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황윤욱,김주혁,김민수 대한설비공학회 2005 설비공학 논문집 Vol.17 No.12
The experimental heat transfer coefficients have been measured for two-phaseconvective boiling in two circular microtubes with inner diameters of 430m and 792m. While the heat transfer was greatly affected by the heat flux in the low quality region, the mass flux played a role in the high quality region. The smaller microtube had greater heat transfer coefficients. When the heat flux is varied from 20kW/m2 to 30kW/m2 at240 kg/m2s, the difference between the average heat transfer coefficients of the test tube A(430m) and the test tube B(792m) changes from 32.5% to 52.1%. At370kg/m2s, the difference between the average heat transfer coefficients changes from 47.0% to 53.8%. A new correlation for the evaporative heat transfer coefficients in microtubes was developed by considering the following factors; the laminar flow heat transfer coefficient of liquid-phase flow, the enhancement factor of the convective heat transfer, and the nucleate boiling correction factor. The correlation developed in this study predicts the experimental heat transfer coefficients within an absolute average deviation of 8.4%.
마이크로핀관의 확관 전후 열전달 및 압력강하 변화 특성에 관한 연구
황윤욱,김민수,Hwang, Yun-Uk,Kim, Min-Su 한국기계연구원 2004 硏究論文集 Vol.34 No.-
The objective of this study is to investigate the pressure drop and heat transfer characteristics of the micro-fin tubes before and after the tube-expansion process. Test tubes are single-grooved micro-fin tubes made of copper with an outer diameter of 9.52 mm before the tube-expansion. The direct heating method is applied in order to make the refrigerant evaporated in the micro-fin tubes. The test ranges of the heat flux, mass flux, and the saturation pressure are 5 to 15kW/$m^2$, 100 to 200 kg/$m^2s$ and 540 to 790 kPa, respectively. The effects of the mass flux, heat flux, and the saturation pressure of the refrigerant on the pressure drop and the heat transfer are presented for the refrigerant R22. In the test conditions of this study, the heat transfer coefficient for the micro-fin tube after the tube-expansion is about 16.5% smaller than that before the tube-expansion because the fin height of micro-fin is reduced and the fin shape becomes flatter. The micro-fin tube after the tube-expansion has about 7.7% greater average pressure drop than that before the tube-expansion process.
황윤욱,김주혁,김민수 대한설비공학회 2006 설비공학 논문집 Vol.18 No.1
The characteristics about the pressure drop in microtubes have been investigated. The test tubes are the circular, seamless, stainless steel tubes with an inner diameter of 0.244, 0.430, and 0.792mm, respectively. R-134a was used as a test fluid. Early flow transition which has been reported in some previous studies is not found in single-phase flow pressure drop tests. The conventional theory between friction factor and Reynolds number predicted the experimental friction factors within an absolute average deviation of 8.9%. The two-phase flow pressure drop increases for higher quality and mass flux, and for reduced inner diameter. The existing correlations fail to predict the experimental data. A new correlation to predict the two-phase flow pressure drop is developed in the form of the Lockhart- Martinelli correlation. The effects of the tube diameter and the surface tension were considered, and the correlation predicted the experimental data within an average absolute deviation of 8.1%.
황윤욱(Yun Wook Hwang),김덕종(Duckjong Kim),김유창(Yu Chang Kim),박상진(Sang Jin Park),허필우(Pil Woo Heo),윤의수(Eui Soo Yoon) 한국유체기계학회 2006 유체기계 연구개발 발표회 논문집 Vol.- No.-
This paper presents the simulation results about the mechanical characteristics of the stereocilia bundle that is modeled as three stereocilia linked with tip-links and side-links. The effects of tip-links and side-links on bundle displacement and stiffness are given with respect to the variation of mechanical properties. The CNT array sensor that mimicks the motion of stereocilia bundle is modeled by considering the interaction between CNTs and fluid. The optimal pitch between CNTs was given for the possible CNT array sensor through geometrical optimization.
황윤욱(Yun Wook Hwang),김덕종(Duckjong Kim),허필우(Pil Woo Heo),박상진(Sang Jin Park),윤의수(Eui Soo Yoon) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.6
The stereocilia bundle has been mechanically modeled as three stereocilia linked with tip-links and side-links. The mechanical characteristics has been studied with respect to the variation of external force exerted on the longest stereocilia. Novel CNT array sensor which simulates the motion of stereocilia bundle has been modeled by considering the interaction between carbon nanotubes and fluid. Two patterns of CNT array such as in-line type and staggered type were examined to give maximum deflection. The optimal pitch between CNTs was given for the possible CNT array sensor through geometric optimization.
황윤욱(Yun Wook Hwang),김덕종(Duckjong Kim),허필우(Pil Woo Heo),박상진(Sang Jin Park),윤의수(Eui Soo Yoon) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11
The mechanical model of the stereocilia bundle has been built using the finite element method. The stereocilia bundle was assumed to be isotropic and linearly elastic in mechanical properties. In the simple structure model, the deflection and the stiffness of the stereocilia bundle were calculated and examined with respect to the variation of the mechanical properties and the geometry. The maximum stiffness value was 6.894 mN/m and the deflections of the stereocilia were 3.55, 2.79, 2.09 ㎚ when the applied force was 10 pN and the Young's modulus of tip-link is 3 GPa. In the FSI model, the deflection of the stereocilia was investigated based upon the interaction between the stereocilia and the lymph fluid. The ratio of the stereocilia deflections was 3.8:2.1:1 which was less than that without considering the interaction between the stereocilia bundle and the lymph fluid.
마이크로핀관의 확관 전후 증발열전달 특성에 관한 실험적 연구
전상희,황윤욱,윤석호,김민수 대한설비공학회 2000 설비공학 논문집 Vol.12 No.10
An experimental study on evaporative heat transfer characteristics in micro-fin tubes before and after expansion process has been performed with R-22. Single-grooved micro-fin tubes with outer diameter of 9.52 mm were used as test sections, and it was uniformly heated by applying direct current to the test tubes. Experiments were conducted at mass flow rates of 20 and 30 kg/hr. For each mass flow rate condition, evaporation temperature was set at 5 and $15^{\circ}C$and heat flux was changed from 6 to 11 kW/$m^2$ The evaporative heat transfer coefficient of micro-fin tubes after expansion is decreased because of the crush of fins and enlargement of inner diameter compared to that before expansion. Convective boiling effect decreased remarkably at higher quality range in the micro-fin tube after expansion, and the difference of the heat transfer coefficient in micro-fin tubes before and after expansion was greater for higher quality region. The evaporative heat transfer coefficient of the micro-fin tube after expansion was 19.9% smaller on the average than that before expansion.