• Title/Summary/Keyword: Phase Stability

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A study of flow oscillations in a upright heated pipe (직립전열관에서의 유체진동에 관한 연구)

  • 박진길;진강규;오세준
    • Journal of Advanced Marine Engineering and Technology
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    • v.8 no.1
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    • pp.85-99
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    • 1984
  • The stability of the two-phase flow in a heated channel is of great importance in the design and operation of the boilers and light water nuclear reactors, because it can cause flow oscillations and lead to a violation of thermal limits with resultant overheating of the channels and cladding. This paper presents a systematic evaluation to the variation effects of the basic four (4) dimensionless parameters in a homogeneous equilibrium model. The flow stability is examined on the ground of static characteristic curves. The complicated transfer function of flow dynamics which gives consideration to the transport lag of density wave is derived, and the transient flow stability is analysed by applying the Nyquist stability criterion in control engineering. The analysis results summed up as follows 1. The coolant flow becomes stable in large friction number and specific flow, while it is unstabale in small friction number and flow. 2. Large phase-change number and Froude number destabilize the two-phase flow, but small numbers stabilize it. The effect to variation of phase-change number is more dominant compared with Froude number. 3. The dynamic analysis is required to hold the sufficient safety of heated channels since only static results does not keep it. The special attention could be payed in the design and operation of heat engines, because the unstaable region exists within the stable boundary at small and middle phase-change number and Froude number.

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A Stability Analysis of the Magnetic Bearing System Subject to Sensor dislocation Error -Discussion on Nonlinear Magnetic Force Model- (센서의 설치 오차에 따른 자기베어링 지지 로터계의 안정도에 관한 연구 -비선형 자기력 모델에 대한 고찰-)

  • 정재일;김종원
    • Proceedings of the Korean Society of Precision Engineering Conference
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    • 1996.11a
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    • pp.799-805
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    • 1996
  • In many cases, the magnetic farce model is linearized at the origin in designing the controller of a magnetic bearing system. However. this linear assumption is violated by the unmodeled nonlinear effect such as sensor dislocation and backup bearing dislocation. Therefore, a direct probe into the nonlinear magnetic force model in an active magnetic bearing system is necessary. To analyze the nonlinear magnetic force model of a magnetic bearing system, phase plot analysis which is to plot the numerical solution of the nonlinear equation in several initial points in the interested region is applied. Phase plot analysis is used to observe a nonlinear dynamic system qualitatively (not quantitatively). With this method, we can get much useful information of the nonlinear system. Among this information, a bifurcation graph that represents stability and locations of fixed points is essential. From the bifurcation graph, a stability criterion of magnetic bearing system is derived.

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Effect of Grain Size and Heat-treating Atmosphere on the Phase Stability of Y-TZP (입자크기와 열처리 분위기 변화에 따른 Y-TZP에서의 상안정성 변화)

  • Chung, Tai-Joo;Ahn, Seung-Su;Song, Eun-Wha;Oh, Kyung-Sik;Lee, Jong-Sook;Kim, Young-Sik
    • Journal of Powder Materials
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    • v.13 no.5 s.58
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    • pp.360-365
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    • 2006
  • The phase stability of tetragonal phase in Y-TZP was investigated in terms of the distribution of grain sizes and heat-treating atmosphere. Y-TZP with various grain sizes were prepared using duration time at $1600^{\circ}C$ as experimental parameter. Accumulated grain size distributions were built from the SEM micrographs and the amount of tetragonal phase were measured using XRD. Both results were compared to determine the critical grain size before and after heat-treatment in vacuum. The critical grain size drastically decreased compared with the small increase of average grain size due to the autocatalytic effect which critically affects the tetragonal to monoclinic phase transformation. After heat-treatment in reductive atmosphere critical grain size relatively increased due to the stabilization of tetragonal phase. The formation of oxygen vacancies during heat-treatment was ascribed to the increase of stability.

An Investigation on Flow Stability with Damping of Flow Oscillations in CANDU-6 heat Transport System (CANDU-6 열수송 계통의 유동 진동감쇠에 의한 유동안정성 연구)

  • 김태한;심우건;한상구;정종식;김선철
    • Journal of KSNVE
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    • v.6 no.2
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    • pp.163-177
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    • 1996
  • An investigation on thermohydraulic stability of flow oscillations in the CANada Deuterium Uranium-600(CANDU-6) heat transport system has been conducted. Flow oscillations in reactor coolant loops, comprising two heat sources and two heat sinks in series, are possibly caused by the response of the pressure to extraction of fluid in two-phase region. This response consists of two contributions, one arising from mass and another from enthalpy change in the two-phase region. The system computer code used in the investigation os SOPHT, which is capable of simulating steady states as well as transients with varying boundary conditions. The model was derived by linearizing and solving one-dimensional, homogeneous single- and two-phase flow conservation equations. The mass, energy and momentum equations with boundary conditions are set up throughout the system in matrix form based on a node-link structure. Loop stability was studied under full power conditions with interconnecting the two compressible two phase regions in the figure-of-eight circuit. The dominant function of the interconnecting pipe is the transfer of mass between the two-phase regions. Parametric survey of loop stability characteristics, i. e., damping ratio and period, has been made as a function of geometrical parameters of the interconnection line such as diameter, length, height and orifice flow coefficient. The stability characteristics with interconnection line has been clarified to provide a simple criterion to be used as a guide in scaling of the pipe.

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Phase Equilibria of the Ferrous Ferrite System of $(Mg_{0.29}-yMnyFe_{0.71})_{3-}\delta$O_4$ ($(Mg_{0.29}-yMnyFe_{0.71})_{3-}\delta$O_4$ 훼라이트계의 상평형)

  • 채정훈;유한일;강선호;강대석;유병두
    • Journal of the Korean Ceramic Society
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    • v.32 no.3
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    • pp.394-402
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    • 1995
  • Electrical conductivity and thermoelectric power of the ferrous ferrite system of (Mg0.29-yMnyFe0.71)3-$\delta$O4 have been measured as function of the thermodynamic variables, cationic composition(y), temperature(T) and oxygen partial pressure(Po2) under thermodynamic equilibrium conditions at elevated temperatures. On the basis of the electrical properties-phase stability correlation, the stability regions of the ferrite spinel and its neighboring phases have been subsequently located in the log Po2 vs. y and log Po2 vs. 1/T planes in the ranges of 0 y 0.29, 1100 T/$^{\circ}C$ 1400 and 10-14 Po2/atm 1. The stability region, Δlog Po2(y, 1/T), of the ferrite spinel single phase widens with increasing Mn-content(y) and the boundaries of each region are linear against 1/T with negative slopes.

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An Estimation on the Stability of W/O Type Emulsified Fuel Using by Capacitance Sensor (정전용량센서를 이용한 W/O형 유화연료의 안정성 평가)

  • Cho, Seong-Cheol;Oh, Yang-Hwan;Im, Seok-Yeon
    • Journal of the Korean Applied Science and Technology
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    • v.28 no.1
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    • pp.95-101
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    • 2011
  • We estimated on the stability of W/O type emulsified fuel using by capacitance sensor, so it concluded the following conclusions. For the first 24 hours, prepared emulsified fuel reveals phase separation ratio of 5%, maintains stable status which verifies the stability of emulsified fuel. Adding more water increases the phase separation ratio rapidly, and adding more surfactant displays stable emulsification. Adding water causes larger size of water droplet diameter, and adding surfactant mixture causes smaller size of water droplet diameter. In conclusion, the size of W/O type emulsified fuel water droplet diameter is directly related to the volume of surfactant, and density of water droplet diameter changes thedistribution according to water contents.

Investigation on the Dimensional Stability of Acetylated Larch (Larix kaempferi) Small Square (아세틸화처리 낙엽송(Larix kaempferi) 소형 각재의 치수안정성 조사)

  • Lee, Won-Hee;Kang, Ho-Yang
    • Journal of the Korea Furniture Society
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    • v.27 no.4
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    • pp.302-308
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    • 2016
  • It has been known that acetylation improves the dimensional stability of wood. Liquid phase acetylation is more popular than gas-phase acetylation for the effectiveness of weight gain of wood. In this study the specimens of domestic red and Korean pines are acetylated in liquid phase and their physical properties, such as density, bending strength, anti-hygroscopicity etc., are analyzed. Acetylation increases the average weights and volume of larch specimens by 11.4% and 3.4%, respectively, and their average oven-dry densities are increased by $0.03g/cm^3$. Acetylation does not influence on Modulus of Rupture (MOR) and Modulus of Elasticity (MOE). The average Percentage Reduction in Hygroscopicity (PRH) and average Percentage Reduction in Water soaking (PRW) of larch specimens are respectively 20.2% and 20.8%. Thus it can be concluded that acetylation improves the dimensional stability of larch specimens.

The Biomechanical Properties of the Shock Absorption Phase during Drop Landing According to Landing Types (드롭랜딩 시 착지형태에 따른 충격흡수구간의 운동역학적 특성)

  • Park, Gu-Tae;Yoo, Kyoung-Seok
    • Korean Journal of Applied Biomechanics
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    • v.25 no.1
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    • pp.29-37
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    • 2015
  • Objective : The purpose of this study was to investigate the biomechanical properties of shock absorption strategy and postural stability during the drop landing for each types. Methods : The motions were captured with Vicon Motion Capture System, with the fourteen infra-red cameras (100Hz) and synchronized with GRF(ground reaction force) data(1000Hz). Ten male soccer players performed a drop landing with single-leg and bi-legs on the 30cm height box. Dependent variables were the CoM trajectory and the Joint Moment. Statistical computations were performed using the paired t-test and ANOVA with Turkey HSD as post-hoc. Results : The dominant leg was confirmed to show a significant difference between the left leg and right leg as the inverted pendulum model during Drop Landing(Phase 1 & Phase 2). One-leg drop landing type had the higher CoM displacement, the peak of joint moment with the shock absorption than Bi-leg landing type. As a lower extremity joint kinetics analysis, the knee joint showed a function of shock absorption in the anterior-posterior, and the hip joint showed a function of the stability and shock absorption in the medial-lateral directions. Conclusion : These findings indicate that the instant equilibrium of posture balance(phase 1) was assessed by the passive phase as Class 1 leverage on the effect of the stability of shock absorption(phase 2) assessed by the active phase on the effect of Class 2 leverage. Application : This study shows that the cause of musculo-skeletal injuries estimated to be focused on the passive phase of landing and this findings could help the prevention of lower damage from loads involving landing related to the game of sports.

Phase Stability and isothermal Phase Transformation of the Yttria-Stabilized Zirconia Powder Prepared from Alkoxide Hydrolysis (알코옥시드 가수분해법으로 합성한 이트리아 안정화 지르코니아 분말의 상안정성 및 등온상전이 특성)

  • Lee, Jong-Kook;Kim, Young-Jeong;Kim, Hwan
    • Journal of the Korean Ceramic Society
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    • v.33 no.6
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    • pp.637-644
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    • 1996
  • Phase stability and isothermal phase transformation during gaging at 25$0^{\circ}C$ were investigated in yttria stabilized zirconia powders prepared from hydrolysis of zirconium isopropoxide. The stability of tetragonal phase at room temperature in zirconia powder was decreased with calcination temperature but increased with the addition of yttria content. During aging at 25$0^{\circ}C$ in humid atmosphere isothermal phase transformation occurred in tetragonal zirconia powder stabilized by constraint effect not by alloying effect and grain size effect. Many twins and microcrackings were found in transformed monomlinic zirconia particles.

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Effects of Wire speed Fluctuation on Arc Stability in GMA Welding (GMAW에서 와이어 송급속도의 변동이 아크안정성에 미치는 영향에 관한 연구)

  • 신현욱;최용범;성원호;장희석
    • Journal of Welding and Joining
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    • v.13 no.4
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    • pp.85-102
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    • 1995
  • Weld quality of GMA welding processes is closely related to arc stability. Although many researches on arc stability have been performed, real-time estimation of arc stability has not been attempted. For instance, Mita proposed a off-line statistical method in which short circuiting and arcing time, and voltage and current wave forms were sampled to assess arc stability. But this method is not suitable to assess arc stability for GMA welder which employ inverter power source due to its controlled current and voltage wave forms. In this paper, the relationship between are stability and wire feed rate fluctuation is analyzed to propose new criterion for inverter power source. When arc voltage and arc current and arcing time are analyzed, we can assess arc stability only for short circuit transfer mode. When wire feed rate is analyzed, we can estimate arc stability udner the condition of spray transfer mode as well. Hence, the wire feed rate is chosen for monitoring process variable to cover possible metal transfer modes in GMAW. Through this research, it has been identified that arc stability in GMA welding processes is closely related to wire fed rate. When inverter power source is used, conventional statistical method of estimating arc stability, such as Mita index, is no longer valid due to its controlled voltage and current wave forms. Arc stability has been also examined in phase plane diagram.

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