Jo Jul 4, 2025
Multistage centrifugal pumps are commonly-used pieces of turbomachinery for applications with high head and flow duty requirements. So, the improvement of their technological indexes is greatly significant for saving energy, ensuring production and raising economic benefit.
Since the axial force in high-pressure multistage pumps reaches tens of tons, it is essential to balance hydraulic axial thrust. Even then, an axial bearing is required; and its size depends largely on the prediction accuracy of axial thrust and on the type of the balancing device. Therefore, the size of bearings and the axial thrust balance systems of multistage pumps are frequently determined on the basis of axial force measurements carried out on each particular type and hydraulic pressure of pumps.
In general, balancing devices are used for balancing the axial thrust acting on the rotors of multistage centrifugal pumps. Balancing devices create forces that oppose the axial thrust generated by the impellers. When dimensioned correctly, balancing devices are able to completely balance the axial thrust, so the required pressure automatically builds up in the radial gap due to an axial displacement of the rotor. Therefore, the axial clearance at the device depends on the axial force acting on the rotor, i.e. on the operation point of the pump.
Balancing devices have two gaps: a radial gap and an axial clearance. The mutual coupling of these two gaps allows automatic balancing of the axial force. According to the variation of operation point of a centrifugal pump, the axial thrust changes. The rotor moves left and right in the axial direction to be in a new equilibrium state, when the axial clearance changes with the movement.
If the axial displacement of the rotor is too large, the balancing device may contact with the casing to cause wear. Thus, the rotor vibrates and the stability of operation is destroyed. Therefore, when balancing device is designed, even small axial displacement must yield large changes in the balancing force. In previous studies, this performance was estimated by sensitivity coefficient.
Sensitivity factor represents the variation of balancing force according to the pressure distribution in front and rear shrouds of impeller, but does not represent variation of balancing force when a rotor moves. Therefore, it can be more correct to estimate the automatic balancing ability of a balancing device by the parameter representing the variation of balancing force when a rotor moves a unit length.
Han Pok Nam, an institute head at the Faculty of Mechanical Science and Technology, has decided leakage through radial and axial clearance, pressure difference and balancing force based on the theoretical analysis of the gap flow of a balancing device. Then, he has defined a new concept of stiffness coefficient and analyzed the relationship between stiffness coefficient and geometrical parameters.
Thus, he has found that the stiffness coefficient decreases with too large or too small sensitivity coefficient, and a reasonable sensitivity coefficient should be chosen around 0.5.
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Jo Jul 3, 2025
It is well known that TEA-CO2 laser is a kind of high-power laser with enough pulse peak output, and study on TEA-CO2 laser has been consistently conducted for a long period of time. The design of electrode structure, which emits field electrons in TEA-CO2 laser, is a question of finding an effective method for increasing laser power by raising the emission efficiency of field electrons and their utilization.
In practice, the tips of cold-cathode are made in the shape of a needle or a blade. Such tips emit field electrons at low discharge voltage by raising the amplification degree of electric field. When the electric field strength at the tips is too high, the lifetime of tips will be shortened owing to the cathode sputtering. The field electron generator that contains cold-cathode provides high vacuum of 10-4Pa.
The Marx generator supplies electric power to the generator of field electrons. Here, such geometric parameters as the height and basal width of tips and the size of anode cells are all related to the amplification degree of electric field. The electric field strength at the tips is affected by not only the shape and size of each tip but also their arrangement. When high voltage is applied to the cathode, the existent electric fields in the vicinity of the tips interact with each other, which obviously influences the electric field strength at the tips.
Pak Sin Hyok, a researcher at the Institute of Nano Science and Technology, has designed an electrode structure of cold cathode and grid anode with a high amplification degree of electric field strength at the tip, in consideration of the tip’s parameters, the arrangement of electrode structure, etc.
He designed the tips as a triangle, and simulated the electric field in the discharge region by the COMSOL Multiphysics software while changing the size and array interstice of tips and the dimension of grid anode corresponding to the tips. On this basis, he found out the optimum design parameters of electrode for raising the amplification degree of electric field strength at a tip to the maximum.
The simulation result shows that the optimum interstice of tips that provides the maximum amplification degree of electric field was 8mm, and the amplification degree of electric field strength was 1.2.
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Jo Jun 2, 2025
The Elephant Rock of Pidan Island is a natural monument located in Sindo County of North Phyongan Province.
When the sea water entered the Korean West Sea area at the end of the fourth glacial period, the rocks on the relatively high hills exposed above the sea formed an extraordinary topography by a long time of abrasion and weathering.
The rock has been called Elephant Rock as it resembles an elephant.
The rock is 40 meters long and 15 meters high, with the top of 8 meters in width.
The “trunk” is 9 meters round.
At the “head” of the rock is a small rock resembling an “ivory” and in the middle of the “trunk” is a small hole. Grass is growing on the rock.
The rock is submerged and exposed by the tide. When it is submerged, it looks like an elephant swimming in the water, and when it is exposed, it looks like being caught in the marshland.
The Elephant Rock, which is of significance in studying the change in the nature after the formation of the Korean West Sea, is a natural monument adding beauty to the beachscape.
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Jo May 11, 2025
Chongbaek Juniper growing in Chongbaek-ri, Kumya County, South Hamgyong Province was planted around 1550. It is 10 meters tall, with the root collar of 5.4 meters round and the canopy of 18 meters from east to west and 21 meters from north to south.
At the height of 50 centimeters, it spreads in three big branches that are 2.6 meters, 2.1 meters and 2.2 meters round, respectively.
It looks like a collection of small clouds from a short distance and like a large furled umbrella from a long distance.
The oldest and largest tree on the eastern coast is a natural monument that is significant for studying the size and age of juniper trees and adds beauty to this area.
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Jo May 7, 2025
The Ancestral Apple Tree is in Ryongjon-ri, Pukchong County of North Hamgyong Province, a place famous for apples.
In October, 1959, President
In June, 2002, Chairman
This long-lived apple tree over the age of 100 is still growing healthily.
Its root collar is 1.2 meters round and the canopy is as wide as 6 meters. It blossoms in May with long and deep green leaves.
This oldest tree in our country is a precious natural monument of significance for judging the maximum age of apples and studying the characteristics of their varieties.
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Jo May 5, 2025
Among the precious and beautiful natural monuments in the DPRK is Kimhwa Elm Tree.
Kimhwa Elm Tree, growing on the shore of the Kumsong River about one kilometer south-east of the seat of Kimhwa County of Kangwon Province, is 500-odd years old.
It is 21 meters tall, with the root collar of 5.7 meters round, the chest-height of 5.2 meters round and the canopy of about 22 meters.
The first branches stretch southwards at the height of 4.5 meters while the second and the third ones stretch westwards and southwards at the heights of 7 and 10 meters, respectively.
It has a total of 12 big branches.
This large old tree adds natural beauty to the surroundings and it is of great significance in the academic research.
The natural monument, Kimhwa Elm Tree, is still growing vigorously, further enriching the treasure house of the natural cultural heritage.
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