• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.7
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• pp.573-580
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• 2021

Paraffin wax is attracting many attentions for promising solid fuel of hybrid rocket because of its higher regression than other fuels. However, even with paraffin fuel combustion, unsteady low-frequency oscillation of combustion pressure is still observed. And, this is related to the formation of liquid layer and the entrainment of fuel droplets entering the axial combustion gas flow. This study investigates the effect of additional combustion of fuel droplets on the occurrence of low-frequency combustion instability. On the other hand, the formation of fuel droplets depends on Weber Number (the ratio of the inertial force to the surface tension of the liquid) and Reynolds Number of the oxidizer flow. Therefore, a laboratory-scale hybrid rocket was used to monitor the occurrence of combustion instability while changing We number. A series of combustion tests were conducted to control We number by changing the oxidizer flow rate or adding LDPE (low density polyethylene) to base fuel. In the results, it was confirmed that there is a critical We number above which the low-frequency combustion instability occurs.

• Proceedings of the Korea Water Resources Association Conference
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• 2020.06a
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• pp.310-310
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• 2020

자연하천에서 식생의 성장은 유속 및 수위 변화에 중요한 영향을 미치며, 따라서 식생으로 인한 흐름저항은 흐름과 유사이송 모델링을 위한 중요 매개 변수가 된다. 즉, 수치 모델을 활용함에 있어 식생의 흐름 저항을 정확하게 추정하는 것이 매우 중요하며, 보다 정확한 추정을 위해 실험 혹은 현장 데이터를 활용한 보정과정이 필수적으로 요구된다. 본 연구에서는 식생 패치를 포함한 인공 수로의 흐름 모의를 위해 실규모 실험 수로에서 측정된 유속 및 수위 데이터를 활용하여 모델보정을 수행하고자 한다. 이를 위해 공간 분포 별로 각기 다른 흐름 저항식의 적용이 가능하며, 식생 저항 공식을 포함하고 있는 Delft3D 모델을 활용하였다. 또한 실규모 수로에서의 유속 및 수위 데이터 수집을 위해 한국건설기술연구원 하천연구센터에서 실험을 수행하였다. 실험 구간의 길이는 약 120 m이고 하폭은 11 m이며, 국내 하천에서 보이는 식생패치의 유사한 형태를 재현하기 위해 하천 내 가장 많이 활착되어있는 버드나무와 유사한 형태의 인공식생을 제작하였다. 인공 식생은 지그재그로 배치되었으며, 식생의 전체 높이는 1.1 m이고, 각 패치 당 23그루의 인공 식생이 총 8개 패치에 식재되었다. 모의 조건은 상류단 유입 유량 2.805 ㎥/s, 하류단수위 98.764 m의 정류 조건을 적용하였다. 또한 식생 패치 구간에서의 흐름 저항 추정을 위해 Delft3D 모델 내에서 선택가능 한 Baptist의 비침수(Non-submerged)식을 적용하였으며, 항력계수 결정을 위해 1과 1.5를 적용하여 측정 수위와 비교하였다. Delft3D의 흐름 모의 결과, 항력계수 1.5를 적용했을 때, 측정 수위와 거의 일치하는 것으로 나타났으며, 항력계수 1.0을 적용했을 경우, 측정 수위에 비해 다소 낮게 모의되는 것으로 나타났다. 또한 항력계수 1.5인 경우 식생 패치 구간에서 평균 0.65 m/s의 유속이 발생하였다.

• Proceedings of the Korea Water Resources Association Conference
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• 2022.05a
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• pp.214-214
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• 2022

감포댐은 2006년 준공하여 각종 시설 및 설비의 노후화가 진행중에 있으며, 최근 감포댐 인근 지역에서 발생한 경주지진(규모 5.8) 및 포항지진(규모 5.4)이 발생하여 지진시 시설물의 안전성에 대해 중요성이 대두되고 있었다. 이에 감포댐 안전성강화 사업을 추진하게 되었고, 주요 사업내용은 취수탑 관리교 내진보강, 댐체 심벽보강을 통한 댐체 안전성확보, 비상방류시설 개선이다. 취수탑 관리교는 교각단면의 변위연성도 및 교량받침에 의한 구조물 안전성 평가를 시행하였고, 단면강도는 모멘트-곡률해석법에서 탄성영역의 단면강도 대비 탄성지진력을 비교하여 안전성을 검토하였다. 검토결과 교량받침의 내진성능은 부족한 것으로 검토되어 유량제어형 멀티펌프와 안전잭을 이용한 변위제어 방식의 교량동시 인상공법이 가능하고 인상정밀도 ±0.5mm이내의 성능을 가진 교량인상공법 적용하여 보강을 하는 것으로 계획하였다. 댐체 심벽보강을 위해 먼저 전기비저항탐사를 수행하여 전기비저항대를 탐사하여 심벽부에서 포화대가 형성된 부위를 조사하였다. 그리고 댐 상·하류에 전극을 설치하고 전류를 발생시켜 측정 지점별 자기장을 측정을 통하여 댐체내 침투에 따른 유로 형성여부 파악을 위한 전자기장탐사를 시행하였다. 마지막으로 심벽 시추조사를 시행하여 공내수를 조사하였다. 조사결과 포화대의 위치 및 이상대가 다양한 것으로 나타났으며, 이는 불특정 지점에 연약대가 존재한다 판단하여 심벽부 보강계획을 전반적으로 적용하는 것으로 결정하였다. 비상방류시설은 최근 15년 실측자료의 홍수기(6~9월) 평균유입량을 적용하여 검토하였으며 검토결과 배제대상 높이 75%까지는 3.7일, 완전배제까지는 11.2일이 걸려 댐설계기준을 충족하는 것으로 검토되다. 다만 감포댐 비상방류시설이 하천제방과 직각방향으로 형성되어 있고, 하천의 폭이 좁아 과거 방류시에도 제방을 월류하는 것으로 조사되어 하천의 세굴을 방지하고 제방 안전성을 확보하기 위하여 비상방류시설에 감세공을 설치하고, 제방에 디플렉터 옹벽을 설치하는 것으로 계획하였다. 감세공은 2번에 걸친 감세효과로 방류시 하류바닥 및 옹벽 보호 효과가 큰 미국 USBR의 TypeVI를 적용하였으며, Flow-3D 모델링을 통하여 제방에 월류가 발생하지 않는 최적의 대안 옹벽규모를 산정하였다.

• Journal of Korea Water Resources Association
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• v.51 no.12
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• pp.1273-1284
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• 2018

The flow and bed change were analyzed using the CCHE2D model, which is a two-dimensional numerical model, at a confluence of the Namhan River and Seom River where deposition occurs predominantly after the "Four Major Rivers Restoration Project." The characteristic of the junction is that the tributary of Seom River joined into the curved channel of the main reach of the Namhan River. The CCHE2D model analyzes the non-equilibrium sediment transport, and the adaptation lengths for the bed load and suspended load are important variables in the model. At the target area, the adaptation length for the bed load showed the greatest influence on the river bed change. Numerical simulation results demonstrated that the discharge ratio ($Q_r$) change affected the flow and bed change in the Namhan River and Seom river junction. When $Q_r{\leq}2.5$, the flow velocity of the main reach increased before confluence, thereby reducing the flow separation zone and decreasing the deposition inside the junction. When $Q_r$>2.5, there was a high possibility that deposition would be increased, thereby forming sand bar. Numerical simulation showed that a fixed sand bar has been formed at the junction due to the change of discharge ratio, which occurred in 2013.

• Clean Technology
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• v.27 no.1
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• pp.61-68
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• 2021

Selective catalytic reduction (SCR) is widely used as a method of removing nitrogen oxide in large-capacity thermal power generation systems. Uniform mixing of the injected ammonia and the inlet flue gas is very important to the performance of the denitrification reduction process in the catalyst bed. In the present study, a computational analysis technique was applied to the ammonia injection system design process of a denitrification facility. The applied model is the denitrification facility of an 800 MW class coal-fired power plant currently in operation. The flow field to be solved ranges from the inlet of the ammonia injection system to the end of the catalyst bed. The flow was analyzed in the two-dimensional domain assuming incompressible. The steady-state turbulent flow was solved with the commercial software named ANSYS-Fluent. The nozzle arrangement gap and injection flow rate in the ammonia injection system were chosen as the design parameters. A total of four (4) cases were simulated and compared. The root mean square of the NH3/NO molar ratio at the inlet of the catalyst layer was chosen as the optimization parameter and the design of the experiment was used as the base of the optimization algorithm. The case where the nozzle pitch and flow rate were adjusted at the same time was the best in terms of flow uniformity.

• Proceedings of the Korea Water Resources Association Conference
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• 2021.06a
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• pp.457-457
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• 2021

인공습지는 자연이 가진 정화기작을 인위적으로 증가시키기 위하여 조성한 자연기반해법에 해당한다. 인공습지는 습지 내 식물, 미생물, 토양 등의 상호기작에 의하여 오염물질이 제거된다. 인공습지의 오염물질저감효율은 시설의 규모와 유량, 유입물질의 부하량 수리학적 부하량, 체류시간 등의 영향을 받게 된다. 일반적으로 인공습지 적정 규모는 유역 및 기상인자의 특성과 조성목적에 고려하여 산정된다. 본 연구는 전국 35개 지역에 설치된 54개 인공습지를 선정하여 모니터링을 수행하였으며, 2011년부터 2018년에 설치된 시설이다. 54개 시설 중 도심지역에 13개, 농업지역 25개, 공업지역 3개, 상업지역 3개, 축산 10개가 설치되어있다. 습지형태는 Cell형 자유수면형 인공습지(Free Water Surface, Cell-FWS), 유로형(Flow) 자유수면형 인공습지(Cell-FWS), Cell과 Flow형이 결합된 Hybrid-FWS, 수직흐름형 인공습지(vertical flow constructed wetland)와 수평지하흐름형 인공습지(vertical flow constructed wetland)가 결합된 HYBIRD 형 습지로 구분된다. 연구결과, 일반적으로 SA/CA 비율이 클수록 오염물질의 저감효율은 증가하는 것으로 나타났다. 오염 물질별 인공습지 규모를 비교할 경우 저감효율 60%에서 인공습지의 규모는 유기물>영양염류>입자상물질 순으로 나타났다. 목표 제거효율 60%에서 SA/CA 비는 BOD에서 약 3.2%, COD에서 2.5%, SS에서 1.9%, TN 2.5%, TP 2.3%로 나타났다. 입자상물질인 SS는 유기물 및 영양염류에 비하여 유역면적 대비 시설면적이 가장 적게 나타났으며, 유기물질 제거에 큰 시설규모가 필요한 것으로 나타났다. 따라서 인공습지 설계시 유역 토지이용 및 강우특성을 고려하여 적정한 수질과 유량모니터링이 필요하며, 이를 토대로 목표 오염물질 선정이 중요한 것으로 나타났다. 또한, 농업지역의 최적화된 인공습지 위치는 임야가 20~30%, 밭이 20% 이하, 논이 10~50%를 포함하는 곳이 적정한 것으로 평가되었다. 도시지역 인공습지는 도시면적이 증가할수록 효율이 크게 변하지 않기에 가용위치가 적정한 위치로 평가된다. 인공습지의 효율은 유역의 세부 토지이용에 크게 의존하는 것으로 평가되었다. 따라서 인공습지 설계시 농업지역에서는 임야, 밭 및 논의 적정면적을 고려하여 인공습지 위치가 결정되어야 하는 것으로 나타났다.

• Journal of Korean Society of Environmental Engineers
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• v.35 no.12
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• pp.877-882
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• 2013

The application of ultrafiltration (UF) and microfiltration (MF) membranes has been increased for drinking water purification. The advantages of UF/MF membrane process compared to conventional treatment processes are stable operation under varying feed water quality, smaller construction area, and automatic operation. Most membrane treatment plants are designed with polymeric membranes. Recently, some studies suggested that the process of treating surface water with ceramic membranes is competitive to the application of polymeric membranes. Higher water flux, less frequent cleaning, and much longer lifetime are the advantages of ceramic membrane comparing to polymeric membrane. Therefore, this research focused on the application of ceramic MF membrane pilot plant at the slow sand filtration plant. The ceramic membrane pilot plant has three trains that used raw water and sand filtered water as a feed water, respectively. For optimizing the pilot plant process, the coagulation with PACl coagulant was used as a pretreatment of ceramic membrane process. In addition, CEB (Chemical Enhanced Backwash) process using $H_2SO_4$ and NaOCl was used for 1.5 days, respectively. The experimental results showed that applying the optimum coagulant dose before membrane filtration showed enhancing membrane fluxes for both raw water and sand filtered water. Also, when using raw water as a feed of membrane, minimum fouling rate was 2.173 kPa/cycle with 25 mg/L of PACl and when using sand filtered water, the minimum fouling rate was 0.301 kPa/cycle with 5 mg/L of PACl.

• Journal of Korean Society of Environmental Engineers
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• v.28 no.3
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• pp.276-285
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• 2006

When polluted stream water was treated with biological aerated filter(BAF) in pilot plant, all operation with 90, 60, 45 and 30 min of EBCT at fixed $0.1m^3air/m^2min$ of aeration showed 80% or higher treatment efficiency of particle materials(SS, turbidity and Chl.-a) and 85% or higher efficiency of ammonia nitrogen removal. It was thought that, in case of BOD, biological stability may sufficiently be assured with BAF because grade III or IV inflow water was changed to grade I for outflow water. In case of $COD_{Mn}$, about 60% of removal efficiency was found. When the mechanism of the result was investigated, about 30% of COD materials was produced by algae clogged in the reactor. There was almost no biological decomposition because specific substrate utilization rate of algogenic organic materials were $0.0245mg{\cdot}COD_{Mn}/mg{\cdot}VSS{\cdot}day$, thus partial backwashing(washing the media in 1 m upper of the reactor once a day) was required. It is thought that elevation of removal rate about 10% of $COD_{Mn}$ and 5.5% of $BOD_5$ could be obtained with partial backwashing resulting in assurance of biologically more stable raw water and that saving backwashing water may be significant.

• Journal of Korea Water Resources Association
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• v.41 no.3
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• pp.305-314
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• 2008

Urban sewer systems are designed to operate in open-channel flow regime and energy loss at circular manholes are usually not significant. However, the energy loss at manholes, often exceeding the friction loss of pipes under surcharge flow, is considered as one of the major causes of inundation in urban area. Therefore, it is necessary to analyze the head loss associated with manholes, especially in surcharge flow. Hydraulic experimental apparatus which can be changed the invert type(CASE A, B, C) and step height(CASE I, II, III) was installed for this study. The range of the experimental discharges were from $1.0{\ell}/sec$ to $5.6\;{\ell}/sec$. As the manhole diameter ratio($D_m/D_{in}$) increases, head loss coefficient increases due to strong horizontal swirl motion. Head loss coefficient was maximum because of strong oscillation of water surface when the range of manhole depth ratios($h_m/D_{in}$) were from 1.0 to 1.5. The average head loss coefficients for CASE A, B, and C were 0.45, 0.37, and 0.30, respectively. Accordingly, U-invert is most effective for energy loss reduction at circular manhole. This head loss coefficients could be available to design the urban sewer system with surcharge flow.

• Journal of Korean Society of Environmental Engineers
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• v.27 no.9
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• pp.978-988
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• 2005

A three-dimensional dynamic model was applied to Lake Paldang, Han River in this study. The model was calibrated and verified using the data measured under different ambient conditions. The model results were in reasonable agreements with the field measurements in both calibration and verification. Utilizing the validated model, we analyzed the spatial and temporal distributions of temperature, current, residence time, and spreading pattern of incoming flows within the lake. Relatively low velocity and high temperature were computed at the surface layer in the southern region of the Sonae island. The longest residence time within the lake was predicted in the southern region of the Sonae island and the downstream region of the South Branch. This can be attributed to the fact that the back currents caused by the dam blocking occur mainly in these regions. Vertical thermal profiles indicated that the thermal stratifications would be occurred feebly in early summer and winter. During early spring and fall, it appeared that there would be no discernible differences at the vertical temperature profiles in the entire lake. The vertical overturns, however, do not occur during these periods due to an influence of high discharge flows from the dam. During midsummer monsoon season with high precipitation, the thermal stratification was disrupted by high incoming flow rates and discharges from the dam and very short residence time was resulted in the entire lake. In this circulation patterns, the plume of the Kyoungan stream with smallest flow rate and higher water temperature tends to travel downstream horizontally along the eastern shore of the south island and vertically at the top surface layer. The model results suggest that the Paldang lake should be a highly hydrodynamic water body with large spatial and temporal variations.