Numerical simulation on soot deposition process in laminar ethylene diffusion flames under a microgravity condition

A numerical study on soot deposition in ethylene diffusion flames has been conducted to elucidate the effect of thermophoresis on soot particles under a microgravity environment. Time-dependent reactive-flow Navier-Stokes equations coupled with the modeling of soot formation have been solved. The model was validated by comparing the simulation results with the previous experimental data for a laminar diffusion flame of ethylene (C₂H₄) with enriched oxygen (35% O₂ + 65% N₂) along a solid wall. In particular, the effect of surrounding air velocity as a major calculation parameter has been investigated. Especially, the soot deposition length defined as the transverse travel distance to the wall in the streamwise direction is introduced as a parameter to evaluate the soot deposition tendency on the wall. The calculation result exhibits that there existed an optimal air velocity for the early deposition of soot on the surface, which was in good agreement with the previous experimental results. The reason has been attributed to the balance between the effects of the thermophoretic force and convective motion. This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Jae Hyuk Choi received his B.S. and M.S. degrees in Marine System Engineering from Korea Maritime University in 1996 and 2000, respectively. He then went on to receive a Ph.D. degrees from Hokkaido university in 2005. Dr. Choi is currently a BK21 Assistant Professor at the School of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. Dr. Choi’s research interests are in the area of reduction of pollutant emission (Soot and NOx), high temperature combustion, laser diagnostics, alternative fuel and hydrogen production with high temperature electrolysis steam (HTES). Junhong Kim received his B.S., M.S., and Ph. D degrees in Mechanical Engineering from Seoul National University in 1998, 2000, and 2004, respectively. His research interests include lifted flames, edge flames, and numerical simulation. Sang Kyu Choi received his B.S. degree in Mechanical Engineering from Seoul National University in 2004. He is a Ph. D student in the School of Mechanical Engineering, Seoul National University. His research interests include edge flames, oxy-fuel combustion, and numerical simulation. Byoung ho Jeon received his B.S degrees in Mechanical Engineering from kangwon University in 1998, and M.S., Ph. D. degrees in Mechanical Engineering from Hokkaido University in 2002, 2008, respectively. Dr Jeon is working at Korea Aerospace Research Institute from 2007. June. as Gasturbine engine developer. Jeon’s research interests are in the area of reduction of pollutant emission (Soot and Nox), High temperature combustion, combustion system (Furnace, Combine Generation system, IGCC, CTL), and Fire safety in building. Osamu Fujita received his B.S., M.S., and Ph. D. degrees in Mechanical Engineering from Hokkaido University in 1982, 1984, and 1987, respectively. Prof. Fujita is currently a Professor at the division of Mechanical and space Engineering at Hokkaido University in sapporo, Japan. Prof. Fujita’s research interests are in the area of reduction of pollutant emission (Soot and Nox), solid combustion, catalytic combustion, high temperature combustion, alternative fuel and fire safety in space. Suk Ho Chung received his B.S. degree in Mechanical Engineering in 1976 from Seoul National University, and his M.S. and Ph. D. degree in Mechanical Engineering in 1980 and 1983, respectively from Northwestern University. He is a professor since 1984 in the School of Mechanical and Aerospace Engineering, Seoul National University. His research interests cover combustion fundamentals, pollutant formation, and laser diagnostics.     

Molecular dynamic simulation of the melting and solidification processes of argon

Molecular Dynamic (MD) simulations have been conducted to look at the melting and solidification of the Lennard-Jones argon (100) interface with small amounts (up to 6.0K) of undercooling and superheating. By combining the fully equilibrated bulk phases of liquid and solid in one simulation box and counting the number of solid-like particles, the interface velocities, i.e. the growth rate or melting rate, were obtained as a function of temperature. The melting temperature, where no growth or melting of crystal particle is expected, is T _m * =0.668 which is close to that of the Gibbs free energy calculation. A linear dependence of growth or melting rate on temperature was found except for high superheating, ΔT > 6K. The high superheating is believed as the main source of slope discontinuity in the rate, not the misuse of initial regime as discussed in the earlier works. This paper was recommended for publication in revised form by Associate Editor Dongsik Kim Jae Dong Chung received his B.S. degree in Mechanical Engineering from Seoul National University, Korea, in 1990. He then received his M.S. and Ph.D. degrees from Seoul National University in 1992 and 1996, respectively. Dr. Chung is currently a Professor at the Mechanical Engineering at Sejong University in Seoul, Korea. He serves as a Director of General Affairs of the SAREK and the thermal division of KSME. Dr. Chung’s research interests include nano-scale heat transfer, phase change, material processing and HVAC&R.     

A numerical study on oil retention and migration characteristics in the heat pump system

In HVAC system, the oil circulation is inevitable because the compressor requires the oil for lubrication and sealing. A small portion of the oil circulates with the refrigerant flow through the system components while most of the oil stays or goes back to the compressor. Because oil retention in refrigeration systems can affect system performance and compressor reliability, proper oil management is necessary in order to improve the compressor reliability and increase the overall efficiency of the system. This paper describes a numerical analysis of oil distribution in each component of the commercial air conditioning system including the suction line, discharge line and heat exchanger. In this study, system modeling was conducted for a compressor, discharge line, condenser, expansion valve, evaporator and suction line. Oil separation characteristics of the compressor were taken from the information provided by manufacturer. The working fluid in the system was a mixture of a R-410A refrigerant and PVE oil. When the oil mass fraction (OMF) was assumed, oil mass distribution in each component was obtained under various conditions. The total oil hold-up was also investigated, and the suction line contained the largest oil hold-up per unit length of all components. This paper was presented at the 7th JSME-KSME Thermal and Fluids Engineering Conference, Sapporo, Japan, October 2008. Min Soo Kim received his B.S., M.S., and Ph.D. degree at Seoul National University, Korea in 1985, 1987, and 1991, respectively. After Ph.D. degree, Prof. Kim worked at National Institute of Standards and Technology (NIST) in U.S.A. for about three years. He is currently a professor at the School of Mechanical and Aerospace Engineering of Seoul National University, Korea. Jong Won Choi received B.S. degree in Mechanical Engineering from Korea University in Seoul, Korea, in 2004, and then received M.S. degrees from Seoul National University in 2006. He is currently a student in Ph.D. course at the School of Mechanical and Aerospace Engineering of Seoul National University in Seoul, Korea. His research interests include refrigeration system, micro-fluidic devices, and PEM fuel cell as an alternative energy for next generation. Mo Se Kim received B.S. degree in Mechanical and Aerospace Engineering from Seoul National University in Seoul, Korea, in 2007. He is currently a student in M.S. course at the School of Mechanical and Aerospace Engineering of Seoul National University in Seoul, Korea. He had studied on the oil migration in the heat pump system, and now he studies on the refrigeration system using an ejector. Baik-Young Chung received his B.S., M.S., and Ph.D. degrees in Mechanical Engineering from Inha University, Korea in 1984, 1986, and 2001, respectively. He is currently a research fellow of HAC Research Center at LG Electronics. He is responsible for the commercial air conditioner group. Sai-Kee Oh received B.S. degree in Mechanical Engineering from Seoul National University, Korea in 1989, and then received M.S. and Ph.D. degrees from KAIST, Korea in 1991 and 1997, respectively. He is currently a principal research engineer of HAC Research Center at LG Electronics. He is responsible for the residential air conditioner group. Jeong-Seob Shin received B.S. degree in Machine Design and Production Engineering from Hanyang University, Korea in 1988, M.S. degree in Mechanical Engineering from KAIST, Korea in 1991, and Ph.D. degree in Mechanical Engineering from POSTECH, Korea in 2004. He has joined HAC Research Center at LG Electronics since 2006 as a principal research engineer.     

Optimal microchannel design using genetic algorithms

This paper presents a novel method of optimizing particle-suspended microfluidic channels using genetic algorithms (GAs). The GAs can be used to generate an optimal microchannel design by varying its geometrical parameters. A heuristic simulation can be useful for simulating the emergent behaviors of particles resulting from their interaction with a virtual microchannel environment. At the same time, fitness evaluation enables us to direct evolutions towards an optimized microchannel design. Specifically, this technique can be used to demonstrate its feasibility by optimizing one commercialized product for clinical applications such as the microchannel-type imaging flow cytometry of human erythrocytes. The resulting channel design can also be fabricated and then compared to its counterpart. This result implies that this approach can be potentially beneficial for developing a complex microchannel design in a controlled manner. This paper was recommended for publication in revised form by Associate Editor Hong Hee Yoo Hyunwoo Bang was born in Korea on June 2, 1978. He received the B.S. degree in mechanical and aerospace engineering from Seoul National University, Seoul, Korea in 2001 and the Ph.D. degree in mechanical and aerospace engineering from Seoul National University in 2007. He did postdoctoral research at University of California Los Angeles, CA that involved the integration of functional biological components into engineered devices with Prof. Jacob J. Schmidt from April 2007 to August 2008. His current research interests include microfluidics based Lab-on-a-chip devices and their design optimization using artificial intelligence. Dong-Chul Han received the B.S. degree from the Department of Mechanical Engineering, Seoul National University, Seoul, Korea, in 1969, and the Dipl.-Ing. and Dr.-Ing. degrees from the Department of Mechanical Engineering, University of Karlsruhe, Karlsruhe, Germany, in 1975 and 1979, respectively. He also received the Habilitation from the Department of Mechanical Engineering, University of Karlsruhe. He had been a professor in the school of Mechanical and Aerospace Engineering at Seoul National University from 1982 to 2008. His research interests include active magnetic bearing systems, mechanical lubrication, Bio-MEMS (MicroElectroMechanical Systems) and nano-fabrication.     

Friction-induced ignition modeling of energetic materials

The heat released during the external frictional motion is a factor responsible for initiating energetic materials under all types of mechanical stimuli including impact, drop, or penetration. We model the friction-induced ignition of cyclotrimethylenetrinitramine (RDX), cyclotetramethylene-tetranitramine (HMX), and ammonium-perchlorate/ hydroxylterminated-polybutadiene (AP/HTPB) propellant using the BAM friction apparatus and one-dimensional time to explosion (ODTX) apparatus whose results are used to validate the friction ignition mechanism and the deflagration kinetics of energetic materials, respectively. A procedure to obtain the time-to-ignition for each energetic sample due to friction is outlined. This paper was presented at the 7th JSME-KSME Thermal and Fluids Engineering Conference, Sapporo, Japan, October 2008. Min-cheol Gwak received his B.S. degree in Mechanical Engineering from Korea Aerospace University, Korea, in 2007. Now he is a graduate student of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. His research interests are ignition of high energy material and combustion phenomena. Tae-yong Jung received his B.S. degree in Mechanical and Aerospace Engineering from Seoul National University, Korea, in 2007. Now he is a graduate student of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. His research interests are solid propellant combustion and phase transformation. Professor J. Yoh received his BSME from UC Berkeley in 1992 and MSME from UCLA in 1995. His doctoral degree is in Theoretical & Applied Mechanics from the University of Illinois at Urbana-Champaign, 2001. His research interest is in high energy system design using high power lasers and condensed energetic materials.     

Investigation of penetration force of living cell using an atomic force microscope

Recently, the manipulation of a single cell has been receiving much attention in transgenesis, in-vitro fertilization, individual cell based diagnosis, and pharmaceutical applications. As these techniques require precise injection and manipulation of cells, issues related to penetration force arise. In this work the penetration force of living cell was studied using an atomic force microscope (AFM). L929, HeLa, 4T1, and TA3 HA II cells were used for the experiments. The results showed that the penetration force was in the range of 2∼22 nN. It was also found that location of cell penetration and stiffness of the AFM cantilever affected the penetration force significantly. Furthermore, double penetration events could be detected, due to the multi-membrane layers of the cell. The findings of this work are expected to aid in the development of precision micro-medical instruments for cell manipulation and treatment. This paper was presented at the 9^th Asian International Conference on Fluid Machinery (AICFM9), Jeju, Korea, October 16–19, 2007.recommended for publication in revised form by Associate Editor Keum-Sik Hong Eun-Young Kwon received her B.S. and M.S degrees in Mechanical Engineering from Yonsei University, Korea, in 2005 and 2007, respectively. Ms. Kwon is currently an Engineer at Digital Printing Division of Samsung Electronics. Her research interests include biotribology, tribology, and electrophotography. Young-Tae Kim received his B.S. in Automotive Engineering from Seoul National University of Technology, Korea, in 2003. He then received his M.S. degree from Yonsei University in Seoul, Korea in 2005. Mr. Kim is currently a Ph. D. candidate at the Graduate School of Mechanical Engineering at Yonsei University in Seoul, Korea. His research interests include biotribology, tribology, and biomechanics. Dae-Eun Kim received his B.S. in Mechanical Engineering from Tufts University, USA, in 1984. He then received his M.S. and Ph.D. degrees from M.I.T. in 1986 and 1991, respectively. Dr. Kim is currently a Professor at the School of Mechanical Engi-neering at Yonsei University in Seoul, Korea. His research interests include tribology, functional surfaces, and micromachining.     

A study on cooling efficiency using 1-d analysis code suitable for cooling system of thermoforming

Thermoforming is one of the most versatile and economical processes available for polymer products, but cycle time and production cost must be continuously reduced in order to improve the competitive power of products. In this study, water spray cooling was simulated to apply to a cooling system instead of compressed air cooling in order to shorten the cycle time and reduce the cost of compressed air used in the cooling process. At first, cooling time using compressed air was predicted in order to check the state of mass production. In the following step, the ratio of removed energy by air cooling or water spray cooling among the total removed energy was found by using 1-D analysis code of the cooling system under the condition of checking the possibility of conversion from 2-D to 1-D problem. The analysis results using water spray cooling show that cycle time can be reduced because of high cooling efficiency of water spray, and cost of production caused by using compressed air can be reduced by decreasing the amount of the used compressed air. The 1-D analysis code can be widely used in the design of a thermoforming cooling system, and parameters of the thermoforming process can be modified based on the recommended data suitable for a cooling system of thermoforming. This paper was recommended for publication in revised form by Associate Editor Dongsik Kim Zhen-Zhe Li received his B.S. degree in Mechanical Engineering from Yanbian University, China, in 2002. He then received his M.S. degree in Aerospace Engineering from Konkuk University, South Korea, in 2005. He then received his Ph.D. degree in Mechanical Engineering from Chonnam National University, South Korea, in 2009. Dr. Li is currently a Researcher of the Department of Mechanical Engineering, Chonnam National University, South Korea. Dr. Li’s research interests include applied heat transfer, fluid mechanics and optimal design of thermal and fluid systems. Kwang-Su Heo received his B.S. degree in Mechanical Engineering from Chonnam National University, South Korea, in 1998. He then received his M.S. and Ph.D. degrees in Mechanical Engineering from Chonnam National University, South Korea, in 2003 and 2008, respectively. Dr. Heo is currently a Post-doctorial Researcher of the Department of Mechanical Engineering, KAIST(Korean Advanced Institute of Science and Technology), South Korea. Dr. Heo’s research interests include applied heat transfer, fluid mechanics and thermal analysis of superconductor. Dong-Ji Xuan received his B.S. degree in Mechanical Engineering from Harbin Engineering University, China, in 2000. He then received his M.S. degree in Mechanical Engineering from Chonnam National University, South Korea, in 2006. He is currently a Ph.D. candidate of the Department of Mechanical Engineering, Chonnam National University, South Korea. His research interests include control & optimization of PEM fuel cell system, dynamics & control, mechatronics. Seoung-Yun Seol received his B.S. degree in Mechanical Design from Seoul National University, South Korea, in 1983. He then received his M.S. degree in Mechanical Engineering from KAIST(Korean Advanced Institute of Science and Technology), South Korea, in 1985. He then received his Ph.D. degree in Mechanical Engineering from Texas Tech University, USA, in 1993. Dr. Seol is currently a Professor of the School of Mechanical and Systems Engineering, Chonnam National University, South Korea. Dr. Seol’s research interests include applied heat transfer, fluid mechanics and thermal analysis of superconductor.     

Micro machining of an STS 304 bar by magnetic abrasive finishing

A magnetic abrasive finishing process is a method of non-traditional precision machining in which the finishing process is completed using magnetic force and magnetic abrasives. In this research, a STS 304 cylindrical workpiece was finished using a magnetic abrasive finishing process at 30,000 rpm, and the roughness, roundness, and changes in the micro-diameter were investigated. The study showed that it is possible to control the micro-diameter and weight of the STS 304 cylindrical workpiece by using a near linear approach. Surface roughness as fine as 0.06 μm (Ry) and roundness as fine as 0.12 μm (LZS) were achievable by using a diamond paste with 1 μm particles. Vibrational motion applied to the workpiece improved the surface roughness. The improvement of the surface roughness was achieved because the vibrational motion effectively removes unevenness in the rotational direction and the direction orthogonal to it. This paper was presented at the 9^th Asian International Conference on Fluid Machinery (AICFM9), Jeju, Korea, October 16–19, 2007.recommended for publication in revised form by Associate Editor Dae-Eun Kim Ik-Tae Im received the B.S., M.S. and Ph.D. degrees in Mechanical Engineering from Hanyang University, Seoul, Korea, in 1993, 1995 and 1999, respectively. He has been a visiting scientist at the Department of Materials Engineering, the University of Tokyo, Japan, where he studied on the film growth during the MOCVD process. His research interests include the numerical modeling on the transport phenomena in various materials processing. He is a professor at the Division of Mechanical Design Engineering at Chonbuk National University in Jeonju, Korea. Sang Don Mun received the B.S. degree and M.S. in Precision Mechanical Engineering from Chonbuk National University, Korea, in 1991 and 1993, respectively. He then received the Ph.D. in Precision Mechanical Engineering at the same university in 1997. Dr. Mun is currently a Professor at the Division of Mechanical Design Engineering at Chonbuk National University in Jeonju, Korea. His research interests include magnetic abrasive finishing, tool wear, and micro machining. Seong Mo Oh received his B.S. degree in Mechanical Engineering from Wonkwang University, Korea, in 1992. He then received his M.S. and Ph.D. degrees from Wonkwang in 1994 and 2000 respectively. Dr. Oh is currently a Lecturer at the Division of Mechanical and Automotive Engineering at Wonkwang University in Jeonbuk, Korea. Dr. Oh’s research interests include tribology, functional surfaces, and micromachining.     

Approximate velocity scale for primary and secondary dual-inlet side-dump flows

Effects of the bulk inlet velocity on the characteristics of dual-inlet side-dump flows are numerically investigated. Non-reacting subsonic turbulent flow is solved by a preconditioned Reynolds-averaged Navier-Stokes equation system with low-Reynolds number k − ɛ turbulence model. The numerical method is properly validated with measured velocity distributions in the head dome and the combustor. With substantial increase in the bulk inlet velocity, general profiles of essential primary and secondary flows normalized by the bulk inlet velocity are quantitatively invariant to the changes in the bulk inlet velocity. This paper was recommended for publication in revised form by Associate Editor Do Hyung Lee Seung-chai Jung received his B.S. degree in Mechanical Engineering from Yonsei University, Korea, in 2001. He then received his M.S. degree in Mechanical Engineering from Yonsei University, Korea, in 2005. Mr. Jung is currently a Ph. D. candidate at Yonsei University, where he is majoring in Mechanical Engineering. Mr. Jung’s research interests include propulsion system and particle-surface collision dynamics. Byung-Hoon Park received his B.S. degree in Mechanical Design and Production Engineering from Yonsei University in 2003. He is currently a Ph.D. candidate in Yonsei University in Seoul, Korea. His research interests include performance design of propulsion systems and nu-merical analysis of instability in multiphase turbulent reacting flow-fields. Hyun Ko received his B.S. degree in Aerospace Engineering from Chonbuk National University, Korea, in 1996. He then received his M.S. degree in Mechanical Design from Chonbuk National University, Korea, in 1998. In 2005, he obtained his Ph.D. degree from Yonsei University, where he majored in mechanical engineering. Dr. Ko is currently a Principal Research Engineer of the MicroFriend Co., Ltd. in Seoul, Korea. His research interests include propulsion related systems and computational fluid dynamics. Woong-sup Yoon received his B.S. degree in Mechanical Engineering from Yonsei University, Korea, in 1985. He then received his M.S. degree from University of Missouri-Rolla in 1989. In 1992, he obtained his Ph.D. degree from the University of Alabama in Huntsville, where he majored in mechanical and aerospace engineering. Dr. Yoon is currently a professor at the School of Mechanical Engineering at Yonsei University in Seoul, Korea. His research interests include propulsion system and particle-related environmental/ thermal engineering.     

NOx emission characteristics in turbulent hydrogen jet flames with coaxial air

The characteristics of NOx emissions in pure hydrogen nonpremixed jet flames with coaxial air are analyzed numerically for a wide range of coaxial air conditions. Among the models tested in simple nonpremixed jet flame, the one-half power scaling law could be reproduced only by the Model C using the HO₂/H₂O₂ reaction, implying the importance of chemical nonequilibrium effect. The flame length is reduced significantly by augmenting coaxial air, and could be represented as a function of the ratio of coaxial air to fuel velocity. Predicted EINOx scaling showed a good concordance with experimental data, and the overall one-half power scaling was observed in coaxial flames with Model C when flame residence time was defined with flame volume instead of a cubic of the flame length. Different level of oxygen mass fraction at the stoichiometric surface was observed as coaxial air was increased. These different levels imply that the coaxial air strengthens the nonequilibrium effect. This paper was recommended for publication in revised form by Associate Editor Haecheon Choi Hee-Jang Moon received his B.S. degree in Aeronautical Engineering from Inha University, Korea in 1986. He then received his M.S. and Doctoral degrees from Universite de Rouen, France in 1988 and 1991, respectively. Dr. Moon is currently a Professor at the School of Aerospace and Mechanical Engineering at Korea Aerospace University in Koyang, Korea. He serves on the Editorial Board of the Korean Society of Propulsion Engineers. His research interests are in the area of turbulent combustion, hybrid rocket combustion and nanofluids. Youngbin Yoon received his B.S. and M.S. degrees in Aerospace Engineering from Seoul National University, Korea in 1985 and 1987, respectively. He received a Ph.D. degree from the University of Michigan in 1994. Dr. Yoon is currently a professor at the School of Mechanical and Aerospace Engineering in Seoul National University, Korea. He is currently on the Editorial board and executive of ILASS-KOREA. The research areas of Dr. Yoon are liquid rocket injectors, combustion instability and control, ram and gas turbine combustor and laser diagnostics.     

Effects of injection type on slot film cooling for a ramjet combustor

A slot film cooling technique has been used to protect a combustor liner from hot combustion gas. This method has been developed for gas turbine combustors. A ramjet combustor exposed to high temperature can be protected properly with a multi-slot film cooling method. An experimental study has been conducted to investigate the change of the first slot angle under recirculation flow and the influence of wiggle strip within a slot, which affects the film cooling effectiveness. The first slot angle has been changed to understand the effect of the injection angle on the film cooling effectiveness in a recirculation zone. The distribution of dimensionless temperature was obtained by a thermocouple rake to investigate the wiggle strip effect, and the adiabatic film cooling effectiveness on downstream wall was measured by a thermochromic liquid crystal (TLC) method. At the first slot position, the film cooling effectiveness decreases significantly because of the effects of recirculation flow. The lip angle of the first slot affects slightly on the film cooling effectiveness. The wiggle strip reinforces the structure of slot and keeps the uniform open area of slot. However, it induces three dimensional flows and enhances the flow mixing between the main flow and the injected slot flow. Therefore, the wiggle strip decreases slightly the overall film cooling effectiveness. This paper was presented at the 7th JSME-KSME Thermal and Fluids Engineering Conference, Sapporo, Japan, October 2008. Kwanghoon Park received his M.S degree in Mechanical Engineering from Yonsei University, Seoul, Korea in 2007. He is currently working for an education of an officer as a drillmaster in Army Consolidated Logistics School. Kang Mo Yang joined the Republic of Korea Army in 2004. He is currently working towards the M.S. degree at the Department of Mechanical Engineering, Yonsei University. Keon Woo Lee received his M.S. degree in Mechanical Engineering from Yonsei University, Seoul, Korea in 2008. In 2008, he joined the Doosan heavy industries & Construction Co, LTD, where he is a member of the IGCC Business Team. Hyung Hee Cho received his PhD degree in Mechanical Engineering from University of Minnesota, Minneapolis, MN in 1992. In 1995, he joined the Department of Mechanical Engineering, Yonsei University, Seoul, Korea, where he is currently a full professor in the School of Mechanical Engineering. His research interests include heat transfer in turbomachineries, rocket/ramjet cooling as well as nanoscale heat transfer in thin films, and microfabricated thermal sensors. Hee Cheol Ham received his PhD degree in Mechanical Engineering from Yonsei University, Seoul, Korea in 2001. In 1984, he joined the Agency for Defense Development, Daejeon, Korea, where he is currently a Principal Researcher. Ki Young Hwang received his Ph.D. degree in Mechanical Engineering from Yonsei University, Seoul, Korea in 1994. In 1979, he joined the Agency for Defense Development, Daejon, Korea, where he is currently a principal researcher in the Airbreathing Propulsion Directorate.     

Numerical study of droplet dynamics in a PEMFC gas channel with multiple pores

The water droplet motion in a PEMFC gas channel with multiple pores, through which water emerges, is studied numerically by solving the equations governing the conservation of mass and momentum. The liquid-gas interface is tracked by a level set method which is based on a sharp-interface representation for accurately imposing the matching conditions at the interface. The method is modified to implement the contact angle conditions on the walls and pores. The dynamic interaction between the droplets growing on multiple pores is investigated by conducting the computations until the droplet growth and sliding motion exhibits a periodic pattern. The numerical results show that the configuration subject to droplet merging is not effective for water removal and that the wettability of channel wall strongly affects water management in the PEMFC gas channel. This paper was presented at the 7th JSME-KSME Thermal and Fluids Engineering Conference, Sapporo, Japan, October 2008. Gihun Son received his B.S. and M.S. degrees in Mechanical Engineering from Seoul National University in 1986 and 1988, respectively, and Ph.D. degree in Mechanical Engineering from UCLA in 1996. Dr. Son is currently a professor of Mechanical Engineering at Sogang University in Seoul, Korea. His research interests are in the area of multiphase dynamics, heat transfer, and power system simulation. Jiyoung Choi received a B.S. degree in Mechanical Engineering from Sogang University in 2005. He is a graduate student of Mechanical Engineering at Sogang University in Seoul, Korea. Choi’s research interests are in the area of PEM fuel cell and microfluidics.     

A study on performance improvement of corrugated type total heat exchanger considering the structure of flow passage on surface

Three types of flow passage structure of a total heat exchanger (perforated type, slit type, and embossed and perforated type) are studied to enhance the heat exchange performance of a heat recovery ventilation system (total heat exchanger). The perforated type has four punched rows of 6mm holes in the flow passage channel, and the slit type has six processed rows of 40mm length. The embossed and perforated type has holes of about 1mm diameter and protrusions of about 0.2mm height on all surfaces. The heat exchange efficiency of the modified total heat exchanger was compared to that of a general total heat exchanger with a smooth surface. The Korean Standard (KS) heat recovery ventilator test condition was applied for tests. In the case of cooling operation based on a typical Reynolds number of 140 (typical air flow rate of 100 m³/hr), the perforated type, slit type, and embossed and perforated type showed temperature efficiency improvement of 1.2%, 2.5%, and 5.0%; latent efficiency improvement of 18.0%, 32.3%, and 24.5%; and enthalpy efficiency improvement of 7.9%, 11.5%, and 11.2%, respectively. The corresponding improvements of heating operation were 3.0%, 3.4%, and 4.0%; 5.0%, 6.6%, and 18.7%; 3.2%, 4.3%, and 7.7%, respectively. On the other hand, the air pressure drop throughout the modified flow passage of the total heat exchanger increased by up to 1.7% at the typical Reynolds number of 140, from the air pressure drop of the regular total heat exchanger. This paper was recommended for publication in revised form by Associate Editor Dae Hee Lee Kyungmin Kwak received his B.S., M.S. and Ph.D. degrees in Mechanical Engineering from Yeungnam University, Korea, in 1993, 1995 and 1999, respectively. Dr. Kwak is currently a Researcher at the Automotive RIC at Kyungil University, Korea. His research interests include heat transfer, refrigeration and air control. Cheolho Bai received his B.S. and M.S. degrees in Mechanical Engineering from Seoul Na-tional University, Korea, in 1984 and 1986, respectively. He then received his Ph.D. from UCLA, USA, in 1992. Dr. Bai is currently a Professor at the School of Mechanical Engineering at Yeungnam University in Kyungsan, Korea. His research interests include heat transfer, refrigeration and air control.     

Optimal design of a variable stiffness joint in a robot manipulator using the response surface method

The response surface method combined with the design of experiment-based design optimization of a variable stiffness joint (VSJ) is presented in this article. A VSJ used in a manipulator of a robot arm to support 1 kg payload at the end is designed by considering the minimization of the total weight as the objective function. Owing to the requirement of large rotational stiffness of the VSJ, over 10 N · m, ring-type permanent magnets are adopted. First, a model composed of two permanent magnets was initially manufactured and tested for comparison with the analysis results. Then, a three-ring-type permanent magnet-based model is suggested and optimized to increase the torque of VSJ. The finite element method is used as a magnetic field analysis method to substitute for the expensive experimental process. Optimization results decrease the weight from 0.899 kg to 0.538 kg, still satisfying the requirement for the rotational stiffness. This paper was recommended for publication in revised form by Associate Editor Tae Hee Lee Jeonghoon Yoo received his B.S. and M.S. degrees in Mechanical Design and Production Engineering from Seoul National University, in 1989 and 1991, respectively. He then received his Ph.D. degrees from the University of Michigan, Ann Arbor, in 1999. Dr. Yoo is currently a Professor at the School of Mechanical Engineering at Yonsei University in Seoul, Korea. Dr. Yoo’s research interests include analysis and design of electromagnetic field systems. Myung Wook Hyun received his B.S. and M.S. degrees in Mechanical Engineering from Yonsei University, Korea, in 1995 and 1997, respectively. While studying for his M.S. degree, Mr. Hyun also studied variable stiffness unit design. He is now working at Samsung Electronics, Co. Ltd.. Jun Ho Choi received his B.S. and M.S. degrees in Mechanical Design from Hanyang University, Korea and his Ph.D. degree from the University of Michigan, Ann Arbor. He is currently a senior research scientist in the Korea Institute of Science and Technology. His research interests include nonlinear control, manipulator control, and safe-joint design. Sungchul Kang received his B.S., M.S., and Ph.D. degrees in Mechanical Design and Production Engineering from Seoul National University, Korea, in 1989, 1991, and 1998 respectively. Dr. Kang is currently a Principal Research Scientist in the Center for Cognitive Robotics Research, Korea Institute of Science and Technology, in Seoul, Korea. Dr. Kang’s research interests include mobility and manipulation of field and service robots and haptics. Seung-Jong Kim received his B.S. degree in Mechanical Engineering from Seoul University, Korea, in 1989, and his M.S. and Ph.D. degrees from KAIST in 1991 and 1998, respectively. Dr. Kim is currently a Principal Research Scientist at the Korea Institute of Science and Technology in Seoul, Korea. Dr. Kim’s research interests include the design, control, and dynamic analysis of mechatronic systems.     

Numerical study of droplet motion in a microchannel with different contact angles

The droplet motion in a microchannel with different contact angles, which is applicable to a typical proton exchange membrane fuel cell (PEMFC), was studied numerically by solving the equations governing the conservation of mass and momentum. The gas-liquid interface or droplet shape was determined by a level set method which was modified to treat the static and dynamic contact angles. The matching conditions at the interface were accurately imposed by incorporating the ghost fluid approach based on a sharp-interface representation. Based on the numerical results, the droplet dynamics including the sliding and detachment of droplets was found to depend significantly on the contact angle. Also, the effects of inlet flow velocity, droplet size and side wall on the droplet motion were investigated. This paper was recommended for publication in revised form by Associate Editor Haecheon Choi Gihun Son received a B.S. and M.S. degree in Mechanical Engineering from Seoul National University in 1986 and 1988, respectively. He then went on to receive his Ph.D. degrees from UCLA in 1996. Dr. Son is currently a Professor of Mechanical Engineering at Sogang University in Seoul, Korea. Dr. Son’s research interests are in the area of multiphase dynamics, heat transfer, and power system simulation. Jiyoung Choi received a B.S. degree in Mechanical Engineering from Sogang University in 2005. He is a graduate student of Mechanical Engineering at Sogang University in Seoul, Korea. Choi’s research interests are in the area of PEM fuel cell and microfluidics.     

Tolerance effects on natural frequencies of multibody systems undergoing constant rotational motion

A general multi-body formulation to analyze the tolerance effects on the statistical property variations of natural frequencies of multi-body systems undergoing constant rotational motion is proposed in this paper. To obtain the tolerance effects, Monte-Carlo simulation method is conventionally employed. However, the Monte-Carlo simulation has serious drawbacks; spending too much computation time for the simulation and achieving very slow convergence around some dynamically unstable regions. To resolve such problems, a method employing analytical sensitivity information is suggested in this paper. To obtain the sensitivities of natural frequencies the eigenvalue problem should be differentiated with respect to a design variable. The sensitivities of mass and stiffness matrices should be calculated at the dynamic equilibrium. By employing the sensitivities of natural frequencies along with the tolerance of the design variable, the statistical property variations of the natural frequencies can be calculated. This paper was recommended for publication in revised form by Associate Editor Seockhyun Kim Seung Man Eom graduated from the Department of Mechanical Engineering at Incheon University in 2005 and received his master degree from the Department of Mechanical Engineering at Hanyang University in 2007. He is currently working as a Researcher of Aircraft Development Team in KIAT(Korea Institute of Aerospace Technology, Koreanair), DaejeonDeajeon, Korea. Bum Suk Kim graduated from the School of Mechanical Engineering at Hanyang University in 2006 and received his master degree from the same department in 2008. He is currently working as a Ph.D. student in the School of Mechanical Engineering in Hanyang University, Seoul, Korea. Hong Hee Yoo graduated from the Department of Mechanical Design and Production Engineering at Seoul National University in 1980 and received his master degree from the same department in 1982. He received his Ph.D. degree in 1989 from the Department of Mechanical Engineering and Applied Mechanics in the University of Michigan at Ann Arbor, U.S.A. He is currently working as a professor in the School of Mechanical Engineering in Hanyang University, Seoul, Korea.     

Topology optimization with displacement-based nonconforming finite elements for incompressible materials

This investigation is concerned with the topology optimization using displacement-based nonconforming finite elements for problems involving incompressible materials. Although the topology optimization with mixed displacement-pressure elements was performed, a displacement-based approach can be an efficient alternative because it interpolates displacement only. After demonstrating the Poisson locking-free characteristics of the employed nonconforming finite elements by a simple patch test, the developed method is applied to solve the design problems of mounts involving incompressible solid or fluid. The numerical performance of the nonconforming elements in topology optimization was examined also with existing incompressible problems. This paper was recommended for publication in revised form by Associate Editor Tae Hee Lee Gang-Won Jang received his M.S. degree in 2000, and Ph.D. degree in 2004, both from the School of Mechanical and Aerospace Engineering, Seoul National University, Seoul, Korea. He is currently an Assistant Professor at the School of Mechanical and Automotive Engineering, Kunsan National University, Jeonbuk, Korea. His current interest concerns topology optimization of multiphysics problems and thin-walled beam analysis. Yoon Young Kim received his B.S. and M.S degrees from Seoul National University, Seoul, Korea, and the Ph.D. degree from Stanford University, Palo Alto, CA, in 1989. He has been on the faculty of the School of Mechanical and Aerospace Engineering, Seoul National University, since 1991. He is also the Director of the National Creative Research Initiatives Center for Multiscale Design. His main research field is the optimal design of multiphysics systems, mechanisms, and transducers. He has served as an editor of several Korean and international journals, and as an organizing committee member of several international conferences.     

Experimental investigation of dynamic responses of a transparent PEM fuel cell to step changes in cell current density with operating temperature

The dynamic responses of a proton exchange membrane fuel cell (PEMFC) are closely related to the novel water management technique used for the efficient operation of automotive PEMFCs. In order to better understand the dynamic water transport during cell transients, this paper presents an experimental investigation of the transient response of a cell under fully humidified conditions. The cell dynamic performance was measured by employing a transparent cell and investigated with visualization images of the water distribution in the flow channels. Furthermore, the effect of the operating temperature on the cell transients was examined. The results show that the cell dynamic behavior for the tested operating temperature (30–50 °C) conditions is mainly governed by water transport characteristics related to cathode flooding. Also, we show that the time needed for the cell to reach steady-state after a current density step increase is retarded due to excessive water accumulation inside the cell at lower operating temperatures. This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Han-Sang Kim received his B.S. and M.S. degrees from the Department of Mechanical Engineering at Seoul National University in 1989 and 1991, respectively. Since 1991, he had worked for the R & D Center of Hyundai Motor Com-pany for ten years. He then obtained his Ph.D. degree from Seoul Natonal University in 2005. He is currently a BK21 associate professor in the School of Mechanical and Aerospace Engineering at Seoul National University. Kyoungdoug Min received his B.S. and M.S. degrees from the Department of Mechanical Engineering at Seoul National University in 1986 and 1988, respectively. He then obtained his Ph.D. degree from M.I.T. in 1994. He is currently a professor in the School of Mechanical and Aerospace Engineering at Seoul National University.     

Modal analysis of a multi-blade system undergoing rotational motion

A modeling method for the modal analysis of a multi-blade system undergoing rotational motion is presented in this paper. Blades are assumed as cantilever beams and the coupling stiffness which originates from the shroud flexibility is considered for the modeling. To obtain general conclusions from the numerical results, the equations of motion are transformed into a dimensionless form. Dimensionless parameters related to the angular speed, the hub radius, and the coupling stiffness are identified and the effects of the parameters on the modal characteristics of the system are investigated. It is shown that the coupling stiffness especially plays an important role to change the modal characteristics of the system. The range of critical angular speed is also obtained through the numerical analysis. This paper was recommended for publication in revised form by Associate Editor Seockhyun Kim Ha Seong Lim graduated from Department of Mechanical Engineering at Hanyang University in 2006 and received his Master’s degree in 2008. He is currently a technical engineer in STX Offshore & Shipbuilding Company, Seoul, Korea. Hong Hee Yoo graduated from the Department of Mechanical Design and Production Engineering at Seoul National University in 1980 and received his Master’s degree from the same department in 1982. He received his Ph.D. degree in 1989 from the Department of Mechanical Engineering and Applied Mechanics at the University of Michigan at Ann Arbor, U.S.A. He is currently a professor in the School of Mechanical Engineering in Hanyang University, Seoul, Korea.     

Optimal operating points of PEM fuel cell model with RSM

The output power efficiency of the fuel cell system mainly depends on the required current, stack temperature, air excess ratio, hydrogen excess ratio, and inlet air humidity. Therefore, the operating conditions should be optimized to get maximum output power efficiency. In this paper, a dynamic model for the fuel cell stack was developed, which is comprised of a mass flow model, a gas diffusion layer model, a membrane hydration, and a stack voltage model. Experiments have been performed to calibrate the dynamic Polymer Electrolyte Membrane Fuel Cell (PEMFC) stack model. To achieve the maximum output power and the minimum use of hydrogen in a certain power condition, optimization was carried out using Response Surface Methodology (RSM) based on the proposed PEMFC stack model. Using the developed method, optimal operating conditions can be effectively selected in order to obtain minimum hydrogen consumption. This paper was recommended for publication in revised form by Associate Editor Tong Seop Kim Dong-Ji Xuan received his B.S. degree in Mechanical Engineering from Harbin Engineering University, China in 2000. He then received his M.S. degree in Mechanical Engineering from Chonnam National University, South Korea in 2006. Currently, he is a Ph.D. candidate of the Department of Mechanical Engineering, Chonnam National University, South Korea. His research interests include control and optimization of PEM fuel cell system, dynamics and control, and mechatronics. Zhen-Zhe Li received his B.S. degree in Mechanical Engineering from Yanbian University, China in 2002. He then received his M.S. degree in Aerospace Engineering from Konkuk University, South Korea in 2005 and his Ph.D. degree in Mechanical Engineering from Chonnam National University, South Korea in 2009. Dr. Li is currently a Researcher of the Department of Mechanical Engineering in Chonnam National University, South Korea. Dr. Li’s research interests include applied heat transfer, fluid mechanics, and optimal design of thermal and fluid systems. Jin-Wan Kim received his B.S. degree in Aerospace Engineering from Chosun University, South Korea in 1990. He then received his M.S. degree in Aerospace and Mechanical Engineering from Korea Aerospace University, South Korea in 2003 and his Ph.D degree in Mechanical Engineering from Chonnam National University, South Korea in 2008. He is currently a Post Doctor of the Department of Mechanical Engineering in Chonnam National University, South Korea. His research interests include control of hydraulic systems, dynamics and control, and mechatronics. Young-Bae Kim received his B.S. degree in Mechanical Design from Seoul National University, South Korea in 1980. He then received his M.S. degree in Mechanical Engineering from the Korean Advanced Institute of Science and Technology (KAIST), South Korea in 1982 and his Ph.D. degree in Mechanical Engineering from Texas A&M University, USA in 1990. Dr. Kim is currently a Professor of the School of Mechanical and Systems Engineering in Chonnam National University, South Korea. Dr. Kim’s research interests include mechatronics, dynamics and control, and fuel cell hybrid electric vehicle (FCHEV) systems.