Performance evaluation and design method for cast-in-place energy piles

박상우 Korea University 2016 국내박사

The ground source heat pump (GSHP) system is one of the most effective heating and cooling systems because it utilizes the constant subsurface temperature regardless of season to improve the thermal performance of heat pump and reduce energy consumption. In order to extract or release thermal energy from or to the ground formation, the GSHP system is coupled with the ground heat exchangers (GHEXs) where heat exchange occurs between a working fluid, circulating through the heat exchange pipe installed underground, and the surrounding ground. Usually, a closed-loop vertical GHEX equipped with a single U-type or a double U-type heat exchange pipe is the most popular in practice. However, the conventional closed-loop vertical GHEX has high construction cost due to the requirement of additional borehole drilling and extra construction site since it should be constructed separately independent of building. Nowadays as an economical alternative to the conventional GHEX, several promising attempts have been made to embody heat exchange pipes in structure foundations such as pile, mat foundation, slurry wall, tunnel lining, etc. An energy pile contains a heat exchange pipe inside the pile and allows a fluid circulating through the pipe inducing heat exchange with the ground formation. By using existing structural foundation, the energy pile can reduces drilling cost, and requires no necessity to seek additional space for installing heat exchangers. Meanwhile, it has been reported that energy piles usually show relatively lower thermal performance than conventional vertical closed-loop GHEXs because the energy piles can utilize only a limited amount of geothermal source due to short installation length. In terms of the improvement of thermal performance, a large-diameter cast-in-place concrete pile can be a promising energy pile type with the high thermal storage capacity of concrete materials and with a large borehole surface area to enhance heat exchange between the heat exchange pipe and the surrounding medium. However, recent researches and demonstration studies on energy pile are largely focused on the conventional piles such as PHC pile and steel pipe pile. Especially in Korea, study related to the large-diameter cast-in-place energy pile is almost absent. Furthermore, existing commercial design programs for GSHP system are only applicable to closed-loop vertical GHEXs and researches on the design of cast-in-place energy pile system is insufficient. In this paper, comprehensive studies on cast-in-place energy piles were carried out depending on three key issues, i.e. thermal performance, thermo-mechanical behavior, and design method. The thermal performance of the cast-in-place energy pile was evaluated by experimental and numerical approaches. Full-scale energy piles were constructed in a test bed with various configurations of heat exchange pipe to experimentally provide the thermal performance and constructability depending on different pipe types. Then, two different field tests, in-situ thermal response test (TRT) and in-situ thermal performance test (TPT), were conducted to estimate the heat exchange capacities of constructed energy piles. In the results of field tests, the larger pipe volume was inserted in same borehole volume, the higher heat exchange rate per pile length occurred due to the larger contact area for heat exchange. However, the heat exchange rate per pile length was not proportional to the installed pipe length, and lower heat exchange rate per pipe length was observed in the energy pile with low pipe volume due to the thermal interference. Based on the results of field tests, the numerical model on heat transfer in energy pile was developed in order to numerically evaluate the thermal behavior of GHEX and surrounding ground depending on various parameters, and to provide the designer with preliminary estimation on the thermal performance of GHEXs. The long-term (i.e. three-month operation) thermal performance of the six cast-in-place energy pile constructed in the test bed and changes in ground temperature during operations were estimated by numerical simulations. In addition, a series of parameter studies was conducted to assess the effect of various boundary conditions (i.e. thermal conductivity of ground, temperature difference between ground and inlet fluid, and flow rate of working fluid) on the thermal performance of cast-in-place energy pile. With the aid of the numerical model, the optimum coil pitch was determined considering long-term heat exchange rate and economic feasibility, and consequently, reducing the coil pitch less than 200 mm was not feasible. In order to investigate the thermo-mechanical behavior of energy pile, a comprehensive measurement of temperature and thermal strain (stress) was carried out. The thermal strain (stress) in the longitudinal direction of the energy pile and temperature variation of ground formation was experimentally monitored during heating and cooling operation. The maximum thermal stress caused by the 15-day continuous operation was 1.4 MPa in heating operation and 2.6 MPa in cooling operation, which are 5% and 10% of design criterion strength of cast-in-place concrete, respectively. In addition, parametric studies were conducted by means of numerical analysis to observe the thermo-mechanical behavior of energy pile with various ground conditions. Finally, a novel design algorithm for the cast-in-place energy pile system was provided. First of all, limitations for applying existing design methods to the cast-in-place energy pile were discussed. Then, in order to provide a novel design algorithm suitable for the cast-in-place energy pile, equations for the borehole thermal resistance and engineering chart to predict the thermal performance of cast-in-place energy pile were developed. Then, the design for cast-in-place energy piles constructed in the test bed was carried out with the developed design algorithm. As the result of design, the maximum heating and cooling load that can be covered by six cast-in-place energy piles was evaluated at 5.59 RT for the cooling operation and 3.24 RT for the heating operation. According to the design result, the GSHP system was constructed in the test bed using six cast-in-place energy piles for the heating and cooling of the office space with the area of 103.74m2. The design algorithm was verified by monitoring the thermal performance of the GSHP system. The heating and cooling of the office was operated during one year in 2015. The COP of the GSHP system was estimated by 2.87 for heating operation and 3.02 for cooling operation. The SPF was 2.76 and 2.95 for heating and cooling operation, respectively. During operation of GSHP system, thermal stress of the energy pile was also measured. The maximum compressive stress was measured as 2.38 MPa in cooling operation, while the maximum tensile stress was measured as 2.29 MPa in heating operation. Based on the thermal performance of cast-in-place energy pile and GSHP system, economic feasibility was analyzed. Consequently, the optimum configuration of heat exchange pipe for the cast-in-place energy pile was evaluated by the coil-type heat exchange pipe considering both the relative economic feasibility and constructability. In addition, cast-in-place energy pile can save the GHEX construction cost by 82.77% and total initial investment cost of GSHP system by 25.31% comparing with the conventional closed-loop vertical GHEX. Compared to the typical heating and cooling system, the investment payback period of the cast-in-place energy pile system was evaluated at 1.78 year.

Nonlinear inelastic thermo-mechanical analysis of steel frames under ambient temperature variations and a fire : Nonlinear inelastic thermo-mechanical analysis of steel frames under ambient temperature variations and a fire

LUU VAN THUC 세종대학교 대학원 2024 국내박사

The main goal of this dissertation is to develop a structural analysis program capable of accurately and efficiently capturing the thermo-mechanical behaviors of steel structures subjected to fire or ambient temperature variations. A fiber beam-column element, which takes into account both geometric and material nonlinearities, has been developed and implemented into a computer program. Two types of nonlinear inelastic analysis developed in the proposed program are: (1) nonlinear inelastic static analysis of steel frames subjected to a fire; and (2) nonlinear dynamic analysis of framed structures under ambient temperature variations. The proposed program can be employed to realistically and accurately assess the behavior of steel frames and their individual components in a direct and computationally efficient manner. For the fiber beam-column element, the cross-sections of members are discretized into fiber elements to formulate an equivalent effective cross-section. The formulation considers the geometric nonlinearity arising from the interaction between axial force and bending moment, and this is accounted for using stability functions. The benefit of employing the stability functions and distributed plasticity model (fiber-based model) is that it can accurately capture the nonlinear effects by using only one or two elements per member. This results in significantly higher computational efficiency compared to the finite element method employing interpolation functions. Furthermore, the phenomenon of plasticity spreading in the cross-section of the element is captured at various integration points along the longitudinal direction of the element, based on the constitutive relations in each fiber. To consider temperature actions, each fiber is continuously assigned different constitutive models in the incremental temperature steps to vary the unique temperature-stress-strain relationship and corresponding expansion. By assigning the different temperatures to each fiber and a division of more fibers in the cross-section, thermal actions will be implemented. A nonlinear thermal incremental-iterative solution scheme based on the standard Newton-Raphson algorithm will be developed in the code work by the Fortran programming language to deal with the nonlinear problems of steel frames due to the thermal expansion and material degradation in a fire or ambient temperature variations. Based on the mentioned elements and algorithms, a computer program is developed. Its accuracy and computational efficiency are verified by comparing predictions with those generated by commercial packages such as Abaqus and other results available in experimental and existing studies. The proposed program has demonstrated its reliability and efficiency through numerous numerical examples and case studies involving large-scale framed structures. It serves as a dependable and efficient tool for daily design use, offering a cost-effective alternative to time- consuming commercial software.

Formation and characterization of Ti-based bulk metallic glasses and bulk metallic glass composites

홍성환 세종대학교 대학원 2017 국내박사

Bulk metallic glasses (BMGs) have shown unique mechanical properties such as high strength and large elastic limit from the absence of lattice defects of crystalline materials. In particular, these materials have exhibited superplastic-like deformation behavior resulting from the significant decrease of viscosity in the supercooled liquid region (SLR) between the glass transition and onset crystallization temperatures (Tg and Tx). The superplastic-like properties on SLR facilitate the net-shape process using thermoplastic forming (TPF). In order to optimize TPF processing condition, the incubation time before crystallization and viscosity value depending on temperature within SLR are very important i.e., the thermoplastic formability strongly depends on processing temperature on SLR. Therefore, it is essential to understand the crystallization kinetics and evaluate viscosity for obtaining the optimal TPF processing condition of the BMGs. The crystallization kinetics and thermo-mechanical behavior of Ti42.6Cu40.8Ni7.65Zr5.4 Sn2.55Si1 BMG are investigated by continuous heating and isothermal DSC analyses with viscosity measurement within SLR. The currently studied BMG presents excellent thermal stability compared with other Ti-based BMGs in terms of activation energy for onset crystallization evaluated from the Kissinger equation. The effective TPF processing condition can be obtained by constructing the TTT diagram obtained from isothermal annealing and the viscosity measurement within SLR. The practicality of solid-to-solid forming of currently studied BMG is confirmed by a laboratory-scale TPF experiment, which exhibits successfully replicated the Korean character on the surface of BMG sample without crystallization of amorphous phase. Based on these results, it is believed that TPF of Ti-based BMGs can be facilitated by exploring the thermoplastic processing condition within SLR. Nevertheless, unfavorable brittleness of the BMGs originated from inhomogeneous deformation by shear localization at room temperature leads to catastrophic failure with no plastic deformation. Moreover, the inhomogeneous shear deformation behavior causes strain softening characteristic of BMGs. These serious drawbacks still limit their access to use an engineering material. In order to overcome these drawbacks, the metallic glass matrix composites containing reinforcing crystalline phases have been developed, which exhibits the improved plasticity through severe interaction between crystalline phases and shear bands during deformation. In following chapter, the mechanical properties and deformation behavior of Ti-based bulk metallic glass composites (BMGCs) containing austenite B2 phases are explored though the detailed investigations on the initiation and propagation behavior of shear bands during deformation. For the compression test, the Ti-based BMGCs containing B2 particles exhibit excellent mechanical properties combining high yield strength and large plasticity. On progress of plastic deformation, the pronounced plastic deformation is attributed to the vigorous shear banding activity. In particular, the shear banding behaviors related with plastic strain are governed by size of B2 particles. For small-sized B2 particles (1 ~ 10 μm), the major shear bands propagate rapidly without hampering. In case of large-sized B2 particles (100 ~ 200 μm), the propagation of shear bands is impeded by B2 particles with severe interaction and multiple primary shear bands were formed. This demonstrates that the B2 particles with distinct length scale play a different role on the stage of plastic deformation. In other word, the role of B2 particles with different length scale on the early stage of plastic deformation is almost same but clearly different at the late stage of plastic deformation (plasticity). The final chapter explores the deformation mechanism of work-hardening behavior on plastic deformation of Ti-based BMGCs with B2 particles. The noticeable work-hardening phenomenon of the BMGCs, at the early stage of plastic deformation, is the result of the individual hardening effect of the both small- and large-sized B2 crystalline particles which are originated from stress-induced martensitic transformation from B2 phase to B19' phase and deformation twinning of crystalline phases. Upon loading, the stress-induced martensitic transformation from austenite B2 phase to martensite B19' phase occurs firstly at the vicinity of the interface between amorphous matrix and B2 particle before yielding. With the increase of strain to 1% plastic deformation stage, martensitic transformed region of B2 phase is gradually extended from interface to inside of B2 particles with deformation twinning of martensite B19' phase. From these observations, we realize that stress-induced martensitic transformation at the interface is attributed to the stress concentration at the interface region between amorphous matrix and B2 particle due to elastic mismatch of amorphous phase and B2 phase. The stress-induced martensitic transformed zone is gradually extended from interface region to inside region of B2 phase depending on the deformation state.

초기재령 콘크리트의 거동 해석 기법

조호진 연세대학교 대학원 2003 국내박사

초기재령 콘크리트의 온도변형 및 수축변형은 콘크리트 구조물에 응력을 발생시키는 주된 요인이고, 이 응력으로 인하여 발생되는 결함은 콘크리트 구조물의 내구성을 감소시킨다. 이러한 초기결함을 제어하기 위해서는 시공전에 초기재령 콘크리트의 거동을 해석적으로 분석할 수 있는 해석기법이 필요하다. 대부분의 기존 해석기법은 초기재령 콘크리트의 열역학적 특성, 역학적 특성 그리고 응력이완 특성을 각각 독립적으로 모델링하여 해석하므로 특성들간의 연관성을 고려하지 못하고, 또 해석에 사용된 특성 모델이 정성적인 수준을 벗어나지 못하는 경우가 많다. 따라서 본 연구에서는 초기재령 콘크리트를 대표하는 열역학적 특성, 역학적 특성, 응력이완 특성 등에 대한 해석모델이 도출되었고, 이를 통합하여 초기재령 콘크리트의 거동을 정량적으로 해석할 수 있는 해석 기법이 개발되었다. 본 연구에서는 먼저 배합조건 및 외부 환경조건의 영향을 고려하여 초기재령 콘크리트의 열역학적 특성을 나타낼 수 있는 해석모델이 도출되었고, 재령과 온도의 영향을 고려하여 초기재령 콘크리트의 역학적 특성 발현을 나타낼 수 있는 해석모델이 도출되었다. 또한 응력이완 현상이 초기재령 콘크리트의 응력 발생에 미치는 영향을 고려할 수 있는 응력이완 특성 해석모델이 도입되었다. 최종적으로 이상의 열역학적 특성, 역학적 특성, 응력이완 특성을 통합하여 초기재령 콘크리트 구조물의 거동을 해석할 수 있는 해석 알고리즘 및 해석 프로그램이 개발되었다. Early-age defects in concrete due to thermal and shrinkage deformation deteriorate the long-term durability of concrete structures. For the construction of durable concrete structures by minimizing the initial defects, it is necessary to analyze the behavior of early-age concrete analytically considering construction conditions in advance of placing. In conventional analysis, both thermo-mechanical and mechanical characteristics of early-age concrete including the stress relaxation characteristic are considered qualitatively as individual behaviors, so the relationships between each behavior are not considered. Therefore, it is necessary to develop analytical models for the thermo-mechanical and mechanical characteristics and an integrated analytical technique which combines the analytical models. In this study, analytical models which characterize the thermo-mechanical behaviors of early-age concrete and which consider different mixtures of the early-age concrete under different external environmental condition are proposed. Then, age and temperature dependent mechanical properties development model, including setting characteristics, strength development, stiffness development, are also proposed. Stress relaxation behavior, which represents gradual decrease of stress at early-age concrete, is considered and evaluated using existing creep models. Finally, an integrated analytical technique which combines the proposed characteristic models for the analysis of early-age concrete behavior is developed. The integrated analytical technique as well as the proposed characteristic models are verified by experiment.

The material behavior during thermo-mechanical processing and the effect of microstructure on the mechanical properties and stretch formability of A1-1. 25 % Mn alloy : 3003

Kwag, Young Jik University of Kentucky 1984 해외박사

Al-1. 25% Mn(3003)합금을 서로 다른 주조과정을 통해 서로 다른 정도의 과포화 상 태를 만든 뒤 열처리와 압연을 적당히 조절하여 서로 다른 미세구조를 갖는 12가지의 시편을 만들었다. 열처리와 냉간 압연시의 미세구조의 변화는 전자현미경(SEM,TEM), X-ray Diffraction, 비저항측정, Hardness 측정등을 통해 자세히 조사되었다. 이렇게 만들어진 12가지의 시편은 서로 다른 과포화도(Supersaturation), Grain Size, 2차상 의 크기와 양을 갖게 되었다. 마지막 시편은 Tensile Test, Pole figure, farmability Test를 통해 물리적 성질 및 성형성이 조사되었고 이러한 성질들과 미세구조를 결정하 는 여러인자들과 어떤 영향이 있는지 조사하였다. 과포화도와 2차상의 분포가 성형성 에 가장 큰 영향을 미치고 있다는 것을 알게 되었다.

다층 모델을 이용한 429EM 스테인리스 강의 열적 기계적 이력 거동에 대한 유한 요소 해석

서동훈 울산대학교 2010 국내석사

자동차 엔진의 배기 매니폴드, 항공기용 가스터빈엔진의 블레이드 등 여러 분야의 핵심적인 부분들이 고온의 조건에서 장시간 작동하게 된다. 고온의 상태에서 작동하던 부품들이 정지하여 온도조건이 저온으로 급격히 변하면 부품은 가열과 냉각에 따른 큰 온도 구배를 겪게 되고 결국 재료에 상당한 열변형과 열응력을 야기한다. 반복적인 열응력은 피로 손상을 일으켜 재료를 파손시키는데 이를 열피로(TMF : Thermo Mechanical Fatigue)라 한다. 본 연구의 목적은 열피로에 대한 매커니즘을 조사하고 재료의 반복 거동에 대한 타당한 모델을 선택하여 열피로에 대한 현상을 정확히 예측하는 것이다. 상용 프로그램에서 재료의 경화 현상을 묘사하는 경우 일반적으로 Chaboche 모델을 많이 사용한다. Chaboche 모델의 경우 복잡한 경화 모델을 이용하므로 재료의 거동을 정확히 표현할 수 있다는 장점을 가지지만 재료의 형상이 복잡한 경우 수렴성이 떨어지는 단점을 가진다. 이에 비해 다층 모델은 재료의 비선형 구간을 간단히 여러 개의 선형을 이용하여 구현한다. 따라서 Chaboche 모델과 비교했을 때 수치적 계산 방법이 간단하여 해석 결과가 정확하면서도 수렴성이 좋으므로 다층 모델을 본 연구의 기본 경화 모델로 채택한다. 상용 유한 요소 프로그램인 ABAQUS에서 제공하는 재료모델을 사용하여 기본적인 재료의 비탄성 영향을 고려한 해석이 가능하지만, 소성변형률 기억 효과나 온도 의존적 재료상수의 처리등을 고려한 해석의 경우 ABAQUS 자체 기능만으로는 불가능하다. 이와 같은 특성을 비탄성 해석 기법에 적용하기 위하여 사용자 서브루틴(UMAT : User subroutine to define a MATerial's mechanical behavior) 기법을 구현하여 수치적 정확성과 안정성을 확립한다. 개발한 ABAQUS 사용자 서브루틴의 검증을 위하여 단축 인장 변형, 저주기 이력 거동 변형과 열적 기계적 하중 상태의 변형에 대한 해석을 실시하고 시험 데이터와 비교하여 그 정확성을 비교한다.