High-speed Imaging Technique Research Articles

Experimental Investigation on the Effects of Injection Parameters on the Air-Assisted Diesel Spray Characteristics

The air-assisted fuel injection (AAFI) system may be installed in the spark ignition aviation piston engine for the atomization of heavy fuels. However, studies on the effects of injection parameters on the spray characteristics are insufficient, which affects the improvement of AAFI engine performance. In this study, air-assisted diesel spray characteristics are investigated experimentally using a high-speed backlit imaging technique. The effects of main air-assisted injection control parameters such as fuel injection pressure, fuel temperature, and fuel injection duration on the spray characteristics are examined. The results show that spray shape changes from “spindle” to “cone” with an increase in fuel injection pressure. As the fuel injection pressure increases to 1.0 MPa, both the spray penetration and spray width increase significantly. “Protrusions” appear on the spray edge at high fuel temperatures. When the fuel temperature drops to 268 K, the spray penetration and spray width increase slightly. The spray shrinks significantly in both the axial and radial directions with an increase in fuel injection duration. Key parameters that directly affect air-assisted spray characteristics include the difference between the fuel-air mixture injection pressure and the ambient pressure, the density of fuel-air mixture in the air-assisted injector premixed chamber, and the kinetic energy density of the fuel. The former two parameters affect the spray penetration while the latter affects the spray width. The study is beneficial for the design of AAFI engines.

Evaluation of resonant scanning as a high-speed imaging technique for two-photon imaging of cortical vasculature.

We demonstrate a simple, low-cost two-photon microscope design with both galvo-galvo and resonant-galvo scanning capabilities. We quantify and compare the signal-to-noise ratios and imaging speeds of the galvo-galvo and resonant-galvo scanning modes when used for murine neurovascular imaging. The two scanning modes perform as expected under shot-noise limited detection and are found to achieve comparable signal-to-noise ratios. Resonant-galvo scanning is capable of reaching desired signal-to-noise ratios using less acquisition time when higher excitation power can be used. Given equal excitation power and total pixel dwell time between the two methods, galvo-galvo scanning outperforms resonant-galvo scanning in image quality when detection deviates from being shot-noise limited.

Coupling model of motion and mass transfer in multicomponent desorption of fine bubbles

Fine bubbles with a diameter less than 1 mm are widely used to intensify the gas–liquid processes since the micro-interface they build can significantly improve the mass transfer of gases to the liquid phase. In this study, a coupled mathematical model of motion and mass transfer of a single fine bubble with multicomponent was established and verified for carbon dioxide desorption. The rising and dissolution behaviors of a single fine bubble with different external pressures and initial diameters were observed using a high-speed imaging technique to evaluate the increasing velocity, Vb, and the mass transfer coefficient, kL. Design indicators such as effective mass transfer height and residence stay time of the bubble have been established to guide the optical selection of initial bubble size and liquid level height (reactor height) for a bubbling reactor's optimal design and operation. The comparison between the theoretical calculations and the experimental results proves that the new model can well describe the movement and multicomponent mass transfer behavior of a single fine bubble in an aqueous solution, and the relative errors are less than 5%.

End-of-injection fuel dribbling dynamics of multi-hole GDI injector

The fuel dribbling at the end-of-injection (EOI) transient is believed to be harmful to the emission performance. However, quantitative characterization and detailed explanation of the fuel dribbling process for GDI injectors are rarely reported, and the mechanism is not thoroughly understood. This study conducted a comprehensive investigation on the fuel dribbling process at the EOI transient for a multi-hole sac-type GDI injector using the high-speed X-ray phase-contrast imaging (XPCI) technique. Simultaneous visualization of needle bouncing, in-nozzle flow, and fuel dribbling directly illustrated that the fuel dribbling process was strongly correlated with needle bouncing. Further quantitative analysis of dribble volume and needle bouncing showed that the fuel dribbling process could be divided into the pre-needle-bouncing stage and the needle-bouncing stage. Distinct mechanisms of the two stages were explained through the dribbling source and the driving force. For the pre-needle-bouncing stage, the dribbling source was the residual fuel in the sac, and the driving force was the pressure difference between the sac and the ambient. For the needle-bouncing stage, the dribbling source was the fuel fed to the sac from upstream, and the driving force was the needle bouncing. Possible control strategies of fuel dribbling were also discussed according to the dribbling mechanisms of the two stages. A high injection pressure tended to increase the dribble amount but to reduce the dribble size and dribbling duration, which was believed to be overall favorable to restrain the adverse effect of fuel dribbling.

Illumination angle correction during image acquisition in light-sheet fluorescence microscopy using deep learning.

Light-sheet fluorescence microscopy (LSFM) is a high-speed imaging technique that provides optical sectioning with reduced photodamage. LSFM is routinely used in life sciences for live cell imaging and for capturing large volumes of cleared tissues. LSFM has a unique configuration, in which the illumination and detection paths are separated and perpendicular to each other. As such, the image quality, especially at high resolution, largely depends on the degree of overlap between the detection focal plane and the illuminating beam. However, spatial heterogeneity within the sample, curved specimen boundaries, and mismatch of refractive index between tissues and immersion media can refract the well-aligned illumination beam. This refraction can cause extensive blur and non-uniform image quality over the imaged field-of-view. To address these issues, we tested a deep learning-based approach to estimate the angular error of the illumination beam relative to the detection focal plane. The illumination beam was then corrected using a pair of galvo scanners, and the correction significantly improved the image quality across the entire field-of-view. The angular estimation was based on calculating the defocus level on a pixel level within the image using two defocused images. Overall, our study provides a framework that can correct the angle of the light-sheet and improve the overall image quality in high-resolution LSFM 3D image acquisition.

Super-Resolution Imaging by Computationally Fusing Quantum and Classical Optical Information

A high-speed super-resolution computational imaging technique is introduced on the basis of classical and quantum correlation functions obtained from photon counts collected from quantum emitters illuminated by spatiotemporally structured illumination. The structured illumination is delocalized—allowing the selective excitation of separate groups of emitters as the modulation of the illumination light advances. A recorded set of photon counts contains rich quantum and classical information. By processing photon counts, multiple orders of Glauber correlation functions are extracted. Combinations of the normalized Glauber correlation functions convert photon counts into signals of increasing order that contain increasing spatial frequency information. However, the amount of information above the noise floor drops at higher correlation orders, causing a loss of accessible information in the finer spatial frequency content that is contained in the higher-order signals. We demonstrate an efficient and robust computational imaging algorithm to fuse the spatial frequencies from the low-spatial-frequency range that is available in the classical information with the spatial frequency content in the quantum signals. Because of the overlap of low spatial frequency information, the higher signal-to-noise ratio (SNR) information concentrated in the low spatial frequencies stabilizes the lower SNR at higher spatial frequencies in the higher-order quantum signals. Robust performance of this joint fusion of classical and quantum computational single-pixel imaging is demonstrated with marked increases in spatial frequency content, leading to super-resolution imaging, along with much better mean squared errors in the reconstructed images.

AN EXPERIMENTAL INVESTIGATION ON LIQUID SHEET BREAKUP DUE TO PERFORATIONS IN IMPINGING JET ATOMIZATION

In this work, atomization of the liquid sheet formed by two like-on-like impinging liquid jets is studied experimentally. The primary focus of this study concerns sheet breakup due to the formation and growth of perforations on the liquid sheet. In this mode of atomization, perforations or holes originate at a location where the liquid sheet becomes thin due to stretching. As time elapses, these perforations will grow in size and move either toward the bottom or toward the rim of the sheet. Depending on the initial location of their formation, these perforations either interact with the rim of the sheet or with other surrounding perforations and disintegrate the sheet into ligaments, which further disintegrate into drops. The phenomenon of perforation formation and growth is captured using a high-speed backlight imaging technique and processed using in-house-developed image processing algorithms based on MATLAB. The effect of liquid properties and Reynolds number on the growth of perforations is studied in this work. Based on the experiments conducted in this study, perforation-based atomization is broadly classified into three different modes. In the first mode, a single perforation interacts with the rim and disintegrates the sheet into ligaments. In the second mode, two or more perforations interact, and their common area of interaction is disintegrated into ligaments and droplets. In the third mode, the liquid sheet breaks up due to perforation interaction with impact waves. Studies on atomization behavior of the liquid sheet at high ambient pressure up to 0.8 MPa revealed improved atomization.

Oblique Collision Experiment and Probability Calculation Model of Collision Coefficient Between Micron Particles and Silicone Rubber Insulators

Studying the collision and rebound characteristics between particles and silicone rubber surfaces is the basis of establishing a scientific contamination deposition model for silicone rubber insulators. In this article, high-speed and microscopic imaging techniques are used to set up an experimental platform for tracking and filming the collision and rebound motion of micrometer particles. Key parameters in the semiempirical calculation formula of critical adhesion velocity and tangential and normal rebound coefficients of particles in the collision issue between particles and silicone rubber surface are also obtained in this article by the parameter optimization method based on experimental measurement results. In addition, a probability calculation model for tangent and normal rebound coefficients of particles is proposed based on the statistical analysis method. Findings show that the critical adhesion velocity of particles decreases with the increase of the particle diameter. Influenced by the roughness distribution of the silicone rubber surface, the tangent rebound coefficient and the ratio between the measurement results and the theoretical calculated values of normal rebound coefficients of particles in the oblique collisions obeys normal distributions. This article is valuable for studying the contamination mechanism of silicone rubber insulators.

Broadband coherent anti-Stokes Raman scattering for crystalline materials

Broadband coherent anti-Stokes Raman scattering (B-CARS) has emerged in recent years as a promising chemosensitive high-speed imaging technique. B-CARS allows for the detection of vibrational sample properties in analogy to spontaneous Raman spectroscopy, but also makes electronic sample environments accessible due to its resonant excitation mechanism. Nevertheless, this technique has only gained interest in the biomedical field so far, whereas CARS investigations on solid-state materials are rare and concentrate on layered, two-dimensional materials such as graphene and hexagonal boron nitride . In this work, we discuss the specific properties of this technique when applied to single-crystalline samples, with respect to signal generation, phase matching, and selection rules in the model systems lithium niobate and lithium tantalate. Via polarized B-CARS measurements and subsequent phase retrieval, we validate the predicted selection rules, unequivocally assign the phonons of the ${\mathrm{A}}_{1}$(TO), E(TO) and ${\mathrm{A}}_{1}$(LO) branches to the detected CARS peaks, and address differences in spontaneous Raman spectroscopy concerning peak frequencies and scattering efficiencies. We thus establish this technique for future investigations of solid-state materials, specifically in the field of ferroelectric single crystals.

Interfacial instabilities in Marangoni-driven spreading of polymer solutions on soap films

HypothesisTuning and controlling the flow behavior of multi-component liquids has been a long-lasting struggle in various technological applications. Here, we studied Marangoni spreading of a polymer-surfactant ternary solution when deposited on a soap film with higher surface tension. The spreading front becomes unstable into a fingering pattern above the entanglement concentration of the polymer solution, indicating that the interplay between the elastic and interfacial properties drives the instability. Balancing these terms results in a critical length scale for the onset of the instability. ExperimentsTo investigate the connection between the rheological characteristics of the samples and the origins of the instabilities, various rheological tests were performed. Elastic and loss modulus of the samples were measured within the linear viscoelastic regime. The spreading behavior of the solutions was studied using high-speed imaging techniques. FindingsAt low concentrations of polymers, spreading dynamics are governed by surface tension gradient and viscous dissipation leading to a stable front growing linearly in time. However, above the entanglement concentration of polymers spreading front destabilizes into a daisy shape pattern suggesting the elastic forces dominating the spreading dynamics. We introduced a length scale that precisely predicts the onset of the instability.

Investigation on droplet transfer behavior during fiber laser-flux cored arc hybrid welding

This paper comparatively investigated the droplet transfer behavior between the flux-cored arc welding (FCAW) and laser-flux cored arc (FCA) hybrid welding for Q235 steel. The High-speed imaging technique assisted with electrical signal analysis was utilized to capture the arc morphology and droplet transfer behavior during FCAW and laser-FCA hybrid welding under different arc current conditions. A parameter of the coefficient of current variation was proposed to evaluate the welding stability. Short-circuiting transfer, globular transfer, and projected transfer were detected as three separate droplet transfer modes during FCAW and laser-FCA hybrid welding. The molten flux was either transferred directly to the molten pool or transferred converging with the molten sheath metal to the pool along the “flux pole” which appears owing to the lagging melt phenomenon. The magnitude and direction of the electromagnetic force and the plasma drag force acting on the droplet played a key part in deciding the droplet transferbehavior. Due to the altered arc force in the short-circuiting mode, the laser facilitated droplet transfer, and hybrid welding maintained a more stable welding process. The laser hampered droplet transfer in both the globular and projected modes, and there was little difference in process stability between hybrid welding and FCAW. In addition, a substantial amount of acicular ferrite was discovered in the laser-FCA weld joint.

The role of 1.8-cineole addition on the change in triglyceride geometry and combustion characteristics of vegetable oils droplets

The use of Crude Vegetable Oils (CVO) in diesel engine faces several problems related to viscosity and combustion characteristics. This research aims to reveal the role of 1.8-cineole on performance characteristics of vegetable oil droplets. In this study, bubble growth, ignition delay, micro-explosion, and flame evolution process with and without 1.8-cineole in CVO has been studied experimentally at atmospheric pressure and room temperature. Droplets are examined using classical high-speed imaging techniques. The result shows that 1.8-cineole decreases all the various ignition delay into uniform time delay except for the non-polar Crude Jatropha Oil (CJO) compounds are more saturated, larger molecule and stiffer. Magnetic field produced in aromatic ring of 1.8-cineole assisted by strong electric chard in its oxygen atom induces electron in fuel molecules to become more reactive. Moreover, the 1.8-cineole compound crooked the molecular chain of triglyceride on Crude Coconut Oil (CCO), Crude Palm Oil (CPO), and Crude Sunflower Oil (CSFO) to bend apart from each other even longer, when compared to that of CJO, therefore, lowering viscosity and increasing volatility and increasing the burning rate. The polarity of the four droplets has high impact on the influence of 1.8-cineole on the performance of oil droplets in the process of evaporation to the ignition.

Elastic breakdown via multi-core high frequency discharge for lean-burn ignition

An advanced ignition technique is developed to achieve multi-event breakdown and multi-site ignition using a single coil for ignition quality improvements. The igniter enables a unique elastic breakdown process embracing a series of high-frequency discharge events at the spark gap. The equivalent electric circuits and current/voltage equations are identified and verified for the first time to explain the working principle that governs such an elastic breakdown process. Benchmarking tests are first performed to compare the elastic breakdown ignition with the conventional and advanced multi-electrode ignition systems. The elastic breakdown and spark events are thereafter analyzed through current and voltage measurements and high-speed imaging techniques. Finally, ignition tests in combustion chambers are performed to examine the effects on the ignition process in comparison with conventional coil-based ignition systems. The experiments show that, the elastic breakdown ignition can distribute multiple high-frequency breakdown events at all electrode pairs of a multi-electrode sparkplug while using only one ignition coil, thereby offering significant cost saving advantage and packaging practicability.

A Technique for High-Speed Microscopic Imaging of Dynamic Failure Events and Its Application to Shear Band Initiation in Polycarbonate

Abstract An experimental technique is reported, which can image the deformation fields associated with dynamic failure events at high spatial and temporal resolutions simultaneously. The technique is demonstrated at a spatial resolution of ∼1 µm and a temporal resolution of 250 ns, while maintaining a relatively large field of view (≈1.11 mm × 0.63 mm). As a demonstration, the technique is used to image the deformation field near a notch tip during initiation of a shear instability in polycarbonate. An ordered array of 10 µm diameter speckles with 20 µm pitch, and deposited on the specimen surface near the notch tip helps track evolution of the deformation field. Experimental results show that the width of the shear band (SB) in polycarbonate is approximately 75 µm near the notch tip within resolution limits of the experiments. The measurements also reveal formation of two incipient localization bands near the crack tip, one of which subsequently becomes the dominant band while the other is suppressed. Computational simulation of the experiment was conducted using a thermomechanically coupled rate-dependent constitutive model of polycarbonate to gain further insight into the experimental observations enabled by the combination of high spatial and temporal resolutions. The simulation results show reasonable agreement with the experimentally observed kinematic field and features near the notch tip, while also pointing to the need for further refinement of constitutive models that are calibrated at high strain rates (∼105/s) and also account for damage evolution.

Understanding diffusion layer agitation by cavitation in electroplating based on high-speed monitoring

Ultrasonic electroplating has gained attention owing to various advantages such as the promotion of mass transport to the substrate surface, improvement of the surface properties of the film, and improvement of limiting current density. However, no studies have clarified the mechanism in diffusion layer agitation caused by cavitation during ultrasonic electroplating. Here, we investigate the main factor of agitation by using a high-speed imaging technique to capture the agitation effect of shock waves and microjets generated from laser-induced cavitation on the diffusion layer of electroplating. The physical parameters of the agitation were characterized using image analysis and a micro-pressure gauge. The results revealed that only microjets affected the agitation phenomenon. The flow velocity was 21 m/s, and the water hammer pressure was low, at least below 0.05 MPa. Our results suggest that the flow velocity, and not the water hammer pressure, plays an important role in the agitation phenomenon on the substrate surface by cavitation.

Wave propagation visualization through ducts using the Schlieren technique for crack localization with the eSHM system.

The effective structural health monitoring (eSHM) system fully exploits the flexibility offered by the 3D printing process for analysis of the structural integrity of additive manufactured parts by integrating a smart technology inside the component. The eSHM system relies on the propagation of pressure waves through capillaries/small ducts embedded in 3D printed metallic components and allows the detection and localization of fatigue cracks. However, the nature and propagation of these waves seem to be determinant for the accuracy of the crack localization system. To achieve a better physical understanding of the propagating waves through the capillaries, computational fluid dynamics simulations are performed and compared with experimental results, obtained by Schlieren flow visualization and high-speed imaging techniques. The presence of propagating shock waves and contact discontinuities is observed in the simulations, as well as a complex reflection mechanism around the leak location. The Schlieren experiments exhibit the same wave shape behavior and complex reflection mechanism around the crack location for the contact discontinuity, and the shock tube analogy is confirmed.

In situ process monitoring and automated multi-parameter evaluation using optical coherence tomography during extrusion-based bioprinting

In three-dimensional (3D) bioprinting, errors during the printing process leads to lower structural fidelity, and the desired structural and functional performances cannot be fully satisfied. Process monitoring with high-speed imaging techniques is helpful for identifying material deposition errors. 3D reconstruction of printed objects enables comprehensive evaluation of structural characteristics, with wide-field full-depth imaging being necessary for large objects. In this study, we demonstrate 3D extrusion-based bioprinter-associated optical coherence tomography (3D P-OCT) to achieve high-speed, wide-field and full-depth imaging, and provide in situ process monitoring and comprehensive evaluation for 3D bioprinting. In 3D P-OCT, a wide field was achieved with a lateral image mosaic using the simplified iterative closest point algorithm, and the extension of imaging depth was achieved with a longitudinal image mosaic using image fusion based on maximum intensity values. During the printing process, 3D P-OCT enables real-time multi-parameter quantification for intelligent feedback. In conclusion, with in situ process monitoring and evaluation, 3D P-OCT contributes to ensure better structural and functional performances of 3D functional tissue constructs.

An experimental investigation of vortex-induced vibration of a curved flexible cylinder

Vortex-induced vibration of a curved flexible cylinder placed in the test section of a recirculating water tunnel and fixed at both ends is studied experimentally. Both the concave and the convex orientations (with respect to the incoming flow direction) are considered. The cylinder was hung by its own weight with a dimensionless radius of curvature of $R/D=66$ , and a low mass ratio of $m^{*} = 3.6$ . A high-speed imaging technique was employed to record the oscillations of the cylinder in the cross-flow direction for a reduced velocity range of $U^{*} = 3.7 - 48.4$ , corresponding to a Reynolds number range of $Re= 165 - 2146$ . Mono- and multi-frequency responses as well as transition from low-mode-number to high-mode-number oscillations were observed. Regardless of the type of curvature, both odd and even mode shapes were excited in the cross-flow directions. However, the response of the system, in terms of the excited modes, amplitudes and frequencies of the oscillations, was observed to be sensitive to the direction of the curvature (i.e. concave vs convex), in particular at higher reduced velocities, where mode transition occurred. Hydrogen bubble flow visualization exhibited highly three-dimensional vortex shedding patterns in the wake of the cylinder, where there existed spatial and temporal evolution of the vortex shedding modes along the length of the cylinder. The time-varying intermittent vortex shedding in the wake of the cylinder was linked to the spanwise travelling wave behaviour of the vortex-induced vibration response. The observed spatially altering wake corresponded to the multi-modal excitation and mode transition along the length of the cylinder.

Modelling of Low-velocity Impacts on Composite Beams in Large Displacement

The paper provides an evaluation of the nonlinear dynamic response of a cantilever beam made of composite material subjected to low-velocity impacts. The structure is assumed to respond in a quasi-static manner and modelled by a continuous beam in large displacement with a lumped mass attached. First, an analytical model was developed to study the free vibrations of a beam, taking into account the nonlinearities due to large displacements and inertia. Then, the analytical findings were compared with experimental test data. The vibration of a real composite beam has been acquired through high-speed imaging technique. The displacements of the beam were extracted by digital image analysis; then, the nonlinear parameters of the analytical model were determined by the Fitting Time History technique. The results obtained by the analytical model and the experimental test are compared with numerical analysis. The validated analytical model was adapted to study a low-velocity impact; the lumped mass was associated with a rigid projectile, whose initial speed represents the impact velocity.

Droplet Transfer Induced Keyhole Fluctuation and Its Influence Regulation on Porosity Rate during Hybrid Laser Arc Welding of Aluminum Alloys

Hybrid laser arc welding (HLAW) features advantages such as higher welding speed and gap tolerance as well as smaller welding deformation and heat-affected zone than arc welding. Porosity in hybrid laser arc weld due to keyhole fluctuation tends to be the initial source of crack propagation, which will significantly diminish the weld performance. A high-speed imaging technique was adopted to record and analyze the droplet transfer and keyhole fluctuation behavior during hybrid laser arc welding of aluminum alloys. A heat transfer and fluid flow model of HLAW was established and validated for a perspective of the evolution process of droplet transfer and keyhole fluctuation. The relationship between keyhole fluctuation and weld porosity was also revealed. During the droplet transfer stage, liquid metal on the top surface of the weld pool flows toward the keyhole originated by globular transfer, and the keyhole fluctuates and decreases significantly, which has a higher tendency to form a bubble in the weld pool. The bubble evolves into porosity once trapped in the mush-zone near the trailing edge of the weld pool. Therefore, globular transfer during HLAW is the principal origin of keyhole fluctuation and weld porosity. Welding current has a significant influence on keyhole fluctuation and weld porosity rate. Droplet transfer frequency, keyhole fluctuation, and porosity rate increase with higher welding current under the globular transfer mode. The porosity rate shows a nearly positive correlation with the standard deviation of keyhole fluctuation.