High-speed Video Camera Observation Research Articles

Combustion characteristics of Ammonia/Air mixture ignited by flame jet with and without hydrogen injection in Pre-Chamber

Ammonia is one of the important fuels that can promote the achievement of carbon neutrality, but its combustion characteristics are not conducive to its application in engines. Injecting hydrogen in the pre-chamber to form a flame jet to ignite the ammonia/air mixture is a method to improve the combustion of ammonia. This ignition method was investigated with the constant volume combustion chamber, high-speed video camera observation and the main-chamber pressure analysis. The flame behavior and combustion characteristics were compared with those ignited by the ammonia flame jet. Hydrogen flame jet significantly enhanced the combustion of the mixture and extended the lean flammability limit. Under conditions of hydrogen injection, the pressure rise delay and combustion duration became shorter, the average heat release rate became higher, and the combustion process was more stable. The hydrogen flame jet rapidly penetrated the main-chamber and impinged on the lower surface, generating intense turbulence. As a result, the combustion pressure took less time to rise from 10% to 50% than it did to go from 50% to 90%. This was different from the situation without hydrogen injection, where time required for both was similar. The hydrogen flame jet was shuttle-shaped when touching the lower surface owing to the rapid combustion speed of hydrogen, while the ammonia flame jet was spindle-shaped with the flame kernel in the center. The combustion process of the flame jet igniting the mixture exhibited two peaks in the heat release rate, reflecting two combustion stages dominated by the flame jet and flame propagation.

The Influence of the Gap Phenomenon on the Occurrence of Consecutive Discharges in WEDM Through High-Speed Video Camera Observation

The Wire Electrical Discharge Machining (WEDM) process is an accurate method for manufacturing high-added-value components for industry. Continuous developments in the process have resulted in specialized machines used in sectors such as aerospace and biomedical engineering. However, some fundamental aspects of the discharge process remain unresolved. This work aims to study the influence of discharge location and bubble expansion on the occurrence of subsequent discharges. A high-speed video camera observation system was constructed to capture images of each discharge. From the acquired images, an algorithm was devised to determine the discharge location based on grayscale analysis. Moreover, the voltage and current waveforms of the discharges and the framing signals of the high-speed video camera were then obtained using an oscilloscope. Synchronizing the observation images and signals allowed for calculating the delay time for each single discharge. The results indicate that most of the discharges occurred near the boundary of the bubble and during bubble expansion. This finding has been observed for a variety of machining conditions and can be explained by the effect of the debris particles concentrated at the bubble boundary. This study provides useful information for better understanding the discharge process in WEDM.

Polycrystalline diamond film fabrication using modulated inductively coupled thermal plasmas at different pressure conditions

This paper investigates pressure influence on polycrystalline diamond formation using sawtooth-waveform modulated induction thermal plasma. Modulated induction thermal plasma was used to promote nucleation of diamond particles in the first stage. The operating pressure was set to 32, 60, and 90 Torr. The deposited diamond films were analyzed by field emission scanning electron microscope and Raman spectroscopic observation. Experimental results indicated that a lower pressure condition provided a higher deposition rate and a qualitatively better diamond film. Spectroscopic and high-speed video camera observation showed that a lower pressure condition expanded the thermal plasma flow axially onto the substrate. Numerical simulation was also made for Ar/CH4/H2 induction thermal plasma to study the influence of pressure on the thermal plasma flow. Calculation results showed that lower pressure involves a higher particle flux of neutral hydrocarbon species on to the substrate surface because of higher convective transport of these species before their ionization. These results imply that neutral hydrocarbon would play a more important role for diamond deposition than hydrocarbon ions in the present thermal plasma diamond film deposition.

Investigating cathode spot behavior in argon alternating current tungsten inert gas welding of aluminum through experimental observation

The behavior of cathode spots in stationary argon alternating current tungsten inert gas welding of aluminum was clarified in this study. The temporal variation in cathode spot distribution during electrode positive (EP) polarity was, for the first time, revealed quantitatively based on high-speed video camera observation with a frame rate of 500 000 fps. In addition, the weld pool surface temperature at the start and end of EP polarity was also measured using the two-wavelength temperature measurement method to clarify the relationship with the cathode spot behavior. As a result, the general tendency of the life cycle of the cathode spot was clarified. The first cathode spot appeared and divided outside the weld pool. Following this, they moved rapidly toward the weld pool center within 100 µs. After that, the cathode spot group became an uneven and ring-shaped distribution, which expanded outward with time with a velocity of approximately 1 m s−1. The cathode spots have a random movement with the average velocities of each cathode spot on the liquid and solid surface being about 142 ± 59 m s−1 and 87 ± 44 m s−1, respectively. The ratio of the number of cathode spots located in the weld pool to the total number of cathode spots decreased from 100% to 37.4% with elapsed time, except right at the beginning. Furthermore, the temperature of the center surface of the weld pool was 1400 K, and this gradually increased to a slightly higher temperature than the melting point of aluminum around the weld pool boundary at the start of EP polarity. The maximum temperature at the center is considered to be linked with absence of a cathode spot. The temperature at the center decreased by 150 K at the end of EP polarity due to the temporal expansion in the ring-shaped distribution of the cathode spot group leading to a decrease in heat input around the center.

Thermal re-ignition processes of switching arcs with various gas-blast using voltage application highly controlled by powersemiconductors

This paper focuses on a fundamental experimental approach to thermal arc re-ignition processes in a variety of gas flows in a nozzle. Using power semiconductor switches in the experimental system, the arc current and the voltage applied to the arc were controlled with precise timing. With this system, residual arcs were created in decaying phase under free recovery conditions; arc re-ignition was then intentionally instigated by application of artificial voltage—i.e. quasi-transient recovery voltage—to study the arc behaviour in both decaying and re-ignition phases. In this study, SF6, CO2, N2, O2, air and Ar arcs were intentionally re-ignited by quasi-TRV application at 20 μs delay time from initiation of free recovery condition. Through these experiments, the electron density at the nozzle throat was measured using a laser Thomson scattering method together with high speed video camera observation during the re-ignition process. Temporal variations in the electron density from the arc decaying to re-ignition phases were successfully obtained for each gas-blast arc at the nozzle throat. In addition, initial dielectric recovery properties of SF6, CO2, air and Ar arcs were measured under the same conditions. These data will be useful in the fundamental elucidation of thermal arc re-ignition processes.

Elucidation of melt flows and spatter formation mechanisms during high power laser welding of pure titanium

This study was undertaken to clarify the relationship among the laser-induced plume ejected from the keyhole, the melt flows in the molten pool, and the formation mechanisms of spatters ejected from the molten pool during 10 kW laser welding of a pure titanium plate. High-speed video camera observation results showed that laser-induced plumes occurred at intervals of about 0.5 ms, and that the maximum plume ejection velocity reached 250 m/s. Three-dimensional X-ray transmission in-situ observation of the weld molten pool with tungsten carbide tracers revealed that the melt flowed mainly along the bottom of the molten pool from the keyhole tip to the rear part and then from the rear to the front near the surface of the molten pool at high speeds, while the melt in front of a keyhole flowed upward along the keyhole wall at a velocity of less than 0.6 m/s and then was accelerated to 2.1 m/s at the height of about 2 mm above the keyhole inlet. One-way upward melt flows were continuously piled up at the tip of the elongated melt, resulting in spattering as droplets from the molten pool due to the strong ejection of laser-induced plumes. Spatters were formed from part of a molten metal elongated around the keyhole inlet, and approximately 20 ms was required to form spatters. About 80% of spatters were generated from melts of the front or sides of the keyhole at the speeds of less than 50 mm/s. When the welding speed increased from 50–100 mm/s to 300 mm/s, the ratio and the size of spatters occurring from the rear part of a keyhole increased from 20% to 80% and became smaller than 1 mm.

Mechanical property and joining characteristics of laser direct joining of CFRP to polyethylene terephthalate

This study was performed to investigate the joining possibility and characteristics of the dissimilar joint between carbon fiber reinforced plastics (CFRP) and amorphous polyethylene terephthalate (PET) plastic using a continuous wave (cw) diode laser with a line-shaped beam. Tensile shear test results demonstrated that strong lap joints with a maximum load of 3200 N could be produced under some proper conditions, and base amorphous PET plastic sheet was elongated without fracture. It was confirmed from SEM observation that the dissimilar lap joint was tightly bonded near the joint interface between two materials, and simultaneously integrated by the interdiffusion and mixing process at the joint interface of two materials through rapid melting, solidification and cooling during laser joining. Through high speed video camera observation, the joining phenomena generating in the laser irradiated part during laser joining of CFRP to PET was obviously identified. Consequently, it was revealed that strong dissimilar lap joint of CFRP to engineering plastic could be possible by a direct laser irradiation without adhesive bonds or mechanical fasteners.

Investigation of Multiphase AC Arc Behavior by High-Speed Video Observation

A stable multiphase ac arc was generated by transformers at a commercial electric system. The arc discharge behavior is investigated by high-speed video camera observation synchronized with voltage waveform analysis. The multiphase ac arc-generating system enables one to obtain wide high-temperature region, which is expected to be suitable for powder heat treatment such as in-flight glass production.

Stability analysis of multi-phase AC arc discharge for in-flight glass melting

A stable multi-phase AC arc was generated to by transformers at a commercial electric system. The arc discharge behavior and the stability were investigated by the high-speed video camera observation synchronized with the voltage waveform analysis. The effect of the number of phase and the flaming gas addition into the arc region on the arc behavior and the arc stability were studied. Results showed that the re-ignition occurred between adjacent electrodes periodically. Two kinds of the stability analysis methods were introduced. The estimated fluctuation degree showed that an increase of the number of the phase leads to the stable multi-phase AC arc. The deviation of the arc voltage from the average arc voltage was estimated to evaluate the uniformity of the multi-phase AC arc. Although an addition of the oxygen flame into the arc region leads to lower uniformity, the modification of the electrode position can improve the uniformity of the multi-phase AC arc.

Relationship of laser absorption to keyhole behavior in high power fiber laser welding of stainless steel and aluminum alloy

This study was undertaken in order to assess absorption of a 10 kW fiber laser beam with tightly focused spot diameter of 200 μm in Type 304 austenitic stainless steel or A5052 aluminum alloy in bead-on-plate welding with a wide range of welding speeds from 17 to 300 mm/s, by using water-calorimetric measurement. The maximum absorptions of the steel and aluminum were 89 and 93% at the lowest speed, respectively. Their absorptions were high over 80% at less than 100 mm/s speeds and were reduced greatly by more than 20% at the highest speed. X-ray transmission in-site observation demonstrated that the center part of an incident beam with a bell-shape profile was delivered directly to a tip of a keyhole below 100 mm/s speed. High-speed video camera observation indicated that an incident beam was more partially exposed out of a keyhole inlet over 100 mm/s, which led to great decrease in laser absorption.

Development of shock proof suspension [disk drive

High speed video camera observation of a slider behaviour after an external shock revealed that the slider jumps first and then re-contacts with the disk at its leading edge. We found an imprint at the re-contact position on the disk. We developed a suspension system that has a roof in the load beam that reduces the re-contact angle and the contact stress. We found that a magnetic disk device incorporating the system sustained no read errors for the external shocks up to 700 G.