• Journal of IKEEE
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• v.15 no.4
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• pp.305-312
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• 2011

A primary interest of this work is to develop an efficient and powerful repetitive pulser system for the application of ultra wide band generation. The important component of the pulser system is a small-sized coaxial type spark gap with planar electrodes filled with SF6 gas. A repetitive switching action by the coaxial spark gap generates two consecutive pulses in less than a microsecond with rise times of a few hundred picoseconds (ps). A set of several parameters for the repetitive switching of the spark gap is required to be optimized in charging and discharging systems of the pulser. The parameters in the charging system include a circuit scheme, circuit elements, the applied voltage and current ratings from power supplies. The parameters in the discharging system include the spark gap geometry, electrode gap distance, gas type, gas pressure and the load. The characteristics of the spark gap discharge, such as breakdown voltage, output current pulse and recovery rate are too dynamic to control by switching continuously at a high pulse repetition rate (PRR). This leads to a low charging efficiency of the spark gap system. The breakthrough of the low charging efficiency is achieved by a parallel operation of two spark gaps system. The operational behavior of the two spark gaps system is presented in this paper. The work has focused on improvement of the charging efficiency by scaling the PRR of each spark gap in the two spark gaps system.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.787-790
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• 2017

This study is a preliminary research on characterization of methane - oxygen combustion used in bipropellant thruster. The limit of combustion stability and flame shape of methane - oxygen non-premixed flame injected by shear coaxial injector in the model combustion chamber Experimental studies have been carried out. A direct image of the flame was photographed using a DSLR camera, and combustion characteristics and flame length were quantified through image post-processing. As a result, it was confirmed that the stabilized flame was generated at the stoichiometric ratio as the oxidizer Reynolds number ($Re_o$) was increased, and the length of the turbulent flame was increased under the same injector diameter condition.

• Journal of the Korean Society of Propulsion Engineers
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• v.24 no.1
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• pp.1-8
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• 2020

In this study, a configuration design of a CH₄/LOx small rocket engine was made and test system was established for the performance evaluation. A coaxial swirl injector was chosen because of its remarkable atomization performance and low combustion instability. Three aspect ratios for the combustion chamber configuration, i.e., 1.5, 1.8, and 2.1 were also set for the comparison of the combustion efficiency. The reliability of the thrust measurement rig was enhanced by pre-and post-calibration process. From the preliminary ground hot-firing test, the measured thrust and specific impulse values were 89.2 N and 181.8 s, respectively, which were 21.6% lower than the ideal values. In addition, the efficiency of characteristic velocity was measured as 84.2%.

• Nuclear Engineering and Technology
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• v.26 no.2
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• pp.212-224
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• 1994

A numerical method has been developed for studying the dynamics of a flexible cylinder in a coaxial cylindrical duct, immersed in inviscid flow. The unsteady inviscid fluid-dynamic force acting on the oscillating cylinder has been estimated more rigorously by means of a spectral collocation method without simplification of governing equations. This numerical approach is applicable to the system haying wider annular gap and/or shorter length of cylinder as compared to existing potential theory. The governing equation of the unsteady flow was obtained from Laplace equation. The equation of cylinder motion coupled with the fluid motion was discretized by Galerkin's method, from which the dynamic behaviour of the system has been evaluated. The effect of the length of the cylinder and the annular gap on the critical flour velocity, where the system loses stability by buckling, was investigated. To validate the numerical method, the potential flow theory developed by Hobson based on thin film approximation has been improved. Typical results of the present numerical theory on the dynamics and stability of the system are compared with those of available existing theory and the present approximate results. Good agreement was found between the results. It was also found that a nondimensional critical flow velocity becomes larger as increasing the annular gap and decreasing the length of cylinder.

• The Proceedings of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.11 no.6
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• pp.104-110
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• 1997

In thes paper, the soung signals due to the collisions against the inside wall by conductint particles with in coaxial cylindrical electrodes, and the corresponding frequency spectrum are analyzed and discribed.Thesesound signals were detected and measured using ultrasonic and vibration sensors attached to the exterior of the GIS enclosure. In the case where a particls is bouncing about between these coaxoal electrodes, the sound signal was found to be more than 10[dB] greater than the background noise due to no particlel. Also, in the case where a particle collides and insulation breakdown caesed by the particle made it possible to determine the condition of the insulation inside the sealed GIS. Lastly, the relationship between the peak amplitude and RMS voltage of the measured signal, the diameter and length of the particle was analyzed. Using thes analysis, it is possible to confirm the size of particle.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.21 no.2
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• pp.46-53
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• 2007

Extension of the mobile communication and the rapidly developing services connected with wireless multimedia(W-CDMA, DMB, Wibro, and so on) need great many base station transceiver systems, antennas, and many signal transmission line to embody these services. The min communication appliances system is exposed always in danger of transient overvoltage by direct stroke or induced lightning surge flowing in through antenna because the antenna of base station transceiver system or relay equipment is combined with expensive communication network equipments through signal transmission line. Now, gas discharge tube(GDT) or narrow band stub arrestor which is used by the surge protector for communication use is exposing limitations as for lightning surge protective means of the communication facility with high speed and wide frequency band. Therefore, for improving performance of surge arrestor, we applied ceramic filter which can reduce lightning surge invading from antenna effectively, and proved that ceramic filter is effective in residual voltage reduction through the experimental research.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.37 no.10
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• pp.975-983
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• 2009

This paper presents the work being carried out in order to deduce hover performance of a small-scale single rotor blade as a preliminary study of a small coaxial rotor helicopter development. As an initial research, a test stand capable of measuring thrust and torque of a small-scale rotor blade in hover state was constructed and fabricated. The test stand consists of three parts; a rotating device, a load measuring sensor and a data acquisition system. Thrust and torque were measured with varying collective pitch angle at fixed RPM. Through this research, hover performance tests were conducted for a small-scale single rotor blade operating in low Reynolds number ($Re\;{\approx}3{\times}10^5$), as well as for verifying the test stand itself for acquiring hover performance.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.13 no.7
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• pp.1272-1278
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• 2009

The present paper is purposeful that I compare the materialization method of the filter of UIR and SIR, then make two filters of different methods, so both are useful. The structure of the method of UIR is simple, so it is used a commercial point of view that the client gets the characteristics what he wants. However, the needs of companies when compared to the same standards, a little more cheaply in price, a little smaller in size, better performance characteristics are expected. In this regard, the method of SIR is smaller than UIR in size, but it shows the similar characteristics of UIR, so it is expected that holds a dominant position in competitive power. In this paper, based on these conditions, I compared the methods of SIR and UIR after the design and production. The standards are Insertion Loss is less than 0.8dB, Return Loss is above 20.0dB, and the Ripple is within 0.3dB. The center frequency is settled 881.5MHzband that TX Band is used by CDMA(SKT). I compared in the same conditions in Bandwidth 25.0MHz.

• Journal of the Korean Society of Propulsion Engineers
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• v.9 no.2
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• pp.97-104
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• 2005

A simple but efficient grid generation technique by using the modified compressible form of stream function has been formulated. Transformation of a physical plane to a streamline plane, the Von Mises Transformation, has been widely used to solve the differential equations governing flow phenomena, however, limitation arises in low velocity region of boundary layer, mixing layer and wake region where the relatively large grid spacing is inevitable. Modified Von Mises Transformation with simple mathematical adjustment for the stream function is suggested and applied to solve the confined coaxial turbulent jet mixing with simple $\kappa-\epsilon$ turbulence model. Comparison with several experimental data of axial mean velocity, turbulent kinetic energy, and Reynolds shear stress distribution shows quite good agreement in the mixing layer except in the centerline where the turbulent kinetic energy distributions were somewhat under estimated. This formulation is strongly suggested to be utilized specially for free turbulent mixing layers in axisymmetric flow conditions such as the investigation of mixing behavior, jet noise production and reduction for Turbofan engines.

• Journal of the Korean Society of Propulsion Engineers
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• v.11 no.3
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• pp.1-6
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• 2007

A 3-tonf-class high pressure sub-scale combustor was designed and manufactured to study the performance improvement of combustor. The combustor consists of a combustion chamber with film cooling, thermal barrier coating and water cooling channels to prevent thermal demage of the hardware and an injector head with 37 coaxial swirl injectors. Hot-firing tests were carried out at the design point with varying flow rate for film cooling. The test result revealed that the increase of film cooling flow rate decreases the combustion performance, but in the cases of similar film cooling flow rates, the combustion performance is dependent on the mixture ratio of main injector excluding the film cooling flow rate.