Planar Laser-induced Fluorescence Method Research Articles

Investigation on mixing performance of the mini-fluidized bed

Mini-fluidized beds (MFBs) can significantly enhance the mass transfer, heat transfer and mixing process. In this study, planar laser induced fluorescence method (PLIF) was used to evaluate the mixing performance in liquid-solid mini-fluidized beds. In contrast to the particle-free tubes, the relative mixing index, mixing length, mixing time, specific power consumption, mixing effectiveness and energy efficiency of mini-fluidized beds with inner diameters of 1–3 mm were analyzed. The relative mixing index of the mini-fluidized beds is 3.07–9.55 times that of the particle-free tubes under the same conditions, and the mixing length and time are reduced by 45.11% ∼ 99.59%. When the bed-to-particle diameter ratio is 13.86, the optimal operating voidage is about 0.76, which corresponds to the maximum mixing effectiveness and mixing energy efficiency. The mixing enhancement of the mini-fluidized bed system was evaluated, which provides a theoretical basis for the new application of microstructures in mini-fluidized bed reactors.

Advancements in laser-based spatiotemporal measurements of flow boiling

Recent advancements in laser and imaging systems, as well as in computational processing capabilities, have made quantitative optical imaging, which is often combined with laser illumination, highly adaptable, robust and reliable. Laser-based diagnostic techniques, such as planar laser-induced fluorescence (PLIF), offer the possibility of simultaneous spatiotemporally resolved measurements of temperature fields in the liquid phase at boiling conditions. In this paper, we examine the applicability of two-colour PLIF (2cPLIF), where the ratio between individual fluorescent emissions from uniformly dispersed dyes is used to take temperature measurements in the liquid phase in the presence of moving vapour-liquid interfaces typical of boiling flow. The implementation of 2cPLIF necessitates uniformity in the concentration of different dyes across the flow field. However, in the case of a multiphase flow such as boiling, thermophoresis can lead to inhomogeneous dye distributions. To overcome this challenge, a single-dye multispectral planar laser-induced fluorescence (SDMS-PLIF) method has been developed, which employs fluorescent emissions in different spectral bands of the same dye (Nile Red). The spectral characteristics of Nile Red were measured using a spectrometer to identify its temperature-sensitive bands over a wide range of dye concentrations, from 0.3 to 30 mg/L. Following this, we demonstrate the measurement capabilities of SDMS-PLIF thermography as applied to a boiling flow in a miniaturised vertical square channel, gaining insight into the thermohydrodynamic interactions between vapour bubbles and a heated wall.

Flame state recognition method of a scramjet based on PLIF image fusion features and an artificial neural network

In this study, a pattern recognition model is proposed to differentiate the flame state of a scramjet using an artificial neural network. The flame images are obtained from a scramjet ground test utilizing planar laser-induced fluorescence (PLIF). By extracting basic features, Hu moments and Zernike moments, the preprocessed flame images are compressed to mine valuable information. In order to reduce redundant input features and improve the efficiency of model operation, the partial least squares (PLS) method is introduced for feature screening and fusion. Then, a back propagation neural network (BPNN) model for multi-flame classification is established and analyzed. Finally, the flame states are determined by comparing the probabilities of different states. In order to optimize the recognition performance, the fusion features are studied and discussed. Experimental results show that when the filtered 11-dimensional features are used as input, the average recognition rate for the four states can reach 97.4%. These results demonstrate the significant potential of integrating PLIF and advanced data analysis methods, thereby broadening their application to intricate combustion fields.

Investigation of the dielectric recovery process of vacuum arc in double breaks by planar laser-induced fluorescence

The multibreak vacuum circuit breaker uses multiple short gaps to interrupt the fault current, greatly improving the dielectric strength, and is a viable method to realize high-voltage interruption. The metal vapor distribution near the current zero is crucial for the dielectric recovery process in the multibreak vacuum circuit breaker. Due to the complicated dielectric construction and the interaction between the breakers, the vacuum arc inevitably deviates from the axisymmetric distribution during the interruption process. The traditional diagnosis method limited to 0D or 1D is not sufficient to study the real distribution of metal vapor near the current zero. To address these issues, we developed a planar laser-induced fluorescence method to measure the 2D distribution of copper vapor near the current zero by detecting 510.6 nm fluorescence intensity. The results indicate that for the butt contacts, the copper vapor is diffused in the gap of the high-voltage break and aggregated on the cathode surface of the low-voltage break. The axial magnetic field and transverse magnetic field affect the 2D copper vapor distribution and eliminate the inconsistency, which is achieved by affecting the motion of charged particles and the ionization-recombination process. Furthermore, the copper vapor density exhibits a positive dependence on the arc current, and the magnetic field impacts the density increase rate and distribution mode.

Impact of pilot flame and hydrogen enrichment on turbulent methane/hydrogen/air swirling premixed flames in a model gas turbine combustor

This work investigates the impact of pilot flame and fuel composition on the structures and stabilization of swirling turbulent premixed methane/hydrogen/air flames in a lab-scale gas turbine model combustor. Simultaneous measurements of the velocity field and OH radicals distribution in the combustor were conducted using particle imaging velocimetry (PIV) and planar laser-induced fluorescence (PLIF) methods, respectively. Flames under stable and close to lean blow-off (LBO) conditions were studied for two fuel mixtures, with a hydrogen mole ratio of 0 and 50 % in the hydrogen/methane mixture, respectively. The studied flames were at a constant Reynolds number of 20,000 with different equivalence ratios. Two pilot-to-global fuel ratios were investigated (2 % and 6 %) while keeping the pilot-to-global air ratio constant at 2 %. Data for non-piloted flames were also acquired for comparison. The pilot flames were shown to extend the operability range. The LBO equivalence ratio of the main flame decreased with increasing fuel mass flow rate in the pilot flames due to the increased amount of hot gases with high concentrations of OH radicals in the outer recirculation zone (ORZ), which significantly enhanced the stabilization of the main flame. The stable flame reaction zone was in the high-speed shear layer between the ORZ and the inner recirculation zone (IRZ). When approaching LBO, the reaction zone was pushed downstream to the IRZ and subsequently decreased the size of IRZ, indicating a strong flow/flame interaction. Hydrogen enrichment was shown to reduce the LBO equivalence ratio. When close to LBO, the OH radicals in the hydrogen-enriched flames were observed in isolated pockets due to differential diffusion, which enhanced resilience to LBO. The flame front curvature, mean progress variable, and flame surface density were calculated from the acquired OH-PLIF data to quantify the impact of fuel composition and pilot flames on the flame structures.

Imaging and investigation with PLIF40 for improved film thickness measurements in annular flow

Gas–liquid annular flow occurs frequently in industrial production processes, and the accurate measurement of film thickness is essential for applying its mass and heat transfer properties. The non-invasive planar laser-induced fluorescence (PLIF) method is widely employed in annular flow investigation. However, both methods of PLIF90 and PLIF70 (the measurement angles between laser and camera are set to 90°and 70°) suffer from total reflection. In this paper, the effect of measurement angle on total reflection, measurement accuracy, and robustness are quantitatively investigated. Accordingly, a PLIF40 method is proposed, which minimizes the influence of total reflection by reducing the measurement angle to 40°. Sub-pixel-based edge detection and liquid film smoothing algorithms are developed to accurately identify liquid films with missing edge details, resulting in a measurement error of 0.09 mm. The flow structure is qualitatively diagnosed by waveform reconstruction and wave height extraction combined with the film thickness distribution. Furthermore, the prediction models for average film thickness are established and compared with the classical prediction models to demonstrate the accuracy and reliability of PLIF40.

Corroboration of the Toms effect from a frictional drag reducing self-polishing copolymer

A novel frictional drag reducing self-polishing copolymer (FDR-SPC) was first developed by the authors. The FDR-SPC is a special derivative of an SPC that was designed to achieve skin frictional drag reduction in turbulent water flow by releasing polyethylene glycol (PEG) into water through a hydrolysis reaction. Thus, the FDR-SPC coating acts as a continuous medium accommodating countless, molecular-level polymer injectors. However, direct evidence of such PEG release has not yet been demonstrated. Here, we report the results of in situ PEG concentration measurement based on the planar laser-induced fluorescence (PLIF) method. Polyethylene glycol methacrylate (PEGMA) was probed by the fluorescent functional material dansyl, and the fluorescence intensity from dansyl-PEG was then measured to quantify the concentration in the flow. The near-wall concentration of dansyl-PEG is observed to range from 1 to 2 ppm depending on the flow speed, which corroborates the existence of a drag reducing function for the FDR-SPC. In the concurrent measurement of skin friction, the present FDR-SPC specimen exhibited a skin friction reduction ratio of 9.49% at the freestream flow speed U=5m/s. In the comparative experiment of dansyl-PEGMA solution injection, the skin friction was found to decrease by 11.9%, which is in reasonable accordance with that for the FDR-SPC.

Predicting buoyant jet characteristics: a machine learning approach.

We study positively buoyant miscible jets through high-speed imaging and planar laser-induced fluorescence methods, and we rely on supervised machine learning techniques to predict jet characteristics. These include, in particular, predictions to the laminar length and spread angle, over a wide range of Reynolds and Archimedes numbers. To make these predictions, we use linear regression, support vector regression, random forests, K-nearest neighbour, and artificial neural network algorithms. We evaluate the performance of the aforementioned models using various standard metrics, finding that the random forest algorithm is the best for predicting our jet characteristics. We also discover that this algorithm outperforms a recent empirical correlation, resulting in a significant increase in accuracy, especially for predicting the laminar length.

Effect of pressure and CO2 dilution on soot formation in laminar inverse coflow flame at conditions close to autothermal reforming

Autothermal reforming (ATR) is an important technology for hydrogen production from natural gas, where soot formation deserves particular attention as it may cause catalyst poisoning. The typical configuration of ATR reactors is that of an inverse diffusion flame (IDF). In this study, soot formation near ATR conditions is investigated experimentally and numerically, focusing on the effects of pressure and CO2 dilution in IDFs with pressure reaching 5 bar. The fuel stream consists of methane diluted with CO2 or N2. The mole fraction of O2 in the oxidant stream is 70%. Polycyclic aromatic hydrocarbons (PAHs) and soot volume fraction are measured by the planar laser-induced fluorescence (PLIF) and planar laser-induced incandesce (PLII) methods. High-fidelity simulations with a detailed soot aerosol model are also performed, and an empirical reactive soot inception model is proposed. A comparison of the results shows that the soot behavior is well predicted by the empirical reactive inception model but not fully captured by the physical inception model based on irreversible dimerization, demonstrating the importance of radical involvement in the soot inception process. Both measurements and predictions show a linear relationship between peak soot volume fraction and pressure, which can be explained by the linear increase of PAH molar concentration with pressure. Simulation analysis indicates that the dimer adsorption and HACA mechanism have a similar quantitative contribution to the soot growth. The CO2 shows stronger suppression effects in soot formation at the investigated pressures compared to N2. The contributions of chemical and thermal effects to soot suppression are numerically analyzed and the results indicate that the chemical effect of CO2 is mainly due to the removal of H radicals through the reaction CO2 + H→CO + OH, providing a soot suppression contribution of two times larger compared to the thermal one.

Experimental investigation on non-buoyant and buoyant flow mixing in a horizontally aligned T-junction at isothermal conditions

Periodically occurring temperature fluctuations in nuclear power piping systems have been linked to cracking and coolant leakage accidents. T-junctions where hot and cold fluids mix are particularly vulnerable. To study thermal mixing flow phenomena, an isothermal operating test bench was built using a sugar respectively a salt solution to model the cold fluid in the T-junction configuration. The experiments aimed to predict the flow phenomena of mixed fluids with different fluid densities. Different types of investigations of non-buoyant and buoyant flow mixing were conducted using qualitative and quantitative measurement techniques, such as flow visualization, planar laser-induced fluorescence method, and wire mesh sensor measurements. Jet-formation and mixing behavior (up- and downstream) were characterized by the momentum ratio and a Richardson number. Additionally, non-buoyant flow conditions as well as buoyant flow with unstable/stable stratification have been summarized and clustered in non-dimensional flow maps. Results showed that wall- and deflecting-jet formations are a risk pattern for thermal fatigue. These were identified based on mean and fluctuating mixing scalar distribution as well as spectral analysis of the mixing scalar fluctuations. The peak frequencies in the non-buoyant case were linked to the axial motion of the Kármán-like vortices, while in the buoyant mixing case, the shear layer vortices resulting from Kelvin-Helmholtz instabilities were responsible.

A detailed experimental evaluation of gas –Liquid film attributes in a horizontal rectangular duct by Planar Laser-Induced fluorescence (PLIF) approach

In the current experimental research, we aimed to discover the interfacial behavior of gas–liquid stratified two-phase flow sub-regime by the Planar Laser-Induced Fluorescence (PLIF) method. The introduced research was conducted in a horizontal stratified flow rectangular channel, where respected flow measurements were studied using some quantitative values established visually by obtained PLIF photographs. Moreover, applied indicators in the current study focus on each of the following factors:wave dynamics, liquid film thickness, flow velocities, and frequencies of film disruption waves. Meanwhile, in order to strain the experimental efforts in the study, film thickness measurements were examined in both space and time domains. In addition, integral data supplemented against loaded spatiotemporally recovered information on interface position and two-phase velocity fields collected using planar laser generated fluorescence (PLIF).The presented study ascertains a novel relationship between dimensionless description of wave velocity, film thickness, and frequency for the liquid film. The study was performed under gas shear with inlet water Reynolds values from 88 to 437, and air Reynolds numbers ranging from 6.95 to 34.8 x103. Experimental studies concluded that different modes of wave generation with a decrement in the wave speed, film thickness, and frequency increased at a specific liquid velocity, with increasing the superficial speed. According to the findings, the experimental and theoretical values of the attendance indicators four recognized stratified flow sub-regimes presented in the results, based on both Navier-Stokes, and Kosky models. The observed flow regimes were the Ripple wave, roll wave, Jeffrey wave, and random disturbance wave. Regarding wave frequency studies, the Azzopardi and Bae models used to compare wave frequency, while the Gawas and Sarkhi models were implemented to compare wave velocity.

Application of Planar Laser-Induced Fluorescence for Interfacial Transfer Phenomena

The present review describes the current achievements in the applications of a planar laser-induced fluorescence (PLIF) method for the diagnostics of liquid films, bubbles, individual droplets, and sprays. Such flows are related with strongly curved interphases, which often results in additional high errors during the PLIF data quantification because of laser light reflection, refraction, and absorption. The present review demonstrates that a two-color PLIF approach and a PLIF modification for regularly structured illumination resolves the reflection- and refraction-caused errors. The latter modification ensures proper phase separation in the measurement cross-section and visualization of the interface dynamics. The former approach provides the accurate evaluation of the local temperature and concentration both in liquid and gaseous phases even in the case of strong variations of the laser sheet intensity. With intensified cameras, the PLIF method is used for multi-parameter diagnostics of the two-phase combustion of sprays in combustion chambers with optical access. It visualizes and quantifies the liquid fuel evaporation and mixing, to measure temperature in the gas and liquid phases and to reveal the regions of pollutant formation. The PLIF technique can also be easily combined with a particle image (or tracking) velocimetry method, to evaluate local heat and mass transfer.

PLIF experiment and verification of boron mixed diffusion model driven by turbulence in nuclear reactor

10B has a large neutron absorption cross section and is commonly used as a neutron poison in the light water reactor (LWR) to adjust the reactivity of nuclear reactors. In this paper, for the first time, a new planar laser-induced fluorescence (PLIF) method with sodium fluorescein as the tracer is used to measure the concentration field of boric acid. In this paper, the injection process is abstracted as a T-tube. The accuracy of LIF is verified by wire mesh sensor. The CFD method is used to simulate the mixing process to come up with proper parameters to consider the influence of turbulence on the flow. The branch velocity of T-tube has a great influence on the diffusion process. The mixing process in the pipeline is mainly driven by turbulence of the side jet. Boric acid is evenly mixed in the pipe within 8 ∼ 10 times of the pipe diameter.

Measurement and Prediction of Droplet Entrainment in Inclined Liquid–Liquid Flow by PLIF Method

Inclined liquid-liquid two-phase flows widely exist in petroleum, chemical, and other important industrial processes. The study of droplet entrainment in inclined flow is of great significance for investigating the heat/mass transfer between phases and optimizing the industrial production processes. In this study, a planar laser-induced fluorescence (PLIF) system is designed to visualize the inclined liquid-liquid flows. The silicone oil (organic phase) and water/glycerol mixture (aqueous phase) are used as the experimental media. The length and amplitude of the interfacial waves are derived from the flow visualizations. The relationship between the critical wavelengths and Froude number is explored to indicate the transition from stratified flow (ST) to stratified flow with mixing at the interface (ST&MI). The wave aspect ratio is derived to characterize the instability and breakage of the interfacial wave. Meanwhile, the entrainment ratios of dispersed droplets in the continuous phases are detected, and the effects of the flow rate and the interfacial shear on the droplet entrainment are studied. Finally, a physical model is developed to predict the droplet entrainment ratios based on the force balance analysis of the interfacial wave. In general, the developed model is effective in predicting the droplet entrainment ratio with mean absolute errors of 0.015 and 0.023 for the organic and aqueous droplets, respectively.

Reactive PLIF Method for Characterisation of Micromixing in Continuous High-Throughput Chemical Reactors

This work aimed to test and optimise reactive Planar Laser-Induced Fluorescence (PLIF) methods for the visualisation of the micromixing regions in chemical reactors using standard PLIF and Particle Image Velocimetry (PIV) equipment with the laser source 512 nm. Two methods were tested: (i) an acid–base reaction with fluorescein as the reaction-sensitive tracer and (ii) Fenton’s reaction, with Rhodamine B as the reaction tracer. Both test-reactions were studied in stopped-flow equipment to define suitable operational conditions, namely the chemical composition of the inflow streams, the concentration of reagents and fluorophore, and suitable excitation light wavelength. The visualisation of the micromixing regions was tested in a continuous flow reactor with a T-jet geometry. A laser light sheet emitted from an Nd:YAG laser illuminated the axial section of the demonstration reactor. The mixing dynamics and the reaction course were visualised with the acid–base reactive PLIF images. Fenton’s reactive PLIF method showed the overall distribution of mixing and reaction regions. The main contribution of this work is benchmarking two methods with costs that enable the visualisation of micromixing regions in continuous high-throughput reactors.

Quasi-3D visualization and local parameter measurement of horizontal and nearly horizontal liquid–liquid two-phase flows by using PLIF method

Horizontal and nearly horizontal liquid–liquid two-phase flows are widely encountered in petroleum, chemical, and other important industrial production fields. The three-dimensional (3D) flow visualization and the local parameter measurement are of great significance for understanding the flow dynamics and revealing the flow-pattern transition mechanism. In this paper, experiments of horizontal and nearly horizontal (+5°) liquid–liquid flow are carried out in an acrylic pipe with an inner diameter of 20 mm. Silicone oil (organic phase) and water/glycerol mixture (aqueous phase) are used as the test fluid. A novel planar laser-induced fluorescence (PLIF) system is designed to detect the flow structures. The incident direction of the laser sheet is along the radial section of the pipe, and the PLIF camera is positioned at 60° to the plane of the laser sheet, referred to as PLIF-60. In the experiment, the two-dimensional (2D) flow images at the pipe radial section are captured by the PLIF-60 system. Considering the effect of the angle between the laser sheet and the PLIF camera, the flow images are corrected to remove the horizontal and vertical distortions. The liquid–liquid stratified interface and the entrained droplets are identified in the corrected flow images. A droplet tracking method is proposed to derive the droplet diameter and its quantity distribution. Quasi-3D flow structures are reconstructed upon the accurate detection of the stratified interface and the entrained droplets. The evolution characteristics of the interface structures and the spatial distributions of droplets are studied. In addition, aqueous phase holdup and droplet entrainment ratios at pipe axial and radial sections are extracted based on the quasi-3D flow visualizations. The effects of the flow conditions and the pipe inclinations on the profiles of the aqueous phase holdup and the droplet entrainment ratio are uncovered.

Hydroxyl radical dynamics in a gliding arc discharge using high-speed PLIF imaging

Plasma discharges can be transient and randomly distributed where a few investigations have been carried out using laser-induced fluorescence to capture snapshots of plasma-produced radicals in the near vicinity of the discharge. Radical distribution dynamics, however, are challenging to study in situ with high spatial and temporal resolution to fully capture the interactions between the discharge and the gas. We here demonstrate a planar laser-induced fluorescence method that can capture molecular distributions of ground state hydroxyl radicals in a discharge plasma and follow how the distribution develops in time with a repetition rate of 27 kHz. The technique is demonstrated by monitoring, in real-time, how the tube-like distribution of ground state OH radicals, surrounding a gliding arc plasma, is affected by flow dynamics and how it develops as the high voltage is turned off at atmospheric pressure. The method presented here is an essential tool for capturing radical-distribution dynamics in situ of chemically active environments which is the active region of the plasma induced chemistry.

Temperature field measurements between a Bunsen flame and flat a cold plate by using PLIF

The present experimental work reports on the temperature measurements in a Bunsen flame, covered by a flat cooled plate from the top. The temperature field is measured based on the planar laser-induced fluorescence method for hydroxyl radical (OH) during the excitation of excitation Q1(8) line of the (1-0) band of the A2Σ+-X2Π electronic system. Thermometry was based on the imaging of the ratio of the fluorescence intensity for the band (2-0) and bands (0-0), (1-1). The paper presents the obtained temperature fields for the distances between the plate and the burner of 1, 2 and 3 calibres. The paper also concludes that the used method can be implemented using a single-pulse laser illumination and is effective for the detection of cold zones.

Rayleigh–Taylor instability in impact cratering experiments

When a liquid drop strikes a deep pool of a target liquid, an impact crater opens while the liquid of the drop decelerates and spreads on the surface of the crater. When the density of the drop is larger than the target liquid, we observe mushroom-shaped instabilities growing at the interface between the two liquids. We interpret this instability as a spherical Rayleigh–Taylor instability due to the deceleration of the interface, which exceeds the ambient gravity. We investigate experimentally the effect of the density contrast and the impact Froude number, which measures the importance of the impactor kinetic energy to gravitational energy, on the instability and the resulting mixing layer. Using backlighting and planar laser-induced fluorescence methods, we obtain the position of the air–liquid interface, an estimate of the instability wavelength, and the thickness of the mixing layer. We derive a model for the evolution of the crater radius from an energy conservation. We then show that the observed dynamics of the mixing layer results from a competition between the geometrical expansion of the crater, which tends to thin the layer, and entrainment related to the instability, which increases the layer thickness. The mixing caused by this instability has geophysical implications for the impacts that formed terrestrial planets. Extrapolating our scalings to planets, we estimate the mass of silicates that equilibrates with the metallic core of the impacting bodies.

Preliminary measurements of the thermal field in a rectangular channel by the Planar Laser-Induced Fluorescence method

This paper describes the preliminary measurements of the temperature field in water channel by the Planar Laser-Induced Fluorescence (pLIF) method in the one-colour mode using commercial LaVision system. The aim of the work is to find suitable parameters of the experiment (concentration of fluorescent dye, laser power, arrangement of the experiment, etc.) for measuring the temperature field in a closed glass channel of rectangular cross-section with a heated bottom-wall and to verify an applicability of this method in general. The intention is to carry out measurements in the transverse plane of the duct such that the specific thermos-convective structures of the Rayleigh-Bénard-Poiseuille convection, i.e. mixed convection, are detected. For simplicity, the initial measurements were performed in the natural convection regime, i.e. the water in the channel was not driven by the circulation pump. In this case, a larger temperature gradient was created near the wall compared to the force convection case when a cold liquid flows continuously into the experimental area. The presented work serves as a methodological guide for future measurements and provides valuable experience for the design of a new channel for investigation of the thermal flow instabilities.