Backlit Imaging Research Articles

High-resolution, highly monochromatic, time-framed, x-ray-backlit-imaging diagnostics with dual-channel toroidal crystals

Here, we developed a high-resolution, dual-channel toroidal-crystal x-ray imager for time-framed x-ray backlit imaging diagnostics using the 4.727 keV helium-like Ti line. We also presented a method for adjusting the dual-channel imager through the self-imaging of a two-dimensional periodic object. Offline x-ray experiments achieved a spatial resolution of ∼5.0 μm in the center and better than 8.0 μm within a field of view (FOV) of ∼2 mm. At the ShenGuang-III prototype laser facility, we obtained imaging results with a spatial resolution of better than 5 μm within an FOV of ±40 µm. This imager thus provides a way of observing with high spatial, temporal, and spectral resolutions to diagnose the behavior of laser-produced plasma.

Effect of droplet spacing on micro-explosion and combustion characteristics of multi-component fuel droplet cluster

The micro-explosion and combustion characteristics of n-butanol/n-pentanol/biodiesel blended fuel droplet cluster are investigated using high-speed backlit imaging technique. The droplet cluster includes 9 droplets with the same diameter, which are arranged in a 3 × 3 matrix equidistantly using the fiber support method. The results indicate that when the normalized droplet spacing (s/d0) is less than 3, the coalescence phenomenon caused by micro-explosions will occur between adjacent two or three droplets in the droplet cluster. For small s/d0 (2 ∼ 4.5), the droplet cluster sustains a fully enveloped flame. Within the range of s/d0 of 5 ∼ 7, two combustion modes emerge: transitioning from a fully enveloped flame to an independent flame, and merging independent flames into enveloped flames, eventually reverting to independent flames. At a s/d0 of 8, each droplet within the cluster maintains an independent flame. In addition, the combustion rate of droplets 1–3 and 7–9 reaches its maximum value at 4 ∼ 4.5 s/d0, about 30 % higher than single droplet. Most importantly, the droplet micro-explosion intensity at the center of the droplet cluster is higher than that of other droplets (similar to a single droplet), which is caused by the “group micro-explosion” effect.

Multi-scale boiling heat transfer investigation on micro-thin aluminum heaters

Nucleate boiling is a highly efficient heat transfer mode distinguished by the liquid-vapor phase change, which occurs through the formation, growth, and detachment of vapor bubbles from a heated surface. Its crucial role in various industrial applications, such as nuclear power plant operation and effective heat management in small electronic devices, has driven significant research efforts. However, despite extensive research dedicated to boiling investigations, there are still substantial knowledge gaps that hinder our ability to accurately predict heat removal rates. These knowledge gaps arise from the complex nature of small-scale boiling phenomena, which are further complicated by their strong dependence on operating conditions and the interactions between walls and fluids. In an effort to address some of these gaps, we conducted multi-scale investigations during pool boiling of de-ionized water on micro-thin aluminum heaters. We captured bubble dynamics through multiple synchronized diagnostic sources, including high-speed backlit imaging to track bubble growth, synchronized high-speed infrared thermometry to capture the corresponding thermal footprint on the boiling surface, and in-house developed fast-response micro-thermocouples to measure temperature at multiple locations within the fluid. Our study reveals peculiar aspects of heat transfer mechanisms occurring at single bubble level (low heat fluxes) and in fully developed nucleate boiling regimes (high heat fluxes).

Investigation on cavitation enhancement on flash boiling atomization using two-dimensional slit nozzles

Flash boiling and cavitation are two important topics in internal phase changing flow and spray atomization studies. They have been proven to be effective in improving spray breakup process, and hypotheses are proposed that these two phenomena could have coupled effects to achieve even better atomization. But few quantitative studies have made direct comparisons on whether the introduction of cavitation into flash boiling will result in finer droplets. This investigation aims to address this issue with optically transparent, two-dimensional slit nozzles with varied geometries. The round corner is introduced to suppress the generation of cavitation at the inlet, and thermal boundary conditions are adjusted to trigger flash boiling. Results from high-speed backlit imaging and phase doppler interferometry unveil that the introduction of cavitation can enhance the flash boiling process throughout the increment of superheat degree, yet the enhancement efficiency is different as thermal conditions change, and is strongly correlated with internal bubble spatial distributions. A model is proposed to explain the governing factor of cavitation enhancement on flash boiling atomization.

Plume Spreading Due to Floor Conditions of a Plunging Liquid Jet Using Stereographic Backlit Imaging

Abstract Plunging liquid jets are a multiphase flow studied to understand how gas is entrained in a liquid and the resulting mixing capabilities. From existing literature, it has been hypothesized that rising bubbles play a noticeable role in the multiphase hydrodynamics of the plunging liquid jet bubble plume and that separating the rising bubbles from the incoming liquid jet can result in a significant increase in the depth of the bubble plume. This study explores the effects of separating the incoming liquid jet from the rising bubble plume through floor interactions and compression effects due to a finite tank depth. This configuration is found in many natural and industrial systems, but not within published literature. Using existing theoretical models of infinite depth plunging liquid jet systems, which align reasonably well with captured baseline data, two models are developed for when floor interactions are present, one theoretical and one empirical. The models show a correlation between plume spread and floor interaction with the incoming plunging liquid jet bubble plume. Data acquired through stereographic backlit imaging over a range of flow rates show a reasonable agreement with the proposed models.

FUNDAMENTAL INVESTIGATION OF LIQUID INJECTION IN SUPERSONIC CROSSFLOW: AN EXPERIMENTAL STUDY

Experimental investigation of the liquid injection into a Mach 2.2 supersonic crossflow through a transverse single circular injector has been carried out in the present study. High-speed visualization techniques such as back-lit imaging, shadowgraphy, and schlieren imaging have been employed to investigate the flow and the liquid jet features. The present study provides a detailed analysis of the breakup behavior of a liquid jet introduced into a crossflow with a Mach number of 2.2 by categorizing it into distinct zones. The liquid jet breakup was induced by surface instabilities, leading to the formation of a protrusion structure that traveled downstream along the jet. The schlieren photographs captured the essential flow dynamics resulting from the liquid injection, such as the bow shock wave and the separation shock wave. Observations indicated that the location where the bow shock wave interacts with the upper wall shifts in the upstream direction as the liquid injection pressure is increased. Furthermore, a parametric analysis was conducted to assess the penetration height of the injected liquid and its variation relative to the injection pressure. The analysis revealed that the penetration of the jet was greatest at an injection pressure of 7 bar, succeeded by 5 bar and 3 bar, respectively. The minimal penetration height was recorded at an injection pressure of 1 bar. In a quantitative analysis, the penetration of a liquid jet was measured at various injection pressures at a normalized axial distance of 5. It was found that the penetration of the liquid jet with an injection pressure of 7 bar was 3.67 times greater than the liquid injection with an injection pressure of 1 bar.

A computational study of a two-fluid atomizing coaxial jet: Validation against experimental back-lit imaging and radiography and the influence of gas velocity and contact line model

Numerical simulations of liquid atomization in a two-fluid coaxial geometry have been performed using a geometric Volume-of-Fluid method. Experimental measurements have been obtained using visible light back-lit imaging and X-ray radiography. Simulations are validated against experiments, using the same geometry and fluid injection rates of air and water, by showing excellent agreement in quantities such as liquid mass distribution in the spray formation region and the liquid jet length statistics and temporal dynamics. At the nozzle exit, the coflowing liquid and gas streams are separated by a cylindrical splitter plate. The liquid is laminar and modeled using a Poiseuille flow while the gas inflow model and the contact line model are varied. For the gas velocity models, the vorticity thickness is shown to have a strong influence on the downstream liquid distribution; the difficulty of its modeling and routes to overcome them are discussed. For the contact line model, pinning the interface to the inner wall of the splitter plate leads to an initial increase in the diameter of the liquid jet just downstream of the nozzle exit. In contrast, pinning to the outer wall of the splitter plate or allowing for a free moving contact line results in a monotonic decrease in the diameter of the liquid jet as the downstream distance is increased, in agreement with the experimental observations and measurements. A sub-grid scale contact line model based on a static contact angle is introduced. The static contact angle is varied in the model, showing that the liquid remains intact longer as the static contact angle is increased.

Study on the effect of injection temperature and nozzle geometry on the flashing transition of liquid ammonia spray

Ammonia is a promising alternative fuel for carbon emission reduction in industry, transport, and power generation. For high-pressure combustion environment applications, direct injection of liquid ammonia (LNH3) is considered. Because of its thermophysical properties, and dependence on the injection and combustion environment conditions, the direct injection of ammonia in its liquid state can lead to flash-boiling, changing drastically the spray characteristics. In this study, we characterized the effect of injection temperature and nozzle geometry on the spray characteristics and transition to flashing using backlit imaging. Flow rates pressure curves and spray angles were obtained in a well-controlled environment and can be used as a database for simulation. Finally, some practical considerations on the design of nozzles are highlighted in the present work.

Effect of near-field characteristics on the two-phase distribution of superheated ammonia spray

The development of low-carbon or zero-carbon fuel-driven engines has become a key technology to help reduce emissions under the background of carbon neutrality. Ammonia fuel has the advantages of low boiling point and high octane rating, but it also has the disadvantages of high self-ignition temperature and slow combustion speed. Direct injection of liquid ammonia has the potential to reduce greenhouse-gas emissions while maintaining engine power output, especially under high replacement ratio conditions. Meanwhile, liquid ammonia has a lower boiling point than traditional fossil fuels and is more likely to enter superheat state with plenty of bubbles nucleated inside the nozzle. After injection, bubble puffing/micro-explosion occurs inside the jet and thus accelerates the atomization and evaporation process of the spray. In this study, a micro-imaging technique was adopted to investigate the near-field characteristics and the gasification process inside the nozzle. Besides, both Schlieren and diffused backlit imaging techniques were applied to characterize the two-phase distribution evolution. By scanning a wide range of flash-boiling conditions, it is found that 3.5% of gasification ratio and 200.0% of near-field volume increment were achieved respectively inside and outside the nozzle at high fuel temperature conditions. Moreover, the linear increment of the gasification ratio accelerates the two-phase distribution area of ammonia spray exponentially. These results would provide insightful information for two-phase evolution and developing simulation models of ammonia sprays.

PLUME ANALYSIS OF PLUNGING LIQUID JETS WITH FLOOR INTERACTIONS USING STEREOGRAPHIC BACKLIT IMAGING

Plunging liquid jets occur when a liquid jet travels through a gas and enters a liquid pool. They are studied in order to understand and model the highly efficient gas-liquid mixing in the resulting bubble plume. Among the aspects of plunging jets studied, compression effects at the bottom or within the pool, while relatively common in application, remain less understood in current literature. The presence of a tank floor spreads the resultant bubble plume which reduces the flow resistance between the rising bubble plume and the incoming liquid jet. The effect of the tank floor on the overall entrainment and mixing is unknown. In order to understand the compression effects of the plume-floor interaction, a liquid-filled tank was designed to simulate multiple tank depths. This work describes the apparatus used to produce a plunging jet with floor effects, as well as the image analysis methodology for identifying and comparing plume formations. Data were collected for multiple liquid flow rates encompassing different entrainment regimes as well as multiple plate positions, varying the level of plume and base interaction. Stereographic backlit images were acquired and several different identification methods are described to better define and compare bubble plume features. Limitations in the image analysis procedures are also described.

MODIFICATION OF AMULTIHOLE INJECTOR TO A SINGLE-HOLE INJECTOR AND SPRAY CHARACTERISTICS OF THE MODIFIED INJECTOR BY TWO DIFFERENT IMAGING TECHNIQUES

In the present days, almost all the diesel engines are fitted with multihole injectors. Spray characterization of the injectors by different optical measurement techniques is very important to under-stand the air-fuel mixing and combustion procedure inside the cylinder. Instead of characterization of the dense sprays from a real multihole diesel injector, often a single plume is used to study the spray parameters. In the present work, spray characteristics are studied for a multihole injector with all nozzle holes open and with a single hole open conditions. Experiments have been conducted in a constant-volume, high-pressure, and high-temperature chamber with various injection pressures and at various ambient conditions. A high-pressure common rail injector with six holes is considered for the study. The penetration and cone angle of the spray from each hole are measured, and the hole-to-hole variation is studied. It has been observed that hole-to-hole variations with respect to spray penetration are prominent in low chamber pressures and injection pressures at the initial stage of injection, though they are minimal at later stages of injection. After that, five holes of the injector are blocked by microwelding, and the remaining single plume was studied and the results compared with the original multiplume spray. The study shows that the penetration length and cone angle of the single plume match well with that of the multiplumes at all chamber pressures and injection pressures. Later, the spray from the modified single-hole injector was characterized by two different imaging techniques, Mie scattering and diffused backlit imaging (DBI), to compare the penetration length and cone angle measured by the two imaging techniques. It is seen that the differences in penetration lengths by two imaging techniques are negligible, whereas the cone angles are greater in the case of backlit shadow imaging, particularly at higher temperatures.

Noninvasive Imaging of a Liquid Jet

Abstract Liquid jets are found in many applications, from printing to manufacturing to entertainment. This study uses three different noninvasive imaging modalities to compare resulting images of a liquid jet operating at three Reynolds numbers that cover laminar, transitional, and turbulent flow. Selected measurement quantities from each image type are also compared. High-speed backlit (BL) imaging is a simple imaging technique found in many laboratories, and this is compared to two high-speed X-ray imaging techniques, white beam (WB) imaging and focused beam (FB) radiography. BL imaging can provide a wide field of view and is easy to implement, but it only shows the presence or absence of liquid. WB imaging can show detailed contours on the surface of the liquid jet, but the imaging region is much smaller. FB radiography produces a point-source measurement and can provide the quantitative, instantaneous local liquid path length, termed the equivalent path length (EPL). All three techniques provide similar measures of jet thickness, with the FB measurements having less variation. FB measurements can also provide detailed cross sections of the average liquid jet thickness at high spatial resolutions.

Micro-explosion characteristics and mechanism of multi-component fuel droplet with high volatility differential

Micro-explosion is considered as one of the effective ways to improve atomization, combustion and reduce emissions. The present study examined the micro-explosion characteristics and mechanism of multi-component fuel droplets using fiber-support method and high-speed backlit imaging technique. The multi-component fuel blends consist of 2, 5-dimethylfuran (DMF) as a lower volatile component, while Jatropha oil is used as the higher volatile constituents. The results show that the micro-explosion modes are categorized as puffing, breakup + puffing, vapor jetting, jetting + puffing according to the single micro-explosion intensity and droplet breakup index. The breakup + puffing mode is the dominant mode, and the jetting + puffing mode only appears in droplets with 25/75 and 50/50 composition. The formation mode of secondary droplets includes short-wave mode and long-wave mode. Two types of bubble formed during Jatropha oil/DMF blended droplets micro-explosion: bubbles caused by DMF superheating and bubbles caused by fatty acid pyrolysis. Moreover, the diameter and the velocity of secondary droplets obey Lorentz distribution and Weibull distribution respectively. The micro-explosion of Jatropha oil droplets experienced bubbles nucleation, bubbles growth, bubbles coalescence, bubbles movement and fragmentation, which are mainly caused by the pyrolysis of fatty acids. However, the micro-explosion of Jatropha oil/DMF blended droplets presents two mechanisms according to the different DMF content. The low DMF concentration leads to formation of a layer of Jatropha oil film on the droplet surface first, and then goes through a similar process with Jatropha oil droplets. The high DMF concentration leads to formation of a layer of DMF film on the droplet surface first. Then go through the same process as high DMF concentration.

Double-spherically bent crystal scheme of stigmatic x ray monochromatic backlit imaging.

We propose an aberration-free monochromatic x ray backlit imaging scheme using a combination of convex and concave spherically bent crystals. This configuration works with a wide range of Bragg angles, satisfying the conditions for stigmatic imaging at a particular wavelength. However, the assembly accuracy of the crystals must meet the Bragg relation criteria for spatial resolution to increase the detection efficiency. Here, we develop a collimator prism with a cross reference line engraved on a plane mirror to adjust a matched pair of Bragg angles as well as the intervals between the two crystals and the object to be coupled with the detector. We explore the realization of monochromatic backlighting imaging with a concave Si-533 crystal and a convex α-Quartz-2023 crystal, obtaining a spatial resolution of approximately 7 µm and a field of view of at least 200 µm. To the best of our knowledge, this is the best spatial resolution of monochromatic images of a double-spherically bent crystal to date. Our experimental results are presented to demonstrate the feasibility of this imaging scheme with x rays.

Micro-explosion enhanced combustion of Jatropha oil/2, 5-dimethylfuran (DMF) blended fuel droplets

The present study investigated the micro-explosive combustion characteristics of Jatropha oil-2, 5-dimethylfuran (DMF) blended fuel droplets using fiber-support and high-speed backlit imaging technique. The flame temperature and soot optical thickness (KL) are measured by two-color pyrometry method. The results show that the combustion of Jatropha oil-DMF blended droplets containing four stages: ignition delay, first micro-explosive combustion, d2 law combustion and second micro-explosive combustion stage. The micro-explosion in second and fourth stage is caused by superheating of DMF and pyrolysis, respectively. The flames of DMF, diesel and Jatropha oil droplets belong to typical diffusion combustion. However, the micro-explosive flame of Jatropha oil-DMF blended droplets belongs to premixed combustion. When the DMF concentration reaches 50%vol, the ignition delay is equivalent to that of diesel. The average burning rate of Jatropha oil-DMF blended droplets increases nonlinearly with air velocity. When the DMF concentration is 75%vol, it reaches the peak, which is 66.4% higher than that of diesel. Furthermore, the effect of micro-explosion on droplet flame deformation is summarized into four modes: slight fluttering flame, local flame separation, large deformation flame, large deformation flame with local flame separation. When there is air flow in the horizontal direction, a “soot shell” forms around the droplets. This is due to the upward buoyancy interacts with the horizontal airflow force to form a rotating airflow force, which concentrates the soot particles around the droplets.

The effects of nozzle taper angle on in-nozzle flow and nozzle tip-wetting under flash boiling conditions

As an important structural design, the taper angle of an injector has been proved to have a significant influence on spray behavior under both subcooled and flash boiling conditions. However, present studies on taper angle mostly focus on the morphology of the far-field spray, the detailed in-nozzle process as well as the influencing mechanism have not been discussed. To illustrate how the taper angles influence the spray characteristics, backlit imaging method has been adopted on two-dimensional optical nozzles with different taper angle designs. Analyses have been made from in-nozzle bubble development to the unique tip-wetting phenomenon at the counterbore of the injector has been discussed in detail. A novel bubble index is proposed in this paper to describe the in-nozzle flow development. It is found that the design of the taper angle would have a significant impact on the in-nozzle pressure distribution, which would then result in different bubbling characteristics even under the same superheat index, and the contact mechanism between phase boundary and counterbore surface would change with different taper angle designs and tend to result in reverse tip-wetting tendencies under flash boiling conditions compared with subcooled conditions.

Modal analysis and interface tracking of multiphase flows using Dynamic Mode Decomposition

Non-invasive visualization techniques for multiphase flows are critical to understanding primary atomization and sprays. We use back-lit imaging to identify the liquid–gas interface of two-phase flows at high temporal and spatial resolutions and employ Dynamic Mode Decomposition (DMD) to study the shape and frequency of instabilities of a liquid jet surrounded by a coaxial annular airblast atomizer. However, DMD is not suitable for interface tracking, so we develop a data-driven two-step approach using the optical sensor data. The method uses DMD on the optical flow field estimated from image snapshot pairs. We demonstrate our method to a representative toy problem of an oscillating drop and on the primary atomization of a numerical planar liquid jet. Finally, we apply our method to the experimental liquid jet from the coaxial airblast atomizer using back-lit imaging. Our method is able to accurately reconstruct and predict the flow and preserves the sharpness of the fluid interface.

Experimental investigation on the effects of fuel–air mixture temperature on the air-assisted kerosene 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 fuel–air mixture temperature on the spray characteristics are insufficient, which affects the design of AAFI engines. This study experimentally investigates the air-assisted kerosene spray characteristics over a wide range of fuel–air mixture temperatures (253–353 K). A high-speed backlit imaging technique was employed to monitor the temporal and spatial evolution of the spray, and a phase Doppler particle analyzer was used to measure the diameter and velocity of droplets. The results indicate that at low temperatures, room temperature, and high temperatures, the spray adopts a slender cylindrical, fusiform, and conical shape, respectively. As the fuel–air mixture temperature increases from 253 K to 353 K, the spray penetration and width first increase and then decrease. Moreover, the diameter and velocity of the droplets follow log-normal distributions. In particular, as the fuel–air mixture temperature increases, the probability that a droplet’s diameter is within the range of 0–10 μm increases from 34.7 % (253 K) to 59.8 % (353 K), while the probability that a droplet’s velocity is within the range of 0–10 m/s decreases from 44.1% (253 K) to 38.1% (273 K), and then increases to 57.5 % (353 K). Accordingly, as the fuel–air mixture temperature rises, the Sauter mean diameter of the droplets decreases, while the mass-average velocity of the droplets first increases and then decreases. The proportion of droplets with Stokes numbers less than 1 is positively correlated with the fuel–air mixture temperature, thus implying that droplets at high temperatures tend to follow the airflow, whereas droplets at low temperatures tend to move along their initial trajectories.

Spatial characterization of the flapping instability of a laminar liquid jet fragmented by a swirled gas co-flow

A liquid jet destabilized by a co-flowing gas jet is subjected to several instabilities that are responsible for its fragmentation into a spray of fine droplets. At the large scale, the difference in the velocities of the gas and liquid phases lead to a meandering motion of the liquid jet called the flapping instability. Flapping is suspected to play a role in the spatio-temporal characteristics of the clouds of droplets further downstream, as it periodically brings a portion of the liquid phase away from the centerline and into the gas jet. While the scaling law determining the frequency associated to flapping has recently been proposed, a study of its spatial characteristics is lacking in the literature. Two-view high-speed back-lit imaging is used to obtain 3D time-resolved measurements of the liquid phase. The trajectories of the bulk of the liquid is computed using barycenter of the liquid presence. Such trajectories are phase averaged over many flapping periods to yield flapping orbits. In the regime of shear break-up atomization, a transition in the shape of the flapping orbits, from planar to circular, is observed along variations of the gas-to-liquid dynamic pressure ratio (also called momentum ratio) as well as the swirl ratio (ratio of the azimuthal to longitudinal components of the gas jet). This is characterized using the shape factor of the flapping orbits, as well as the flapping amplitude. The latter is found to decrease with dynamic pressure ratio but is larger when swirl is added to the gas jet. The transition appears to be related to the formation of bags along the liquid jet, that originate from interfacial instabilities and opposes the existence of a preferential flapping direction.

Measuring Shape Parameters of Pearls in Batches Using Machine Vision: A Case Study.

To solve the problem of low precision of pearl shape parameters’ measurement caused by the mutual contact of batches of pearls and the error of shape sorting, a method of contacting pearls’ segmentation based on the pit detection was proposed. Multiple pearl images were obtained by backlit imaging, the quality of the pearl images was improved through appropriate preprocessing, and the contacted pearl area was extracted by calculating the area ratio of the connected domains. Then, the contour feature of the contact area was obtained by edge tracking to establish the mathematical model of the angles between the edge contour points. By judging the angle with a threshold of 60° as the candidate concave point, a concave point matching algorithm was introduced to get the true concave point, and the Euclidean distance was adopted as a metric function to achieve the segmentation of the tangent pearls. The pearl shape parameters’ model was established through the pearl contour image information, and the shape classification standard was constructed according to the national standard. Experimental results showed that the proposed method produced a better segmentation performance than the popular watershed algorithm and morphological algorithm. The segmentation accuracy was above 95%, the average loss rate was within 4%, and the sorting accuracy based on the shape information was 94%.