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얇은 히터 thermal activity의 올바른 결정을 위한 가열 방법에 따른 실질 가열 두께 연구
정우현(Woo Hyun Jung),조항진(HangJin Jo) 대한기계학회 2020 대한기계학회 춘추학술대회 Vol.2020 No.12
In various industrial field such as nuclear engineering (fuel cladding) and electronics (chip cooling), a critical heat flux of thin heater are getting a lot of interests due to its unusual characteristic, lower CHF value than the thick heater. In case of the thin heater, generally the critical heat flux has a tendency to decrease as the thickness of heater decreased. This phenomenon is interpreted as the thermal property effect of heater, represented by the thermal activity parameter that is composed of heater thickness, density, thermal capacity, and thermal conductivity. In this paper, it is claimed that the heater thickness parameter could be changed according to the heating method; Joule heating method (direct heating) or conduction heating methods (direct heating). By solving the conduction problem of heater numerically with the dry patch mechanism concept for the CHF, giving thinner thickness for the Joule heating method as an effective heating thickness showed consistency on the numerical results.
접촉각에 따른 기포 직경 및 임계열유속 변화에 관한 연구
노현우(Hyunwoo Noh),조항진(HangJin Jo),김설하(Seol Ha Kim),박현선(Hyun Sun Park),김무환(Moo Hwan Kim) 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11
A pool boiling experiment was conducted to investigate the relation among contact angle, bubble diameter, and critical heat flux. Infrared(IR) thermometry was employed to measure the bubble diameter directly in pool boiling experiment conducted on Si and Teflon surface by visualizing the liquid-vapor interface through IR-transparent silicon substrate. Critical heat flux on Si and Teflon surface were about 1,200kW/m² and 160kW/m², respectively. It was observed that bubble behaviors, i.e., nucleation site density, bubble frequency, and bubble diameter, were drastically changed with respect to contact angle conditions at the same heat flux condition. The bubble diameter decreases as the applied heat flux increases and as contact angle decreases.
광섬유 분산온도센서를 이용한 관내 막응축 열전달 및 유동양식 평가
이용남(Yongnam Lee),조항진(HangJin Jo) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11
The film condensation heat transfer under low-mass-flux condition in horizontal tube was experimentally studied. The condensation experiment was performed under the conditions of a steam pressure of 0.5MPa and a mass flux of 50kg/㎡s with all steam quality range. An optical fiber sensor was used as a distributed temperature sensor (DTS), and the outer tube wall temperature data with intervals of 2.6 mm in the axial direction were measured. In two representative flow patterns (annular and stratified flow), the local heat flux and local heat transfer coefficient in the radial direction were measured and compared with the average radial heat transfer coefficient. Based on the experimental results, it was confirmed that the flow pattern of condensate in tube could be measured only by the tube wall temperature distribution, and using this, the possibility of application in conditions that cannot be visualized such as high temperature or high pressure was suggested.
노현우(Hyunwoo Noh),조항진(HangJin Jo),유동인(Dong In Yu),박현선(Hyun Sun Park),모리야마 키요후미(Kiyofumi Moriyama),김무환(Moo Hwan Kim) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
Pool boiling experiments were conducted on flat and structured SiO₂ surfaces to investigate the effects of surface structures on boiling performances. A micro-structured surface was prepared with the MEMS fabrication methodology, and has regularly distributed micro-scale pillar array. Critical heat flux was enhanced on micro-structured surface up to ~1.7 ㎿/㎡, compared to flat surface with ~0.8 ㎿/㎡. Heat transfer coefficient of the micro-structured surface was higher than that of flat one at high heat flux region, and the difference became larger as applied heat flux increased. Droplet and bubble dynamics were studied and the results presented pinning behavior of liquid film and smaller bubble departure diameters on the micro-structured surface. At high heat flux region, liquid path was observed only on micro-structured surface, which guarantees consistent liquid supply rate to the heating surface.
다공성 매질 방법을 이용한 PCHE의 유동분배 및 열전달 성능 분석
정성석(Seong Seok Chung),조항진(HangJin Jo) 대한기계학회 2023 대한기계학회 춘추학술대회 Vol.2023 No.11
In this study, the flow distribution and heat transfer performance of the Printed Circuit Heat Exchanger (PCHE) were evaluated under different header conditions, including various outlet header shapes, using the porous media method in ANSYS CFX. The PCHE model consists of 7 plates, each etched with 8 straight-type channels. To enhance predictability concerning pressure drop and heat transfer correlations, the simulation was conducted with various outlet header shapes, as the outlet headers contribute significantly to pressure drop. Flow distribution was quantified using CoV (Coefficient of Variance) values, which indicate deviations in mass flow rates in the PCHE. As the pressure drop ratio between the PCHE core and the header increased, flow distribution improved, and the heat transfer coefficient converged to the global heat transfer coefficient when assuming uniform flow distribution.
개질된 표면을 이용한 풀비등 임계열유속 증진에 관련한 실험적 연구
강순호(Soonho Kang),안호선(Hoseon Ahn),조항진(Hangjin Jo),김무환(MooHwan Kim),김형모(Hyungmo Kim),김준원(Joonwon Kim) 대한기계학회 2009 大韓機械學會論文集B Vol.33 No.11
In the boiling heat transfer mechanism, CHF(critical heat flux) is the significantly important parameter of the system. So, many researchers have been struggling to enhance the CHF of the system in enormous methods. Recently, there were lots of researches about enormous CHF enhancement with the nanofluids. In that, the pool boiling CHF in nanofluids has the significantly increased value compared to that in pure water because of the deposition of the nanoparticle on the heater surface in the nanofluids. The aim of this study is the comparison of the effect of the nanoparticle deposited surface and the modified surface which has the similar morphology and made by MEMS fabrication. The nanoparticle deposited surface has the complex structures in nano-micro scale. Therefore, we fabricated the surfaces which has the similar wettability and coated with the micro size post and nano structure. The experiment is performed in 3 cases : the bare surface with 0.002% water-ZnO nanofluids, the nanoparticle deposited surface with pure water and the new fabricated surface with pure water. The contact angle, a representative parameter of the wettability, of the all 3 cases has the similar value about 0 and the SEM(scanning electron microscope) images of the surfaces show the complex nano-micro structure. From the pool boiling experiment of the each case, the nanoparticle deposited surface with pure water and the fabricated surface with pure water has the almost same CHF value. In other words, the CHF enhancement of the nanoparticle deposited surface is the surface effect. It also shows that the new fabricated surface follows the nanoparticle deposited surface well.
부하추종 운전전략수립을 위한 시스템 코드 기반 질소 브레이튼 사이클의 출력제어 과도해석
정성균(Sungkun Chung),최재현(Jae Hyun Choi),조항진(HangJin Jo) 대한기계학회 2022 대한기계학회 춘추학술대회 Vol.2022 No.11
Recently, control strategies of power conversion systems are getting important due to coupling energy systems including renewable and nuclear sources. In this study, a power-control transient analysis of a nitrogen Brayton cycle which is one of the realistic and near-term options for a power conversion system of sodium-cooled fast reactors is performed to confirm the flexibility and safety of the system. The transient calculation is performed with a heat source variation scenario which is varied from 100% to 55% determined from a reference. The transient results showed that net power could be controlled from 100% to 0% with a turbine bypass valve. For safe operation of the system, recuperator recirculation and inventory tank valves are used to avoid the local frozen in a heat exchanger and choking events in compressors. From the power-control transient analysis, it is concluded that the nitrogen Brayton cycle is applicable for coupling energy systems with simple control strategies.