대류유동조건에서 나노입자를 포함한 콜로이드의 유효열전도율

나영수(Young Su Na) 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11

The effective thermal conductivity of a convective flowing colloidal system, called as the dynamic thermal conductivity, was measured, and it was compared with the effective thermal conductivity of a stationary colloid, called as the static thermal conductivity. Nano-sized alumina particles suspended in water are under the fully developed laminar tube flow with uniformly heating and cooling condition. In the heating condition, the dynamic thermal conductivity is lower than the static thermal conductivity, and it increases with the increment of the Reynolds number. In opposition to the heating condition, the dynamic thermal conductivity in the cooling condition is higher than the static thermal conductivity, and it decreases with the increment of the Reynolds number. This contrary trend of the dynamic thermal conductivity between heating and cooling conditions can be explained by thermophoresis that determines the behavior of nano-sized particles. The concentration of nano-sized particles suspended in water will be coarse or dense locally near the heating or cooling surface, respectively, and the thermophoretic velocity of dispersed particles depends on the temperature regarding the Reynolds number.

뜬 마이크로 디바이스를 이용한 Ge-Si<SUB>x</SUB>Ge<SUB>1-x</SUB> Core-Shell Nanowires 의 열전도율 측정

박현준(Hyun Joon Park),나정효(Jung hyo Nah),Emanuel Tutuc,설재훈(Jae Hun Seol) 대한기계학회 2015 大韓機械學會論文集B Vol.39 No.10

나노선에서 코어-셸 헤테로 구조를 도입함으로써 열 전도율을 낮출 수 있으며, 이로 인해 열전 효율(ZT)을 향상시킬 수 있다는 것이 이론 연구를 통해 제안되었다. 본 논문에서는 코어-셸 나노선의 열전도율 감소를 실험적인 방법을 통해 확인하였다. 화학증기 증착법을 통해 만든 게르마늄-규소 x 게르마늄 1-x(Ge?SixGe1-x) 코어-셸 나노선의 열전도율을 마이크로 크기의 뜬 디바이스를 이용하여 측정하였다. 셸에서 측정된 실리콘의 함유율(x)는 0.65 로 확인하였으며, 게르마늄은 코어와 셸 사이에서, 격자 불일치(lattice mismatch)에서 비롯된 결점(defect)와 같은 역할을 한다. 또한, 4-point I-V 측정실험에, 휘트스톤 브릿지 실험을 추가 진행함으로써 측정 민감도를 강화하였다. 측정된 열전도율은 상온에서 9~13 W/mK 으로써, 비슷한 지름을 가지는 게르마늄 나노선과 비교하였을 때, 열전도율이 약 30 % 낮아졌음을 확인하였다. Theoretical calculations suggest that the thermoelectric figure of merit (ZT) can be improved by introducing a core-shell heterostructure to a semiconductor nanowire because of the reduced thermal conductivity of the nanowire. To experimentally verify the decrease in thermal conductivity in core-shell nanowires, the thermal conductivity of Ge?SixGe1-x core-shell nanowires grown by chemical vapor deposition (CVD) was measured using suspended microdevices. The silicon composition (Xsi) in the shells was measured to be about 0.65, and the remainder of the germanium in the shells was shown to play a role in decreasing defects originating from the lattice mismatch between the cores and shells. In addition to the standard four-point current? voltage (I-V) measurement, the measurement configuration based on the Wheatstone bridge was attempted to enhance the measurement sensitivity. The measured thermal conductivity values are in the range of 9? 13 W/mK at room temperature and are lower by approximately 30 than that of a germanium nanowire with a comparable diameter.

저전도율 유체 측정에서 소형 전자기유량계의 신호 특성

임기원(Ki Won Lim),정성수(Sung Soo Jung) 대한기계학회 2016 大韓機械學會論文集B Vol.40 No.9

유체의 전도율(conductivity)이 전자기유량계의 특성에 미치는 영향을 평가하기 위해 소형 전자기 유량계를 설계 제작하였다. 유량계의 측정관(measuring tube)은 4 mm x 3 mm의 직사각형 단면을 가지며 길이는 9 mm이다. 전극은 직경이 1.5 mm인 점전극과 단면이 2 mm x 3 mm인 면전극으로 설계하였다. 코일의 열 발생에 영향을 미치는 자화주파수, 회전수 및 굵기를 변화시키면서 최적값을 찾았다. 최적화된 측정관은 자속밀도(magnetic flux density) 0.04 T (400 gauss)를 발생시키며 안정되고, 선형적인 유량신호를 얻을 수 있었다. 유량계를 특성을 평가하기 위해 중량식 유량계 교정 장치를 제작하였다. 유동율 범위는 최대 1.17 x 10<SUP>-5</SUP>㎥/s(700cc/min) 이며 ISO규격에 따라 유동량 결정의 불확도를 평가하였으며 크기는 0.06 % 이내이었다. 유체의 전도율은 3 - 11 μS/cm 사이에서 조절하였다. In order to scrutinize the fluid conductivity effects on the electromagnetic flowmeter(EMF) characteristics, a small scale EMF was designed and fabricated. The measuring tube has a 3 mmⅹ4 mm rectangular cross-section, 9 mm length, and a 2 mmⅹ3 mm plate electrode and a Ф 1.5 mm point electrode. The design parameters, such as the magnetizing frequency and the number of coil turns, and the diameter were optimized. The EMF was tested with a gravimetric calibrator and showed good linearity in the range of 0 to 1.17ⅹ10<SUP>-5</SUP> ㎥/s. The fluid conductivity was varied between 3 and 11 μS/ cm , and the magnitude of the flow signal was proportional to the fluid conductivity and the wetted area of the electrode. The design information and the test results provide flow measurement techniques for very low flowrate.

기존자료 분석을 통한 열전도율의 비교 연구

서현수,공성훈,안창환,최영식 대한건축학회지회연합회 2008 대한건축학회지회연합회 학술발표대회논문집 Vol.2008 No.1

Thermal conductivity is important to cooling/heating load calculation. Related load calculations and energy consumption rates, there are many results of thermal conductivity. So we need a survey of material conductivity in this study. The purpose of this study is to analyzing thermal conductivity by existing material data book. The major results are as follow. 1) The range of aluminum alloy is 199.864∼0244.02(W/m*K). 2) The range of marble is 1.2782∼3.3698(W/m*K). 3) The range of Mortar is 1.0807∼1.743(W/m*K).

단열성능향상 재료를 사용한 구조용 콘크리트의 열전도 특성

박영신,강민기,김정호,지석원,전현규 한국건축시공학회 2016 한국건축시공학회지 Vol.16 No.1

The part of a building with the biggest energy loss is the exterior and many studies are actively conducted toreduce the energy loss on that part. However, most studies consider the window frames and insulation materials, butmany studies do not discuss the concrete that takes more than 70% of the exterior. In order to minimize the energyloss of buildings, it is necessary to enhance the concrete's insulation performance and studies need to be conducted onthis. Therefore, this study used a micro foam cell admixture, calcined diatomite powder, and lightweight aggregates asa part of a study to develop a type of concrete with improved insulation performance that has twice higher thermalconductivity compared to concrete. It particularly secured the porosity inside concrete to lower thermal conductivity. As a result of the experiment, the slump and air capacity showed fair results, but all mixtures containing microfoaming agent showed 14.3~35.1% lower mass per unit of volume compared to regular concrete. Compressive strengthdecreased slightly due to the materials used to improve the insulating performance, but it all satisfied this study'starget strength(24MPa). Thermal conductivity was up to twice higher than that of regular concrete. 건물에서 에너지 손실이 가장 큰 부위는 외피로서, 이부분의 에너지 손실을 감소하기 위한 연구가 활발히 진행되고 있으나 이는 대부분 창호 및 단열재를 사용한 연구이며 건물 외피의 70% 이상을 차지하고 있는 콘크리트에 대한 연구는 미미한 실정이다. 따라서 건물의 에너지 손실을최소화하기 위해서는 콘크리트 자체에서 단열성능을 확보할 수 있어야 하며 이에 대한 연구가 필요하다. 이에 본 연구에서는 보통콘크리트보다 열전도율을 2배 이상 개선시킨 구조용 단열성능향상 콘크리트 개발 연구의 일환으로마이크로기포제, 규조토 미분말, 경량골재를 사용하였으며, 콘크리트 내부공극을 다량확보하여 열전도율을 낮추고자 하였다. 실험결과, 슬럼프와 공기량은 양호한 결과를 나타내었으며, 단위용적질량에서는 마이크로기포제를 사용한 모든 배합에서 보통콘크리트보다 14.3∼35.1 % 감소된결과를 나타내었고 압축강도는 단열성능 향상 재료를 사용하여 다소 감소하는 경향을 나타내었으나 본 실험의 목표강도(24MPa)를 모두 만족하였다. 또한 열전도율은 보통콘크리트 대비 최대 2배 이상 개선된 결과를 나타내었다.

베어링의 열전도율이 평행 슬라이더 베어링의 윤활성능에미치는 영향

박태조,이원석,박지빈 한국트라이볼로지학회 2018 한국트라이볼로지학회지 (Tribol. Lubr.) Vol.34 No.6

Temperature rise due to viscous shear of the lubricating oil generates hydrodynamic pressure, even if the lubricating surfaces are parallel. This effect, known as the thermal wedge effect, varies significantly with film-temperature boundary conditions. The bearing conducts a part of the heat generated; hence, the oil temperature varies with the thermal conductivity of the bearing. In this study, we analyze the effect of thermal conductivity on the thermohydrodynamic (THD) lubrication of parallel slider bearings. We numerically analyze the continuity equation, Navier-Stokes equation, energy equation including the temperature-viscosity and temperature- density relations for lubricants, and the heat conduction equation for bearing by creating a 2D model of the micro-bearing using the commercial computational fluid dynamics (CFD) code FLUENT. We then compare the variation in temperature, viscosity, and pressure distributions with the thermal conductivity. The results demonstrate that the thermal conductivity has a significant influence on THD lubrication characteristics of parallel slider bearings. The lower the thermal conductivity, the greater the pressure generation due to the thermal wedge effect resulting in a higher load-carrying capacity and smaller frictional force. The present results can function as the basic data for optimum bearing design; however, the applicability requires further studies on various operating conditions.

양생조건에 따른 초기재령 시멘트 모르타르의 전자기 특성에 대한 실험적 연구

권성준(Kwon Seung-Jun),송하원(Song Ha-Won),Maria Q. Feng 대한토목학회 2008 대한토목학회논문집 A Vol.28 No.5A

최근 들어 비파괴기술이 발달함에 따라, 콘크리트 구조물의 건전성 평가에 전자기 특성을 이용한 평가기법이 적용되고 있다. 시멘트 건설재료와 같은 절연성 재료는 고유한 유전상수 또는 전도율을 가지므로 특성화될 수 있는데, 이러한 전자기 특성은 물-시멘트비, 단위 시멘트 량과 같은 배합조건에 따라 변화하게 된다. 실내조건에 노출된 시멘트 모르타르는 일반적으로 수분에 포화되지 않으므로, 공극률이 전자기특성에 큰 영향을 미치는데, 이러한 공극률은 주로 초기재령에서 결정되어지며, 양생조건에 따라 매우 민감하게 변화한다. 본 연구에서는 4가지 종류의 물시멘트비를 가진 시멘트 모르타르를 대상으로, 전자기 특성(유전상수, 전도율)을 광범위 대역인 0.2 ㎓~20 ㎓ 범위에서 측정하였다. 각 시편은 배합 후 0일에서 28일까지 총 5가지의 다른 수중양생기간을 가지도록 하였으며,28일 이후, 실내노출상태에서 전자기 특성을 측정하였다. 한편 28일 재령시, 수은압입법을 통하여 공극률을 분석하였으며, 실내상태의 수분손실을 측정하여 포화도를 평가하였다. 양생초기부터 변화하는 공극률을 평가하기 위해, 초기재령 콘크리트의 거동 평가 프로그램을 이용하여 시멘트 모르타르의 공극률 변화를 분석하였으며, 측정된 전자기 특성의 변화를 분석하였다. 전자기 특성을 영향인자(재령, 물-시멘트비)와 쉽게 비교하기 위해서, 5 ㎓~20㎓ 영역의 값을 하나의 평균값으로 도출하였다. 초기재령에서 평균화된 유전상수와 전도율은 물-시멘트비의 감소에 따라서 선형으로 증가하였으며,4주의 양생기간동안 물-시멘트비에 관계없이 양생기간의 제곱근에 비례하여 증가하였다. Recently, NDTs (Non-Destructive Techniques) using electromagnetic(EM) properties are applied to the performance evaluation for RC (Reinforced Concrete) structures. Since nonmetallic materials which are cement-based system have their unique dielectric constant and conductivity, they can be characterized and changed with different mixture conditions like W/C (water to cement) ratios and unit cement weight. In a room condition, cement mortar is generally dry so that porosity plays a major role in EM properties, which is determined at early-aged stage and also be affected by curing condition. In this paper, EM properties (dielectric constant and conductivity) in cement mortar specimens with 4 different W/C ratios are measured in the wide region of 0.2 ㎓~20 ㎓. Each specimen has different submerged curing period from 0 to 28 days and then EM measurement is performed after 4 weeks. Furthermore, porosity at the age of 28 days is measured through MIP (Mercury Intrusion Porosimeter) and saturation is also measured through amount of water loss in room condition. In order to evaluate the porosity from the initial curing stage, numerical analysis based on the modeling for the behavior in early-aged concrete is performed and the calculated results of porosity and measured EM properties are analyzed. For the convenient comparison with influencing parameters like W/C ratios and curing period, EM properties from 5 ㎓ to 15 ㎓ are averaged as one value. For 4 weeks, the averaged dielectric constant and conductivity in cement mortar are linearly decrease with higher W/C ratios and they increase in proportion to the square root of curing period regardless of W/C ratios.

환경 조건에 따른 열전도율 변화에 관한 연구

서은석 ( Seo Eun-seok ),김봉주 ( Kim Bong-joo ),유남규 ( You Nam-gyu ),홍상훈 ( Hong Sang-hun ),김해나 ( Kim Han-nah ),정의인 ( Jung Ui-in ) 한국건축시공학회 2019 한국건축시공학회 학술발표대회 논문집 Vol.19 No.2

Insulated buildings are exposed to the external environment due to aging and construction problems, resulting in a decrease in building energy efficiency. Therefore, the purpose of this paper is to provide a material for the change in thermal conductivity of the insulation when it is exposed to various external environments. In the experiment, five types of heat insulating materials were selected, stored under different environmental conditions, and the thermal conductivity was measured periodically to confirm the change in thermal conductivity. As a result, the thermal conductivity of all the insulating materials except the PF board increased with the passage of time. This is because thermal insulation absorbs atmospheric moisture under all environmental conditions and the thermal conductivity increases, and in the case of thermal insulation stored indoors in environmental conditions, the temperature differs from the thermal insulation stored outside. It is considered that there is little evaporation of moisture absorbed constantly, and the change in thermal conductivity is large.

STS 304 중공 원통의 비정상 열전달 특성

이상철(Sang Chul Lee),김영근(Young Geun Kim),심규진(Kyu Jin Shim),배강열(Kang Youl Bae),정한식(Han Shik Chung),정효민(Hyo Min Jeong) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.4

This paper introduced about characteristics on the non-steady heat transfer of STS 304 hollow cylinder. In the non-steady state, the specific heat and conductivity are depended on the temperature variations, and these properties affect to the governing equation on heat conduction. But the most of numerical analysis on heat conduction is assumed to constant properties which is conductivity and specific heat. Assuming that the properties are reacted sensitively, the numerical results can have the difference of between constant properties with non-constant properties. The main parameters are specific heat and conductivity. The temperature distributions of the STS 304 hollow cylinder became in steady state after 4 minutes in case of the constant properties. As the conductivity is varied with temperature, the temperature distributions became in steady state after 15 minutes. Therefore, a numerical analysis of the non-steady state heat transfer will has to apply that conductivity varied with temperature.

친수성 표면개질의 에어로겔을 혼입한 시멘트 페이스트의압축강도 발현 및 열전도율 평가

안태호,박종범,윤현섭,양근혁 한국건축시공학회 2018 한국건축시공학회지 Vol.18 No.1

The objective of the present study is to examine the feasibility on the development of high-insulation concrete usingaerogels with hydrophilic surface treatment. To prevent the segregation and enhance the dispersibility of agerogels in thecement pastes, the substrate of aerogels was modified to be hydrophobic property using surfactant. The modified aerogelswere added from 0% to 100% of the cement volume at the interval of 25% under the constant cement content. Somecement pastes showed segregation phenomenon and flocculation of aerogels during mixing phase. The addition of aerogelsdecreased the compressive strength of cement pastes but enhanced the thermal conductivity. The thermal conductivity ofpastes with 100% aerogels was lower by 43% when compared with that measured in the conventional paste. To improvethe compressive strength and insulation capacity of concrete containing aerogels, a reliable surface treatment method ofaerogels needs to be further investigated. 이 연구의 목적은 친수성 표면개질의 에어로겔을 활용한높은 단열 콘크리트 재료 개발의 기초 평가이다. 시멘트계재료에서 에어로겔의 재료분리 방지 및 분산성 향상을 위하여 소수성 에어로겔의 표면은 계면활성제를 이용하여 친수성을 부여하였다. 시멘트 페이스트에서 에어로겔의 첨가양은 시멘트 부피 대비 0%에서 100%로 변화하였다. 일부 시멘트 페이스트는 배합단계에서 재료분리 및 에어로겔의 뭉침현상이 나타났다. 에어로겔의 혼입은 시멘트 페이스트의압축강도를 저하시키는 반면 열전도율은 향상시켰다. 에어로겔을 100% 첨가한 페이스트의 열전도율은 일반 페이스트대비 약 43% 낮았다. 에어로겔 혼입 콘크리트의 압축강도와단열성 향상을 위해서는 에어로겔의 최적 표면개질에 대한연구가 요구되었다.