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정재동(Jae Dong Chung) 대한설비공학회 2010 대한설비공학회 학술발표대회논문집 Vol.2010 No.6
This paper deals with approximate integral solutions to the one-dimensional model describing the charging process of stratified thermal storage tanks considering mixing region. Temperature is assumed to be the form of Fermi-Dirac distribution function, which can be separated to two sets of cubic polynomials for each hot and cold side of thermal boundary layers. Proposed approximate integral solutions are compared to the previous works of the approximate analytic solutions and show reasonable agreement. The approach, however, has benefits in mathematical difficulties, complicated solution form and unstable convergence of series solution founded in the previous analytic solutions. Contrary to the results of no mixing region, thermocline thickness becomes thicker as flow rate goes up because of promoted flow mixture and increased mixed region. There is an optimal flow rate depending on the relations between flow rate and the size of mixed region.
정재동(Jae Dong Chung),이경태(Kyung Tae Lee) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.11
A model for the phonon dispersion relationship for cubic zinc sulfide structure, for example SiC, is developed in terms of two unknown force constants. Born model that incorporates bond bending and bond stretching, is used for the force constants. The force constants are determined by fitting to experimental data. Using only the nearest-neighbor coupling results in 6×6 sized dynamic matrix. The eigenvalues of dynamics matrix for each wavenumber in 3-D κ space correspond to frequencies, 3 for optical phonon and 3 for acoustic phonon, which is so-called dispersion relation (κ?w). The density of state is determined by counting the states for each frequency bin, and the properties such as specific heat and thermal conductivity can be obtained. The specific heat is estimated on this model and compared with experiment and other models, i.e. Debye model, Einstein model and combined Debye-Einstein model. In spite of the simple bond potential model, reasonable agreements are found.