Jo May 8, 2023
Pak Yong Ho, a researcher at the Nano Physics Engineering Institute, by using a polymer jet fluid model by CFD, has calculated the velocity field distribution in it to determine a uniform velocity profile and determined a new curved profile of the collector to obtain uniform fiber in an arched multi-nozzle electrospinning device.
Electrospinning is a method of producing micro fibres by injecting polymer solutions in electrostatic fields. Microfibers obtained by electrospinning have widely been applied in various fields, due to their superior properties such as very small diameter, high porosity, large specific surface area, etc.
Recently, highly productive electrospinning methods such as multi-nozzle electrospinning and nozzle-free electrospinning have been proposed. Multi-nozzle electrospinning is a method of overlapping many nozzles, so it is difficult to realize stable and continuous spinning as nonuniform distribution of electric fields at the nozzle tip creates corona discharge by strong electrical interaction between the nozzles.
Based on the calculation of the flow rate in angular jet flow through a CFD simulation of the jet flow in an electrostatic field, he determined the contour shape of the collector plate obtained by the uniform velocity plane. Then, he drew the curve equation of the curved collector which can obtain uniform diameter electrospun fiber in the multi-nozzle electrospinning system where an arched multi-nozzle and a curved collector are combined. After that, he found the curve equation of the curved collector to obtain uniform electrospun nanofiber with the diameter of 131.3nm, which is suitable for preparing a nanofiber ionexchange membrane for electrodialysis.
The curve equation of the collector profile for obtaining the electrospun nanofiber (131.3nm in diameter) in an arched multi-nozzle electrospining device is y=-2-12x3+0.002x2-2-8x-91.991.
It means that a new methodology to obtain uniform electrospun fibers with arbitrary diameters in a multi-nozzle electrospinning device without additional means was developed.
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Jo May 5, 2023
Ro Myong Gum, a researcher at the Nano Physics Engineering Institute, has investigated the effects of laser power on the reduction of graphene oxide by 355nm UV impulse laser.
Graphene oxide, which is chemically exfoliated from graphite, a precursor of graphene, is a suitable material for producing graphene in large quantities. However, reduction is necessary because graphene oxide is an insulator.
In the past, a chemical method, heating treatment, microwave and plasma, etc. were applied to the reduction of graphene oxide, but nowadays graphene oxide reduction by laser is under study.
She performed some experiments on the thermodynamic change of a graphene oxide film when graphene oxide is irradiated by 355nm UV impulse laser, and confirmed the impact factors of laser on the reduction.
As compared with conventional reduction methods, the proposed method is simple in process and enables reduction and mask at the same time, so it will prove the most suitable for manufacturing micro-electronic devices.
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Jo May 2, 2023
U Yong Nam, a section head at the Robotics Institute, has made a study of the mobile properties of an omnidirectional AGV with Mecanum wheels.
A Mecanum wheel, which is designed to move a vehicle in any direction, consists of a series of rollers attached to its circumference.
ADAMS was used for the simulation, where he analyzed the motion in X and Y directions, turning round on the position and oblique.
The simulation result showed that the velocity error in the X-direction movement and turning round on the position of the AGV is small and stabilization is fast, and vice-versa in the Y-direction movement and oblique.
The results verified that an AGV with four Mecanum wheels can operate in a static state.
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Jo Apr 28, 2023
A research team led by Kim Sung Ri, a section head at the Faculty of Mechanical Science and Technology, has carried out a CFD simulation on a vertical turbine flowmeter with air and hydrogen and compared the meter factors. Then, they have conducted research on improving its starting characteristic in a low flowrate domain.
Generally, the meter factors of a turbine flowmeter for different densities of gases are different.
To fix this problem, they carried out a CFD simulation on a vertical turbine flowmeter that employs air and hydrogen as working media, and analyzed the effect of the density of gas on meter factors.
First, they created a geometrical model and mesh, and performed a CFD simulation by ANSYS Fluent using an SST κ-ω turbulence model. The result showed that as the density of air is much higher than hydrogen, the zero-load rotational speed of air is much higher than that of hydrogen for the same flowrates, and in contrast, the meter factor of air is smaller than that of hydrogen. It also showed that the meter factor of hydrogen had nonlinearity in a low flowrate domain, while the meter factor of air was almost constant over the whole flowrate domain.
Second, they carried out CFD simulations on vertical turbine flowmeters with 2 and 6 inlet channels, respectively, and analyzed the effect of the number of inlet channels on the starting characteristic. The result showed that in case of 2 channels, the zero-load rotational speed in a low flowrate domain was much higher than in case of 6 channels and the meter factor had linearity.
In conclusion, adjusting the area of an inlet channel is a good method for improving the performance of gas flowmeters, and in particular, this method is the most effective for measuring low-density gas in a low flowrate domain.
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Jo Apr 27, 2023
Many crashing accidents of helicopters and multicopters are attributable to the propellers plunging into a vortex ring state. Therefore, safe operation of a helicopter or a multicopter needs a thorough study on the vortex ring of a propeller and the vortex ring state.
Though many studies have been conducted on a vortex ring and its state, few of them deal with the cause of a vortex ring and determination of the critical descent velocity.
Myong Song Nam, an institute head at the Faculty of Mechanical Science and Technology, has determined the cause of the vortex ring of a descending propeller and the critical descent velocity by CFD method.
He used the MRF(Moving Reference Frame) method of Fluent for adequate prediction of the wake flows of a propeller.
The simulation showed that the origin of the vortex ring is not the vorticity line, but the interaction between the wake stream tube and the surrounding air, and that as the position of the vortex ring core varies with the increase in the descent velocity, the vortex ring approaches the propeller and consequently, the propeller enters into a vortex ring state and the vortex ring sharply moves above the propeller. Then, he predicted the boundary value of the descent velocity at which the propeller enters into a vortex ring state. The vortex ring state relates closely to the position of a vortex ring core or the thrust, which starts to vary suddenly at about 4.2m/s of descent velocity.
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Jo Apr 26, 2023
U Yong Nam, a section head at the Robotics Institute, has developed a tower clock.
The tower clock is composed of a driving ball-worm gearing box (1:84) and a clock gearing box (reduction ratio of the hour hand and the minute hand 1:12). The accuracy of the clock is ensured by the side clearance and the motion smoothness of the driving ball-worm gearing box.
When the worm shaft rotates in the driving ball-worm gearing box, the rolling balls in the circular workspace between the worm and the worm wheel transfer torque to the ball-worm wheel. Then, the balls circulate along the ball recirculation passage through 2~3 teeth. At the input and output of the passage is a piece to guide the balls back smoothly. And there is a ball-bush to stop the balls from dispersing.
The control system is composed of a main clock and terminal clocks calibrated by the national standard time. The control device consists of a digit I/O part, a serial communication part, a display, control buttons, and a power circuit.
The terminal clocks receive time pulses through RS-485 communication every 30s from the main clock and send them to the drive module to drive the motor.
The large-size tower clock with a ball-worm gearing box has several advantages.
First, it is much easier to achieve the dimensional accuracy and the assembling accuracy of the gearing box than a clock with a planetary gearing box.
Second, its service life is semipermanent or permanent.
Third, it ensures a high degree of accuracy and it is easy to operate.
It will find use at the front gates of factories and enterprises, in the waiting rooms of stations, and at public places.
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