High-speed Photography System Research Articles

Experimental Study on the Influence of Microwave Energy Pulse Width and Duty Cycle on Evaporation and Ignition Characteristics of ADN-Based Liquid Propellant Droplets

This study investigates the evaporation and ignition characteristics of a single droplet of ammonium dinitramide (ADN)-based liquid propellant utilizing a waveguide resonant cavity device, in conjunction with a high-speed photographic imaging system and testing system. Experimental methods are employed to analyze the impact of microwave pulse width and duty cycle on the puffing and meicro-explosion phenomena of the droplet, as well as the delay time and duration of ignition. The experimental findings reveal that increasing the duty cycle enhances the ignition success rate and diminishes flame development time. Specifically, elevating the microwave duty cycle from 60% to 80% reduces the ignition delay time of the droplet from 132.8 ms to 88.1 ms, and the ignition duration from 23.1 ms to 19.9 ms. Furthermore, an increase in microwave energy pulse width expedites the combustion process of the flame and influences plasma generation. Increasing the pulse width of microwave energy from 20 µs to 40 µs prolongs the ignition delay time from 140.3 ms to 200.5 ms and extends the ignition duration from 56.7 ms to 77.8 ms. Additionally, it is observed that a higher duty cycle leads to a more pronounced puffing phenomenon that initiates earlier. In contrast, a higher pulse width results in a more pronounced puffing phenomenon that commences later. This study provides a thorough investigation into the microwave ignition mechanism of ADN-based liquid propellants, offering theoretical insights into the ignition and combustion stability of such propellants in microwave-assisted ignition systems.

Water entry of hollow cylinders with fronts of different fillet radii: A visualization study

In the present study, we investigate the water entry of three hollow cylinders with fronts of different fillet radii of 0, 5, and 10 mm. Particular attention is given to the effect of fillet radius of front on cavity dynamics. To interpret the mechanism of cavity evolution, a high-speed photography system is employed to capture instantaneous impact phenomena and behaviors of the hollow cylinders. Unique characteristics of the internal jet and ring-rupture pinch-off of the cavity are described and discussed. The results show that both dynamic patterns and characteristic parameters of the cavities differ with the fillet radius of the front. For the flat-front hollow cylinder, a fully developed streamlined gas cavity is formed, and the expansion rate and size of the cavity are substantially greater than those associated with slanted fronts. The cavity trajectory remains nearly vertical, and the internal jet is the most extended. With increasing fillet radius of the front, underdeveloped cavities with relatively small size are produced and the collapse time of the cavity is shortened. Furthermore, the cavity trajectory is characterized by lateral deflections, and the maximum attainable height of the internal jet is reduced.

Experimental study on the cavity dynamics of a sphere entering flowing water

This paper delves into the dynamics of a sphere entering flowing water at varying impact velocities and flow speeds. Using a high-speed photography system and image processing, we track the cavity evolution and trajectory. Flowing water is observed to tilt the cavity and postpone its detachment from the free surface. Beyond surface sealing, we identify a flowing-induced pinch-off phenomenon during water entry, marking a transition in closure regimes. This transition establishes a threshold impacting cavity tilt angle and pull-away length. By mapping the phase diagram of flow Reynolds number (Rew) against impact Froude number (Fr), we classify partial surface seal, pinch-off, and surface seal into distinct regimes. The Fr1/3 law effectively predicts the rising trend of cavity depth (H) and pinch-off depth (Hp) in flowing water. However, the Hp/H ratio differs from that reported in existing literature.

Experimental study on the effect of unloading rate on gneiss rockburst

Rockburst are often encountered in tunnel construction due to the complex geological conditions. To study the influence of unloading rate on rockburst, gneiss rockburst experiments were conducted under three groups of unloading rates. A high-speed photography system and acoustic emission (AE) system were used to monitor the entire process of rockburst process in real-time. The results show that the intensity of gneiss rockburst decreases with decrease of unloading rate, which is manifested as the reduction of AE energy and fragments ejection velocity. The mechanisms are proposed to explain this effect: (i) The reduction of unloading rate changes the crack propagation mechanism in the process of rockburst. This makes the rockbursts change from the tensile failure mechanism at high unloading rate to the tension-shear mixed failure mechanism at low unloading rate, and more energy released in the form of shear crack propagation. Then, less strain energy is converted into kinetic energy of fragments ejection. (ii) Less plate cracking degree of gneiss has taken shape due to decrease of unloading rate, resulting in the destruction of rockburst incubation process. The enlightenments of reducing the unloading rate for the project are also described quantitatively. The rockburst magnitude is reduced from the medium magnitude at the unloading rate of 0.1 MPa/s to the slight magnitude at the unloading rate of 0.025 MPa/s, which was judged by the ejection velocity.

A compact flexible sub-nanosecond framing photographic system.

A novel high-speed multi-frame photographic system is presented in this paper. The system demonstrates exceptional compactness and flexibility, requiring only the introduction of a cavity comprising multiple beam-splitters in the optical path to enable multi-frame imaging of sub-nanosecond events. The number and temporal delay of frames can be easily adjusted by adjusting the distance and angle between beam-splitters. These capabilities are demonstrated by observing the laser ablation process, highlighting the great potential for application in capturing ultrafast time-evolving events such as optical breakdown, the evolution of laser-produced plasmas, and the propagation of shock waves.

Improved flow boiling characteristics in minichannel with open-cell porous ribs

Porous metal has been proven to exhibit significant heat transfer enhancement in minichannel. Focusing on fully utilizing the advantages of porous metal in heat transfer and reducing pressure penalty, a porous ribs minichannel (PRM) heat sink employing open-cell porous copper as rib was designed and fabricated. PRM heat sink has 19 minichannels that are 1 mm in width and 2 mm in depth. Commercial open-cell porous copper with a porosity of 0.85 and 120 PPI (Pore per Inch, PPI) was selected. Flow boiling experiments were carried out with ethanol as the working fluid. The flow patterns of two heat sinks were recorded with a high speed photograph system and comparative analyzed. High-frequency periodic motion of vapor core boundary in width direction of porous ribs minichannel was recorded. This is considered a fluctuation of density wave along the channel width direction. Cooperating with capillary force, movement of vapor core boundary induces mass transfer between adjacent channels, compensating for uneven flow distribution between channels. The results show that PRM heat sink has better heat transfer performances and flow stability with a slight increment in pressure drop compared with solid ribs minichannel (SRM) heat sink. The porous ribs minichannel heat sink provides a new way to relieve flow insatiability for heat exchangers and thermal management systems.

Parallel water entry: Experimental investigations of hydrophobic/hydrophilic spheres

This study aims to experimentally investigate the vertical parallel water entry of two identical spheres (in geometry and material) with different surface wettability (hydrophilic or hydrophobic) pairings. The spheres simultaneously impact the water surface with velocities ranging from 1.71 to 4.32 m s−1. The corresponding ranges of the impact Froude, Weber, and Reynolds numbers are 3.87–9.75, 816–5167, and 38.5×103 to 96.8×103, respectively. The spheres' lateral distances vary from 1.0 to 5.0 times the diameter. A high-speed photography system and image processing technique analyze the event dynamics, focusing on air-entrainment cavity behavior (shapes, closure, shedding), water flow features (Worthington jets, splashes), and sphere kinetics. Results for hydrophobic/hydrophobic cases show that even at the maximum lateral distance, a slightly asymmetric cavity forms, but deep-seal pinching occurs at a single point, similar to a single water entry scenario. As the lateral distance decreases, the spheres significantly influence each other's behavior, leading to the formation of a highly asymmetric air cavity and an oblique Worthington jet. In the case of a hydrophobic/hydrophilic pairing, vortices generated behind the hydrophilic sphere influence the air cavity development of the hydrophobic sphere. This can cause a secondary pinch-off, especially at low lateral distances. This effect becomes more pronounced at higher impact velocities. Additionally, at higher impact velocities and minimum lateral distance (direct contact between the spheres), a smaller cavity detaches from the hydrophobic sphere's cavity, attaches to the hydrophilic sphere, and moves with it. These different regimes result in varying descent velocities for the spheres.

Experimental study on the effect of water absorption level on rockburst occurrence of sandstone

To investigate the mechanism of rockburst prevention by spraying water onto the surrounding rocks, 15 experiments are performed considering different water absorption levels on a single face. High-speed photography and acoustic emission (AE) system are used to monitor the rockburst process. The effect of water on sandstone rockburst and the prevention mechanism of water on sandstone rockburst are analyzed from the perspective of energy and failure mode. The results show that the higher the absorption degree, the lower the intensity of the rockburst after absorbing water on single side of sandstone. This is reflected in the fact that with the increase in the water absorption level, the ejection velocity of rockburst fragments is smaller, the depth of the rockburst pit is shallower, and the AE energy is smaller. Under the water absorption level of 100%, the magnitude of rockburst intensity changes from medium to slight. The prevention mechanism of water on sandstone rockburst is that water reduces the capacity of sandstone to store strain energy and accelerates the expansion of shear cracks, which is not conducive to the occurrence of plate cracking before rockburst, and destroys the conditions for rockburst incubation.

Investigation of factors enhancing droplets spreading on leaves with burrs.

Spread effect is one of the aspects on deposition quality evaluation of pesticide droplets. It could be affected by many factors such as the microstructure of the target plant leaf surface, physical features of the droplets, and the concentration of spray additives. In this study, using a high-speed photography system, 2.3% glyphosate ammonium salt solution with different concentration of the additive was applied to investigate the impact process of single droplet deposition on the plant leaf surface with burrs. Effect of droplet sizes and velocities on spreading area and dynamic deposition procedure was analyzed using image processing programs. The diffusion factor in the process of droplet spreading was changed over time. The occurrence of bubbles in the droplets was observed in the results. With the bubble generation, the droplet diameter expands and a better diffusion effect is obtained. As a result, better spreading effect was obtained as the droplet diameter was expanded with the generation of bubbles. The significant effects of each physical property of droplets on droplet spreading and the interaction effects between the influencing factors were analyzed. A significant correlation was found between additive concentration, droplet impact velocity, droplet diameters and droplet spreading area. All interactions of concentration:velocity, concentration:diameter, velocity:diameter, and concentration:velocity:diameter had a significant effect on the spreading area of droplets. The study of the factors influencing the process of pesticide droplet impact on the leaf surface contributes to the efficient use of pesticides. Thus, the consumption of pesticides and the resulting impact on the environment can be reduced.

Experimental observation of the combustion modes based on a novel multistage pre-chamber turbulent jet ignition (TJI) system

Transient jet flame propagation under newly proposed multistage pre-chambers is studied in a constant-volume combustion chamber with a high-speed schlieren photography system. Various combustion behaviors, including the flame tip velocity, jet emergence timing, projected flame area, pressure, and heat release rate, are investigated under different pre-chamber structures. The present work will provide constructive insight into the design, manufacture, and application of turbulent jet ignition engines. It is shown that the pre-chamber structure determines the main chamber flame development by influencing the flame development inside the pre-chamber. As the flame is accelerated by an obstacle in the pre-chamber, faster exit velocity of hot jet and intense turbulence are observed in the main chamber. In addition, the overall development of the jet flame in the main chamber can be separated into two stages, the former of which is dominated by jet flows, while the latter stage is controlled by the chemical reaction under different excess air coefficients, presenting turbulent combustion characteristics. In this work, six ignition modes under ultra-lean conditions are observed, including (1) jet ignition occurrence on the entire jet surface due to the sufficiently high reactivity; (2) local ignition in the middle of the hot jet; (3) local multipoint ignition and ignition at the jet tip; (4) ignition induced by delayed burning at the jet root; (5) jet tip ignition with backward flame propagation; and (6) global extinction. For the effect of initial pressure, it is found that under stoichiometric conditions, the initial pressure has a minor influence on flame tip propagation, while it significantly influences pressure evolution and heat release rate. However, the increase in initial pressure can improve flame propagation and pressure evolution under lean conditions. Under near-extinction conditions, the ignition mode could be switched from unstable ignition to stable ignition. A numerical simulation is also conducted to reveal the flame development inside the pre-chamber under different pre-chamber structures.

Study on the Oscillating Phenomenon of Welding Pool with Different Penetration States in Short-Circuiting GMAW

The oscillating phenomenon of the weld pool has a direct physical correspondence with its penetration state, which has broad application prospects in the monitoring of weld penetration states. This article studied the characteristics of the oscillating phenomenon of the weld pool in short-circuiting gas metal arc welding (S-GMAW) with different penetration states based on a welding high-speed photography system. The results show that the form of the liquid bridge electric explosion impact on the weld pool is “surface impact” in S-GMAW. The oscillation frequency of the weld pool decreases as the size of the weld pool increases. Compared with the partially penetrated weld pool, the boundary conditions of the fully penetrated weld pool had different boundary conditions, resulting in higher oscillation amplitude and lower oscillation frequency. In S-GMAW, no difference in welding voltage signal, caused by the difference in amplitude of the weld pool oscillation with different penetration states, was observed.

The influence of flexible/rigid obstacle on flame propagation and blast injuries risk in gas explosion

ABSTRACT The interaction between flame, shock wave and obstacle is an important content in the study of fuel combustion and safety. In this paper, the performance difference between flexible and rigid obstacles in the process of explosion flame propagation was studied. Five barriers with different blocking rates（BR） were tested in the pipeline for methane premixed explosion. The high-speed photography system collects the flame image, the flame front velocity is calculated, and the dynamic pressure changes in the upstream and downstream pipelines during flame propagation are recorded by the high-frequency pressure acquisition system. The results show that the obstacle affects the unburned flow field and generates compression waves and shear layers. The explosion promotion degree of the two obstacles presents different trends, and the peak pressure decreases. At the same time, obstacles reflect pressure waves, causing instability in the front of the flame. With the increase of plugging rate, the difference of peak pressure rises to 30% at BR = 0.4. The flame velocity of flexible obstacles is also more flat, and the peak velocity at BR = 0.2 and BR = 0.4 is close to 43 m/s. During the study, it was found that the flexible material can effectively absorb the reflected wave, and the pressure oscillation is absorbed. The average amplitude of the rigid oscillation rises from 4 kPa to 10 kPa, and the flexible oscillation decreases to nearly zero. The difference of barrier materials makes the impact of shock wave on flame and primary explosive injury (PBI) lighter, which is more conducive to assessing the explosion risk of combustible gas.

Experimental studies on rock failure mechanisms under impact load by single polycrystalline diamond compact cutter

Percussive drilling shows excellent potential for promoting the rate of penetration (ROP) in drilling hard formations. Polycrystalline diamond compact (PDC) bits account for most of the footage drilled in the oil and gas fields. To reveal the rock failure mechanisms under the impact load by PDC bits, a series of drop tests with a single PDC cutter were conducted to four kinds of rocks at different back rake angles, drop heights, drop mass, and drop times. Then the morphology characteristics of the craters were obtained and quantified by using a three-dimensional profilometer. The fracture micrographs can be observed by using scanning electron microscope (SEM). The distribution and propagation process of subsurface cracks were captured in rock-like silica glass by a high-speed photography system. The results can indicate that percussive drilling has a higher efficiency and ROP when the rock fractures in brittle mode. The failure mode of rock is related with the type of rock, the impact speed, and the back rake angle of the cutter. Both the penetration depth and fragmentation volume get the maximum values at a back rake angle of about 45°. Increasing the weight and speed of falling hammer is beneficial to improving the rock breaking effects and efficiency. The subsurface cracks under the impact load by a single PDC cutter is shaped like a clamshell, and its size is much larger than the crater volume. These findings can help to shed light on the rock failure mechanisms under the impact of load by a single PDC cutter and provide a theoretical explanation for better field application of percussive drilling.

Experimental study on dynamic mechanical properties of mortar-sandstone composite under impact load

Mortar-rock composite structures are very common in underground engineering. Their mechanical behaviour and failure modes are much more complicated than those of pure mortar or rock structures. Understanding the energy and damage evolution of composite structures under impact loads is important for the stability of engineering structures. In this paper, dynamic compression tests of different grades of mortar-sandstone composite specimens were carried out by using the Hopkinson pressure bar system and a high-speed photography system to investigate the effects of impact velocities and mortar strength on the mechanical properties, energy dissipation and failure modes of the composite specimens. The results showed that the dynamic peak stress, energy consumption characteristics, impact toughness index and fragmentation blocks of the mortar-sandstone composite were obviously affected by strain rate. The mortar-sandstone composites had a progressive failure mechanism, and the strengths of the end and the junction of the mortar specimens were less than those of other regions. Cracks formed in the composite specimens mainly from both ends of the mortar, and the specimens were dominated by splitting failure. Increasing the mortar strength grade improved the dynamic compressive strength and the elastic energy storage of the samples, resulting in larger bearing capacity and less damage. With increasing impact velocity, the degree of fragmentation of the composite specimens increased, the average fragmentation blocks decreased, and the mortar specimens were damaged more severely than the sandstone. The mortar specimens finally broke into powder debris, while the sandstone finally broke into fragments.

Effect of insulation material on the firing performance of TaN@(Al/Ni)

In order to improve the ignition ability of reactive film, in this paper, the effects of copper oxide and silicon nitride on the flame height of the TaN@(Al/Ni) energy exchanger were investigated by using a high-speed photographic system. The insulating layer material was applied to the preparation TaN@(Al/Ni) by the MEMS process. The experimental results showed that the flame height of (Al/Ni)–TaN energy exchanger was obviously higher than that of silicon nitride when copper oxide was used as the insulation layer, which indicates that the oxygen-containing insulation layer could increase the flame height of the energy exchanger.

The Mechanism of Plugging Open-Pit Mine Cannon Holes and the Modification of Plugging Materials

Step blasting is an important part of open-pit mining, which is accompanied by hazards such as large blasting blocks, flying stone splashing, blasting noises, and blasting dust during the blasting process. In order to reduce the harm caused by blasting, this paper uses impact dynamics and rock dynamics to explain the deformation damage and motion law caused by detonation of the material blocked by the gun hole. By simulating the motion of the blocked material in the gun hole, the motion and failure characteristics of the blocked material in the gun hole are revealed. In this paper, geological polymer is introduced into the field of open-pit mine blasting, and 700 g rock powder, 200 g slag, 40 g NaOH solution (30%), and 140 g water glass with a modulus of 3.2 and 80 g of water are selected to prepare geological polymer-modified plugging materials to change rock powder blockage from bulk to solid, and improve the plugging performance. Finally, a field test was carried out in the open-pit mine explosion area, and a comparative test was carried out through the high-speed photography system; it is demonstrated that the modified blocking material could improve the blockage ability of the gun hole, reduce the large block rate of the upper part of the step, reduce the amount of dust, reduce the amount of flying stone, and improve the production efficiency and safety.

Dynamic response characteristics and damage rule of graphite ore rock under different strain rates

In the process of mining graphite mine, rock mass is often subjected to dynamic loads such as blasting or mechanical crushing, which involves dynamic responses of different strain rates, and blasting and crushing effect are affected by the rock dynamic properties and damage specials. The dynamic response characteristics and damage rule of graphite ore rock under different strain rates are very important but rarely studied in the past. To study these issues and provide support for graphite ore rock mining, the dynamic compression tests of graphite ore rock under five kinds of impact pressures were designed and carried out by using the Split Hopkinson Pressure Bar (SHPB) test system, combining with the high-speed photography system and crushing screening tests. The dynamic characteristics, crushing process, crushing mode, crushing form and fragmentation distribution of the graphite ore rock under different strain rates were analyzed. The results show that the dynamic characteristics of the graphite ore rock have obvious strain rate effect. The hardening coefficient (DIF) is positively correlated with the cubic root of strain rate, and the softening factor (K) is negatively correlated with the cubic root of strain rate. Shear failure mainly occurs in the graphite ore rock under impact load, and the crushing process can be divided into five stages, they are compaction, crack initiation, crack expansion and penetration, fragmentation collision and fragmentation fall. In addition, the crushed blocks are mainly triangular pyramid (or cone-like) fine granular and powder. The broken fragments of the graphite ore rock are in accord with the fractal geometry characteristics. That is, the average broken particle size (dS) decreases linearly with the increase of strain rate, and the fractal dimension (Da) increases weakly with the increase of strain rate. Based on D-P fracture criterion and Weibull distribution model, the dynamic damage constitutive model of the graphite ore rock was established, and the correlation between strain rate and Weibull distribution parameters (m and F0) was used to reasonably modify the damage constitutive model. The modified damage constitutive model curve is in good agreement with the experimental curve, which can basically reflect the strain rate effect of the dynamic characteristics of the graphite ore rock and the evolution characteristics of the dynamic stress–strain curve at different stage.

AP-HTPB propellant combustion under strain conditions with laser absorption spectroscopy.

Understanding the combustion behaviors of solid propellant with different levels of strains is of practical interest. In this work, an experimental study of the effects of static and dynamic strains on the burning rate, temperature, CO, and C O 2 formation of aluminized ammonium perchlorate (AP)-hydroxyl terminated poly-butadiene (HTPB) propellant combustion was presented at initial pressures of 0.1MPa, 0.2MPa, and 0.5MPa. The strains were being applied onto solid propellant by exerting static and cyclic loadings. The propellant burning rate was acquired by a 4kHz high-speed photography system, and the combustion temperature, CO, and C O 2 column densities were measured at 10kHz through laser absorption spectroscopy (LAS). At atmospheric pressure, it was demonstrated that the propellant burning rate increased with tensile stress and decreased with compressive stress. The measured flame temperature showed a similar correlation with strains as compared to the propellant burning rate. At elevated pressures, the increase of the propellant burning rate due to tensile stress was more evident, while the difference in combustion temperatures was less significant. For the cyclic strain condition, the variations of the measured C O 2 and CO column densities were consistent with the static strain condition.

Suppression characteristics of multi-layer metal wire mesh on premixed methane-air flame propagation

Metal wire mesh is widely used in the energy industry for its excellent protective properties as a fire stopping and explosion isolating material. In this study, the suppression characteristics of different layers of metal mesh on the dynamic behavior of premixed methane-air flame propagation were studied experimentally. A high-speed photographic schlieren system was used to photograph the explosion process to capture the changes in the microstructure of the flame, and high-frequency pressure sensors and micro-thermocouple measurements were used to capture the flame explosion pressure and temperature. The experimental results show that the suppression effectiveness of wire mesh is a reflection of the coupling of explosive flame propagation behavior and combustion state in the pipe. Increasing the number of mesh layers and mesh density can destroy the microstructure of the premixed methane-air flame front and hinder the progress of flame propagation. Increasing the number of wire mesh layers will delay the peak time of premixed flame propagation speed and reduce the peak speed values of flame propagation. Wire mesh has a pronounced attenuation effect on premixed flame temperature and explosion overpressure. The maximum flame temperature attenuation rate is 34.99%–60.95%, and the maximum explosion overpressure attenuation rate is 33.70%–74.02%. And the suppression effect is greatly enhanced as the increase of mesh layers.

Experimental study on effect of freeze-thaw cycles on dynamic mode-Ι fracture properties and microscopic damage evolution of sandstone

The investigation of dynamic fracture characteristics of sandstone after freeze-thaw (F-T) treatments is of great significance for the optimization of blasting parameters and stability analysis of rock masses engineering in cold or high-elevation regions. In the present study, the P-wave velocity, porosity, T2 spectrum curve, and micromorphology of sandstone specimen after different F-T cycles were measured using nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) techniques. The F-T damage factor was defined according to the reduction in wave impedance. Moreover, the dynamic mode-Ι fracture test was carried out using notched semicircular bend (NSCB) sandstone specimen. Meanwhile, the fracture process of NSCB specimen was captured by adopting a high-speed photography system. Test results show that F-T cycles can irreversibly deteriorate the microscopic pore structure and the cementation between mineral particles, which is manifested by an increase in the proportion of macropores and mesopores. The dynamic fracture pattern of sandstone specimen is affected by the coupling of F-T cycle number and loading pressure. Furthermore, the dynamic fracture toughness of sandstone constantly increases with the increase of loading rate at a specified F-T cycle number, while the F-T cycle treatment has a significant weakening effect. There has a negative exponential correlation between dynamic fracture toughness and F-T damage factor.