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다공성 매질에서 Local Thermal Equilibrium에 관한 연구
장석필,김성진,Jang, Seok-Pil,Kim, Seong-Jin 대한기계학회 2002 大韓機械學會論文集B Vol.26 No.8
In the present study a general criterion for local thermal equilibrium is presented in terms of parameters of engineering importance which include the Darcy number, the effective Prandtl number of fluid, and the Reynolds number. For this, an order of magnitude analysis is performed for the case when the effect of convection heat transfer is dominant in a porous structure. The criterion proposed in this study is more general than the previous criterion suggested by Carbonell and Whitaker, because the latter is applicable only when conduction is the dominant heat transfer mode in a porous medium while the former can be applied even when convection heat transfer prevails. In order to check the validity of the proposed criterion for local thermal equilibrium, the forced convection phenomena in a porous medium with a microchanneled structure subject to an impinging jet are studied using a similarity transformation. The proposed criterion is also validated with the existing experimental and numerical results for convection heat transfer in various porous materials that include some of the parameters used in the criterion such as a microchannel heat sink with a parallel flow, a packed bed, a cellular ceramic, and a sintered metal. It is shown that the criterion presented in this work well-predicts the validity of the assumption of local thermal equilibrium in a porous medium.
나노유체 : 화학적 환원법의 혼합방법이 주석 나노유체의 입자 크기에 미치는 영향
장석필 ( Seok Pil Jang ),이승윤 ( Seoung Youn Lee ),이승현 ( Seung Hyun Lee ),김상훈 ( Sang Hoon Kim ) 한국액체미립화학회 2012 한국액체미립화학회 학술강연회 논문집 Vol.2012 No.-
being changing as alternative materials instead of tin-lead solder ball as following development of ecofriendly industry. However, one of the adverse effect of this change is tin-alloys have higher melting temperature (232oC) compared with tin-lead (~183oC). For this reason, electronic assembly has disadvantages such as warpage, electrical energy consumption, thermal stress and thermal cracking. Therefore studies of lowering meting temperature of tin are intensively investigating as a fundamental study of reducing tin-alloys melting temperature. In previous researches, the melting temperature of nanoparticle is non-linearly decreased with decreasing particle size which is the most dominant effect to determine the melting temperature1). Therefore decreasing the size of particle is core parameter to reduce the melting temperature of tin nanoparticle. Furthermore, to use these lead-free tin nano solder ball practically in industrial part, cost problem should be accompany with the investigation of lead-free solder ball. So, not only decreasing of the particle size but also improving of the production efficiency has to be considered. In this paper, tin nanoparticles are synthesized by chemical reduction method in ethylene glycol using stirrer and ultra-sonic wave bath respectively. Tin (II) acetate, Sodium borohidride and Polyvinylpyrrolidone (PVP) are used as precursors, reducting agents and surfactants respectively. Transmission electron microscopy (TEM) and particle size analyzer (PSA) are used to characterize particle size, shape and distribution of Sn particle and surfactant. Fig. 1, 2 show the well-made Sn nanoparticle manufactured with stirring method and ultra-sonic energy respectively. The stirring method and ultra-sonic energy method both well manufactured 45 ± 5 nm and 10 ± 5 nm average diameters sized tin nanoparticle respectively. The using of ultra-sonic energy is able to make much smaller sized tin nanoparticle than the using of stirrer and it has alternative advantages. As follows, Benefits of using ultrasonic energy are large quantity production due to shorten manufacturing time that only 5 minute and better dispersion stability. While the stirring method need at least two or more hours for one case of experiment. Fig.3 shows the manufacturing time effect on the particle size with ultra-sonic energy. Except for the time problem, the main prohibited problem to manufacture large quantity of Sn nanoparticle is dispersing of surfactant. It was identified that the suspension stability of surfactant effect on particle size and dispersing of surfactant is not easy in bulky amount of base fluid2). By so using of ultrasonic energy comparatively solved these problems not only prohibit aggregation of surfactant during reacting but also easily control large quantity of experimental specimens. Fig.4 shows the 0.01 vol. % of the optimum volume fraction of Tin (II) acetate respect on decreasing of the particle size. Additionally, the using of ultrasonic energy is helpful to reduce 80% of the integrating watt which is 25kWh and 125kWh for using of ultrasonic energy and using of stirrer respectively for each experiment.
장석필(Seok Pil Jang) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.4
Investigators have been perplexed with the thermal phenomena behind the recently discovered nanofluids, fluids with unprecedented stability of suspended nanoparticles although huge difference in the density of nanoparticles and fluid. For example, nanofluids have anomalously high thermal conductivities at very low fraction, strongly temperature-dependent and size-dependent conductivities, and three-fold higher critical heat flux than that of base fluids. Traditional conductivity theories such as the Maxwell or other macroscale approaches cannot explain why nanofluids have these intriguing features. So in this paper, we devise a theoretical model that accounts for the fundamental role of dynamic nanoparticles in nanofluids. The proposed model not only captures the concentration and temperature-dependent conductivity, but also predicts strongly size-dependent conductivity. Furthermore, we physically explain the new phenomena for nanofluids. In addition, based on a proposed model, the effects of various parameters such as the ratio of thermal conductivity of nanofluids to that of a base fluid, volume fraction, nanoparticle size, and temperature on the thermal conductivities of nanofluids are investigated.
장석필(Seok Pil Jang),이병호(Byeong Ho Lee),황교식(Kyo Sik Hwang),Stephen U.S. Choi 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5
This paper investigated effects of the nanoparticles velocity induced by AC electric field on the thermal conductivity enhancement of nanofluids. Using the Langevin equation, it is shown that the velocity of a Al₂O₃-nanoparticle dispersed in the water is increased about 30% at the AC 300 voltage under electric field compared with Brownian velocity. Also, the thermal conductivity of Al₂O₃-water nanofluids is experimentally measured. Based on the results, we report that the thermal conductivity of the nanofluids is increased about 10% at 10㎐ under the fixed AC voltage, AC 300V.
김준호(Jun Ho Kim),장석필(Seok Pil Jang),최철진(Chul Jin Choi),황교식(Kyo Sik Hwang),이호재(Ho-Jae Lee) 대한기계학회 2009 大韓機械學會論文集B Vol.33 No.8
In this paper, drying mechanism is analyzed for improving the energy efficiency of an electric clothes dryer which consumes more electric power than other appliances. For the purpose, characteristic curves of the dryer such as temperature, relative humidity, evaporation rate, mass transfer coefficient, remaining moisture content curves are experimentally obtained. Based on the experimental results and analysis of drying mechanism, the effect of power of a heater and heat loss on the power consumption of an electric clothes dryer are systematically presented. These results demonstrate the feasibility of controlling heat loss at the heater as well as the backduct component to decrease the power consumption of an electric clothes dryer.