High-speed Visualization Research Articles

MEASUREMENT OF THE VELOCITY OF PULSED PLASMA FLOW AT THE PW-7 INSTALLATION

The paper considers two independent methods for measuring the velocity of the plasma flow generated in the PV-7 pulsed plasma accelerator: a method based on observation and evaluation of the Doppler shift of spectral lines, and a method of high-speed visualization of plasma motion. To record the plasma flow radiation spectrum, a monochromator M833 was used. High-speed video recording was carried out at 640,000 fps using a Phantom VEO710S CMOS camera. The results of measurements of the average flow velocity obtained at a working gas pressure of 2⋅10-2 Torr, capacitance and voltage of the capacitor bank of 400 μF and 4 kV are presented. The results obtained by two independent methods were compared with each other. Argon was used as the working gas in the experiments. It is shown that the value of the plasma flow velocity estimated by the first method is 12.5 m/s, and the value of the plasma flow velocity estimated by the second method is 16.7 m/s. From these data the measured flow velocity values have a small discrepancy. Thus, it has been established that high-speed video recording and Doppler shift methods make it possible to obtain comparable estimates of flow velocity within the measurement errors. Determining the magnitude of the plasma flow velocity is of great practical importance.

A comparative study of flow boiling performance in smooth and multi-stage enhanced open microchannels

Flow boiling in open microchannels is currently one of the research focuses in the field of phase-change heat transfer. The present paper described a comparative and experimental research on flow boiling heat transfer of SF-33, an environmentally friendly working fluid, in smooth surfaces open microchannels (SOM) and multi-stage enhanced open microchannels (MSEOM). The study investigated the flow boiling performance of SF-33 in two open microchannels with different mass fluxes, heat fluxes and inlet subcooling. The different boiling flow patterns in two open microchannels were analyzed with high-speed visualization. The results indicated that under different inlet cooling conditions, the MSEOM improved the flow boiling heat transfer performance significantly and improved the wall temperature uniformity. The visualization images indicated that the flow pattern transition was different due to the different surface structures of two open microchannels. The SOM experienced plug-stratified flow at high heat flux, while MSEOM transformed into churn flow due to its sintered copper powder and wire mesh micro-nanocomposite structures. The multi-stage enhanced structures exhibited excellent hydrophilicity and wicking capability, leading to a less pressure drop in the MSEOM compared with that in SOM. The pressure drops for both open microchannels decreased as inlet subcooling increased.

Three-dimensional simulation of cavitating flow in reciprocating positive displacement pumps with fluid-actuated valves

Abstract An in-house compressible three-dimensional (3D) finite volume Computational Fluid Dynamics (CFD) method, with statistical turbulence model and moving grid capabilities, is presented and applied to the suction stroke of a simplex plunger positive displacement pump. The approach utilizes a pressure-based implicit solution algorithm and a mass transfer cavitation model. Fluid-actuated valve dynamics are approximated using a fluid-structure interaction algorithm. The closed valve and early phase of valve opening are approximated by a permeable wall. A circular segment model is introduced, significantly reducing computing time. Experimental validation is performed by time-resolved pressure and flow rate measurements, as well as high-speed visualizations of the valve dynamics. A speed variation is conducted to investigate harmless advanced and erosive distinctive partial cavitation. The simulation reproduces the delay of cavity collapse, observed with increasing speed, and reveals distinctive void patterns related to the chamber pressure time progression. These void patterns are fundamental for understanding the cavitation dynamics and potential erosion risk in the system. Deviations from data remain in the flow phase subsequent to the collapse due to overestimated wave reflection at the inlet boundary in the suction pipe.

Experimental investigation into thermal characteristics of subcooled flow boiling in horizontal concentric annuli for cooling ultra-fast electric vehicle charging cables

In an effort to reduce the charging time of electric vehicles (EVs), the thermal management of the charging system has emerged as one of the most urgent issues. The charging rate has been restricted to avoid thermal failure especially in charging cable. Recently, a direct contact cooling technique that utilizes subcooled flow boiling for charging cables has been proposed and has shown promising thermal management performance. However, there has been a lack of clear explanation regarding its thermal characteristics due to its intrinsically complicated flow features. This study investigates the thermal characteristics of subcooled flow boiling in a horizontally aligned concentric annular tube intended to simulate the direct contact cooling technique employed for charging cables. For the experimental investigation, the wall temperature measurements and high-speed flow visualization images acquired from the transparent test module annulus, with inner and outer diameters of 6.35 mm and 22.0 mm, were utilized. Three key axial thermal characteristics of subcooled flow boiling in annulus have been analyzed, and they are flow regime transition, variation of heat transfer mechanisms, and occurrence of critical heat flux (CHF). The transition of flow regimes and the variation of dominant heat transfer mechanisms in the axial direction are mutually influencing due to the coupled effects between the hydrodynamic and thermal characteristics of simultaneously developing flows. The flow regime changes from partially developed boiling (PDB) to fully developed boiling (FDB) as the intensities of nucleate boiling and bubble recondensation vary in the axial direction, while the dominant heat transfer mechanism shifts from single-phase convection to nucleate boiling, corresponding to the flow regime transition from PDB to FDB. The departure from nucleate boiling (DNB) type CHF is observed, manifested by the wavy interface propagating from the far downstream to the upstream region, and the intensities of nucleate boiling and bubble recondensation are also identified as the dominant parameters in determining the occurrence of CHF. Charging time estimation reveals that the proposed method can achieve an 80 % charge for 100-kWh EV batteries in 98 s, while maintaining the cable wire temperature below the safety limit of 80 °C. This strongly supports its potential as a promising thermal management technique for future ultra-fast charging systems.

Microscale heat transfer and phase change mechanisms of carbon dioxide at near-critical conditions

Recent studies highlight the potential benefits of two-phase carbon dioxide (CO2) systems operating near their thermodynamic critical point. Significant heat transfer coefficients and resistance to critical heat flux from improved vapor phase properties can enable heat transfer and reliability enhancements. However, the physical mechanisms of phase change that govern heat transfer behavior in the near-critical regime of reduced pressures (0.85<PR<1, where PR=P/Pcrit) are not well understood. In the present study, subcooled microscale (hydraulic diameter, Dh=500μm) flow boiling experiments are conducted to elucidate the heat transfer behavior near the critical point and the underlying mechanisms that influence it. Experimental data reveals a deviation from the trends predicted by the Cheng correlation (Cheng, L et al., 2008, “New prediction methods for CO2 evaporation inside tubes: Part II—An updated general flow boiling heat transfer model based on flow patterns.” Int. J. Heat Mass Transfer), in particular, displaying a decreasing peak heat transfer coefficient as pressure increases. Heat transfer in the near-critical regime can be further partitioned into two regions: near-critical boiling (0.85<PR<0.96) that exhibits heat transfer highly dependent on surface temperature, and supercritical-like behavior (0.96<PR<1) exhibiting a strong heat transfer dependence on mass flux. High-speed side-view flow visualization reveals the dominant nucleation mechanism (heterogeneous or homogeneous) governs the heat transfer behavior and is adequately predicted using flow inlet conditions. The present study provides a method for predicting nucleation behavior, which, in turn, facilitates the prediction of heat transfer near the critical point. The continuous alteration of heat transfer in the near-critical regime as a function of PR is underscored by this mechanistic insight, bridging the gap between typical flow boiling (PR<0.85) and supercritical heat transfer (PR>1).

Determining pressure from velocity via physics-informed neural network

This paper describes a physics-informed neural network (PINN) for determining pressure from velocity where the Navier-Stokes (NS) equations are incorporated as a physical constraint, but the boundary condition is not explicitly imposed. The exact solution of the NS equations for the oblique Hiemenz flow is utilized to evaluate the accuracy of the PINN and the effects of the relevant factors including the boundary condition, data noise, number of collocation points, Reynolds number and impingement angle. In addition, the PINN is evaluated in the two-dimensional flow over a NACA0012 airfoil based on computational fluid dynamics (CFD) simulation. Further, the PINN is applied to the velocity data of a flying hawkmoth (Manduca) obtained in high-speed schlieren visualizations, revealing some interesting pressure features associated with the vortex structures generated by the flapping wings. Overall, the PINN offers an alternative solution for the problem of pressure from velocity with the reasonable accuracy and robustness.

High-Speed Three-Dimensional Scanning Force Microscopy Visualization of Subnanoscale Hydration Structures on Dissolving Calcite Step Edges.

Hydration at solid-liquid interfaces plays an essential role in a wide range of phenomena in biology and in materials and Earth sciences. However, the atomic-scale dynamics of hydration have remained elusive because of difficulties associated with their direct visualization. In this work, a high-speed three-dimensional (3D) scanning force microscopy technique that produces 3D images of solid-liquid interfaces with subnanoscale resolution at a rate of 1.6 s per 3D image was developed. Using this technique, direct 3D images of moving step edges were acquired during calcite dissolution in water, and hydration structures on transition regions were visualized. A Ca(OH)2 monolayer was found to form along the step edge as an intermediate state during dissolution. This imaging process also showed that hydration layers extended from the upper terraces to the transition regions to stabilize adsorbed Ca(OH)2. This technique provides information that cannot be obtained via conventional 1D/2D measurement methods.

Bubble behavior parameters extraction and analysis during pool boiling based on deep-learning method

The nucleate pool boiling plays an important role in thermal and chemical engineering applications. Analyzing bubble dynamics at nucleation site is crucial to improve the understanding of boiling heat transfer mechanism. Quantitative extraction of bubble parameters from high-speed visualized images is a labor-intensitive and time-consuming task making it necessary for automatically detect single bubble growth and measure boiling characteristic parameters.In the present work, we proposed a deep learning based self-adaptive statistical algorithm for extraction of bubble behavior parameters quickly and automatically from numerous high-speed visualization images looking from the side view of a boiling chamber. A dataset was constructed for training and performance evaluation based on experimental data of saline solution pool boiling. The StarDist and U-Net convolutional neural network were combined in the algorithm so that more exact segmentation of the bubbles can be identified. Based on the segmentation results, a post-processing program was developed to extract the sequential variation of bubbles during consecutive cycles at nucleation sites. The dynamic characteristic parameters that affect heat transfer, such as nucleation density, bubble departure diameter, departure frequency, and wait time under different heat flux were obtained quantitatively. The comparison of automatic extraction algorithm and manual processing proves the reliability and superiority of our method. This work indicates that the proposed method has great potential to be widely applied as an efficient and universal tool for processing different types of bubble shadowgraph images.

A novel coaxial air-R134a spray cooling for heat transfer enhancement of laser dermatology

Efficient cooling is essential in laser dermatology to prevent thermal damage, but current methods are often hindered by thermal barrier film deposition. This study presents coaxial air-R134a spray cooling for heat transfer enhancement and deposited-film mitigation. Surface temperature was measured using a TFTC thermocouple, while heat flux was estimated through the improved Duhamel theorem. High-speed camera visualization with the Mie-scattering technique captured the spray and film behavior. The results showed uniform and effective cooling via transient surface heat transfer and film behaviors. The boiling curve identified three distinct cooling phases: transition boiling, nucleate boiling, and single-phase film deposition. Airflow adjustments not only enhance the heat transfer coefficient but also control spray dynamics, droplet rebounding, and film spreading. Self-organizing maps revealed that medium-to-high nozzle spacing (y/D) reduces surface temperature and increases the heat transfer coefficient. Film resistance time is minimized by either high airflow and low y/D or low airflow and high y/D. For critical heat flux, low y/D and airflow are optimal. A y/D = 32 and 60 L/min airflow provided appropriate operating conditions. However, airflow increased frost deposition during off-duty, suggesting moisture-free air in the future.

Coaxial air blast atomization of a particulate gel suspension jet

The objective of this study is to experimentally and theoretically explore the impact of particle volume fractions and nozzle thread structure on the coaxial air blast atomization of particulate gel suspension jets through the high-speed flow visualization technology. The four breakup modes of particulate gel suspension jets are confirmed, namely oscillation mode, membrane-type breakup, fiber-type breakup, and superpulsating submode. However, the fiber-type breakup of jets disappears as the particle concentration reaches 40%, primarily attributed to the numerous particles sharply promoting the instability on the free jet surface. The experimental statistical results demonstrate that an increase in particle concentration and the adoption of the threaded nozzle both result in an expansion of the jet spray angle. The breakup length exhibits a reduction trend with the increase of particle concentration. However, it remains nearly the same value as that in the case of volume fraction 20% when the particle concentration continues to increase, contributed by the surge of viscous dissipation counteracting the enhanced instability on the jet interface caused by a large number of particles. The breakup length of a pure gel jet is predicted through the linear instability analysis, which is further modified by the viscosity model of particle suspensions to derive the breakup lengths of particulate gel suspension jets with different concentrations. The theoretical predictions align well with the statistical results in the experiment. The research is poised to have potential implications for numerous industrial processes and engineering applications including gel propellants.

Evaluation of the jet distortion by sediment erosion in the needle in the Pelton turbines

Abstract In order to succeed in the transition to a more renewable energy sector, the hydropower industry must overcome multiple challenges in the coming decades, including the need for faster and reversible turbines to support emerging renewable energy sources. In addition, the location of new projects in tropical and semi-tropical areas poses unique considerations, as does the increase in sediment. Evaluating sediment erosion in turbines is a critical challenge, which may partly be addressed by analyzing the behavior of water jets from eroded nozzles in Pelton turbines. This paper presents an evaluation of the quality of the jet under various hydraulic conditions, focusing on needle erosion. The study is part of a research project conducted in the Waterpower Laboratory at NTNU, focusing on Pelton turbines. Furthermore, this paper investigates erosion patterns caused by sediments on the needle and explores the impact on jet performance. Through high-speed visualization and image processing, the behavior of a jet from the needle tip to a representative distance downstream has been investigated. The investigation revealed discernible differences between jets with eroded and uneroded needles in size and edge variation.

Effects of doors on an open cavity flow at Mach number 6

Open cavities with different door-opening angles are investigated using high-speed schlieren visualization and dynamic pressure measurements in hypersonic flow with a freestream Mach number of 6. With the help of numerical simulations, the shear layer deformation and pressure increase in the cavities due to the impingement of the door-leading-edge shocks are identified via comparison with those in the cavity without doors. As the door-opening angle decreases from 90°, the shear layer above the forepart of the cavity is gradually raised by the high pressure in the cavity. When the door-opening angle decreases to 30°and 15°, the boundary layer on the upstream flat plate of the cavity separates, and separation shock is observed. The doors enhance the instability of the cavity flow and increase the pressure fluctuations in the cavities. A new oscillation pattern, referred to as coupled oscillation, is observed in the cases with separation on the upstream flat plate, in which the separation shock oscillates at the same dominant frequency as the flow inside the cavity. Compared with the cavity without doors, this coupled oscillation causes a lower oscillation frequency and a larger overall sound pressure level. Cross-correlation analyses between pressure signals indicate that the disturbances generated at the trailing edge of the cavity can propagate to the separation on the upstream flat plate and cause coupled oscillation of the separation shock. The fundamental frequencies of the coupled oscillations can be normalized to approximately the same Strouhal number as that of the cavity without doors. These findings support that the oscillation mechanisms of hypersonic cavities without and with doors are primarily dominated by acoustic feedback.

Characterization of a pulsating water jet for rain erosion testing of blade coatings: Flow visualization, pressure investigation, and damage analysis

In the past decade or so, blade coatings have become state-of-the-art technology for the protection of wind turbine blades against particle and rain erosion. The present work considers the application of a high-frequency pulsating water jet, based on a passive acoustic-type nozzle, for accelerated rain erosion evaluation of blade coatings. Using high-speed image visualization and impact pressure investigation, the flow properties were systematically characterized, providing both qualitative and quantitative understanding of the key flow parameters. Continuous, pulsating and expanding flow zones, at velocities from 99 to 143 m/s, were classified for standoff distances within 120 mm. For a reference blade coating, the pulsating jet produced more realistic erosion patterns than the continuous flow. Furthermore, high-frequency impacts at around 7500 impacts/s led to fast fatigue accumulation, and enhanced lateral jetting effects, all of which shortened the coating erosion lifetime. In summary, for this work, a high-frequency pulsating water jet, for accelerated erosion studies on wind turbine blade coatings, was constructed and applied.

Forward projected background oriented schlieren for study of sparks in internal combustion engines

The use of renewable fuels, such as hydrogen, in internal combustion engines requires novel combustion strategies that require a deep understanding of their ignition phenomena. This leads to the need for improved or new measurement techniques that can provide further insight into these phenomena. As part of a study to better understand the ignition phenomenon, this paper presents a modified Background Oriented Schieren method developed to study the heat transfer from sparks generated by spark plugs. This new method is called Forward Projected Background Oriented Schlieren or FPBOS, and this is the first time such a method has been introduced to the best of the authors’ knowledge. FPBOS provides a flexible and more powerful alternative to other BOS techniques, especially for use in micro-calorimetry applications. It can be used similarly to any other BOS method and with different correlation algorithms. This study presents a detailed account of this technique and its use to obtain high resolution, high-speed visualisation of the heat transfer from the spark to the environment and the interaction of the spark with the flow field.

High-Speed Schlieren Imaging And PIV Of Unsteady Free Jet Injection With Plume Formation

This work aims to investigate Schlieren image correlation velocimetry for flow field quantification of gaseous jet injection in non-manipulable apparatus like flow chambers. Such flow cases often struggle using tracer-based methods: Seeding particles may locally cause technical problems, or the plant design prevents the principal opportunity of tracer adding, both of which lead to a reduced method repertory. This is specifically the case under flow conditions, where contamination of apparatus internals by seeding particles should be avoided completely. Therefore, in this paper, we explore a seedless, refraction-based method to quantify the velocity field of a jet plume. As an experimental flow case, we examine a transient jet release into a closed compartment under nearly atmospheric conditions. For simplification and greater clarity, the laboratory model geometry is kept essentially two-dimensional, thus we chose a flat-jet nozzle as injection device. The experimental investigations cover high-speed Schlieren imaging and laser sheet visualization, followed by digital image correlation. Combining the traditionally qualitative Schlieren approach for flow visualization with Fast Fourier Transform and cross-correlation algorithms, we calculated the velocity vector field of an unsteady jet plume formation. Furthermore, we determined the axial jet velocity profile at steady-state conditions. Our research findings highlight the applicability of Schlieren image correlation velocimetry to quantify gaseous jet formation at encased flow sections spatially. Concluding, the results coincide with data from particle image velocimetry measurements and point out the potential of expanding flow diagnostics to hitherto hardly explored (industrial) flow cases.

Kelvin-Helmholtz instability as one of the key features for fast and efficient emulsification by hydrodynamic cavitation

The paper investigates the oil–water emulsification process inside a micro-venturi channel. More specifically, the possible influence of Kelvin-Helmholtz instability on the emulsification process. High-speed visualizations were conducted inside a square venturi constriction with throat dimensions of 450 µm by 450 µm, both under visible light and X-Rays. We show that cavity shedding caused by the instability results in the formation of several cavity vortices. Their rotation causes the deformation of the oil stream into a distinct wave-like shape, combined with fragmentation into larger drops due to cavitation bubble collapse. Later on, the cavity collapse further disperses the larger drops into a finer emulsion. Thus, it turns out that the Kelvin-Helmholtz instability is similarly characteristic for hydrodynamic cavitation emulsification inside a microchannel as is the Rayleigh-Taylor instability for acoustically driven emulsion formation.

Cavitating flow around a low aspect ratio NACA0012 hydrofoil with regular grooves

Abstract The purpose of this work is to study the influence of the periodic grooves applied to the surface of the hydrofoil as passive control method on the cavitation dynamics and development. The cavitating flow in the slit channel around NACA 0012 hydrofoil is investigated. An experimental study of the flow was carried out using the high-speed visualization followed by image processing of acquired data. Attached cavity length is substantially reduced by 25% - 30% by using grooves on hydrofoil surface. However, cavitation inception for grooved hydrofoil occurs at almost two-time lower flowrate.

Questioning the ASTM G32-16 (stationary specimen) standard cavitation erosion test

Cavitation erosion is one of the most severe problems encountered in hydraulic turbomachinery. When testing the materials, the engineers usually rely on standardized procedures. The most common one being the vibratory ASTM G-32 test, which offers two possibilities of performing the test – the direct, where the specimen is attached to the ultrasonic device and the indirect, where the specimen is stationary and exposed to the ultrasonic horn, positioned just 0.5 mm from it. The erosion rates from the two are significantly different and a question may be asked if they are at all comparable and further on are they comparable to the “real-life” hydrodynamic cavitation which occurs in turbomachinery.In this study we performed erosion tests on a stationary specimen where the gap between the specimen and the horn was varied from 0.3 to 4 mm. In addition, we used high speed visualization to observe the cavitation in the gap.We observed that the cavitation erosion rate strongly depends on the gap. From visualization we see that the cavitation dynamics significantly changes in a small gap, leading to a large, but 2-dimensional cavitation bubbles which collapse very slowly, compared to the small spherical ones in a larger gap.We investigated the probability of shock wave occurrence and derived a very simple model, which gives accurate qualitative predictions of experimental data.Finally, the study puts into question the validity of ASTM G32 test – the most common approach used in engineering today.

Aerodynamic breakup of gel suspension droplets loaded with aluminum particles

The persistent and frequent space explorations are affected by the performance of propellants, among which the closely related gel propellants have been widely used due to their unique characteristics. Currently, research efforts primarily concentrate on the metallization and atomized combustion of gel propellants, emphasizing the combustion performance of metallized gel propellants. However, little attention has been given to the secondary breakup process that occurs just before combustion. By utilizing high-speed flow visualization technology, this study examines the key aerodynamic breakup process of gel suspension droplets loaded with micro aluminum particles, experimentally and theoretically investigating the droplet dynamics induced by particle volume fraction. The experimental confirmation of the three breakup behaviors exhibited by gel suspension droplets, namely oscillation mode, bag-stamen breakup, and hemline breakup, is presented along with the construction of a phase diagram. The impact of heterogeneous effects resulting from an increase in particle volume fraction on the critical breakup Weber number is revealed, with a concentration of 9 % as the turning point. Within the range of moderate Weber numbers, the tail flick deformation of suspension droplets in airflow is observed and reported for the first time. At different particle concentrations, there keeps a linear correlation between the dimensionless tail length and Weber number. Spherical suspension droplets will deform into thin disks in the airflow, with a nearly constant deformation rate regardless of changes in particle concentration. Additionally, the relationships between characteristic times and droplet parameters are discussed through statistical analysis of the results. As Ohnesorge number and particle volume fraction increase, the characteristic times both show a monotonically increasing trend. Moreover, a model of unstable growth rate associated with the particle volume fraction is established through linear instability analysis, accurately predicting the variation in critical Weber number for gel suspension droplet breakup. The congruence between our theoretical framework and experimental findings not only enhances comprehension of the impact of micro aluminum particles on the secondary breakup of gel suspension droplets but also provides valuable guidance for the aerospace applications of metallized gel propellants.

Experimental Study of the Cavitating Flow on an Independently Heated Venturi Nozzle

Abstract Despite the observation of change in the cavitation regime on a heated surface, the specific section of the wall surface that plays a more dominant role in this transition phenomenon remains unknown. This study experimentally investigated the effect of surface temperature of different regions on the cavitating flow in terms of the cavitation regime. The experiments were conducted using a convergent-divergent Venturi nozzle comprising two parts that could be heated independently. The Venturi nozzle could be fully or selectively heated at either the front, where the leading edge of the cavity sheet was located, or the rear, where the cavity sheet developed. The cavitation behavior under different heating conditions was investigated using high-speed visualization and fluctuating pressure measurements. Compared with the non-heated case, which exhibited a sheet-cloud cavitation regime, the cavitation regime on the completely heated Venturi nozzle exhibited transient cavitation. The same transition phenomenon was observed when only the front of the Venturi nozzle was heated. A liquid film was observed beneath the cavity sheet of transient cavitation when only the front portion was heated. In contrast, heating the rear part alone did not induce a change in the cavitation regime. Thus, it appeared that the transition of the cavitation regime on a heated surface was mainly influenced by the temperature increase at the leading edge of the cavity sheet.