Double Optical Probe Research Articles

A Novel Micro-Displacement Sensor Based on Double Optical Fiber Probes Made through Photopolymer Materials.

In this paper, a novel micro-displacement sensor with double optical fiber probes is proposed and designed, which can realize the highly sensitive sensing of longitudinal or lateral micro-displacements. The optical fiber probes are made through photopolymer formulation, and the effects of reaction time and optical power on the growth length of the probe are illustrated. The relationship between light intensity and longitudinal micro-displacement is a power function in the range of 0–100 μm at room temperature with a correlation coefficient of 98.92%. For lateral micro-displacement, the sensitivity is −2.9697 dBm/μm in the range of 0–6 μm with a linear fit of 99.61%. In addition, the linear correlation coefficient decreases as the initial longitudinal distance increases, and the function of these correlation coefficients is also linear with a linearity of 96.14%. This sensor has a simple manufacturing process, low cost, high sensitivity, and fast response speed. It is suitable for harsh environments such as strong electromagnetic interference and corrosivity, and has a broad application prospect in the field of micro-displacement sensing.

A double signal optical probe composed of carbon quantum dots and Au@Ag nanoparticles grown in situ for the high sensitivity detection of ellagic acid

Carbon quantum dots (CQDs) were prepared from trifolium leaves via hydrothermal method, a fluorometric and colorimetric double signal probe based on CQDs-gold nanoparticles (AuNPs)-AgNO3 system was constructed for the detection of ellagic acid (EA). In this system, Ag+ ions were reduced by EA to Ag atoms, which deposited on the surfaces of AuNPs to form Au@AgNPs in situ, along with an obvious color change from pink to yellow. Simultaneously, the fluorescence of CQDs was quenched by Au@AgNPs, attributing to inner filter effect (IFE). Under the optimal conditions, sensitive assay of EA was achieved, with the limit of detection (LOD) of 0.41 μM for fluorescence analysis and 0.22 μM for colorimetric assay. The changes of fluorescence and absorbance correlated well with EA concentration in the range of 0.50–25.0 μM. In addition, the double signal probe also exhibited high selectivity toward EA. The feasibility of the probe was confirmed in commercial cosmetics analysis with satisfactory results.

Simulation of hydrodynamics in gas-solid bubbling fluidized bed with louver baffles in three dimensions

In this study, the flow characteristics of Geldart A particles in the bubbling fluidized bed with louver baffles were simulated by CFD method for the first time. A new drag model for bubbling fluidized bed with louver baffles was developed after our early work [Yang et al. Chem. Eng. J. 259 (2015) 338–347]. In this drag model, the baffled fluidized bed was treated as a series connection of freely bubbling fluidized beds with the louver baffle as the gas distributor. The hydrodynamics of each freely bubbling fluidized bed were simulated using the structure-based drag model with the modification of the bubble diameter equation to adapt the change of bubble size after passing each louver baffle. Through analyzing bubble diameters measured by double optical fiber probe, it was found that the averaged diameter of bubbles regenerated from baffles was nearly equal to the distance between two adjacent vanes in the louver baffle. By this method, the bubbling fluidization can be correctly predicted and the simulation results showed good agreements with the experimental data of the radial and axial solid volume fraction distributions. In the simulation results, it can be seen that the variation of axial solid volume fraction across the louver baffle was fluctuant, which was different from other types of baffles. Meanwhile, the lower particle concentration at the position of louver baffle indicates the particles back-mixing inhibited by louver baffles. The guiding effect of louver baffles on the gas and solid flow can be captured in the gas and solid velocity profiles as well.

Annular Gap Bubble Column: Experimental Investigation and Computational Fluid Dynamics Modeling

This paper investigates the countercurrent gas–liquid flow in an annular gap bubble column with a 0.24 m inner diameter by using experimental and numerical investigations. The two-phase flow is studied experimentally using flow visualizations, gas holdup measurements, and double fiber optical probes in the following range of operating conditions: superficial air velocities up to 0.23 m/s and superficial water velocities up to −0.11 m/s, corresponding to gas holdups up to 29%. The flow visualizations were used to observe the flow patterns and to obtain the bubble size distribution (BSD). The gas holdup measurements were used for investigating the flow regime transitions, and the double fiber optical probes were used to study the local flow phenomena. A computational fluid dynamics (CFD) Eulerian two-fluid modeling of the column operating in the bubbly flow regime is proposed using the commercial software ansys fluent. The three-dimensional (3D) transient simulations have been performed considering a set of nondrag forces and polydispersity. It is shown that the errors in the global holdup and in the local properties are below 7% and 16%, respectively, in the range considered.

Experimental investigation of counter current air-water flow in a large diameter vertical pipe with inners

Counter current two phase flow is encountered in a wide variety of industrial applications. This paper describes the experimental results obtained in a circular column of 240 mm diameter with two inner pipes. The counter current flow studied concerns an upward flow of air and a downward flow of water at ambient temperature and pressure. The following range of operating conditions was analysed: superficial air velocities up to 23 cm/s and superficial water velocities up to – 11 cm/s, corresponding to a global air volume fractions (gas holdup) up to 29%. The experimental investigation concerned (i) flow visualization, (ii) local data from a double fibre optical probe and (iii) gas holdup measurements. The images obtained from an optical camera were used to observe the general flow patterns. The data obtained from the double fibre optical probe were used to study the local flow characteristics. In particular, the local void fractions, the bubble velocities, the bubble mean diameters and the bubble diameter distributions and are presented and discussed. The bed expansion technique was used to obtain the gas holdup measurements for every operating condition. The gas holdup measurements are discussed, compared with existing correlations and used for investigating the flow regime transitions. Finally, the gas holdup and the local void fraction measurements data are compared and used for understanding the local hydrodynamics.

Experimental study of interfacial parameter distributions in upward bubbly flow under vertical and inclined conditions

Radial distribution of interfacial parameters for air–water bubbly flow in a circular tube (i.d.50mm) was investigated experimentally under vertical and inclined conditions with the help of double optical fiber probe method, including the local Interfacial Area Concentration (IAC), bubble frequency and void fraction. The ranges of superficial gas and liquid velocities are 0.002∼0.037m/s and 0.072∼0.569m/s, respectively. The inclination angles are 5°, 15° and 30°. The core peak, wall peak, intermediate peak and transition distribution types were observed for vertical upward flow. Under inclined condition, bubbles congregate toward the upper part of the tube, resulting in the asymmetrical distributions of the local interfacial parameters. Comparing with that of vertical flow, the central broad peak leans to the upper part of the tube, and its value increases as the inclination angle increases. Accordingly, with increasing of the inclination angle, the peak near the lower wall weakens gradually, even disappears, whereas, the one near the upper wall of the tube boosts up. The interfacial parameters increase diametrically from ri/R=−0.84 to the peak location in upper part of the inclined pipe, of which higher slope is observed as the inclination angle increases. The Sauter mean diameter is influenced by the gas and liquid velocities as well as the inclination angles. The asymmetrical distributions of interfacial parameters under inclined condition are induced by the radial component of buoyancy force.

Unsteady Cavitation Analysis Using Phase Averaging and Conditional Approaches in a 2D Venturi Flow

The present study refers to a cavitating Venturi type section geometry characterized by a convergent angle of 18° and a divergent angle of about 8° where the sheet cavity presents typical self-oscillation behavior with quasi-periodic vapor clouds shedding. This work is an extension of previous works concerning void ratio measurements and velocity fields using double optical probe and constitutes a complete analysis of the two-phase structure of unsteady cavitating flow. This paper provides a new method based on conditional and phase averaging technique with wall pressure signal to treat experimental data in order to evaluate more precisely time-averaged and rms values of the void ratio and instantaneous velocity fields. Conditional analysis shows a different behavior of the two-phase flow dynamics leading to highlight high void ratio events linked to the break-off cycle. Unsteady phase averaging of the optical probe signal gives the evolution of the void ratio at each studied location in the venturi and shows that the fluctuations close to the wall (where the re-entrant jet is predominant) are in phase with the upper part of the cavity instead of the thickness of the cavity which is unchanged.

Investigations in an external-loop airlift photobioreactor with annular light chambers and swirling flow

Photosynthetic microorganisms could serve as valuable compounds, but also for environmental applications. Their production under controlled conditions implies to design specific reactors, named photobioreactors, in which light supply is the main constraint. This paper was devoted to an original external-loop airlift photobioreactor (PBR) with annular light chambers in which a swirling motion was induced. The aim was to characterize this novel geometrical configuration in terms of gas–liquid hydrodynamics, and to test its potentiality for algal cultures. This PBR consisted of two identical columns connected by flanges defining tangential inlets, each column being made of two transparent concentric tubes (6 L in liquid volume, 50 m −1 in specific illuminated area). Firstly, the global flow characteristics (circulation and mixing times) were determined by a tracer method and modelled by an axial dispersed plug flow with complete recirculation (Péclet number). By means of a double optical probe, both local and global time-averaged parameters of the gas phase were measured, namely void fraction, bubble velocity, frequency and size. The gas–liquid mass transfer were also characterized, in tap water and in culture medium, by measuring overall volumetric mass transfer coefficients. In a second time, cultures of the microalga Chlamydomonas reinhardtii were run in batch mode. The variations of biomass concentration and pigment content with time from inoculation were successfully obtained. All these findings highlighted: (i) some significant differences in terms of gas–liquid hydrodynamics between the present PBR and the usual airlift systems, (ii) the interest of this configuration for algal cultures, even if complementary studies and technological improvements are still required for definitively validating its scale-up.

Experiments and modeling of cavitating flows in venturi: attached sheet cavitation

Correlated experimental and numerical studies were carried out to analyze cavitating flows and to describe the two-phase flow structures of attached sheet cavitation in Venturi geometries. New double optical probe measurements were performed and special data processing methods were developed to estimate void ratio and velocity fields for cold water flows. By applying a computational method previously developed in LEGI ( Laboratoire des Ecoulements Géophysiques et Industriels, Grenoble, France) based on the code Fine TM/Turbo and on a barotropic approach, several steady calculations were performed in cold water cavitating flows. Local and global analyzes based on comparisons between experimental and numerical results were proposed.

Experimental determination of the drag coefficient in a swarm of bubbles

Simultaneous measurements of liquid velocity by laser Doppler velocimetry and bubble velocity, diameter, and void fraction by a double optical probe are performed in a bubble column to study the influence of the void fraction on the relative velocity of a swarm of gas bubbles. Bubble diameters d b vary from 2 to 10 mm and local void fractions α loc can reach 35%. It is found that, for α loc < 15 % , the relative bubble velocity is determined by the hindrance effect and consequently decreases with the void fraction. Beyond this critical value, the aspiration of bubbles in the wake of the leading ones dominates the hindrance effect and the relative velocity thus increases suddenly. The contribution of the bubble diameters to this evolution is also determined. Finally, a drag correlation, valid for the whole range of void fraction and for pure water–air systems, is proposed.

X-ray measurements within unsteady cavitation

The purpose of this work is to describe the two-phase flow structure of cloud cavitation. The experimental study is performed in a cavitation tunnel equipped with a Venturi-type test section. The geometry studied is characterized by a convergent angle of 18° and a divergent angle of 8°. It leads to regular large vapour cloud shedding. The flow is investigated by means of an X-ray attenuation device. Twenty-four collimated detectors enable the instantaneous volume fraction of the vapour phase to be measured at different locations of the two-phase flow. The X-ray intensity measurement mode enables fast data acquisition (1,000 Hz). The results are compared with double optical probe measurements. The influences of the velocity and the size of the cavitation area on the time–space distribution of the vapour fraction are studied. The convection velocity of the vapour structures is estimated. The mean distribution of the volume fraction of the vapour phase during the shedding cycle is computed using the phase- averaging method.

Measurements of Bubble Characteristics: Comparison Between Double Optical Probe and Imaging

AbstractThe objective of this study is to evaluate the accuracy of double optical probe technique in local two‐phase flow measurements. Able to provide both bubble velocity, chord length, and gas hold‐up, this technique involves nevertheless delicate signal treatment procedures and, being intrusive, depends on probe tips configurations and sizes. During probe‐bubble interactions, bubble surface is occasionally deformed and this can complicate measurements analysis. The current study is focussed on the rise of single bubbles of known sizes in still water (equivalent diameter of 2.15mm and 4.5mm). Bubble parameters obtained from digital image processing method and from a double optical probe are compared statistically.

Experimental study on the number density distribution function in turbulent bubbly flows with coalescence and break-up

The population balance equation considers the change of the bubble size and the bubble number density due to coalescence and break-up in bubbly flows. Although a number of theoretical models exist for coalescence and break-up rates, nearly no experimental data for the validation of these models exist for turbulent flow with high void fraction. To overcome this lack, vertical air–water pipe flows were examined. A comprehensive validation of the population balance equation is based on the knowledge of the development of the number density distribution function and of the liquid flow turbulence along the pipe flow. The experimental and analytical procedure to obtain these parameters is presented. The void fraction, interfacial area density and mean bubble volume were calculated as statistical moments of the number density distribution function. For its measurement a combination of single and double fibre optical probes was used that allowed measurement in high void fraction flows. X-hotfilm probes were used to measure the liquid flow turbulence level of the two-phase flow. The flow inlet condition was influenced with the aid of turbulence grids. The results show that the number of coalescence and break-up events depend strongly on the number of bubbles per unit volume and the on bubble volume that differ even for the same gas and liquid superficial velocities. From the axial development of the number density distribution function coalescence and break-up events could be identified within certain bubble volume ranges and the source terms of the population balance equation could be quantified. In a first step the data were used to check two existing theoretical models with respect to coalescence and break-up rates.

Measurement of local flow characteristics in buoyancy-driven bubbly flow at high void fraction

The sedimentation of a homogeneous dispersion of gas bubbles in water is studied, using double optical probe and hot-film anemometry, for high bubble Reynolds number (300–500), and high void fraction α (0–0.3). An experimental device has been specially designed to produce a uniform swarm of monodisperse bubbles without recirculating motions in the liquid phase. It is found that the evolution of the rise velocity of the bubbles is very similar to the sedimentation of particles in dense regimes, the hindrance function being characterized by scaling in α 1/3. The fluctuating kinetic in the liquid, once scaled by the mean relative gas velocity, exhibits a remarkable linear evolution with the void fraction. Moreover, the energy associated with the agitation of the bubbles reaches a maximum value when the distance between bubbles decreases.

Measurements of local interfacial area: application of bi-optical fibre technique

Local mean interfacial area in gas–liquid bubbly flow is experimentally investigated; data is obtained from the experiments carried out with a double optical sensor intrusive probe and analysed by statistical methods. Local variables are measured in the riser of an external loop airlift reactor in air/water medium. The results are presented in the form of the radial profiles of void fraction, bubble diameter and local average interfacial area at superficial gas velocities ranging from 0.003 to 0.11 m/ s . The axial variations of the same parameters are also investigated. The values of local interfacial area obtained from the classical equation ( a i t=6ε g/ d 32 ) and from bubble velocity statistical treatment methods agree reasonably well. This shows that the double optical probe is a reliable method for local measurements of interfacial area. Eventually, the maximum angle α 0 between the probe vertical direction and bubble velocity vector at the piercing point is evaluated and the influence of gas superficial velocity on α 0 is demonstrated.

Measurements within unsteady cavitation

A current method used to characterize the phase distribution within gas (or vapor)/liquid two-phase flows consists of using local probes. Two sensors placed in series enable void fraction, velocity, and chord length measurements. The choice of the data treatment method depends strongly on the unsteady behavior of the flow: the higher the velocity fluctuations, the more complex and time consuming the data treatments. Three different methods have been developed to study various cavitating flow conditions. They have been applied to characterize two-phase flows within sheet cavities using a double optical probe.

Single and double optical probes in air-water two-phase flows: real time signal processing and sensor performance

To progress in the understanding of phase detector capabilities, a first requirement has been fulfilled by manufacturing reproducible sensors. A second objective, investigated in this paper, is to diminish the sensitivity of the processing technique to the various criteria required to perform the signal analysis. A new real-time signal processing technique, based on optical probe responses during probe–interface interactions, is presented, and its objectiveness is demonstrated. When used with mono-fiber or dual-fiber sensors, it provides local void fraction and gas velocity measurements. The performance of these measuring chains is evaluated in various air–water two-phase flows for different probes. Possible mechanisms responsible for void fraction uncertainties are discussed. It is also shown that monofiber probes with a conical–cylindrical–conical extremity are well suited to measure gas velocities in dispersed two-phase flows.

Local measurements in turbulent bubbly flows

Local measurements methods in bubbly flows are discussed. Concerning liquid velocity measurement, problems linked to HFA and LDA are first analysed. Then simultaneously recorded velocity signals obtained by both anemometers are compared. New signal processing is developed for the two techniques. Bubble sizes and velocities measurements methods using intrusive double optical sensor probe are presented. Plane bubbly mixing layer has been investigated. Local measurements using the described methods are presented as examples.

Two-phase flow structure of sheet cavitation

An experimental study of flow within sheet cavities is performed in a cavitation tunnel equipped with a Venturi-type test section. The flow is investigated by means of a double optical probe allowing void fraction, velocity, and chord length of the vapor structures to be measured. Laser velocimetery, wall pressure measurements, and visualization techniques are also used to characterize the liquid flow around the cavity. The consistency of the experimental results was checked though mass and momentum balances. The effects of Reynolds and cavitation numbers are analyzed. Special attention is given to the dynamic behavior of the flow, and to the vapor flow rate within the cavities. The measurements show a complex two-phase flow characterized by the presence of an extended reversed flow occurring along the solid surface and a regular decrease in void fraction along the cavity. The phase transitions seem to be mainly restricted by the dynamic of the bubbles and thermodynamic effects.

Experiments on unsteady cavitation

The unsteady behaviour of cloud cavitation is obviously influenced by its internal flow pattern. The main purpose of this work is to investigate such a two phase flow during a cavitation cycle. The tests are carried out with a convergent divergent nozzle. Observations are made by using a classical video set in combination with a stroboscopic light sheet. The use of a double optical probe enables void fraction and velocity to be measured inside the two phase flow structure. Data acquisition is governed by a pressure signal measured near the cavity closures to follow their evolution during the shedding process. Special care has been taken in validating the experimental techniques because they have not been used in such flows. The measurements show an extended reversed flow occurring along the solid surface. It plays a significant function in the vapour cloud shedding process.