echo and liquid film thickness, we use the twin-crystal ultrasonic probe to measure the local thickness of liquid film. In view of the non-uniform circumferential distribution of the liquid film, three ultrasonic probes are installed on the same pipe section to measure the circumferential distribution of liquid film thickness. Then a high speed data acquisition system suitable for multi-channel and a data transmission system based on gigabit network are designed. The experiments of gas-liquid two-phase flow and oil-gas-water three-phase are carried out in a vertical pipe with a diameter of 20 mm. The fluctuation of local liquid film thickness has been measured by the distributed ultrasonic sensor, and the average thickness of liquid film of slug flow and churn flow under different flow conditions is obtained. The result indicates that the average liquid film thickness of slug flow and churn flow gradually thickens with the increase of liquid superficial velocity, the distributed ultrasonic sensor can accurately measure the change of film thickness.
... The authors confirmed the theory of Benjamin [12] defining a critical Reynolds number beyond which wavy instabilities appear on the film surface. De Olivera et al. [13] used an optical technique based on light attenuation to measure the liquid film thickness in an open channel flow. This nonintrusive technique, available for translucent fluid, allowed recordings with approximately an error of 7%. ...
This work is devoted to a non-intrusive experimental approach, based on Laser Multi-reflection technique, in the investigation of thickness distribution variations and wave’s dynamics of a liquid film flowing over an inclined plane. This investigation was first founded on the needs of quantifying the liquid film thickness and on minimizing, as much as possible, some drawbacks pointed out, in the literature, throughout the experimental techniques available. Moreover, the technique could be applied to transparent, opaque as well as particle laden liquid films. The technique is validated and evaluated using two approaches according to the flow case: stable or instable. In case of stable flow, the comparison was made using Spectroscopic Ellipsometry and theoretical prediction established by the Nusselt model. For a wavy interface a setup, especially devoted to that purpose, was used to validate the accuracy of the measurements. In both cases the uncertainties were within 5%. The experiments are discussed hereafter including the accuracy of the results. Some experimental data, for plane inclination ranging from 1° to 10°, are reported. The data takes into account the film thickness at various positions. The instability threshold is also reported.
... The development of techniques to measure liquid film thickness has been an important topic in the area of two-phase flow for a long time. Many methods based on different physical principles have been constantly proposed, such as acoustic method [10,11], conductance-based method [12], capacitance-based method [13,14], optical method [15][16][17], nucleonic technique [18,19] and so on. Among the listed methods, the conductance-based method with simple equipment and convenient operation is widely used, which can be divided into two categories, indirect method and direct method. ...
... Liquid film thickness in microchannels was found to be an exponential function of the capillary number and became zero when the capillary number approached zero. Limitations of the optical methods in the liquid film measurements exist due to the intensified refraction and reflection effects when the scale is reduced (de Oliveira et al. 2006), as well as expensive equipment. Compared with those methods, electrical technique generally has high time resolution and spatial resolution (D'Aleo et al. 2013). ...
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.
... Optical techniques are usually preferred for measuring thin film thickness as they are more accurate, non-destructive and require little or no sample preparation. Laser based techniques are described by Oliveira et al. (2006), Wegener and Drallmeier (2010). ...
... where C 1 and C 2 were the experimental constants to be determined; S o the signal from oscilloscope, measured in mV; and l f the liquid film thickness, in µm. De Oliveira et al. (2006) provides all necessary information relating to the calibration process. After calibration, the optical fibers were positioned in the nozzles in three regions. ...
The liquid film thickness inside twin-fluid atomizers, specially at their exit ports, is a fundamental parameter that determines the mean drop diameter and distribution in the sprays outside the atomizers. A series of experiments has been carried out in order to measure the interfacial wave velocity and liquid film thickness inside Y-Jet atomizers, working with air and water, using an optical apparatus designed and assembled specially for this purpose. The atomizers were manufactured using a 3:1 scale to allow easier access to the inside flow without perturbing it. In order to maintain a comparison basis with geometrical and operational parameters used in the literature, a non-dimensional analysis based on the Weber number for the air flow inside the mixing duct was conducted. The optical measurements were based on the attenuation signal analysis of a laser beam through the liquid film for its thickness measurement and on the cross-correlation of two signals for the the interfacial wave velocity measurement. The results of the interfacial wave velocity indicated that it grows as it flows along the mixing duct. The results for the liquid film thickness are compatible with other results from the literature and showed some dispersion because they are strongly influenced by the two-phase flow pattern inside the atomizers, namely annular-dispersed and wispy-annular. The high entrainment fraction of these flow patterns and the intense deposition of drops and ligaments and atomization of the wave crests have inevitably influenced the liquid film thickness measurement technique used.
