• Journal of the Korean Institute of Telematics and Electronics
• /
• v.25 no.9
• /
• pp.1108-1114
• /
• 1988

Usng the P.Yeh's analytic method, we derive the condition for oscillation of a linear phase conjugate oscillator which consists of photorefractive medium and two conventional mirrors. From this general oscillation condition, we obtain the threshold oscillation conditon of a single phase conjugate resonator and the self-oscillation condition of photorefractive medium, then in special case (phase shift =90\ulcorner: no external dc electric field), oscillation conditions of the linear phase conjugate oscillator for any cavity length are derived. The results indicate that, unlike in a phase conjugate oscillator with Kerr-like medium, oscillation cannot occur at special cavity length for given couplinyg strengths.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.21 no.2
• /
• pp.126-130
• /
• 2009

The present study describes an investigation into the characteristics of combustion oscillation and its suppression instability of a silo type gas turbine combustor in commercial power plant. Combustion oscillation is occurred the combustor in near full load during operation. As a result of FFT analysis of the combustion dynamics, the frequency of the oscillation is analyzed as the 1'st longitudinal mode of acoustic resonance of the combustor. For suppress of the instability, combustion tuning with adjust of fuel valve schedule is carried out, which changes equivalent ratio of each burners. As the result, the oscillation is successfully reduced with meeting the level of NOx emission regulation.

• Proceedings of the KIPE Conference
• /
• 2017.07a
• /
• pp.150-151
• /
• 2017

This paper deals with the suppression of subharmonic current oscillation of V/F controlled high speed induction motor drive in turbo blower applications. High speed induction motor has a relatively low stator impedance for high speed operation and therefore the dc link voltage ripple may result in subharmonic current oscillation at the steady state in high speed operation. This paper shows that the subharmonic oscillation is caused by the dc link voltage ripple and also proposes the suppression method, which generates the compensation voltage through the proportional current controller to reduce the subharmonic current oscillation. The simulation and experimental results show that the proposed suppression method reduces the subharmonic current oscillation to the acceptable low level in turbo blower applications.

• Communications of the Korean Mathematical Society
• /
• v.19 no.3
• /
• pp.495-506
• /
• 2004

Some oscillation criteria are given for second order nonlinear differential equations by means of integral averaging technique.

• Journal of Mechanical Science and Technology
• /
• v.17 no.10
• /
• pp.1507-1519
• /
• 2003

The two-phase flow patterns for both non-loop and loop type oscillating capillary tube heat pipes (OCHPs) were presented in this study. The detailed flow patterns were recorded by a high-speed digital camera for each experimental condition to understand exactly the operation mechanism of the OCHP. The design and operation conditions of the OCHP such as turn number, working fluid, and heat flux were varied. The experimental results showed that the representative flow pattern in the evaporating section of the OCHP was the oscillation of liquid slugs and vapor plugs based on the generation and growth of bubbles by nucleate boiling. As the oscillation of liquid slugs and vapor plugs was very speedy, the flow pattern changed from the capillary slug flow to a pseudo slug flow near the annular flow. The flow of short vapor-liquid slug-train units was the flow pattern in the adiabatic section. In the condensing section, it was the oscillation of liquid slugs and vapor plugs and the circulation of working fluid. The oscillation flow in the loop type OCHP was more active than that in the non-loop type OCHP due to the circulation of working fluid in the OCHP. When the turn number of the OCHP was increased, the oscillation and circulation of working fluid was more active as well as forming the oscillation wave of long liquid slugs and vapor plugs in the OCHP. The oscillation flow of R-142b as the working fluid was more active than that of ethanol and the high efficiency of the heat transfer performance of R -142b was achieved.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2006.11a
• /
• pp.295-298
• /
• 2006

Experiments in methane-air low strain rate counterflow diffusion flames diluted with nitrogen have been conducted to study the behavior of flame extinction and edge flame oscillation in which lateral conduction heat loss in addition to radiative heat loss could be remarkable at low global strain rates. Onset conditions of edge flame oscillation and flame oscillation modes are also provided with global strain rate. It is seen that flame length is closely relevant to lateral heat loss, and this affects flame extinction and edge flame oscillation. Edge flame oscillations in low strain rate flames are categorized into three: a growing oscillation mode, a decaying oscillation mode, and a harmonic oscillation mode. The regime of flame oscillation is also provided at low strain rate flames.

• 한국연소학회:학술대회논문집
• /
• 2005.10a
• /
• pp.275-280
• /
• 2005

At previous study, nitrogen oxide emission was decreased with decreasing pressure index. This tendency was explained by the flame oscillation with changing combustor pressure. In this study, the characteristics of flame oscillation with changing combustor pressure were investigated. It can be found that flame length is extended and flame width is narrowed by decreasing combustor pressure. It can be observed that pilot flame and the surrounding air converge on the inner flame in the $P^{\ast}{\geqq}1$ conditions and that surrounding air and flow pattern was widely dispersed in the $P^{\ast}<1$ conditions. In the respect of average flame length, low fluctuation was shown in the $P^{\ast}<1$ conditions. On the other hands, large fluctuation was shown in the $P^{\ast}<1$ conditions. Flame oscillation are observed from $P^{\ast}=$ 0.98 in the condition of $P^{\ast}<1$ and the amplitude of flame oscillation becomes larger when $P^{\ast}$ is lowered. These results demonstrate that low NOx phenomenon was caused by flame oscillation with changing combustor pressure.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2009.11a
• /
• pp.359-362
• /
• 2009

Damping characteristics of pressure oscillation induced by an orifice in fluid feeding line for are investigated numerically and experimentally. Assuming small pressure oscillation, acoustic damping effect of orifice is confirmed experimentally, and with the mean flow, damping characteristics of an orifice are investigated numerically. When an orifice is installed at the node of pressure oscillation corresponding to the anti-node of velocity oscillation, suppression of pressure oscillation is maximized and with the mean flow, the resonant frequency is decreased. And, it is found that the optimal position of an orifice for damping shouled be changed.

• Atmosphere
• /
• v.22 no.2
• /
• pp.187-197
• /
• 2012

This study investigates the relationship between the Korean temperature and the atmospheric circulation such as Arctic Oscillation, Siberian High and Aleutian Low during the winter (December-January) for the period of 1970-2011. It is found that all indices to represent aforementioned circulations are significantly correlated with Korean winter temperature for the period of 1970 - 2011. There are marked contrasts in such relationship, however, before and after the mid-1980s when a significant regime shift of Korean winter temperature occurred. While Korean winter temperature has a close relationship with Arctic Oscillation after the mid-1980s, its relationship with Siberian High and Aleutian Low is weakened. The composite analysis between a positive and negative phase of Arctic Oscillation before and after the mid-1980s is conducted to examine a recent strengthening of Arctic Oscillation-Korean winter temperature relationship. It is found that the structural changes of low-level wind and the geopotential height at 500 hPa between the two phases of Arctic Oscillation are more effective to influence Korean winter temperature after the mid-1980s. This may induce a close relationship between the Korean winter temperature variability and Arctic Oscillation after the mid-1980s compared to before the mid-1980s.

• Journal of Welding and Joining
• /
• v.21 no.3
• /
• pp.85-91
• /
• 2003

Narrow gap welding has many advantages over conventional V-grooved butt welding such as high productivity, small deformation and improved mechanical property of joints. With narrower groove gap, less arc heat input is expected will all the other advantages of narrow gap welding. The main defects of narrow gap welding include the lack of root fusion, convex bead surface and irregular surface, all of which have negative effects on the next welding pass. This paper suggests an up-and-down torch oscillation for ultra narrow gap welding with gap size of 5mm and investigates the proper welding conditions to fulfill the reliable and high welding quality. First, GMA welding model was suggested for ultra narrow gap welding system with Halmoy's model referenced for wire melting modeling. And the arc length in ultra narrow gap was defined. Secondly, based on the experimental results of up-and-down torch oscillation welding, phase shift of current and wire extension length were simulated for varying oscillation frequency to show that weld the bead shape in ultra narrow gap welding can be predicted. As the result, it was confirmed that reliable weld quality in ultra narrow gap welding can be achieved with up-and-down torch oscillation above 15Hz due to its ability to provide uniform heat input along the sidewall of gap.