성층화된 축열조의 1차원모델에 대한 해석적인 해

유호선,박이동,Yoo, H.,Pak, E.-T. 대한설비공학회 1995 설비공학 논문집 Vol.7 No.1

In order to establish a theoretical basis for the analyses of transient behaviors in stratified thermal storage tanks, analytical approaches to an improved one-dimensional model are made. In the present model the storage tank is treated as a finite region with an adiabatic tank exit, whereas it has been considered as a simple semi-infinite region previously. Application of the Laplace transformation and the Inversion theorem to the governing equations makes it possible to obtain an exact infinite-series solution, which is convergent only at sufficiently large time. Accordingly a complementary solution which is available for short times, i.e., the time range of this study is sought by an approximate method. The approximate solution which is rigorously validated through the examination of neglected terms in the solution procedure agrees quite well with the exact one. Moreover, it is simpler to use and more convenient to interpret the physical meaning of the solution. Comparison of the present solution with the previous ones shows relatively large difference near the tank bottom, which results from the more realistic boundary condition adopted in the present model. Some representative results by the approximate solution including effects of the Peclet number on temperature distrbutions are illustrated to show the utility of this study. In consequence, it is expected that the present results based on the improved model replace the foregoing ones as a new theoretical reference for studies of thermal stratification fields.

열전도가 주도적인 삼차원 접촉융해에 대한 비정상 해석

유호선,Yoo, Hoseon 대한기계학회 1999 大韓機械學會論文集B Vol.23 No.8

This work reports a set of approximate analytical solutions describing the initial transient process of close-contact melting between a rectangular parallelepiped solid and a flat plate on which either constant temperature or constant heat flux is imposed. Not only relative motion of the solid block tangential to the heating plate, but also the density difference between the solid and liquid phase is incorporated in the model. The thin film approximation reduces the force balance between the solid weight and liquid pressure, and the energy balance at the melting front into a simultaneous ordinary differential equation system. The normalized model equations admit compactly expressed analytical solutions which include the already approved two-dimensional solutions as a subset. In particular, the normalized liquid film thickness is independent of all pertinent parameters, thereby facilitating to define the transition period of close-contact melting. A unique behavior of the solid descending velocity due to the density difference is also resolved by the present solution. A new geometric function which alone represents the three-dimensional effect is introduced, and its properties are clarified. One of the representative results is that heat transfer is at least enhanced at the expense of the increase in friction as the cross-sectional shape deviates from the square under the same contact area.

초기 및 중등도 치주염에서 임상지수, Interleukin-1, MMP-8, 치주원인균 감소에 대한 전동칫솔의 효과

유호선,채경준,홍지연,최성호,김종관,Yoo, Ho-Sun,Chae, Gyung-Joon,Hong, Ji-Youn,Choi, Seong-Ho,Kim, Chong-Kwan 대한치과의사협회 2008 대한치과의사협회지 Vol.46 No.12

이 연구의 목적은 초기 및 중등도 치주염 환자에서 전동칫솔을 사용할 경우 임상 지수의 향상 정도와 치주원인균의 정량적 감소 효과를 12주의 연구 기간 동안 평가하는 것이다. $25{\sim}55$세의 환자 80명을 대상으로 12주 동안 진행하였으며, 치태지수 0.5 이상, 치은지수 0.5 이상을 나타내는 대상에서 일반 칫솔 혹은 전동칫솔 ($Sonicare^{(R)}$ Elite, Philips Oral Healthcare Inc., Snoqualmie, Washington, USA) 사용 군을 임의로 선정하였다. 하루 2회, 매 회 2분 간 사용하고, 각 군의 칫솔 사용을 교육하였다. 임상지수는 치태지수 (PI; Silness & $L{\ddot{o}}e$), 치은지수 (GI; $L{\ddot{o}}e$ & Silness), 탐침 후 출혈 부위 (%), 치주낭 깊이 부착소실을 초진 1, 4, 12주에 측정하였다. Interleukin-1 (IL-1), MMP-8과 치은연하치태샘플에서 채취한 4 종류의 치주원인균 (Actinomyces visco년, Porphyromonas gingivalis, Streptococcus sanguis, Tannerella forsythensis)에 대한 16S rRNA test는 초진, 1주, 12주에 측정하였다. 측정 결과 전동칫솔과 일반 칫솔 모두 임상지수의 유의한 감소가 나타났으며, 치은지수는 일반칫솔에 비해 전동칫솔에서 감소효과가 통계적으로 더 우수하게 나타났다 (p<0.001). 탐침 후 출혈의 감소는 전동칫솔에서 76.73%, 일반칫솔에서 44.57% 정도로 전동칫솔 군이 더 우수하게 나타났다. 치주낭 깊이 감소는 초진에 비해 전동칫솔 군에서 18.55%, 일반칫솔 군에서 14.81% 정도로 나타났으며, 초진과 비교하였을 때 부착수준의 향상 정도는 전동칫솔 25.24%, 일반 칫솔 16.94% 정도로 두 군 모두 통계적으로 유의하게 개선되었다 (p < 0.001). 두 군 모두 IL-1 beta, MMP-8 농도의 감소가 있었으며, 치주원인균 중 A.viscosus, P.gingivalis, T.forsythensis 역시 두 군 모두에서 초진에 비해 12주에 유의한 감소를 나타내었으나, S.sanguis는 전동칫솔 군에서만 12주에 유의한 감소가 있었다. 이상의 결과에서 12주 간의 연구 기간 동안 초기 및 중등도 치주염 환자에서 소니케어 전동칫솔의 사용은 임상지수 및 치주 원인균 감소에 통계적으로 유의한 개선 효과를 나타내었다.

완전혼합영역을 갖는 성층축열조의 충전과정에 대한 이론적인 해석

유호선,박이동,Yoo, H.,Pak, E.T. 대한설비공학회 1995 설비공학 논문집 Vol.7 No.2

A theoretical one-dimensional model for the charging process in stratified thermal storage tanks is established presuming that the fluid ensuing from the tank inlet creates a perfectly mixed, layer above the thermocline. Both the generic and asymptotic closed-form solutions are obtained via the Laplace transformation. The asymptotic solution describes the nature of the charging pertaining to the case of no thermal diffusion, whereas the generic solution is of practical importance to understand the role of operating parameters on the stratification. The present model is validated through comparison with available experimental data, where they agree well with each other within a reasonable limit. An interpretation of the exact solution entails two important features associated with the charging process. The first is that an in-crease in the mixing depth $h_m$ causes a relatively slow temperature rise in the perfectly mixed region, but on the other hand it results in a faster decay of the overall temperature gradient across the thermocline. Next is the predominance of the mixing depth in the presence of the prefectly mixed region. In such a case the effect of the Peclet number is marginal and there-fore the thermal characteristics are solely dependent on the mixing depth paticularly for large $h_m$. The Peclet number affects significantly only for the case without mixing. Variation of the storage efficiency in response to the change in the mass flow rate agrees favorably with the published experimental results, which confirms the utility of the present study.

시멘트-모래-파라핀 왁스 혼합물을 이용한 축열에 관한 연구

유호선,노승탁,Yoo, Ho-Seon,Ro, Sung-Tack 대한설비공학회 1980 설비저널 Vol.9 No.3

In order to enhance the thermal energy storage capacity of cement mortar and to improve the effective thermal conductivity of paraffin waxes, cement- sand- paraffin wax mixture was investigated. By means of finite difference method, the transient temperature distribution in a hollow cylinder with phase change using average composite properties was obtained, and compared with experimental results. It was shown that the heat absorbed by mixture with $25\%$ paraffin fraction was as much as $50\%$ more than either a concrete mortar or pure paraffin wax in the case of ${\Delta}T=\;18.25^{\circ}C$.

변온유입 성층축열조의 충전과정에 대한 해석적 접근

유호선,Yoo, Hoseon 대한설비공학회 1997 설비공학 논문집 Vol.9 No.1

This paper presents an approximate analytical solution to a two-region one-dimensional model for the charging process of stratified thermal storage tanks with variable inlet temperature in the presence of momentum-induced mixing. Based on the superposition principle, an arbitrary-varying inlet temperature is decomposed into inherent discontinuous steps and continuous intervals approximated as a finite number of piecewise linear functions. This approximation allows the temperature of the upper perfectly-mixed layer to be expressed in terms of constant, linear and exponential functions with respect to time. Applying the Laplace transform technique to the model equation for the lower thermocline layer subject to each of three representative interfacial conditions yields compact-form solutions, a linear combination of which constitutes the final temperature profile. A systematic method for deriving solutions to the plug-flow problem having polynomial-type boundary conditions is also established. The effect of adiabatic exit boundary on solution behaviors proves to be negligible under the actual working conditions, which justifies the assumption of semi-infinite domain introduced in the solution procedure. Finally, the approximate solution is validated by comparing it with an exact solution obtained for a specific variation of inlet temperature. Excellent agreements between them suffice to show the necessity and utility of this work.

하악 대구치부위의 고정성 보철물에서 2개의 장폭경과 3개의 표준 임프란트의 비교

유호선,소성수,한동후,조규성,문익상,Yoo, Ho-Sun,So, Sung-Soo,Han, Dong-Hoo,Cho, Kyoo-Sung,Moon, Ik-Sang 대한치주과학회 2002 Journal of Periodontal & Implant Science Vol.32 No.3

Osseointegrated implants have been established as the standard treatment modality for full/partial edentulous patients since the 1960's, and the long term results for full edentulous patients have proven to be successful. Based on these results osseointegrated implants are now widely used for partial edentulous patients. There has been an increased interest towards the efficacy of wide implants, despite many reports mentioning the lower success rate of wide implants compared to regular implants. Recently, mandibular molar area defects are commonly restored using 2 wide implants, but it is not determined whether which treatment modality-3 regular implants or 2 wide implants-shows superior success rate. In this study, 2 wide implants and 3 regular implants used for the restoration of mandibular molar area are used to compare the survival rate of 1-4 years, and to analyze and compare the failure factors. The following conclusions could be drawn from this study. 1. Wide implants and regular implants showed 94.5% and 97,6% of survival rate respectively. After prosthodontic work, the survival rate was 100% and 98.1% for wide implants and regular implants respectively. 2. 5 failed implants have been removed. 2 wide implants and 1 regular implant have been removed due to failure of osseointegration. 1 wide implant was removed due to abscess formation caused by over-heating, and 1 regular implant was removed due to mechanical failure caused by over-loading within the first year of function. 3. No statistically significant difference was observed with respect to the amount of marginal bone loss of wide and regular implants.(P>0.05) In conclusion, restoration of the mandibular molar area using 3 regular implants was found to be a good treatment modality, and 2 wide implants could he considered a good treatment modality when success factors are taken into account.

빙축열조의 방냉과정에 대한 해석

유호선,김영인,Yoo, H.S.,Kim, Y.I. 대한설비공학회 1989 설비공학 논문집 Vol.1 No.1

This paper presents an analysis to predict thermal behaviors of water in ice storage tank during the coldness release process. To deal with complicated transient phenomena due to ice-water phase change and the density inversion, a theoretical model which consists of initial perfectly mixed, stratified and thermal diffusion state was introduced and a criterion on the growth of thermal boundary layer was developed. The analysis includes considerations on the type of ice-making heat exchanger, refrigerator on/off and tank arrangement. Also, discussions on the various parameters and operating conditions which have influence on the performance of the system were made. Finally, simulated results were shown, which agreed with experiments in trends reasonably.

등열유속에 의한 평판위 비정상 접촉융해에 대한 근사적 해석해

유호선,Yoo, Hoseon 대한기계학회 1998 大韓機械學會論文集B Vol.22 No.12

This paper focuses on the unsteady close-contact melting phenomenon occurring between a phase change material kept at its melting temperature and a flat surface on which constant heat flux is imposed. Based on the same simplifications and framework of analysis as the case of constant surface temperature, an approximate analytical solution which depends only on the liquid-to-solid density ratio is successfully derived. In order to keep consistency with the known solution procedure, both the dimensionless wall heat flux and the Stefan number are properly redefined. The obtained solution proves to agree quite well with the published numerical data and to be capable of resolving the fundamental features of unsteady close-contact melting, especially in the presence of the solid-liquid density difference. The density ratio directly affects the film growth rate and the initial value of solid descending velocity, thereby controlling the duration of unsteady process. The effects of other parameters can be evaluated readily from the steady solution which is implied in the normalized result. Since the dimensionless surface temperature for the present boundary condition increases from zero to unity along the evolution path of the liquid film thickness, the unsteady process lasts longer than that for the case of isothermal heating.

대류가열 비정상 접촉융해에 대한 해석해

유호선,Yoo, Ho-Seon 대한기계학회 2000 大韓機械學會論文集B Vol.24 No.3

This study deals with the unsteady close-contact melting of solid blocks on a flat surface subject to convective heating. Normalizing the model equations in reference to the steady solution successfully leads them to cover constant heat flux and isothermal limits at small and large extremes of the Biot number, respectively. The resulting equations admit a compactly expressed analytical solution, which includes the previous solutions as a subset. Based on the steady solution, the characteristics of close-contact melting can be categorized into constant heat flux, transition, and isothermal regimes, the boundaries of which appear to be nearly independent of the contact force. The unsteady solutions corresponding to Biot numbers in the transition regime show intermediate behaviors between those of the two limits. With a proper approximation, the present solution procedure can cope with the case of variable fluid temperature and heat transfer coefficient. Regardless of imposed conditions, the mean normalized Nusselt number during the unsteady process asymptotically approaches to a constant value as the Biot number comes close to each limit.