Browsing by Author "Sheu, LJ"

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An analysis for measurement of thermal diffusivity components of anisotropic platelike samples by AC calorimetric method
(2000-07-01)
Sheu, LJ
;
Lin, JD
;
機械工程學系
;
Department of Mechanical Engineering
This work examines the effects of anisotropy and transparency on measurements of thermal diffusivity components with an ac calorimetric method associated with laser heating. Analytical results indicate that the region where the two-dimensional effect occurs increases with the decrease of the ratio of cross-plane to in-plane thermal diffusivity. The region also increases with the optical thickness of the sample. The linear relations, as indicated by a decay constant from which the cross-plane thermal diffusivity is deduced, are not obtained for media of both optically moderate and thin thickness, while at sufficiently large optical thickness, anisotropy and two-dimensional effects are found insignificant.
Numerical analysis on hot spot in reactive chemical storage
(1998)
Sheu, LJ
;
Lin, JD
;
Chen, JR
;
機械工程學系
;
Department of Mechanical Engineering
In an exothermic reactive chemical storage, a thermal explosion can occur when the heat generation within the storage is greater than heat removal from the storage. In a solid system, the existence of an inflection point in the temperature versus time curve is often used as a criterion for thermal explosion. In this study the applicability of this criterion to a fluid storage is examined by investigating the hot spot in the system. Transient natural convection of an exothermically reactive fluid in the vertical cylindrical storage tank with isothermal walls is investigated numerically. The axisymmetric 2-D Navier-Stokes equations governing the now fields are reduced by introducing the stream function-vorticity formulation and solved by the alternating-direction-implicit (ADI) technique. The reactive heat sources are represented by a zeroth order rate expression with a Arrhenius type rate constant. It is found that the hot spot is no longer located in the center of the tank due to the buoyancy effect. In a particular range of Rayleigh number and Frank-Kamenetskii number, the hot spot is found to move periodically. A stable steady regime, stable oscillatory regime and thermal explosion regime can be specified based on the dependence of the critical Frank-Kamenetskii number on the Rayleigh number. In the stable oscillatory regime multiple inflections in temperature versus time are found. Therefore, the existing criterion which defines thermal explosion by the appearance of an inflection in the temperature versus time curve is no longer applicable in reactive fluid systems. The results of nonconvective systems do, however, provide a preliminary and conservative estimate for thermal explosion of a reactive storage.
Simultaneous measurement of spectral optical properties and thickness of an absorbing thin film on a substrate
(1999-10-08)
Chen, JJ
;
Lin, JD
;
Sheu, LJ
;
機械工程學系
;
Department of Mechanical Engineering
This paper describes a technique and also illustrates an application of the technique for simultaneously measuring both the spectral complex refractive index (n(1),k(1)) and the thickness d(1) of an absorbing thin solid layer on a thick substrate. The contours of constant reflectance R and transmittance T in the (n(1), k(1)), (n(1), d(1)) and (k(1), d(1)) planes are examined for a thin layer on a substrate, which is exposed to oblique unpolarized incidence, under various conditions in order to facilitate an optimal choice of the combination of measured quantities for the inverse estimation of parameters. Theoretical analysis illustrates that optimal choices would include measurement of R at a large angle of incidence, combined with measurements of normal T and near normal R so as to reduce erroneous solutions or nonconvergence. And any additional measurement R at any angle of incidence may be used to prevent the multiple solutions. For the inverse estimation of parameters, we also present a technique in association of the least squares method to extract the thickness and optical constants characterizing the thin absorbing film from measurements of R and T. The method is then applied for experimental measurement of the optical properties and thickness of a polyvinylalcohol (PVA) film placed upon a substrate of ZnSe. These results provide useful information for analyses on the applications of PVA in textile sizing, adhesives, polymerization stabilizers, and paper coating. (C) 1999 Elsevier Science S.A. All rights reserved.