Backlight Imaging Research Articles

Point Spread Function And Spatial Resolution Analysis Of Multi-Focus Plenoptic Cameras

With the recent development of plenoptic cameras, light field imaging has become a reliable solution for 3D metrology. Plenoptic cameras allow recovering the 3D information of a scene with a single acquisition and only one camera. These advantages make light field imaging particularly interesting for the metrology of two-phase flows. The measurement depth of a back-light imaging setup can be determined by the calibration of the Point Spread function (PSF) of the optical system. This study provides a method adapted to multi-focus plenoptic cameras to measure the PSF width from reconstructed images. We used a calibration target composed of multiple calibrated opaque disks and a diffuse backlight illumination set-up. The PSF width is estimated by measuring the grayscale gradient at the interface of the images of the disks. Coupled with the depth information provided by the plenoptic camera, we obtained the evolution of the PSF at different depth planes. The information on the PSF enables us to obtain the focus criterion, which sets the limits of the measurement volume. The spatial resolution of the reconstructed images is obtained from the PSF. The results obtained for the resolution as a function of virtual depth are consistent with results from previous studies.

Experimental study on generation and growth of bubbles in external flash-boiling spray regime

High-performance liquid propulsion systems, such as hypersonic scramjets and liquid rockets, utilize regenerative cooling by employing fuel as a coolant to shield components from thermal damage. The flash-boiling spray, resulting from superheated liquid injection and prevalent in regenerative cooling, has received considerable research interest for enhancing atomization and combustion efficiency. In this study, superheated water jets at various temperatures between 30 and 90 ℃ were injected into a vacuum chamber at different pressures above 5 kPa, and their spray patterns were acquired through diffusive backlight imaging. Images were analyzed to determine key parameters, such as bubble nucleation flux, bubble growth rate, and spray angle, in an external flash-boiling spray regime. The estimated nucleation flux agreed well with the predictions the model based on nucleation theory in the region of sufficiently high superheat. The dynamics of bubble growth was investigated and explained using a bubble growth model. The jet-expansion energy was computed by measuring the spray angle and compared with the available energy. By mapping the evolution of flash-boiling spray in relation to relevant nondimensional numbers, the study underscores the necessity of accounting for the pressure inside the injector to adequately model bubble generation inside the injector. Furthermore, this study substantiates the feasibility of visually recognizing and analyzing individual bubbles in the external flash-boiling regime, thereby enabling the generation of experimental data crucial for the validation of bubble nucleation and growth models.

Catalytically promoted green fuel with hydrogen peroxide: Effect of hypergolic combustion on atomization and flow characteristics using impinging jets

This study explores the jet impingement of a catalytically promoted hypergolic green fuel with High Test Peroxide (HTP). Experimental investigations were conducted using simultaneous high-speed visible, infrared and backlight imaging. Two major aspects were investigated. First, a study exploring the potential of decoupling the combustion phenomenon through the utilization of the fuel without a catalyst as a simulant, enabling a comparative analysis of the atomization process with the authentic hypergolic pair undergoing combustion. A Reynolds versus Weber numbers diagram was obtained for jets with equal momentum in the steady-state flow regime, and a novel breakup mode was observed. The named Reactive Foamy Segregation mode was found as a two phenomenological regime, where a reacting foam exists together with a segregation stream. An analysis of the liquid film velocity formed by impinging jets indicated that the hypergolic pair exhibited significantly lower speeds and corroborates with the introduced breakup mode. Second, an analysis about the transient and steady-state jet flow effects, revealing the existence of separate planes for the reacting foam and plumes, regions with different oxidizer to fuel ratios. This natural effect was intensified by the force generated from the exothermic reaction of hydrogen peroxide decomposition. Notably, the central region of the sheet exhibited the lowest temperature, indicating that the liquid-phase mixture and propellants’ residence time were insufficiently effective in decomposing the peroxide. These findings contribute to a deeper understanding of the complex fluid dynamics involved in catalytically promoted hypergolic reactions applied in liquid rocket engines.

A MAXIMUM ENTROPY PRINCIPLE MODEL FOR THE INITIALIZATION OF EULERIAN–LAGRANGIAN SPRAY SIMULATIONS

NOx emission regulations have become more and more restrictive for internal combustion engine-powered vehicles, especially for road transport applications. To minimize emissions and comply with regulations, selective catalytic reduction (SCR) systems are the most efficient deNOx technology thanks to the injection of a urea-water solution (UWS). State-of-the-art computational fluid dynamics (CFD) techniques employ Eulerian-Lagrangian frameworks to deal with the two phases of such problems. Still, the associated low velocities of UWS applications make it difficult to use standard breakup models (Kelvin-Helmholtz, Rayleigh-Taylor, Taylor analogy breakup) to generate initial drop size distributions. Hence, these specific studies end up needing experimentally characterized drop size distributions to initialize the CFD simulations or using expensive Eulerian-Eulerian simulations to obtain the outcomes of the primary breakup of the liquid jet. The maximum entropy principle (MEP) allows generating a droplet size-velocity probability distribution function (PDF) from initial injection conditions and injector characteristics while satisfying conservation equations. The most probable PDF curve is determined by the distribution that maximizes the entropy of the problem. A critical Weber number has been proposed to select which droplets will break up subsequently after the initial droplet break up. The model has been validated against experimental results obtained by high-resolution laser backlight imaging. Comparable results have been found and realistic tendencies were achieved, decreasing the expected droplet size with increasing injection pressures. The proposed model could help with introducing alternative breakup models for low-velocity applications without the need for prior droplet size knowledge.

Energy exchange during the early phase of droplet impact onto a dry surface

This study investigates droplet impact dynamics on a rigid substrate using droplets of water and water–glycerol mixtures. Experiments are conducted in the Weber number range of 179–929, and the corresponding Reynolds numbers are in the range of 5253–12 090. The profile of the droplet interface is studied. Assuming self-similarity in the internal flow at the interface of the droplet, the profiles are scaled using appropriate scales. It is observed that the profiles of the droplet interface (at different points of time) collapse close to each other when plotted in this manner. This observation serves as an experimental evidence to the self-similarity of the internal flow in this phase. Based on this fact, we make an estimate of kinetic energy at early stages of droplet impact. The surface energy is also estimated by assuming axisymmetry in the droplet profile obtained through back-light imaging. We observe that a considerable amount of the kinetic energy is lost during the early stages of droplet impact. We believe that an accurate estimation of this deficit in kinetic energy at an early stage of impact will be necessary to account in the energy based models for droplet impact.

Development of a monochromatic crystal backlight imager for the recent double-cone ignition experiments

We developed a monochromatic crystal backlight imaging system for the double-cone ignition (DCI) scheme, employing a spherically bent quartz crystal. This system was used to measure the spatial distribution and temporal evolution of the head-on colliding plasma from the two compressing cones in the DCI experiments. The influence of laser parameters on the x-ray backlighter intensity and spatial resolution of the imaging system was investigated. The imaging system had a spatial resolution of 10 μm when employing a CCD detector. Experiments demonstrated that the system can obtain time-resolved radiographic images with high quality, enabling the precise measurement of the shape, size, and density distribution of the plasma.

Machine Vision Method for Quantitative Statistics Analysis of Industrial Product Images

To address the problems of unstable accuracy, low efficiency, and subjective influence of manual counting, a machine vision-based method to count the quantity of tobacco shreds is proposed for the first time. In this paper, the complex tobacco shred image is obtained by backlight imaging. The adaptive threshold segmentation method is used to segment tobacco shreds. The pixel area of the tobacco shred area is calculated by connected domain labelling. Second, independent tobacco shreds and adhesive tobacco shreds were identified based on the pixel area, and the quantity of segmented tobacco shreds was counted for the first time. Subsequently, in complex scenarios (such as tobacco shreds adhesive and overlapping), an image is usually obtained by manually drawing the contours of the adhesive and overlapping tobacco shreds on the basis of primary statistics. Finally, different individuals are distinguished, segmentation is completed, and tobacco shred quantity statistics are realised. The experimental results show that the average accuracy is 100.0 % for quantitative statistics of independent tobacco shred images. For tobacco shred images with adhesive and overlapping interference, the minimum accuracy is 90 %, and the accuracy increases with the increase in tobacco shred quantity. Furthermore, the efficiency of the tobacco shred quantity statistics conducted by the method in this paper was only affected by complex scenarios. Compared to artificial processing, the efficiency was increased by more than 100 %. The work in this paper can provide the technical basis for measuring the dimensions of tobacco shreds.

Size measurement and filled/unfilled detection of rice grains using backlight image processing.

Measurements of rice physical traits, such as length, width, and percentage of filled/unfilled grains, are essential steps of rice breeding. A new approach for measuring the physical traits of rice grains for breeding purposes was presented in this study, utilizing image processing techniques. Backlight photography was used to capture a grayscale image of a group of rice grains, which was then analyzed using a clustering algorithm to differentiate between filled and unfilled grains based on their grayscale values. The impact of backlight intensity on the accuracy of the method was also investigated. The results show that the proposed method has excellent accuracy and high efficiency. The mean absolute percentage error of the method was 0.24% and 1.36% in calculating the total number of grain particles and distinguishing the number of filled grains, respectively. The grain size was also measured with a little margin of error. The mean absolute percentage error of grain length measurement was 1.11%, while the measurement error of grain width was 4.03%. The method was found to be highly accurate, non-destructive, and cost-effective when compared to conventional methods, making it a promising approach for characterizing physical traits for crop breeding.

Controlled spherical deuterium droplets as Lagrangian tracers for cryogenic turbulence experiments.

The study of the smallest scales of turbulence by (Lagrangian) particle tracking faces two major challenges: the requirement of a 2D or 3D optical imaging system with sufficiently high spatial and temporal resolution and the need for particles that behave as passive tracers when seeded into the flow. While recent advances in the past decade have led to the development of fast cameras, there is still a lack of suitable methods to seed cryogenic liquid helium flows with mono-disperse particles of sufficiently small size, of the order of a few micrometers, and a density close enough to that of helium. Taking advantage of the surface tension, we propose two different techniques to generate controlled liquid spherical droplets of deuterium over a liquid helium bath. The first technique operates in a continuous mode by fragmenting a liquid jet, thanks to the Rayleigh-Taylor instability. This results in the formation of droplets with a diameter distribution of 2 ± 0.25DN, where DN is the diameter of the jet nozzle (DN = 20μm in the present experiment). This method offers a high production rate, greater than 30kHz. The second technique operates in a drop-on-demand mode by detaching droplets from the nozzle using pressure pulses generated using a piezoelectric transducer. This approach yields a much narrower diameter distribution of 2.1 ± 0.05DN but at a smaller production rate, in the range 500 Hz-2kHz. The initial trajectories and shapes of the droplets, from the moment they are released from the nozzle until they fall 3mm below, are investigated and discussed based on back-light illumination images.

Experimental study on bubble dynamics and heat transfer of pool boiling at sub-atmospheric pressures

Pool boiling, as an efficient phase-change heat transfer method, has attracted extensive attention in recent years. However, the bubble dynamics behavior (BDB) and heat transfer characteristics of pool boiling at sub-atmospheric pressure (sub-ATM) are significantly different from those at or above standard atmospheric pressure (std-ATM). This paper presents experimental results of BDB and heat transfer characteristics of pool boiling at sub-ATM in the range of 30–120 kPa. High-speed backlight imaging technique was adopted. The results show that with the decrease of boiling pressure, the diameter and near circularity coefficient of bubble departure increase, and the frequency and aspect ratio decrease. With the decline in pressure, the critical heat flux (CHF) of pool boiling on the surface of the electric heater decreases. Specifically, the CHF at 120 kPa is 23.2% higher than that at 40 kPa. At sub-ATM, the boiling heat transfer coefficient (HTC) grows with the increase of liquid height. However, the HTC hardly changes with liquid height at std-ATM and higher.

Backlight and Spotlight Image Enhancement Based on Von Kries Model

Backlight and Spotlight Image Enhancement Based on Von Kries Model

Breakup mode of continuous-type liquid-liquid pintle injector

A pintle injector is necessary for the reuse of a launch vehicle. Significant studies have been conducted on the breakup process in impinging or swirl type injectors. However, the breakup process in liquid-liquid pintle injectors from injection to the atomization of liquid films has rarely been investigated. Therefore, in this study, the breakup mode and process quantified characteristics of the pintle injector were investigated. The backlight imaging technique was applied to capture the spontaneous phenomenon, and a longitudinal section view of the spray on the pintle injector was obtained using the 2P-SLIPI technique. The breakup modes were classified into three categories based on the spray shape (hollow cone, fishbone, and branch). The atomization process was investigated with high-speed and SLIPI images in chronologicalorder. Schematics of the process were also prepared to understand the breakup mechanism. The dynamic characteristics, such as breakup frequency and peak amplitude, and static characteristic (breakup length) were investigated using the flow Weber number. Furthermore, the dynamic characteristics are dependent on the radial flow, and the static characteristics are influenced by the annular jet flow. This study will serve as a reference for the breakup mechanism and characteristics of a liquid-liquid pintle injector design.

Detachable image decomposition and illumination mapping search for low-light image enhancement

Low-light image enhancement is a fundamental low-level computer vision task that has a wide range of applications, such as night monitoring, all-weather automatic driving, back-light imaging, and so on. In this paper, we propose a novel four-stage detachable framework for low-light image enhancement, where each module can be trained and fine-tuned separately. More importantly, the proposed framework can effectively be adapted to unseen datasets. In the first stage, we propose an image decomposition network that can extract a pair of a reflectance map and an illumination map given an image with arbitrary lighting conditions. The proposed loss function ensures that images of the same scene with different lighting conditions share the same reflectance map. Instead of using gamma correction, we establish an automatic searching scheme in the second stage to find an explicit mapping relationship between illumination maps in different lighting conditions. In the third stage, we use an efficient and effective unsupervised training method to find the best parameter set for the mapping. Finally, a normal-light image is obtained by composing together its reflectance map and transformed illumination map. A series of experiments are conducted on several popular datasets, and the results demonstrate the superiority of our proposed method on unseen datasets. Our framework surpasses other state-of-the-art methods and is applicable to enhance images in a wide range of lighting conditions.

Assessment of injector-flow characteristics of additised and renewable diesel blends through high-speed imaging

The influence of additives inducing viscoelasticity in diesel fuel, on the in-nozzle cavitation evolution and the expelled spray morphology has been quantified by high-speed, diffused back-light and schlieren imaging applied to two single-hole true-scale transparent injectors of straight and tapered orifice layouts (so-called Spray C and D of the engine Combustion Network), as well as a five-hole configuration (Spray M). More specifically, the in-nozzle cavitating flow and its effect on near-nozzle spray formation of a non-Newtonian diesel fuel sample treated with Quaternary Ammonium Salt (QAS) additives and exhibiting viscoelastic effects, as well as biodiesel (FAME), are compared against conventional diesel fuel for the first time. The operating conditions corresponded to injection and ambient pressures in the range of 500–900 bar and 1–20 bar, respectively. It was found that viscoelasticity has an overall suppressing effect on wall-attached, or so-called geometrical, cavitation. Furthermore, the investigation revealed that the action of viscoelastic additives has the capability to enhance the magnitude of well-established longitudinal vortices, with the subsequent after-effect of leading to increased cone angles of the expelled spray. On the contrary, it tends to suppress turbulence-induced transient instabilities in a manner similar to turbulence suppression.

Micro-explosive combustion and flame spread of Jatropha oil/2, 5-dimethylfuran (DMF) blended fuel droplet array

The micro-explosive combustion and flame spread characteristics of a multi-component fuel droplet array are investigated using high-speed backlight imaging technique. The multi-component blends consist of 2, 5-dimethylfuran (DMF) as a lower volatile component, while Jatropha oil is used as the higher volatile constituents. Three droplets (droplet array) with the same diameter are horizontally arranged at equal intervals using fiber support method. The effects of normalized droplet spacing (S/d0) and DMF concentration on multiple droplets micro-explosive combustion and flame spread are also analyzed. The results show that the interaction between multiple droplets combustion can not be ignored under small S/d0. It increases with the increase of DMF concentration and the decrease of S/d0. When the S/d0 is constant, the interaction between droplets depends on the duration of simultaneous combustion of multiple droplets and the number of droplets surrounded by the same flame. Due to the micro-explosion, the average flame spread velocity of Jatropha oil-DMF blended fuel droplet array decreases monotonously with S/d0 in normal gravity, which is different from that in microgravity. Four flame spread modes of droplet array containing micro-explosion are found: full-envelop flame, sequential full-envelop flame, semi-envelop flame to independent flame and independent sequential flame.

Orifice Flow Dynamics in a Rocket Injector as an Excitation Source of Injector-Driven Combustion Instabilities

To investigate a hypothesis of the orifice flow-induced instability in rocket engine thrust chambers, a single liquid oxygen (LOX) injector with an optically accessible orifice module was used for experiments, with water as a simulant for LOX. The unsteady pressure downstream of the orifice was measured using high-speed piezoelectric sensors under cavitating and non-cavitating intra-injector flow conditions. The cavitating orifice flows were directly visualized via backlight imaging with a high-speed camera through the optically accessible orifice module. Cavitation initiated at the cavitation number of 2.05, and the downstream bubble cloud formation started below 1.91. The unsteady pressure spectrum arising from cavitation comprises multiple peaks over a broad frequency range, which can cause low- and high-frequency instabilities. The dominant frequencies from cavitation decrease with increasing pressure drop, while the frequencies during non-cavitating flow increase. The non-cavitating orifice flow excites the second longitudinal acoustic mode of the injector tube. The acoustic mode excited by the non-cavitating flow becomes stronger when the pressure peak in the range of whistling phenomenon is close to the first longitudinal acoustic mode. In conclusion, the excitation mechanisms of the orifice-induced instability for the cavitating and non-cavitating flows were well identified, despite the limitations of water as a simulant for LOX.

Evaporation and Autoignition Characteristics of JP-10 Droplets with Hyperbranched Polyester as Additive

It was found in our previous work that hyperbranched polyester (HPE) can generate radicals and accelerate the chemical reactions of hydrocarbon fuels used as initiators. In this work, the evaporation and autoignition characteristics of JP-10 droplets with or without HPE were investigated using the high-speed backlight imaging technique in detail. The results indicate that the puffing and micro-explosion phenomena of HPE-blended JP-10 droplets can accelerate fuel evaporation and autoignition. When a 0.1% mass concentration of HPE was used, the droplet lifetime was reduced by 16.5% in evaporation at 850 K and 18.0% in autoignition at 900 K. A mechanism of HPE that promotes puffing and micro-explosions was proposed by analyzing droplet images of combustion and SEM images of combustion residues. Overall, this study provides a method for improving the evaporation and autoignition performance of JP-10.

Experiments on Slug Flow Dynamics in Helically Coiled Tubes

Slug flow characteristics in helically coiled tubes were investigated in experiments. Pressure loss, slug velocity, and slug passing frequency were measured using pressure transducers and backlight imaging of dyed water under illumination. For a 20-mm-diameter tube, parametric dependencies on gas and liquid volume flow rates for total superficial velocities of up to 6 m/s with three different radii of curvature (R = 0.270 m, R = 0.375 m, and R = infinity/straight tube) were explored. The main experimental results obtained are (1) the bubbly flow regime shrinks because of centrifugal acceleration from the coiled geometry, (2) the liquid slug length remains unchanged regardless of changes in gas and liquid flow rates, and (3) the pipe friction factor decreases with slug passing frequency.

ECML driven geographical location of utility poles in smart grid: Data analysis and high-definition recognition

The construction of smart grid is inseparable from the appropriate deployment of utility poles. China has many poles; if they are fully utilized, can they bring more power to the smart grid? Fortunately, thanks to advances in technologies such as artificial intelligence and the Internet of Things, can we use Google Earth to recognize utility poles and optimize the deployment of power facilities? Fundamentally, Google Earth generates static images. Faced with many static images, we need further image processing for high-definition recognition of utility poles. In addition to the current chaotic distribution of poles, we also need to optimize the deployment of power facilities. However, due to the resolution of Google Earth and the aerial photography angle, many backlight phenomena are not conducive to the recognition of utility poles. Therefore, this paper proposes a backlight image enhancement algorithm based on convolutional neural networks (CNN) and constructs a novel network architecture that integrates decomposition, restoration, and adjustment to recognize poles in high-definition under backlight. Furthermore, to solve the problems of the overflow of CNN parameters and unclear training effect, particle swarm optimization (PSO), the evolutionary computing-based machine learning (ECML) is used to search CNN parameters automatically and seek the optimal solution to achieve the optimization of the overall model. The experiments prove that the CNN-based image enhancement algorithm effectively recognizes the utility poles under different illumination and backlight. At the same time, the experimental results show that the PSO-based optimization method can optimize CNN parameters obviously, and the classification accuracy is increased.

Two-phase flow pattern identification in horizontal gas–liquid swirling pipe flow by machine learning method

Gas–liquid two-phase swirling flow has been widely used in nuclear industry. Its flow pattern is fundamental to investigate the two-phase flow. Although flow patterns of non-swirling flow in a horizontal pipe have been investigated for a long history, flow patterns of swirling flow in the pipe are rarely reported. In this paper, gas–liquid two-phase flow patterns of swirling flow generated by a vane-type swirler inside a horizontal pipe were investigated by a visualization experiment. Five swirling flow patterns were observed and recorded by the backlight imaging method. Then image processing method was used to obtain void fraction, and the statistical analysis (CDF and PDF) of void fraction for each swirling flow patterns was performed. Owing to the distinguished and stable feature of PDF signals, four parameters describing the characteristics of PDF signals have been proposed as the indicator of machine learning method. Finally, five algorithms of machine learning method have been used to identify the swirling flow patterns, and RUSBoost tree algorithm performs best with an accuracy of 97.4%.