• 대한기계학회논문집
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• 제14권3호
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• pp.716-724
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• 1990

본 연구에서는 2차원 경사충돌분류에 대한 실험을 수행하였고, 이 결과를 수] 치해석에 의한 계산치와 비교하였다. 실험에서는 난류특성을 구명하기 위하여 열선 풍속계를 사용해 충돌각의 변화에 따른 평균속도, 각 난류응력성분, 압력등을 측정하 였다.수치해석을 위한 난류모델로는 표준 K-.epsilon.모델을 사용하였다.

• 한국식품과학회지
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• 제20권2호
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• pp.272-279
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• 1988

대표적인 재료로서 무를 선정하여 실제 건조과정을 이용한 dynamic test에 의하여 최적화에 사용할 수 있는 ascorbic acid파괴와 갈변의 kinetics를 구했다. 다양하게 변하는 건조조건에서 무우가 건조될 때의 수분함량과 온도의 변화과정을 이용하여 가정된 kinetic model을 적분하여 실제 측정된 ascorbic acid와 갈변의 data에 가장 접근하도록 parameter를 반복계산에 의하여 찾았다. Ascorbic acid 파괴와 갈변은 각각 1차와 0차반응으로, 두 반응의 온도의존성은 Arrhenius식으로 나타내었고 수분의존성의 가능한 여러 수학적 함수관계가 model의 단순성과 측정치와의 만족도면에서 비교되었고 타당한 품질변화 model을 분석하였다. 얻어진 kinetic model들은 실험치를 잘 예측하였고 건조공정의 최적화에 이용될 수 있는 것으로 생각되었다. Ascorbic acid 파괴는 건조초기 고수분함량에서 낮고 건조가 진행되어 수분 $9{\sim}12g/g$ dry solid부근에서 최대속도를 보이다가 더 이상 수분이 낮아지면서 감소하였다. 갈변은 고수분함량에서 낮다가 수분감소와 함께 증가하여 수분함량 $4{\sim}6g/g$ dry solid부근에서 최대를 나타내고 수분함량이 더욱 감소하면 계속 감소하였다.

• 대한원격탐사학회:학술대회논문집
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• 대한원격탐사학회 2005년도 Proceedings of ISRS 2005
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• pp.415-417
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• 2005

In order to retrieve the latent and sensible heat fluxes, high-resolution airborne imageries with visible, near infrared, and thermal infrared bands and ground-base meteorology measurements are utilized in this paper. The retrieval scheme is based on the balance of surface energy budget and momentum equations. There are three basic surface parameters including surface albedo $(\alpha)$, normalized difference vegetation index (NOVI) and surface kinetic temperature (TO). Lowtran 7 code is used to correct the atmosphere effect. The imageries were taken on 28 April and 5 May 2003. From the scattering plot of data set, we observed the extreme dry and wet pixels to derive the fitting of dry and wet controlled lines, respectively. Then the sensible heat and latent heat fluxes are derived from through a partitioning factor A. The retrieved latent and sensible heat fluxes are compared with in situ measurements, including eddy correlation and porometer measurements. It is shown that the retrieved fluxes from our scheme match with the measurements better than those derived from the S-SEBI model.

• Journal of Advanced Marine Engineering and Technology
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• 제21권4호
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• pp.430-436
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• 1997

Time-dependent Flow characteristics of a two-dimensional lid-driven square cavity flow of six high Reynolds numbers, $10^4 2{\times}lO^4., 3{\times}l0^4, 5{\times}lO^4, 7.5{\times}lO^4$ and $10^5$ were investigated. A convection conservative difference scheme based upon SOLA to maintain the nearly 2nd-order spatial accuracy was adopted on irregular grid formation. Irregular grid number is $80{\times}80$ and its minimum size is about 1/400 of the cavity height(H) and its maximum is about 1/53 H. The result shows that at Re= $10^4$, periodic migration of small eddies appearing in corner separation region and its temporal sinusoidal fluctuation are represented. And another critical Reynolds number which shows the commencement of flow randomness emerging from the periodic fluctuation is assumed to be around Re= $1.5{\times}10$. At five higher Reynolds numbers ($2{\times}lO^4., 3{\times}l0^4, 5{\times}lO^4, 7.5{\times}lO^4$ and $10^5$), an organizing structure of four consecutive vortices similar to a Moffat vortex at two lower corners is revealed from time-mean flow patterns.

• Journal of Electrical Engineering and Technology
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• 제13권1호
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• pp.60-67
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• 2018

The presence of induction motor loads in a power system may cause the phenomenon of delayed voltage recovery after the occurrence of a severe fault. A high proportion of induction motor loads in the power system can be a significant influence on the voltage stability of the system. This problem referred to as FIDVR(Fault Induced Delayed Voltage Recovery) is commonly caused by stall of small HVAC unit(Heating, Ventilation, and Air Conditioner) after transmission or distribution system failure. This delayed voltage recovery arises from the dynamic characteristics associated with the kinetic energy of the induction motor load. This paper proposes the UVLS (Under Voltage Load Shedding) control strategy for dealing with FIDVR. UVLS based schemes prevent voltage instability by shedding the load and can help avoid major economic losses due to wide-ranging cascading outages. This paper review recent topic about under voltage load shedding and compare decentralized load shedding scheme with conventional load shedding scheme. The load shedding strategy is applied to an actual system in order to verify the proposed FIDVR mitigation solution. Simulations demonstrate the effectiveness of the proposed method in resolving the problem of delayed voltage recovery in the Korean Power System.

• Journal of Electrical Engineering and Technology
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• 제12권6호
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• pp.2099-2105
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• 2017

In a power system that has a high wind penetration, the output power fluctuation of a large-scale wind turbine generator (WTG) caused by the varying wind speed increases the maximum frequency deviation, which is an important metric to assess the quality of electricity, because of the reduced system inertia. This paper proposes a stable power-smoothing scheme of a doubly-fed induction generator (DFIG) that can suppress the maximum frequency deviation, particularly for a power system with a high wind penetration. To do this, the proposed scheme employs an additional control loop relying on the system frequency deviation that operates in combination with the maximum power point tracking control loop. To improve the power-smoothing capability while guaranteeing the stable operation of a DFIG, the gain of the additional loop is modified with the rotor speed and frequency deviation. The gain is set to be high if the rotor speed and/or frequency deviation is large. The simulation results based on the IEEE 14-bus system demonstrate that the proposed scheme significantly lessens the output power fluctuation of a WTG under various scenarios by modifying the gain with the rotor speed and frequency deviation, and thereby it can regulate the frequency deviation within a narrow range.

• Journal of Electrical Engineering and Technology
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• 제11권1호
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• pp.86-92
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• 2016

Upon detecting a frequency event in a power system, the stepwise inertial control (SIC) of a wind turbine generator (WTG) instantly increases the power output for a preset period so as to arrest the frequency drop. Afterwards, SIC rapidly reduces the WTG output to avert over-deceleration (OD). However, such a rapid output reduction may act as a power deficit in the power system, and thereby cause a second frequency dip. In this paper, a hybrid reference function for the stable SIC of a doubly-fed induction generator is proposed to prevent OD while improving the frequency nadir (FN). To achieve this objective, a reference function is separately defined prior to and after the FN. In order to improve the FN when an event is detected, the reference is instantly increased by a constant and then maintained until the FN. This constant is determined by considering the power margin and available kinetic energy. To prevent OD, the reference decays with the rotor speed after the FN. The performance of the proposed scheme was validated under various wind speed conditions and wind power penetration levels using an EMTP-RV simulator. The results clearly demonstrate that the scheme successfully prevents OD while improving the FN at different wind conditions and wind power penetration levels. Furthermore, the scheme is adaptive to the size of a frequency event.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집D
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• pp.697-702
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• 2001

Numerical simulations of 1,2 dichloroethane(EDC) pyrolysis are conducted to understand the process on the production of the vinyl chloride monomer(VCM) and by-products. A chemical kinetic mechanism is developed, the adopted scheme involving 44 gas-phase species and 260 elementary forward and backward reactions. Detailed sensitivity analyses and the rates of production analysis are performed on each of the reactions and the various species, respectively. The concentrations of EDC, VCM, and HCI predicted by this mechanism are in good agreement with those deduced from experiments of commercial and laboratory scale. The mechanism is found to accurately predict the VCM yield and the production of by-products by varying the ranges of pyrolysis temperature, residence time, and pressure which impact on the pyrolysis of 1,2 dichloroethane. The influence of reactions related to H atom on the relative sensitivity of EDC becomes important as the residence time increases. The pyrolysis of EDC mainly occurs through $C_{2}H_{4}Cl_{2}+Cl=CH_{2}ClCHCl$.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.783-788
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• 2001

Choosing the most suitable mathematical model and relating this to turbulent tangential tensions model are very important in the investigations of turbulent two-phase flow. This paper considers two-fluid scheme. According to it, two phases have their own densities, velocities, and temperatures at any spatial point and at any moment. The equations of motion and heat transfer for each phase are linked with the forces of interaction between two phases. These forces are considered as predominant for the flow. As a closure in the system of motion equations, one modification of $K - {\epsilon}$ turbulent model is worked out. The modification uses two equations for turbulent kinetic energy of the phases and one - for the turbulent energy loss of main phase. This model can be set as a $K_g - K_p -{\epsilon}$ model. The modified model has been tested for both a two-phase non-isothermal flat jet and axially symmetrical jet. The numerical results are compared with the reference data revealing a good agreement between them.

• 한국해양공학회지
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• 제11권4호
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• pp.122-129
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• 1997

Flow characteristics of two-dimensional closed square cavities near unsteady critical Reynolds numbers were studied numerically at five Reynolds numbers : 8${\times}10^3$, 8.5${\times}10^3$, 9${\times}10^3$, 9.5${\times}10^3$ and $10^4$ were investigated. A convection conservative difference scheme based upon SOLA to maintain the nearly 2nd-order spatial accuracy was adopted on irregular grid formation. Irregular grid number is 80${\times}$80 and its minimum size is about 1/400 of the cavity height(H) and its maximum is about 1/53 H. The result shows that the critical Reynolds number indicating the emergence of flow unsteadiness exists near Re=8.5${\times}10^3$ and their flow patterns reveal periodic fluctuation during transient and fully-developed stages.