원뿔형 분사층 반응기의 다상유동 및 열전달 특성 연구

김효성 연세대학교 대학원 2019 국내석사

연약지반 개량을 위한 이중코어 PBD 성능연구 및 현장 적용성 검토

양정훈 고려대학교 대학원 2009 국내석사

본 연구의 계측 및 시료 채취는 부산신항 북컨테이너 터미널의 시험 시공현장에서 수행되었다. 시험 시공현장은 준설토를 매립하여 조성된 지반으로 연약층의 두께가 40m~50m의 대심도의 점토층으로 구성되어 있으며, 이를 개량하기 위하여 연직배수공법을 적용하여 40m 깊이까지 PBD(plastic board drain)를 타설하여 적용하고 모래매트 및 재하하중을 성토를 하였다. 연직배수 공법은 연약지반의 배수거리를 단축하여 압밀을 촉진시키는 공법으로, 주변지반으로부터 간극수가 배수재 내로 유입되고 배수재 내부의 종방향 유로를 통하여 간극수를 배출하게 된다. 시공현장에 타설된 PBD는 기존의 샌드 드레인에 비하여 경제적인 이점이 있으나, 얇은 판 모양의 단면 구조로 인하여 지반의 침하에 따른 PBD의 절곡, 폐색현상, 토압에 의한 단면감소 등으로 인한 통수능 감소 현상이 쉽게 발생할 가능성이 있다. 통수능 감소가 지속적으로 발생하여 단위시간당 유입되는 간극수량이 배수재의 통수능력보다 크게 유입되었을 때, 원활한 간극수 배출이 방해받게 되는데 이러한 현상을 배수저항이라 정의한다. 배수저항은 측압, 동수경사, 세립자의 침투 등의 지반조건과, 배수재의 길이, 단면적, 재질 등의 배수재 조건에 의해 영향을 받게 된다. 특히 본 연구 대상과 같이 대심도 연약지반에 설치한 배수재의 경우 연약지반을 개량하기 위하여 배수재의 길이가 증가하고, 대심도에서는 큰 횡 방향 토압이 발생하여 배수저항에 의한 압밀지연의 가능성이 우려되었다. 이러한 배수저항 요인을 극복하기 위하여 통수단면적과 강성을 증가시킨 이중코어 PBD를 시험현장에 타설하여 기존의 단일코어 PBD와 비교 분석하고, 현장시험 결과로부터 대심도 연약지반 개량을 위한 이중코어 PBD 적용 필요성을 검토하였다.

연약지반 개량을 위한 순환골재다공질 콘크리트 말뚝과 쇄석말뚝으로 보강된 복합지반의 거동특성

김세원 고려대학교 대학원 2009 국내석사

최근 건설량의 증가로 인하여 부족한 산업부지 확충과 새로운 건설부지 확보가 필요한 실정이다. 이를 위해 국내에서는 연약한 해안점토 및 연약지반에 대한 활발한 연구가 진행되고 있다. 이러한 연약지반은 상부 구조물을 지지할 수 없는 상태의 지반으로 유기질토가 많이 포함되어 있는 고함수비, 저응력 그리고 큰 변형률 등의 특성을 가지고 있다. 따라서 큰 침하와 전단변형을 유발하게 되어 사면파괴와 지반지지력 부족 등 안전성에 영향을 주게 되므로 지반 개량을 통한 안전성 확보가 필수적이라 하겠다. 연약지반개량에는 개량 원리에 따라 여러 가지 공법들이 있으며, 국내에서는 모래다짐말뚝(sand compaction pille, SCP) 및 쇄석다짐말뚝(Granular compaction pile) 공법 등을 사용하고 있다. 이러한 공법은 원지반과 다짐으로 인하여 밀도가 조밀해진 모래 및 쇄석에 의해 복합지반을 조성하여 강도의 증가 및 압축성 즉, 침하저감 등의 공학적 성질을 개선시킨다. 그러나 이러한 공법들은 천연재료의 부족으로 인한 재료비의 급증과 환경파괴라는 쟁점으로 인해 그 적용의 한계에 다다르고 있는 실정이다. 또한 이러한 공법들은 말뚝 선단부의 팽창파괴(Bulging Failure)나 말뚝표면의 간극막힘(Clogging) 현상 등의 결함이 우려된다. 따라서 이러한 기존 공법들의 단점들을 극복하기 위해서는 재료적인 측면과 기능성 측면에서의 새로운 공법 개발이 시급한 실정이다. 최근 우리나라에서는 건축 구조물의 노후화로 인한 재건축 등으로 기존의 콘크리트 구조물의 해체 폐기가 활발하게 진행되고 있다. 매년 기하급수적으로 발생되고 있는 폐콘크리트 등의 건설폐기물 처리가 새로운 사회적 문제로 부각되고 있는 실정이다. 건설폐기물의 90%정도가 폐콘크리트, 폐아스팔트, 토사로 이루어져 있기 때문에 적절한 분리, 선별의 처리과정을 거친다면 새로운 골재자원으로 이용할 수 있다. 특히, 이러한 과정에서 얻을 수 있는 순환골재는 그 활용도가 양호한 것으로 평가되고 있으며, ‘건설폐기물의 재활용촉진에 관한 법률’에 따라 대형 공사현장에서는 순환골재를 의무적으로 사용해야 하기 때문에 순환골재를 이용한 연약지반 개량공법의 그 활용도가 양호할 것으로 판단된다. 이러한 배경으로 최근 건설재료의 수급으로 인해 고갈되는 천연자원을 보호하고 건설재료의 품귀현상을 극복하기 위하여 건설 폐기물인 순환골재를 연약지반개량 분야에 적용함과 동시에 기존 다짐말뚝공법들의 재료적 측면과 기능적 측면의 단점을 개선하는 새로운 공법인 순환골재 다공질 콘크리트말뚝에 의한 지반 개량효과를 실내모형실험을 통하여 분석하였다. 실내모형실험에서는 개량된 복합지반의 침하량과 과잉간극수압 및 연직응력증분을 관측하였으며 실험 결과로부터 시간의 경과에 따른 복합지반의 거동 특성을 확인하였다. 또한 본 연구에서는 유한요소법을 이용한 ABAQUS 상용 프로그램을 사용하여 실내모형실험을 비교 검토하였다. 또한 이러한 말뚝이 타설된 현장의 복합지반의 개량효과를 검증하기 위해 지반 내에 연약한 암염층(Sabkha)이 존재하는 Arabian Gulf 연안의 Kayan, Saudi 지역에 쇄석다짐말뚝이 시공된 지반의 재하시험 결과를 검토하였다. Priebe가 제안한 복합지반 침하예측식으로 설계된 현장의 침하량을 정재하시험 계측값과 비교하고, ABAQUS 프로그램을 이용한 수치해석을 수행하여 검증하였다.

지열 냉난방 시스템에서 지중 열교환기의 열전달 거동 연구

길후정 고려대학교 대학원 2009 국내석사

Under the present international energy situation, it is necessary to develop new and renewable energies which will substitute fossil energies, and the government has a big interest in development and supply of new energies, and it also encourages a research to keep pace with the advanced countries. Among new and renewable energies, the geothermal energy is a new energy source that is not only inexhaustible but also eco-friendly. The most common example using geothermal energy is ground heat exchanger in a geothermal heat pump system (GHP). Up to the present this heating system is popularly constructed in many advanced countries including the USA, and this construction is increasing gradually in Korea, too. The heat exchanger according to its establishing method can be classified into four modes such as a vertical mode, a horizontal mode, a surface water mode, and an underground water mode. A closed-loop vertical ground heat exchanger is widely used in Korea because of the installing site problem. In this study, a series of numerical analysis was accomplished on the thermal performance and sectional efficiency of a closed-loop vertical ground heat exchanger (U-loop) in a geothermal heat pump system (GHP) involving soil and grout. ABAQUS which is finite element analysis program was employed to evaluate the temperature distribution on the cross section of the U-loop system involving HDPE pipe/grout/formation. And It was compared the sectional efficiency between the conventional U-loop and a new latticed HDPE pipe system. Especially, the latticed pipe is equipped with a thermal insulation zone in order to reduce thermal interference between the inflow pipe and the outflow pipe. Also, analysis of thermal stress was evaluated in this numerical analysis along with ABAQUS. FLUENT which is 3-D finite volume analysis program that can analyze coupled system between heat transfer and fluid flow, was used in simulating the operating process of the closed-loop vertical ground heat exchanger by considering the effect of the thermal properties of grout, rate of circulation pump, distance between the inflow pipe and the outflow pipe, and the effectiveness of the latticed HDPE pipe system. Also, series of analysis about heat exchanger in tunnel(Energy textile) was accomplished.

표층처리된 초연약지반 거동에 관한 연구

이종선 고려대학교 대학원 2009 국내석사

It is necessary to develop a rational design method for surface reinforcement of very soft ground because most current design works rely on merely crude empirical correlations. In this study, the mechanical behavior of very soft ground that is reinforced on the surface was investigated with the aid of a series of numerical analyses. Key material properties of each dredged soft ground, reinforcement and backfill sand mat have been parametrically estimated in the numerical analysis. The result of numerical analysis was compared with those of the laboratory model test. Through back-analyses by matching the numerical and experimental result, it is possible to determine representative material properties of the dredged soft ground, reinforcements and backfill sand mat. These verified material properties permit to evaluate the stiffness of reinforcement and the thickness of sand mat on the overall deformation of the reinforced soft ground. Also, The effect of surface desiccation confirmed the result of numerical analyses based on these numerical model. Along with the result of the study previously performed, a series of in-situ loading conditions and settlement due to reinforcement operation by construction vehicles such as Low Ground-Pressure Dozer, PBD machine, has been numerically simulated. These result have been used to evaluate the limit bearing capacity for the unreinforced and reinforced soft ground. In addition, Bearing Capacity Ratio() are proposed to shows the influence of surface reinforcement. The numerical result obtained in this research were compared with Yamanouchi`s empirical correlation for the limit bearing capacity. The series of engineering charts for estimating the limit bearing capacity will provide field engineers with preliminary design tool for surface reinforcement of very soft ground.

수평형 지중 열교환기 되메움재의 열특성에 관한 연구

이강자 高麗大學校 大學院 2010 국내석사

This thesis presents the results of a laboratory study on the thermal conductivity of silica sand and residual granite soil used as backfilling materials for horizontal ground heat exchanger. It is necessary for as horizontal ground heat exchanger to enhance its thermal effectiveness and reduce initial construction cost. Most of soil characteristics such as water content and dry density, soil size, shape, etc. influence on thermal conductivity. Therefore, the thermal conductivity-water content-dry density relationship is required to be quantitatively studied in the heat transfer process considering various soil moisture conditions such as saturated, partially saturated and dried soils. A series of laboratory experiments have been performed to evaluate the thermal conductivity of the proposed backfill materials with the aid of the TP02 System and the QTM-500 System. Test results show that high dry density(or low porosity) and water content in soils increase thermal conductivity. The residual granite soil's thermal conductivity higher than silica sand due to higher quartz content and well-graded grain distribution . Additionally, A variety of efforts intends to develop a specific thermal conductivity model for backfilling soils correlating thermal conductivity, dry density and water content. To propose a new model, the thermal conductivities of the silica sand and the residual granite soil were compared with the popular empirical prediction models such as Kersten(1949), Johansen(1975), etc. In case of silica sand, the Kersten model was slightly modified to fit all of the measurements. However, the Kersten model could not properly represent the thermal conductivity relationship of the residual granite soil such that a new simply model has been proposed for it. The results show that the new models provide accurate approximations of soil thermal conductivity for the two type of soils. From the proposed model herein, a set of engineering charts were provided, which can be used to estimate the thermal conductivity of the two soil types for constructing horizontal ground heat exchangers.

이차원 방사방향 축대칭 비선형 유한변형 압밀이론에 대한 실험적 고찰

안용훈 高麗大學校 大學院 2010 국내석사

Many fine-grained dredged materials undergo strains greater than 50% during self-weight consolidation that requires a great amount of time. Therefore, it is of importance to predict the time rate of settlement for dredged materials. Vertical drains are commonly used to accelerate the consolidation process of soft soils, such as dredged materials. The installation of vertical drain provides a horizontal drainage path to water in the deposit soil in addition to the vertical direction. The estimation of time rate of settlement is considerably complicated when vertical drains are installed to enhance consolidation process of dredged material because the vertical drains are commonly installed before self-weight consolidation is ceased. In this thesis, the vertical drain theory developed by Barron(1948) is applied to analyze the non-linear consolidation behavior considering horizontal drainage. The overall average degree of self-weight consolidation of the dredged soil under the condition that the water is drained in both horizontal and vertical directions is estimated using the Carillo(1942) formula. In addition, the Morris(2002) theory and the one-dimensional non-linear finite strain numerical model(PSDDF : Primary Consolidation, Secondary Compression, and Desiccation of Dredged Fill) are applied to analyze the self-weight consolidation in case of only the vertical drainage is considered. Two methods are applied to predict the self-weight consolidation behavior considering the vertical drain. One is to combine the Morris theory for self-weight consolidation considering only the vertical drainage and the Barron theory for the horizontal drainage. The combination of the two separate methods is conducted the Carillo formula. The other one is that the results of PSDDF analysis for the one-deimensional self-weight consolidation is combined with the Barron theory by adapting the finite strain coefficient of consolidation () for each void ratio that is obtained from the PSDDF analysis. The Carillo formula is also used in this method. A series of self-weight consolidation tests of 100cm height with and without a vertical drain in laboratory are performed to compare with the consolidation behavior predicted by the analytical and numerical solutions. Finally, the effect of installation time of vertical drains during the self-weight consolidation is examined by simulating an imaginary dredged material placement site with 10m and 20m thickness. The installation time is det to 10%, 30%, 50%, 70% and 90% of the average degree of consolidation of the dredged material at the sites. The method proposed herein can simulate properly the time rate of the self-weight consolidation of dredged materials that is facilitated with vertical drains.

준설매립지반의 자중압밀을 고려한 2차원 축대칭 비선형 유한변형 압밀 모델 개발

곽태훈 高麗大學校 大學院 2011 국내석사

Vertical drains have been commonly used to increase the rate of the consolidation of dredged material. The installation of vertical drains additionally provides a radial flow path in the dredged foundation. In practice, vertical drains are usually installed in the process of self-weight consolidation of a dredged soil deposit which makes conventional analytical or numerical models difficult to quantify the consolidation behavior. The objective of this study is to develop a partial differential equation and to develop a numerical model for 2-D axisymmetric non-linear finite strain consolidation considering self-weight consolidation to predict the behavior of a vertical drain in dredged foundation which is installed during the self-weight consolidation. The non-linear relationship between the void ratio and effective stress and permeability during consolidation are taken into account in the model. In order to verify the developed model in this research, the results of the numerical analysis are compared with that of the lab-scaled self-weight consolidation test in which an artificial vertical drain is activated at any designated time(for example, 50% and 70% degree of self-weight consolidation). In addition, the model verification has been carried out by comparing with the simplified method proposed by An et al.(2010) witch is uncoupled method to consider an effect of a vertical drain during the self-weight consolidation in a dredged deposit. The comparisons show that the developed numerical model can properly simulate the consolidation of the dredged material with the vertical drains installed. Finally, the effect of installation schedule of the vertical drains and of pre-loading during the self-weight consolidation is examined by simulating an imaginary dredged material placement site with thicknesses of 10 m and 20 m. The vertical drains were installed in the simulation at 30%, 50% and 70% of the average degree of consolidation of the dredged material. This simulation infers an applicability of the proposed method in this research in designing a soil improvement in a soft dredged deposit when vertical drains and pre-loading are performed before the self-weight consolidation is ceased.

Performance of ground heat exchangers for civil infrastructures

이철호 Graduate School, Korea University 2012 국내박사

The need for developing alternative energy sources in Korea is dramatically increasing in order to prepare the era of hefty oil prices because the shortage of energy and resources enforces us to import about 97% of fossil fuels used in Korea. Among the various types of new and renewable energy resources, geothermal energy which is defined as thermal energy generated and stored in the Earth is considered to be a relatively clean and inexhaustible resource. Recently, ground-source heat pump (GSHP) systems have been increasingly used around the world, because they are among the most energy- and cost-efficient heating and cooling systems for residential and commercial buildings. A typical GSHP system mainly consists of a conventional water-source heat pump unit coupled with a group of ground heat exchangers (GHEXs) where heat exchange occurs between a working fluid circulating through the GHEX and the ground formation. A GHEX, where heat extraction or injection of thermal energy occurs from/into the ground, is an important device that determines the performance of a GSHP system and its initial installation cost. In this study, conventional system (borehole heat exchanger or closed-loop vertical ground heat exchanger) is examined by performing field test, laboratory test and numerical analysis to improve thermal capability of GHE with consideration of construction cost. And two possible thermo-active energy structures, i.e., tunnel and cast-in-place pile are also employed to compare with conventional system for heat exchange rate. Field tests and numerical analyses are used to evaluate performance of two thermo-active structures. To evaluate the effective ground thermal conductivity and investigate the thermal efficiency of the GHEX, six test boreholes were constructed with different installation conditions: such as different grouting materials (cement vs. bentonite), different shape of pipe-sections (conventional U-loop type vs. new 3 pipe-type), and different additives (silica sand vs. graphite), at a test bed in Wonju, South Korea. Thereafter, the thermal efficiency of each borehole is evaluated by conducting a series of in-situ thermal response tests. Lastly, the design length of GHEXs for an artificial heating/cooling load for an imaginary building is estimated based on the in-situ TRT results of each borehole. A series of cost analyses has been carried out to show the applicability of the cement grouting, the graphite additive, and the new 3-pipe type of heat exchange pipe. A new geothermal energy source obtained from a tunnel structure has been studied in this study. The geothermal energy is extracted through a textile-type ground heat exchanger named "Energy Textile" that is fabricated between a shotcrete layer and a guided drainage geotextile. To evaluate the thermal performance of the energy textile, a test bed was constructed in an abandoned railway tunnel located in Seocheon, South Korea. A series of laboratory experiments was performed to measure the thermal conductivity of tunnel wall materials (i.e., shotcrete, lining material). A test bed was then constructed to facilitate an evaluation of the thermal performance of the energy textile under the field condition. To observe a long-term thermal behavior, the difference in the temperatures of the inlet and outlet fluid of the energy textile is monitored using a constant-temperature water bath (CWB) and a real simulation system for both cooling and heating operating. Based on the field experiment results, numerical analyses considering field conditions such as arrangement of pipes, property of lining material and temperature of the air in tunnel are also conducted to evaluate the performance of the geothermal energy extraction system that utilizes the energy textile under the field condition. In addition, thermal stress due to the change of temperature is estimated because original function of the tunnel lining is a structural stability and finishing structure of the tunnel therefore additional condition, i.e. energy textile, should avoid affecting negative influence to the tunnel lining. Energy piles set up the heat exchange pipe inside the pile foundation and allow circulating fluid through the pipe leading to heat exchange with the subsurface ground formation. In this study, a cast-in-place concrete energy pile is considered, which functions not only as a structural foundation but also as an underground heat exchanger. The energy pile is a large-diameter drilled shaft equipped with two types of heat exchange pipe installation, i.e., a W-shape and an S-shape. The drilled shaft reaches to the depth of 60 m whilst the heat exchange pipes are inserted to about 30 m deep from the ground surface. The W-shape and S-shape pipes were installed in the opposite sections in the drilled shaft. In-situ thermal performance on the cast-in-place concrete energy pile was evaluated by carrying out a series of in-situ thermal response tests (TRTs) and CFD numerical analyses. TRTs were separately performed for both the W-shape and an S-shape heat exchange pipes, and the heat exchange rate was relatively compared each other. In addition, two closed-loop vertical ground heat exchangers of 40 and 60 m in depth were symmetrically constructed adjacent to the cast-in-place concrete energy pile at the distance of 1.5 m, respectively. Based on the results of the numerical simulations, it is shown that CFD analysis can be used to simulate heat exchange behavior of energy pile and to optimize the design factors by performing parametric studies.

Analytical interpretation of slug test in a vertical cutoff wall

임지희 Graduate School, Korea University 2012 국내석사

An analytical solution for a partially penetrating well in backfill materials of the vertical cutoff wall is developed by applying the method of the images. There are typically two types of interpretation method: the method of successive steady states and the method using the transient solution. In this paper, both of them are considered respectively. As for first method, a steady-state shape factor for evaluating the hydraulic conductivity of the vertical cutoff wall by the slug test was estimated using the analytical solution. Meanwhile, for the transient solution, the analytical solution of cutoff wall was developed. Two types of boundary conditions were used to account for the properties of the soil surrounding the cutoff wall: interfaces with or without the existence of filter cakes, i.e. constant-head boundary and no-flux boundary conditions. As for the method of successive steady states, the constant-head and no-flux boundary conditions yield higher and lower shape factors, respectively, than those for the infinite aquifer that shows the significance of proper boundary conditions at the cutoff wall. By comparing the results considering the compressibility of the backfill material with that from the analytical solution developed in this study, the analytical solution was proved to correspond to the case of incompressible backfill materials. A case study was re-analyzed and the results were compared to the reported results to show the applicability of the current solution in practice. Thy hydraulic conductivity values for the case study obtained from our method are about 20% to 70% higher than those estimated by the conventional Bouwer and Rice method. As for the transient solution, type curves are constructed from the currently derived analytical solution and compared with those of a partially penetrated well within an aquifer. The constant head boundary condition provides faster hydraulic head recovery curve than the aquifer case. On the other hand, no flux boundary condition leads to slower hydraulic head recovery. The greater the shape factor and deviation of the well, and the smaller the width of the vertical cutoff wall, the greater the effect of the boundary condition that was observed. The type curves obtained from the analytical solution for a cutoff wall are similar to those made by the numerical method in the literature. In the case study, the analytically estimated hydraulic conductivities of vertical cutoff walls that were reanalyzed in this paper are from 4.8% to 44.6% smaller than those estimated from the type curve method using numerical analysis and the difference was bigger for the no-flux boundary condition.