Orientation effects on bubble dynamics and nucleate pool boiling heat transfer of graphene-modified surface

Kim, TaeJoo,Kim, Ji Min,Kim, Ji Hoon,Park, Su Cheong,Ahn, Ho Seon Elsevier 2017 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.108 No.2

<P><B>Abstract</B></P> <P>Modified surfaces with graphene coatings were evaluated experimentally to enhance nucleate boiling performance on an orientated surface, from upward to downward (0°, 45°, 90°, 120°, 135°, 150°, 160°, and 170°). Two-dimensional (2D) laminate and three-dimensional (3D) porous graphene were prepared for surface modification, and their boiling heat transfer coefficient (BHTC) and critical heat flux (CHF) values were evaluated according to orientation angle. Moreover, their boiling structures were observed and analyzed using high-speed visualization. The results showed that the surfaces coated with 2D and 3D graphene had enhanced BHTC and CHF performance in comparison with the bare surface with no graphene modification. The 2D graphene surface showed increased nucleate site density and early onset of nucleate boiling, resulting in enhanced BHTC, by 40%, in the downward condition. The enhancement ratio of CHF was 40% on 2D graphene and 20–25% on 3D graphene.</P> <P><B>Highlights</B></P> <P> <UL> <LI> 2D and 3D graphene film enhanced pool boiling performance both boiling heat transfer coefficient (BHTC) and critical heat flux (CHF) on various orientation angles from upward (0°) to downward (180°). </LI> <LI> On downward-facing (120–170°), graphene film showed early onset of nucleate boiling and increased nucleate site density, which led early mature of nucleate boiling regime, i.e. slug flow, resulting in enhanced BHTC. </LI> <LI> The graphene film could delay CHF trigger efficiently even though it was covered with larger slug vapor. </LI> </UL> </P>

중성자 영상법을 이용한 PEMFC 에서의 물 배출 특성에 관한 실험적 연구

김태주(TaeJoo Kim),김종록(JongRok Kim),김무환(MooHwan Kim),심철무(CheulMuu Sim),이승욱(SeungWoo Lee),전진수(JinSoo Jeon) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.6

The neutron imaging technique was used to investigate the water discharge characteristics at PEMFC. The feasibility test apparatus was consisted of pressurized air and water in order to simulate the actual operating PEMFC. The feasibility tests have been performed at two kinds of flow field type (1-parallel and 3-parallel) with 100 ㎠ active area and different flow rate (200, 400, and 850 ㏄/min). Although the water and pressurized air was supplied only into cathode, the water movement into anode was found by neutron image. The water at cathode is well discharged, but except the 850 ㏄/min at 3-parallel flow field type, the water at the anode was not removed by pressurized air. The total water volume variations at each condition were calculated from the neutron images. This technique can be used for detailed studies on water distribution and transport in the PEMFC.

하나로(HANARO) NRF 와 중성자 영상법을 이용한 PEM fuel cells 내의 물 배출 특성에 관한 연구

김태주(TaeJoo Kim),김종록(JongRok Kim),심철무(CheulMuu Sim),김무환(MooHwan Kim) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.10

An investigation into the water discharge characteristics of PEM fuel cells is carried out by using a feasibility test apparatus and the Neutron Radiography Facility (NRF) at HANARO. The feasibility test apparatus was composed of a distilled water supply line, a compressed air supply line, heating systems and single PEM fuel cells which was a 1-parallel serpentine type with a 100 ㎠ active area. Three kinds of methods were used; compressed air supply only, heating only, and a combination of the methods of a compressed air supply and heating, respectively. The resultant water discharge characteristics are different according to the applied methods. The compressed air supply only is suitable for removing the water at a flow field and a heating only is suitable for water at the MEA. Therefore in order to remove all the water at PEM fuel cells, the combination method is needed at the moment.

헬리콥터 무베어링 로터 허브 복합재 유연보 피로 안전수명 해석

김태주(Taejoo Kim),기영중(Youngjoong Kee),김덕관(Deog-kwan Kim),김승호(Seung-ho Kim) 한국항공우주학회 2013 韓國航空宇宙學會誌 Vol.41 No.7

7,000lb 급 헬리콥터를 위한 무베어링 로터 허브 시스템을 설계한 후, 무베어링 로터허브 시스템의 주요 구성품 중 하나인 유연보에 대해 요구수명 8,000시간을 만족하는지 확인하기 위한 피로해석을 수행하였다. 2차원 탄성 보 모델에 대한 단면 구조해석 방법을 적용하였으며, 정적구조해석을 통해 피로손상에 취약할 것으로 예상되는 두 단면에 대한 피로해석을 수행하였다. 구조해석을 위해 VABS를 사용하여 유연보 단면 형상에 대한 인장, 굽힘 및 뒤틀림 강성을 계산하였고, wohler equation을 적용하여 유연보를 구성하는 두 가지 복합재 소재에 대한 S-N 곡선을 생성하였다. CAMRAD II를 통해 무베어링 로터 시스템의 하중해석을 수행하였으며, 하중해석 결과를 HELIX/FELIX 표준하중 스펙트럼에 적용하여 무베어링 로터 시스템의 하중 스펙트럼을 생성한 후, 이를 통해 피로해석을 수행하였다. After we designed Bearingless rotor hub system for 7,000lb class helicopter, flexbeam fatigue analysis was conducted for validation of requirement life time 8,000 hours. sectional structural analysis method applying elastic beam model was used. Fatigue analysis for two sections of flexbeam which were expected to weak to fatigue damage from result of static analysis was conducted. Extension, bending and torsion stiffness of flexbeam section shape was calculated using VABS for structure analysis. S-N curve of two composite material which composed flexbeam was generated using wohler equation. Load analysis of bearingless rotor system was conducted using CAMRAD II and load analysis result was applied HELIX/FELIX standard load spectrum to generate bearingless rotor system load spectrum which was used flexbeam fatigue safe life analysis.

중성자속 및 선형 흡수 계수 보정을 고려한 중성자영상법을 이용한 PEMFC 내의 물 배출 특성에 관한 실험적 연구

김태주(TaeJoo Kim),김종록(JongRok Kim),김무환(MooHwan Kim),심철무(CheulMuu Sim) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5

The neutron imaging technique was used to investigate the water discharge characteristics at PEMFC. Prior to investigation of water discharge characteristics, the linear attenuation coefficient for water at Neutron Radiography Facility (NRF) was calibrated. The feasibility test apparatus was consisted of pressurized air and water in order to simulate the actual operating PEMFC. The feasibility tests have been performed at 1-parallel serpentine type with 100 ㎠ active area and different air flow rate (1, 2, and 4 lpm). The total water volume variations at each condition were calculated from the neutron images. The water at channel is well discharged as soon as supplying the pressurized air into the PEMFC. However, because the water at MEA isn’t removed the total water volume is constant after 150. Therefore more effective method is needed in order to discharge water at MEA, and the neutron imaging technique is helpful for it.

중성자 영상법을 이용한 PEMFC에서 유동 방향과 차압에 따른 성능 및 물 분포에 관한 실험적 연구

김태주(TaeJoo Kim),김종록(Jongrok Kim),심철무(CheulMuu Sim),김무환(MooHwan Kim) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5

In this investigation, we prepared a 3-parallel serpentine single PEMFC which has the active area of 25㎠ and the flow channel cross section of 1×1㎜. Distribution and transport of water in an operating PEMFC were observed by varying the flow directions (co-current and counter-current) in each channel and the differential pressures (100, 200, 300 ㎪) applied between the anode and cathode channels. This investigation was performed at the neutron imaging facility at the NIST. It has been observed that the differential pressure affects the performance of PEMFC while the flow direction affects spatial distribution of water when the neutron images were analyzed for several different operating conditions. More specifically, the amount of water production in the fuel cell increased as the partial pressure increases at a given current density and the water production was more uniform for the counter current than the co-current case. It is shown that the neutron imaging technique is a powerful tool to visualize the PEMFC.

기본임부를 수행하는 기동헬기에 적용될 무베어링 허브 복합재 구성품 피로수명 해석

김태주(Taejoo Kim),기영중(Youngjoong Kee),김덕관(Deog-kwan Kim),김승호(Seung-ho Kim) 한국소음진동공학회 2013 한국소음진동공학회 학술대회논문집 Vol.2013 No.4

Rotor system is a very important part which produces lift, thrust and control force in helicopter. Component of rotor system must endure various flight load for the required life. In helicopter rotor system, bearingless rotor system is the highest technology rotor system compare with articulated and hingeless rotor system. Baaringless rotor system is not include mechanical flap hinge, lag hinge and pitch bearing. Bearingless rotor component flexbeam which made by composite material has conduct hinge and bearing role instead of mechanical flap hinge, lag hinge and pitch bearing. These characteristics has less part number and lass weight than others. In this paper, conduct safe life analysis of bearingless composite component flexbeam and torque tube applying to utility helicopter load condition.

헬리콥터 무베어링 로터 허브시스템의 개념 연구

김덕관(Deog-Kwan Kim),김민환(Min-Hwan Kim),윤철용(Chul Yong Yun),김태주(Taejoo Kim),김승호(Seungho Kim) 한국소음진동공학회 2011 한국소음진동공학회 학술대회논문집 Vol.2011 No.4

In this paper, it was described the current technology status of bearingless rotor hub system for helicopter which is one of major rotor hub system. Also, a conceptual study on the new bearingless rotor hub system of helicopter was described. First, the advantages and disadvantages of major helicopter rotor hub system are described in comparison to each other types of rotor hub system. The unique characteristics of bearingless rotor hub system are described compared to other types of rotor hub systems. Next, the main function, role and characteristics of the sub-components of bearingless rotor hub system are described. Recent helicopters which adopt this bearingless rotor hub system are described and introduced. This conceptual study shows that double-H sectional construction and rectangular construction of flexbeam are the most effective candidates of this new bearingless rotor system. This bearingless rotor hub system can be used for 7,000lbs class helicopter. Now, a further trade-off study will show.

EXPERIMENTAL APPROACHES FOR WATER DISCHARGE CHARACTERISTICS IN PEMFC USING NEUTRON IMAGING TECHNIQUE AT CONRAD, HMI

TAEJOO KIM,김종록,CHEULMUU SIM,SUNGHO LEE,YOUNGJIN SON,MOOHWAN KIM 한국원자력학회 2009 Nuclear Engineering and Technology Vol.41 No.1

In this investigation, we prepared a 1 and 3-parallel serpentine single PEMFC, which has an active area of 100 cm2 and a flow channel cross section of 1x1mm. Distribution and transport of water in a non-operating PEMFC were observed by varying flow types and the flow rates (250, 400, and 850 cc/min). This investigation was performed at the neutron imaging facility at the COld Neutron RAdiography facility (CONRAD), HMI, Germany of which the collimation ratio and neutron fluence rate are 250, 1 106 n/s/cm2, respectively. The neutron image was continuously recorded by a scintillator and lens- CCD coupled detector system every 10 seconds. It has been observed that although the distilled water was supplied into the cathode channel only, the neutron image showed a water movement from the cathode to the anode channel. The water at the cathode channel was completely discharged as soon as the pressurized air was supplied. But the water at the anode channel was not easily removed by the pressurized air except for the 3-parallel serpentine type with 850 cc/min of air flow rate. Moreover, the water at the MEA wasn’t removed for any of the cases.

Effect of heterogeneous wetting surface characteristics on flow boiling performance

Kim, Jin Man,Kim, TaeJoo,Yu, Dong In,Noh, Hyunwoo,Kim, Moo Hwan,Moriyama, Kiyofumi,Park, Hyun Sun Elsevier 2018 The International journal of heat and fluid flow Vol.70 No.-

<P><B>Abstract</B></P> <P>A hydrophobic surface promotes bubble nucleation due to hydrophobicity. Thus, a hydrophobic surface has a higher boiling heat transfer coefficient (BHTC) than a hydrophilic surface. In contrast, a hydrophilic surface supplies liquid to a heating surface. This mechanism enhances the critical heat flux (CHF). In this respect, there is a trade-off between a hydrophobic and a hydrophilic surface. In this study, we examined the effect of heterogeneous wetting surfaces on flow boiling performance. We designed four types of hydrophobic stripes; there are two directions, parallel and crossed to the flow, and the width (the pitch) of the hydrophobic stripes in each direction is 3 or 1 mm. In the macro-channel, the flow boiling performance on the surfaces depended on the patterns. The parallel striped surfaces had higher CHFs than the crossed striped surfaces. In addition, the narrow patterns in each direction had higher CHFs than the wide patterns. The difference in BHTC among the parallel striped surfaces was not large, but the difference in BHTC among the crossed striped surfaces was considerable. A visualization technique revealed that the merging and confinement of bubbles were key factors in explaining the boiling characteristics. Considering the drag coefficient and bubble breakup, we suggest appropriate designs of the hydrophobic pattern for the improvement of BHTC and CHF in the flow boiling performance.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Parallel or crossed hydrophobic stripes were designed with widths of 3 or 1 mm. </LI> <LI> The parallel striped surfaces showed higher CHFs than the crossed surfaces. </LI> <LI> The crossed striped surface having a width of 1 mm showed the highest BHT. </LI> <LI> The boiling characteristics were explained by the merging and breakup of bubbles. </LI> </UL> </P>