• Nuclear Engineering and Technology
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• 제53권6호
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• pp.1796-1809
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• 2021

The subchannel of a research reactor used to generate high power density is designed to be narrow and rectangular and comprises plate-type fuels operating under downward flow conditions. Critical heat flux (CHF) is a crucial parameter for estimating the safety of a nuclear fuel; hence, this parameter should be accurately predicted. Here, machine learning is applied for the prediction of CHF in a narrow rectangular channel. Although machine learning can effectively analyze large amounts of complex data, its application to CHF, particularly for narrow rectangular channels, remains challenging because of the limited flow conditions available in existing experimental databases. To resolve this problem, we used four CHF correlations to generate pseudo-data for training an artificial neural network. We also propose a network architecture that includes pre-training and prediction stages to predict and analyze the CHF. The trained neural network predicted the CHF with an average error of 3.65% and a root-mean-square error of 17.17% for the test pseudo-data; the respective errors of 0.9% and 26.4% for the experimental data were not considered during training. Finally, machine learning was applied to quantitatively investigate the parametric effect on the CHF in narrow rectangular channels under downward flow conditions.

• Korean Chemical Engineering Research
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• 제59권1호
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• pp.138-151
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• 2021

The effect of a reactant ratio on the growth of a buoyancy-driven instability in an irreversible A+B→C reaction system is analyzed theoretically and numerically. Taking a non-stoichiometric reactant ratio into account, new linear stability equations are derived without the quasi-steady state assumption (QSSA) and solved analytically. It is found that the main parameters to explain the present system are the Damköhler number, the dimensionless density difference of chemical species and the ratio of reactants. The present initial grow rate analysis without QSSA shows that the system is initially unconditionally stable regardless of the parameter values; however, the previous initial growth rate analysis based on the QSSA predicted the system is unstable if the system is physically unstable. For time evolving cases, the present growth rates obtained from the spectral analysis and pseudo-spectral method support each other, but quite differently from that obtained under the conventional QSSA. Adopting the result of the linear stability analysis as an initial condition, fully nonlinear direct numerical simulations are conducted. Both the linear analysis and the nonlinear simulation show that the reactant ratio plays an important role in the onset and the growth of the instability motion.

• Geomechanics and Engineering
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• 제30권4호
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• pp.393-399
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• 2022

The Segregation Potential (SP) is one of the most widely used predictors of frost heave in cold regions. Laboratory step-freezing tests determining a representative SP at the onset of the formation of the last ice lens (near the thermal steady state condition) can predict susceptibility to frost heave. Previous work has proposed empirical semi-log fitting for determination of the representative SP and applied it to several fine-grained soils, but considering only frost-susceptible soils. The presence of fines in coarse-grained soil affects frost susceptibility. Therefore, it is required to evaluate the applicability of the empirical semi-log fitting for both frost-susceptible and non-frost-susceptible soils with fines content. This paper reports laboratory frost heave tests for fines contents of 5%-70%. The frost susceptibility of soil mixtures composed of sand and silt was classified by the representative SP, and the suitability of the empirical semi-log fitting method was assessed. Combining semi-log fitting with simple laboratory frost heave testing using a temperature-controllable cell is shown to be suitable for both frost-susceptible and non-frost-susceptible soils. In addition, initially non-frost-susceptible soil became frost susceptible at a 10%-20% weight fraction of fines. This threshold fines content matched well with transitions in the engineering characteristics of both the unfrozen and frozen soil mixtures.

• Nuclear Engineering and Technology
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• 제56권2호
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• pp.762-769
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• 2024

The TMSR-SF0 simulator is an integral effect thermal-hydraulic experimental system for the development of thorium molten salt reactor (TMSR) program in China. The simulator has two heat transport loops with liquid FLiNaK. In literature, the 95% level confidence uncertainties of the thermophysical properties of FLiNaK are recommended, and the uncertainties of density, heat capacity, thermal conductivity and viscosity are ±2%, ±10, ±10% and ±10% respectively. In order to investigate the effects of thermophysical properties uncertainties on the molten salt heat transport system, the uncertainty and sensitivity analysis of the heat transfer characteristics of the simulator system are carried out on a RELAP5 model. The uncertainties of thermophysical properties are incorporated in simulation model and the Monte Carlo sampling method is used to propagate the input uncertainties through the model. The simulation results indicate that the uncertainty propagated to core outlet temperature is about ±10 ℃ with a confidence level of 95% in a steady-state operation condition. The result should be noted in the design, operation and code validation of molten salt reactor. In addition, more experimental data is necessary for quantifying the uncertainty of thermophysical properties of molten salts.

• 한국추진공학회지
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• 제3권4호
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• pp.93-103
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• 1999

소, 중형 산업용 항공기나 초등 훈련기용으로 많이 이용되고 있는 터보프롭 엔진의 정상상태 성능해석 프로그램을 개발하고 성능진단을 위해 선형 GPA 기법을 적용하였다. 정상상태 성능해석 프로그램의 검증을 위해 상용 정상상태 해석 프로그램인 TURBOMARCH 해석결과와 비교하였다. 지상정지조건에서의 성능 및 비행마하수에 따른 출력 둥을 비교한 결과 각 구성품의 입,출구 온도 및 압력, 출력 등에서는 약 3%이하의 오차율을 보였으며, 마하수 변화에 따른 출력 비교에서도 최대오차율 2.4 % 이내로 프로그램의 신뢰성을 확인하였다. 선형 GPA 기법의 계측변수의 선정에 따른 정확성을 알기 위해 종속변수의 선정을 다르게 하여 오차율을 알아보았다. 성능저하 원인으로는 압축기에만 오염이 발생하였을 경우, 압축기와 압축기 터빈에 각각 오염과 부식이 발생하였을 경우, 압축기 터빈과 동력터빈에 동시에 부식이 발생하였을 경우, 압축기, 압축기 터빈, 동력터빈이 모두 오염과 부식이 발생하였을 경우를 가정하였다. 해석결과 계측기 변수가 많을수록 RMS 오차가 적었으며 같은 수의 계측기 변수라 하더라도 어떠한 변수를 계측하는가에 따라 오차율이 달라짐을 알 수 있었다. 비교적 오차율이 적으면서 경제성이 있는 경우는 8개의 측정변수를 이용한 경우로 최적의 계측기 수 및 계측기 변수 선정이 중요함을 알 수 있었다.

• 한국음향학회지
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• 제7권4호
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• pp.54-76
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• 1988

본 연구의 제 1 부에서는 통일된 행렬표현 기법을 통하여 여러가지 블럭적응 여파기 구현방법들을 도출할 수 있음을 보였다. 제 2 부에서는 여러 주파수영역 블럭적응 여파기들 중에서도 수렴속도가 매우 빠른 self-orthogonalizing 알고리즘과 계산량이 대폭 감소되는 비제약 알고리즘의 수렴특성들을 overlap-save 및 overlap-add 블럭데이타 분할방법에 대해서 분석한다. 먼저, 수렴인자가 상수일 때와는 달리, 앞에서 언급한 두 주파수영역 여파기들이 공통의 자기상관행렬의 지배를 받기 때문에 수렴특성 분석에 있어서 서로 밀접한 관련이 있음을 보인다. 다음으로 여파기 계수의 수효가 충분히 클 때, 주파수영역 블럭적응 여파기는 계수적응 알고리즘에서 제약의 유무에 관계없이 동일한 최적해를 가짐을 보인다. 그리고 나서 비제약 알고리즘의 계수들은 적절한 조건하에서 원래의 제약알고리즘과 같이 동일한 최적해에 수렴함을 증명한다. 이에 반하여, 최소자승오차 관점에서의 성능분석 결과는 제약을 풀었을 경우에 정상상태에서 약간의 성능저하가 있음을 밝혀낸다. 한편으로 계수의 수효가 작을 때는 원래의 제약 알고리즘은 심한 성능저하를 초래하는 반면에 비제약 알고리즘은 제약의 제거를 통해 상대적으로 계수의 수효가 증가한 효과 대문에 훨씬 좋은 수렴특성을 가짐을 보인다. 또한 self-orthogonalizing 주파수영역 블럭적응 여파기의 자기상관행렬이 주파수 영역에서 대각행렬로 됨을 보여 줌으로써 효율적으로 수렴시간을 단축시키는 구현방법임을 뒷받침한다.

• 한국지반환경공학회 논문집
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• 제16권3호
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• pp.5-13
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• 2015

해일에 의한 바닷물의 월류는 방조제 구조물에 피해를 유발시킨다. 지금까지 발생된 마운드 형태 방조제의 파괴 유형 조사 결과, 육지 쪽 마루부 파손과 선단부의 세굴이 대표적인 파괴 사례로 지목되었다. 이와 같은 파괴를 유발하는 가장 큰 원인은 월류에 의해 발생된 압력과 유속이다. 본 연구에서는 에너지 관점에서 마운드 형태 방조제에서 압력과 유속을 산정할 수 있는 이론해를 제안하고 검증하였다. 이를 위해 방조제 마루부와 선단부에 흐름을 유사정적비회전류로 보고 동심원유선이 형성된다고 가정하였다. 한계흐름조건과 베르누이정리를 이용하여 방조제 마루부와 배면 선단부에서의 작용하는 압력 및 유속 산정식을 유도하였다. 이들 식을 이용하여 동심원유선 및 월류고를 가정하여 마루부와 선단부에서 압력과 유속을 산정하였다. 그 결과 마루부에서는 부의 압력이 선단부에서는 양의 압력이 각각 크게 작용하는 것으로 나타났으며 유속에 의한 전단응력도 작용하는 것을 확인하였다. 또한 제안된 이론해의 적용 한계에 대한 고찰도 이루어졌다.

• Environmental Sciences Bulletin of The Korean Environmental Sciences Society
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• 제1권2호
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• pp.125-136
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• 1997

Digestion of a municipal wastewater sludge by the anaerobic sequencing batch reactor(ASBR) was investigated to evaluate the performance of the ASBR process at a critical condition of high-solids-content feed. The reactors were operated at an HRT of 10 days with an equivalent loading rate of 0.8-1.5 gVS/L/d at $35^{\circ}C.$ The main conclusions drawn from this study were as follows: 1. Digestion of a municipal wastewater sludge was possible using the ASBR in spite of high concentration of settleable solids in the sludge. The ASBRS with 3- and 4-day cycle period showed almost identical high digestion performances. 2. No adverse effect on digestion stability was observed in the ASBRS in spite of withdrawal and replenishment of $30\%\;or\;40\%$ of liquid contents. A conventional anaerobic digester could be easily converted to the ASBR without any stability problem. 3. Flotation thickening occurred in thicken step of the ASBRS throughout steady state, and floating bed volume at the end of thicken period occupied about $70\%$ of the working volume of the reactor. Efficiency of flotation thickening in the ASBRS could be comparable to that of additional gravity thickening of a completely mixed digester. 4. Solids were accumulated rapidly in the ASBR during start-up period. Solids concentrations in the ASBRS were 2.6 times higher than that in the completely mixed control reactor at steady state. Dehydrogenase activity had a strong correlation with the solids concentration. Dehydrogenase activity of the digested sludge in the ASBR was 2.9 times higher than that of the sludge in the control reactor, and about 25 times higher than that of the subnatant in the ASBR. 5. Remarkable increase in equivalent gas production of $52\%$ was observed at the ASBRS compared with the control reactor in spite of similar Quality of clarified effluent from the ASBRS and control reactor. The increase in gas production from the ASBRS was believed to be combined results of accumulation of microorganisms, higher driving force applied, and additional long-term degradation of organics continuously accumulated.

• 에너지공학
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• 제8권4호
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• pp.592-604
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• 1999

대면적 용융탄산염 연료전지 운전 시 스택에서 발생하는 스택내 온도차와 극간 차압의 문제점을 해결하기 위하여 정상상태특성 및 동특성을 분석하였다. 두가지 용융탄산염연료전지 스택(5 kW(3,000$\textrm{cm}^2$, 20장), 3kW(6,000$\textrm{cm}^2$, 5장) 의 운전결과를 통하여 스택성능에 중요한 영향을 미치는 조작변수(전류밀도 , 연료 이용률, 공기 이용률)와 조절변수(스택 온도차, 연료극 차압, 공기극 차압)를 결정하였다. 조작변수들에 대한 조절변수들의 변화량인 스택의 정상상태 이득률을 구하여 시스템의 안정성을 분석하고 동특성을 나타내는 전달함수를 구하였따. 전달함수는 3$\times$3 행렬로 시간 지연항이 없는 전형적인 1차 시스템으로 표현되었다 동특성 분석에 의해 대면적 용융탄산염 연료전지 시스템의 최적 운전 조건을 설정할 수 있었다. 5 kW 스택의 경우 전류밀도가 150mA/$\textrm{cm}^2$ 일 때 스택 출구 온도를 68$0^{\circ}C$ 이하로 유지하기 위해서는 공기극 가스 recycle 에 의한 가압 운전이 필요하였으며, 효과적으로 용융탄산염 연료전지 시스템을 제어하기위해서는 조작변수의 조절변수상호간의 연계성을 제거하기위한 다중 입.출력 제어 및 연계 제거기가 필요함을 확인하였다. 이 결과는 향후 대면적 용융탄산염을 연료전지 시스템의 제어구조 설계와 운전모드설정에 중요한 자료로 사용될 것이다.

• 한국농공학회지
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• 제37권5호
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• pp.53-61
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• 1995

Analyses of subsurface drainage effects of farmland with respect to pipe and envelop material are made by the laboratory experiments using soil box to give basic information for the subsurface drainage system planning and design. Three different diameter PVC perforated pipes and a mesh pipe are used with envelop materials such as sand, rice bran, and crushed stone. Steady state subsurface drainage flow rate increased as envelop material changed from sand to rice bran and crushed stone. This indicates that as the hydraulic conductivity of the envelop material increases, the drainage flow rate increases. On the other hand, for a given envelop material, the mesh pipe which has the largest openning area shows the largest flow rate while small diameter PVC pipes show small flow rates. This tells that as the openning area and pipe diameter increase, the flow rate increases, too. Therefore, selection of pipe and envelop material should be made in accordance with the design drainage flow rate. Unsteady state subsurface drainage flow rate with respect to time differs for different envelop material. In case the sand was used as an envelop material, the small diameter PVC pipes show larger flow rates than the large diameter PVC pipe and mesh pipe. When the rice bran was used, the mesh pipe shows the largest flow rate, while small diameter pipes show smaller flow rates. In case the crushed stone was used as an envelop material, the large diameter PVC pipe and mesh pipe show larger flow rates, while small diameter pipes show a little bit smaller flow rates. However, the variation of flow rates among different pipes is the smallest when the crushed stone is used. The flow rate curve with respect to the pipe changes little for the crushed stone envelop which has a large hydraulic conductivity, while that changes much for the sand and rice bran envelops. However, it is difficult to draw a consistent relationship between the drainage flow rate and pipe for all the envelop materials. Since the subsurface drainage experiments are made only under the restricted laboratory condition in this study, further study including field experiment is required.