High-speed Observation System Research Articles

A new high-speed observation system for evaluating the spark location in WEDM of Inconel 718

Abstract Wire electrical discharge machining (WEDM) is a good machining method for nickel-base superalloy, which is a key material in aero-engine, rocket engine and nuclear reactor. Experimental data indicate that the spark location along the thickness direction is usually uneven in WEDM. This phenomenon will be harmful to the usability and reliability of the parts machined by WEDM, especially in aerospace. However, the spark location is very difficult to observe directly with high precision due to the narrow cutting kerf. In view of this situation, a new high-speed observation system is proposed to evaluate the spark location during the continuous discharge process for the first time. The discharge spark, in turn, passes through heat-resistant glass, optical filter, focusing lens and digital high-speed videocamera. A series of image processing methods and centroid methods are utilized to acquire the spark location in this system. It can be observed that the number of discharge sparks of all regions in decreasing order is: middle region, lower region and upper region. The maximal standard deviation (St) of spark location can reach up to 0.137. Thereafter, a group of discharge experiments are carried out to analyze the relations between the process parameters and spark location evenness. An optimal combination of process parameters for even spark location is obtained, which can produce a standard deviation (St) of spark locations as low as 0.039. Eventually, the relation between spark location and machining characteristics is also illustrated. Ra can be decreased by 32.6% without reducing machining efficiency based on the process parameter optimization for even spark location.

High-speed time measurement system based on binary optical representation

Photo-mechanics is an important experiment means to study the structural mechanics behaviour. At present, joint high-speed observation system has become an important method with multiple high-speed cameras, but unknown startup delays among cameras will decrease the system's accuracy. To address the problem caused by the asynchronization of multi-camera, a high-speed time measurement system is proposed to locate camera capturing time. Binary optical representation of time is adopted to design and implement the system. In order to reduce error in the system operating, three theorems are presented and proved to illustrate the essence of binary code. Based on that, an optimized algorithm is developed to transform optical representation to time information, and then the errors are evaluated mathematically. The experimental results show that the proposed system successfully locates time at microsecond level with a shutter time of 9.615 us and the optimized approach reduces the maximum error and standard deviation of error by 722 and 52 times respectively. The systematic method proposed in this paper contributes to practical applications efficiently and accurately.

Experiment and Simulation Study on the Breakdown Current, Velocity, and E-Field in Air Gap Discharge

The breakdown current, current impulse velocity, and E-field in air gap discharge are of interest among researchers on high-voltage engineering and lightning study. It is the breakdown current and current impulse velocity that govern the radiated E-field property. The current impulse velocity, however, is not well addressed in the literature. In this paper, we use a self-developed high-speed observation system, a Rogowski coil, and a transient E-field sensor to measure the current impulse propagation velocity, breakdown current, and vertical E-field in the rod-to-plane discharge after the air gap breakdown under dc voltage. From the time difference of arrival of light signals in different receiving channels, the resultant current impulse propagation velocity is measured as 6.81 ± 0.57 × 106 m/s. Meanwhile, the breakdown current at the bottom of the grounded rod electrode has a peak value of approximately 1.3 kA and 10%–90% rise time of 40 ns, and the transient vertical E-field at ground level has an amplitude of approximately 15 kV/m and 10%–90% rise time of 64 ns. In addition, according to the measured breakdown current and resultant current impulse velocity, the vertical E-field is computed by using the finite-difference time-domain method in the two-dimensional cylindrical coordinate system. The computed vertical E-field agrees well with the measured E-field, validating our measured current impulse velocity. Besides, the computed horizontal E-field and azimuthal H-field are also analyzed. Finally, the influence of current impulse velocity on the vertical and horizontal E-field and azimuthal H-field is discussed.

Current Impulse Velocity After the Air Gap Breakdown at Atmospheric Pressure

The characteristics of the air gap discharge are of interest among researchers on high voltage and lightning study. This letter uses a self-developed high-speed observation system to measure the propagation velocity of the current impulse after the air gap breakdown in the rod-to-plane discharge with dc voltage. In addition, the discharge channel is captured by a still camera. From the time difference of arrival of light signals in different receiving channels of high-speed observation system, it shows that the propagation velocity of the current impulse is in the range from 4.3 ± 0.36 × 106 to 1.1 ± 0.09 × 107 m/s.

Investigation of erosion mechanism of tungsten-based electrode in multiphase AC arc by high-speed visualization of electrode phenomena

Electrode phenomena in a multiphase AC arc were successfully visualized using a high-speed observation system with a bandpass filter system to understand the erosion mechanisms of tungsten-based electrodes due to the droplet ejection and electrode evaporation. The obtained results indicated that both droplet ejection and electrode evaporation contributed to the electrode erosion in the multiphase AC arc. The erosion by droplet ejection mainly occurred during the cathodic period, while electrode evaporation mainly occurred during the anodic period. The rates of erosion by droplet ejection and evaporation were estimated to be 6 and 3 g/min, respectively, when the arc current was 100 A. The results of an evaluation of the possible forces acting on the electrode tip suggested that the electromagnetic force was the dominant force in the cathodic period, resulting in droplet ejection.

Flame propagation characteristics and combustion mechanism of FeOOH-coated zirconium particles

Zirconium (Zr) particles are coated with ferric oxyhydroxide (FeOOH) nanoparticles to form a core–shell structure, and this process alters the combustion performance of the zirconium particles. This study explored the flame propagation characteristics and combustion mechanism of FeOOH-coated zirconium particles. A high-speed observation system was used to reveal the flame propagation velocities and temperatures, and scanning electron microscopy, thermogravimetry (TG), and X-ray diffraction were used to examine and elucidate the combustion mechanism of FeOOH-coated zirconium particles. The results showed that when the concentration was increased, the flame propagation velocities of pure zirconium dust cloud were higher than those of FeOOH-coated zirconium dust cloud with the molar ratio of 1:6, which in turn were higher than the molar ratio of 1:3 due to the zirconium content. With increasing the concentration from 0.313 to 0.938 kg m−3, the temperatures of FeOOH-coated zirconium dust cloud with the molar ratio of 1:6 and 1:3 both were no peak. Moreover, the temperatures of FeOOH-coated zirconium dust cloud with the molar ratio of 1:6 were always higher than those with the molar ratio of 1:3. There were three phases of mass loss FeOOH-coated zirconium particles by TG test. The first phase of mass loss was because of evaporation of the FeOOH-coated zirconium particles, and the second phase of mass loss was the generation of Fe2O3 because of dehydroxylation of FeOOH. The third phase of mass loss was the residual of thermal decomposition due to Fe3O4 being formed from Fe2O3. It was also concluded that a replacement reaction occurred between Zr and Fe2O3 in the combustion process, and pure Fe is oxidized to Fe3O4 at high temperatures.

Flame propagation through zirconium particles coated with different ratios of Fe3O4

This study investigated the effects of Fe3O4 coating ratios on the characteristics of flame propagation in dust clouds of 38-μm zirconium particles. A high-speed observation system, fine thermocouples, scanning electron microscopy and X-ray photoelectron spectroscopy were used to reveal the flame propagation behaviors, velocities, temperatures and reaction mechanisms in detail. The results showed that the propagating flames emitted a strong white light, with a thin yellow light zone gradually appearing in front of the white light zone. As the concentration was increased, the flame propagation velocities of zirconium particles coated with an Fe3O4 ratio of 9:1 were greater than those of zirconium particles coated with an Fe3O4 ratio of 3:1 due to the zirconium content and the contact area between zirconium particles and the oxygen. The temperatures of zirconium particles coated with an Fe3O4 ratio of 9:1 and zirconium particles coated with an Fe3O4 ratio of 3:1 did not peak in the experimental range. But the temperatures of zirconium particles coated with an Fe3O4 ratio of 9:1 were always higher than those of zirconium particles coated with an Fe3O4 ratio of 3:1. The scanning electron microscopy and X-ray photoelectron spectroscopy demonstrated that a replacement reaction occurred between Zr and Fe3O4 during the combustion process.

Improvement of high-speed observation system for studies on dynamics of microbubbles beside rigid wall

Improvement of high-speed observation system for studies on dynamics of microbubbles beside rigid wall