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THE EFFECT OF MICRO/NANOSCALE STRUCTURES ON CHF ENHANCEMENT
안호선,김무환 한국원자력학회 2011 Nuclear Engineering and Technology Vol.43 No.3
Recently, many research studies have investigated the enormous critical heat flux (CHF) enhancement caused by nanofluids during pool boiling and flow boiling. One of the main reasons for this enhancement is nanoparticle deposition on the heated surface. However, in real applications, nanofluids create many problems when used as working fluids because of sedimentation and aggregation. Therefore, artificial surfaces on silicon and metal have been developed to create an effect similar to that of nanoparticle deposition. These modified surfaces have proved capable of greatly increasing the CHF during pool boiling, and good results have also been observed during flow boiling. In this study, we demonstrate that the wetting ability of a surface, i.e.,wettability, and the liquid spreading ability (hydrophilic surface property), are key parameters for increasing the CHF during both pool and flow boiling. We also demonstrate that when the fuel surface in nuclear power plants is modified in a similar manner, it has the same effect, producing a large CHF enhancement.
평판형 히터를 이용한 알루미늄과 타이타늄 산화물 나노유체의 풀비등 임계열유속에 관한 실험적 연구
안호선(HoSeon Ahn),김형대(Hyung Dae Kim),조항진(Hangjin Jo),강순호(Soonho Kang),김무환(Moo Hwan Kim) 대한기계학회 2009 大韓機械學會論文集B Vol.33 No.10
Pool boiling heat transfer and critical heat flux (CHF) of water-based nanofluids with alumina and titania nanoparticles of 0.01% by volume were investigated on a disk heater at saturated and atmospheric conditions. The experimental results showed that the boiling in nanofluids caused the considerable increase in CHF on the flat surface heater. It was revealed by visualization of the heater surface subsequent to the boiling experiments that a major amount of nanoparticles deposited on the surface during the boiling process. Pool boiling of pure water on the surface modified by such nanoparticle deposition resulted in the same CHF increases as what boiling nanofluids, thus suggesting the CHF enhancement in nanofluids was an effect of the surface modification through the nanoparticle deposition during nanofluid boiling. Possible reasons for CHF enhancement in pool boiling of nanofluids are discussed with surface property changes caused by the nanoparticle deposition.
평판형 히터를 이용한 나노 유체 풀비등 임계열유속의 실험적 연구
안호선(Ho Seon Ahn),김선태(Seontae Kim),조항진(Hangjin Jo),제준호(JunHo Je),김무환(Moo Hwan Kim) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5
The flat plate experimental apparatus supporting only thermal heating was developed to perform the CHF enhancement experiment with nanofluids in pool boiling. Nickel and copper surface was used as heater surfaces and nanofluids was created with A1₂O₃, TiO₂ nanoparticles and deionized water by 2-step methods. And CHF in deionized water enhance on nanoparticles deposited surface. So, CHF enhancement in nanofluids is also achieved by the surface effect. CHF enhancement quantities are different as a kind of surface. Copper surface have more CHF enhancement than nickel in same nanofluids. And boiling in A1₂O₃, TiO₂ nanofluids made different surface shape. Surface investigation (SEM) about the nanoparticles coated surface was performed. And these different surfaces induced a different CHF enhancement ratio.
마이크로 기둥 구조가 있는 표면에서의 물 액적의 겉보기 접촉각 및 젖음성 특성에 대한 연구
유동인,박현선,김무환,안호선 한국기계기술학회 2014 한국기계기술학회지 Vol.16 No.1
Generally, in the previous researches, it is found that a water droplet is respectively in Wenzel and Cassie-Baxter states on hydrophilic/hydrophobic rough surfaces. And Wenzel and Cassie-Baxter equations are used to estimate the apparent contact angle on the surfaces. However, difference between measured apparent contact angle and estimated apparent contact angle with the equations is recently reported and new model to estimate apparent contact angle on rough surfaces is proposed. In this study, wetting state and apparent contact angle on the surfaces with micro-pillars should be investigated to find solution of this argument. Using the high resolution microscope, the wetting state of the D.I.water droplet on the surface with micro-pillars was visualized and apparent contact angle of the D.I.water droplet was measured. On the basis of experimental data, the equations to estimate apparent contact angle were verificated and the general wetting characteristics on the surfaces with micro-pillars are finally classified.
메주균을 달리한 숙성 된장의 유리아미노산 , 유리당 및 유기산 조성의 비교
이택수,안호선,배정설 한국농화학회 1987 Applied Biological Chemistry (Appl Biol Chem) Vol.30 No.4
Three lots of mejues were prepared with three different strains of Aspergillus oryzae, Bacillus natto, Bacillus subtilis and one was made by conventional method. The four different soy bean pastes were analyzed for compositions of free amino acids, sugars and organic acids during the period of fermentation. Amino nitrogen contents in the samples of A. oryzae were higher than others throughout the aging period. The amounts of each amino acids were varied markedly among the samples after 20-days, while glutamic acid, aspartic acid, lysine and phenylalanine were dominant. in all samples after 90-days. Glucose contents were found to be in the range of 0.46∼2.66% and other sugars of fructose, sucrose, rhamnose and maltose were less than 0.35%. The levels of total free sugar were relatively higher in the samples prepared with B. natto than others. Curie, lactic, malic, acetic and oxalic acids were identified, and the content of lactic acid was higher in the samples of A. oryzae, whereas citric acid was higher in conventional method.
STUDY OF LEIDENFROST MECHANISM IN DROPLET IMPACTING ON HYDROPHILIC AND HYDROPHOBIC SURFACES
김설하,강준용,안호선,조항진,김무환 대한설비공학회 2013 International Journal of Air-Conditioning and Refr Vol.21 No.4
Water droplets, 2mm in diameter, were allowed to fall freely onto hydrophobic and hydrophilic heated surfaces, and their impacts were imaged using high-speed cameras to investigate the droplet dynamics and heat transfer. As the heating power increased, the water droplets evaporated faster,eventually hovering over the surface due to the formation of a boiling ¯lm when the Leidenfrost point (LFP) was reached. The heat transfer from the surface into the droplet was evaluated, and LFP transition phenomena were investigated using time-resolved imaging of both side and bottom views. The hydrophilic surface showed a higher heat transfer rate and a higher LFP than the hydrophobic surface did. Furthermore, the droplet dynamics revealed very different shapes depending on the surface wettability; vigorous bubble nucleation and growth was observable for the hydrophilic surface, but not the hydrophobic surface. The rebound behavior of the droplets was analyzed based on the droplet free energy, including kinetic, potential, and surface energy terms.
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.