High-speed Digital In-line Holography Research Articles

Spray trajectory and 3D droplets distribution of liquid jet in crossflow with digital inline holography

Liquid jet in crossflow (LJCF) has a wide application in the actual engine engineering. This article presents an experimental investigation of a water jet transversely injected into a subsonic crossflow through digital inline holographic imaging, and presents the relationship between the column trajectory and the downstream droplet distribution. A phenomenological analysis based on high-speed digital inline holography (DIH) with the frequency of 25 kHz is presented to interpret the source of droplets with different sizes in the bag breakup mode and the shear breakup mode. High-resolution DIH with a spatial resolution of 5.5μ m is applied to measure breakup point, droplet size, and location under 30 mm downstream of the orifice. The experiment is carried out under normal temperature and pressure. Gas Weber number Weg varies from 11 to 67, and the liquid to gas momentum ratio q changes from 10 to 28, which are mainly under bag breakup and multi-mode breakup regime. Crossflow velocity profiles are also measured. Liquid penetration were obtained and fitted through spray pattern under x/dj<10 and through downstream droplet statistics under 12<x/dj<70 separately. Under the cases studied, spray penetration evaluated by droplet statistics is larger than that evaluated through spray pattern. In addition, droplet size is relative large in the core region of the spray under higher Weg, possibly due to the presence of vortices in the core region and stronger aerodynamic effects at the periphery of spray. This research presents the quantification of the primary breakup process and corresponding three-dimensional (3D) downstream SMD distribution simultaneously, which helps improve the understanding of spray evolution in crossflow.

Probing into gas leakage characteristics of ventilated supercavity through bubbly wake measurement

The stability of ventilated supercavitation is strongly influenced by gas leakage characteristics of the cavity. In this study we conduct a systematic investigation of such characteristics under different closure conditions including re-entrant jet (RJ), quad vortex (QV), twin vortex (TV), and pulsating twin vortex (PTV), generated from different tunnel speeds and ventilation conditions. Using high speed digital inline holography (DIH), all the individual bubbles shed from the cavity are imaged downstream and are used to quantify the instantaneous gas leakage from the cavity. In general, the supercavity gas leakage exhibits significant fluctuations under all closure types with the instantaneous leakage rate spiking up to 20 times of the ventilation input under RJ and QV closures. However, the magnitude and occurrence rate (frequency) of such excessive gas leakage (above ventilation input) vary substantially across different closures, tunnel speeds and ventilation conditions. Particularly, as the supercavity transitions from RJ, to QV, TV, and PTV with increasing ventilation or decreasing tunnel speed, the relative excessive gas leakage decreases sharply from RJ to QV, plateaus from QV to TV, and drops again from TV to PTV. Correspondingly, the occurrence frequency of such excessive leakage first exhibits a double peak distribution under RJ, migrates to a single peak mode under QV and TV, and eventually transitions to a distribution with a broadened peak at a higher frequency under PTV. These trends can be explained by the flow instabilities associated with three gas leakage mechanisms, i.e., re-entrant jet impingement leakage, vortex tube gas leakage, and cavity pulsation induced bubble pocket shed-off, whose relative significance changes under different closure and flow conditions. Subsequently, two metrics are introduced to quantify the relative change of ventilation needed to compensate the change of extra gas loss and the predictability of the occurrence of excessive gas leakage, respectively. Based on these metrics, we suggest that the supercavity operating under TV closure with moderate ventilation is optimal for ventilation-based controls of supercavity stability.

Aerodynamic fragmentation of water, ethanol and polyethylene glycol droplets investigated by high-speed in-line digital holography

Digital holography is a remarkable interferometric technique for determining the size, shape, distribution, and spatial location of particles. Therefore, when combined with high-speed imaging technology, it represents a relevant technique for the study of the aerodynamic fragmentation phenomenon wherein the aerodynamic strengths counterbalance the cohesion forces of the drops causing thus their decomposition. We discuss in this paper the droplets fragmentation of water, ethanol and polyethylene glycol 300 solutions interacting with a shock wave and flow generated by an open-ended shock tube. In this context, a shock tube device was set up, and both the released flow as well as the shock wave generated were characterized by ultrafast imaging shadowgraphy and high speed pressure measurement. Millimeter diameter average drops are fragmented and imaged by high-speed digital in-line holography at a rate of 24800 fps, and then the image retrieved numerically. The Weber numbers are determined for each liquid and the observed fragmentation patterns are consistent with the description of the ‘chaotic regime’ reported in the literature by Pilch and Erdman (1987) [1].

Effects of surface roughness on splashing characteristics of large droplets with digital inline holographic imaging

Ice accretion on aircraft wings due to Supercooled Large Droplet (SLD) has a significant impact on flight safety. The dynamic behaviours of large droplet, such as deformation, breakup and splashing, have significant effects on icing accretion. The corresponding studies have been carried out experimentally and numerically, and empirical models to predict the dynamic behaviours of large droplets have been proposed. However, few empirical models involve surface roughness effects, especially its effects on the secondary droplets during icing accretion. In this paper, an experimental investigation on the impact of droplets onto surfaces with different roughness was conducted by using the high-speed Digital Inline Holography (DIH). The rapid splashing processes of droplets with Weber number around 570 and Mundo impact parameter K around 240 were captured and reconstructed based on a wavelet-based algorithm. Results show that DIH is suitable to diagnose the secondary droplets with a wide size range and record their velocities and locations. The roughness of the impact surface has a positive effect on the dynamics of splashing droplets, and the parameters including diameter, velocity, splash angle, splash height and mass-loss rate of splashing droplets rise with the increase of roughness. The new empirical correlations of the splashing characteristics of secondary droplets were fitted and developed, which could be used to update the current icing simulation models to take into account the effect of impact surface roughness on large droplets dynamics. This fundamental study may also be applied to the industrial applications including wind turbine icing, inkjets, sprays, as well as the erosion and formation of ice layers.

Quantification of the size, 3D location and velocity of burning iron particles in premixed methane flames using high-speed digital in-line holography

Due to its low emission and high energy density, iron powder has been proposed as a promising recyclable metal fuel for a future low-carbon society. The comprehensive understanding of combustion behavior of iron particles is crucial for studying fundamental mechanisms, developing suitable combustion technologies, and designing efficient iron powder combustor. In this work, iron particles are combusted in a modified Bunsen burner with a stable metal powder supplying system. As a versatile three-dimensional (3D) imaging technique, high-speed digital in-line holography (DIH) is employed to reconstruct the 3D particle field and simultaneously quantify the size, 3D location and velocity of burning iron particles. The statistical results of three cases with oxygen volume fraction varying from 24% to 40% are obtained and compared at different heights above the burner. Along the height, some typical features of the burning iron particles were observed. The violent combustion of iron particles accelerates the ejection of the particles radially outward from the central region of the flame, resulting in non-uniform spatial distribution of the particles and reducing the particle number density in the measurement volume. Such trend is enhanced with increased oxygen concentration. Besides, the observed particle size enlarged as the height increases, which validates the swelling phenomenon of iron particle oxidation. The results demonstrate that DIH is a powerful tool for in-situ, quantitative characterization of particle dynamics in flames.

Investigation of particle density on dust cloud dynamics in a minimum ignition energy apparatus using digital in-line holography

Solids industries have inherent dust explosion hazards. Process risk assessment parameters include overpressure, deflagration index, and minimum ignition energy. These parameters are influenced by dust dynamics, such as turbulence and concentration. Cloud dynamics are affected by particle morphology, density, size, and polydispersity, which affect explosion parameters. Influence of properties such as density during explosion parameter measurement is often overlooked. This study examines particle density influence on dust clouds in the Kühner MIKE3. Two clouds of particles with different density, but similar particle size and morphology were examined with high-speed digital in-line holography, providing velocity and concentration measurements prior to ignition. Results show velocity and concentration of denser dust is lower compared to lighter dust after ignition delay. The results highlight the density effect on cloud dynamics and make a case for particle density to be considered in setting ignition delay in order to get consistent cloud dynamics and test results.

High-speed digital in-line holography for in-situ dust cloud characterization in a minimum ignition energy device

Precise measurement of the minimum ignition energy (MIE) of combustible dust is crucial to safety in the process industries. The development of comprehensive methods for MIE dust cloud characterization is therefore highly desirable. The objective of this study is to investigate the application of high-speed digital in-line holography (DIH) for volumetric and in-situ characterization of dust clouds near the ignition zone of a Kühner MIKE3 MIE device. The dispersion behavior of borosilicate glass and soda-lime glass dust clouds are studied at 20 kHz. Size measurements, with successful detection down to 13 μm, are compared with Beckman Coulter particle size analyzer results. The 5-ms hologram videos also yield new transient particle volume, concentration, and velocity measurements. Sample sizes of 15–150 mg of dust powder are sufficient for each high-speed run. This work showcases new diagnostic capabilities that are accessible to dust explosion research and identifies areas for future research.

Primary Fragmentation Behavior Investigation in Pulverized Coal Combustion with High-Speed Digital Inline Holography

Comprehension on the primary fragmentation of coal particles is important in understanding and optimizing practical coal combustion and creating credible combustion models. High-speed digital inlin...

Comparison of simulation and experiments for multimode aerodynamic breakup of a liquid metal column in a shock-induced cross-flow

While the mechanisms that drive breakup and aerodynamic dispersion of traditional liquids such as water have been extensively studied, it is not yet clear if models for traditional liquids can be used to accurately describe the behavior of molten metals. In this paper, multiphase simulations with the interface-capturing combined level-set volume-of-fluid approach are used to provide time-resolved morphology and breakup data for a liquid column subject to a shock-induced cross-flow. For the first time, numerical simulation of the behavior of a liquid metal (Galinstan alloy composed of gallium, indium, and tin) is compared to the well-documented behavior of water. Simulations consider a gas cross-flow Weber number between 10 and 12, which produces a multimode breakup morphology consisting of multiple baglike structures. Up to bag breakup, we confirm that the deformation rate of Galinstan follows the same dependence on the gas cross-flow Weber number as ordinary liquids when time is nondimensionalized by including the liquid-gas density ratio. Moreover, we determine that the appearance of a central stem along the column upstream surface in multimode bag breakup is consistent with the occurrence of Rayleigh-Taylor instability. We also resolve bag stretching and fragmentation, to the full extent allowed by our computational resources, and carry out a direct comparison with the measurements of size and velocity of secondary droplets from high-speed digital inline holography. For Galinstan, we illustrate the differences between simulation and experiment that emerge because of the modification of the surface properties of the metal exposed to air.

Evolution of volatile cloud in pulverized coal combustion with high-speed digital inline holographic visualization

The coal devolatilization plays a significant role in the combustion of pulverized coal particles. The evolution of volatile cloud during devolatilization of pulverized coal particles (105–125 μm) is studied in a high-temperature flat-flame burner by combining high-speed photography with high-speed digital inline holography (DIH). By the high-speed holographic visualization, the evolution of volatile cloud of pulverized coal from volatile release to soot aggregation generation can be divided into four stages. Effects of coal type on volatile cloud evolution are investigated using three different coals, i.e., Shanxi bituminous coal, Ximeng lignite and Yinni lignite. The results show that both the Shanxi bituminous coal and Ximeng lignite produce soot aggregation during devolatilization, which is rarely observed for Yinni lignite. Moreover, Shanxi bituminous coal has a higher potential in soot cluster formation for its higher coal rank than Ximeng lignite. The high-speed reconstructed image sequences are analyzed to measure the velocity slip between the parent particle and volatile cloud. Compared with Shanxi bituminous coal, Ximeng lignite exhibits a larger slip velocity. This work also demonstrates that high-speed DIH has the powerful capacity of directly observing the evolution of volatile cloud, and helps to gain a deep understanding of pulverized coal combustion.

Investigating particle and volatile evolution during pulverized coal combustion using high-speed digital in-line holography

Devolatilization is an important process in pulverized coal combustion because it affects the ignition, volatile combustion, and subsequent char burning and ash formation. In this study, high-speed digital in-line holography is employed to visualize and quantify the particle and volatile evolution during pulverized coal combustion. China Shanxi bituminous coal particles sieved in the range of 105–154 µm are entrained into a flat flame burner through a central tube for the study. Time-resolved observations show the volatile ejection, accumulation, and detachment in the early stage of coal combustion. Three-dimensional imaging and automatic particle extraction algorithm allow for the size and velocity statistics of the particle and stringy volatile tail. The results demonstrate the smaller particle generation and coal particle swelling in the devolatilization. It is found that the coal particles and volatiles accelerate due to the thermal buoyancy and the volatiles move faster than the coal particles. On average, smaller particles move faster than the larger ones while some can move much slower possibly because of the fragmentation.

Multi-point vibrometer based on high-speed digital in-line holography.

This paper describes a digital holographic setup based on in-line holography and a high-speed recording to get a multipoint vibrometer. The use of a high-speed sensor leads to specificities that enable the in-line configuration to be used. The case of transient vibrations is investigated through a full simulation of the holographic process. The simulation shows that the first instants are critical since distortion may occur, resulting in errors in the phase measurement. Experimental results are provided by exciting an aluminum beam with a transient signal. A comparison with the velocity measured by a pointwise vibrometer is provided. Frequency response functions are extracted and the experimental results confirm the ability of the method to provide full-field contactless measurements at the high-speed time scale evolution of the vibration.

Dynamics of cavitation clouds within a high-intensity focused ultrasonic beam

In this experimental study, we generate a 500 kHz high-intensity focused ultrasonic beam, with pressure amplitude in the focal zone of up to 1.9 MPa, in initially quiescent water. The resulting pressure field and behavior of the cavitation bubbles are measured using high-speed digital in-line holography. Variations in the water density and refractive index are used for determining the spatial distribution of the acoustic pressure nonintrusively. Several cavitation phenomena occur within the acoustic partially standing wave caused by the reflection of sound from the walls of the test chamber. At all sound levels, bubbly layers form in the periphery of the focal zone in the pressure nodes of the partial standing wave. At high sound levels, clouds of vapor microbubbles are generated and migrate in the direction of the acoustic beam. Both the cloud size and velocity vary periodically, with the diameter peaking at the pressure nodes and velocity at the antinodes. A simple model involving linearized bubble dynamics, Bjerknes forces, sound attenuation by the cloud, added mass, and drag is used to predict the periodic velocity of the bubble cloud, as well as qualitatively explain the causes for the variations in the cloud size. The analysis shows that the primary Bjerknes force and drag dominate the cloud motion, and suggests that the secondary Bjerknes force causes the oscillations in the cloud size.

Ultra-high-speed tomographic digital holographic velocimetry in supersonic particle-laden jet flows

Ultra-high-speed tomographic digital holographic velocimetry is used to measure the three-component, three-dimensional velocities and trajectories of micron-sized particles in a supersonic underexpanded jet flow. In high-speed digital in-line holography, the depth resolution or depth of field is severely restricted by the limited resolution of the digital recording array, which leads to significant elongation of the reconstructed particles in the depth direction. It is shown that by applying tomographic digital holography, this limitation imposed by the high-speed recording array is relaxed and that the accurate reconstruction of 3D particle intensities is possible without the depth-of-field problem. This method is demonstrated by measuring 110 μm solid particles suspended in a ‘mildly underexpanded’ jet with a nozzle pressure ratio of 2.0. The interference pattern produced by both the suspended particles and the density gradient field is simultaneously recorded by two digital high-speed cameras at a frame rate of 500 000 fps with an exposure time of 250 ns. Individual object fields are reconstructed by tomographic holography and the velocity of the micron-sized particles is analyzed by cross-correlation particle tracking velocimetry. The accuracy of the particle velocity measurements is estimated to be within 5 m s−1 or approximately 4.5% of the particle velocity. In addition, the method is used to study the particle–flow interactions by means of coherent imaging, which reveals a complex interaction between the micron-sized particles and the flow structure of this high-speed 3D unsteady turbulent particle-laden flow.