Chuen-Lin Tien

Bio: Chuen-Lin Tien is an academic researcher from Feng Chia University. The author has contributed to research in topics: Thin film & Lens (optics). The author has an hindex of 25, co-authored 127 publications receiving 4525 citations. Previous affiliations of Chuen-Lin Tien include National Central University & University of California, Berkeley.

Boundary and inertia effects on flow and heat transfer in porous media

Abstract: The present work analyzes the effects of a solid boundary and the inertial forces on flow and heat transfer in porous media. Specific attention is given to flow through a porous medium in the vicinity of an impermeable boundary. The local volume-averaging technique has been utilized to establish the governing equations, along with an indication of physical limitations and assumptions made in the course of this development. A numerical scheme for the governing equations has been developed to investigate the velocity and temperature fields inside a porous medium near an impermeable boundary, and a new concept of the momentum boundary layer central to the numerical routine is presented. The boundary and inertial effects are characterized in terms of three dimensionless groups, and these effects are shown to be more pronounced in highly permeable media, high Prandtl-number fluids, large pressure gradients, and in the region close to the leading edge of the flow boundary layer.

Heat transfer mechanisms during short-pulse laser heating of metals

Abstract: This work studies heat transfer mechanisms during ultrafast laser heating of metals from a microscopic point of view. The heating process is composed of three processes: the deposition of radiation energy on electrons, the transport of energy by electrons, and the heating of the material lattice through electron-lattice interactions. The Boltzmann transport equation is used to model the transport of electrons and electron lattice interactions. The scattering term of the Boltzmann equation is evaluated from quantum mechanical considerations, which shows the different contributions of the elastic and inelastic electron-lattice scattering processes on energy transport. By solving the Boltzmann equation, a hyperbolic two-step radiation heating model is rigorously established. It reveals the hyperbolic nature of energy flux carried by electrons and the nonequilibrium between electrons and the lattice during fast heating processes. Predictions from the current model agree with available experimental data during subpicosecond laser heating. 20 refs., 7 figs., 2 tabs.

Femtosecond laser heating of multi-layer metals—I. Analysis

Abstract: Multi-layer metal films such as metallic coatings on metal substrate are important elements in modern engineering applications. Specifically, gold-coated metal mirrors are widely used in high-power laser systems. This work studies microscopic energy deposition and transport processes during short-pulse laser heating of multi-layer metals: the absorption of radiation energy by free electrons and the energy exchange between electrons and the lattice. The results show that multi-layer metals present very different thermal responses from single-layer metals during the heating process. In a gold and chromium multi-layer film, although laser energy is absorbed by free electrons in the top gold coating layer, most of the absorbed energy is converted into lattice energy not in the gold layer but rather in the underlying chromium layer. The underlying chromium layer reduces the lattice-temperature rise of the top gold layer significantly during short-pulse laser heating, suggesting a new way to increase the resistance of mirrors to thermal damage in applications of high-power lasers.

Boundary and inertia effects on convective mass transfer in porous media

Abstract: The present work consists of a numerical and experimental investigation of the effects of the presence of a solid boundary and inertial forces on mass transfer in porous media. Particular emphasis is placed on mass transfer through the porous medium near an impermeable boundary. The local volume-averaging technique has been used to establish the governing equations. The numerical solution of the governing equations is used to investigate the mass concentration field inside a porous medium close to an impermeable boundary. In conjunction with the numerical solution, a transient mass transfer experiment has been conducted to demonstrate the boundary and inertia effects on mass transfer. This is accomplished by measuring the time and space-averaged mass flux through a porous medium. The results clearly indicate the presence of these effects on mass transfer through porous media.

Femtosecond laser heating of multi-layer metals. ii: experiments

Abstract: Femtosecond thermoreflectivity experiments are performed to investigate energy deposition and transport during the very early period of short-pulsed laser heating of gold and chromium multi-layer metal films. The chromium layer underneath the top gold layer is found to produce significant effects on the laser-energy deposition process. Experimental results show that radiation absorption by free electrons and the subsequent heating of the lattice occur not only at different times but also at different locations in a multi-layer metal film. The conventional radiation heating model fails to predict these results, and a more rigorous two-step model agrees well with the measured data.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Abstract: A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

A Unified Field Approach for Heat Conduction From Macro- to Micro-Scales

Abstract: A universal constitutive equation between the heat flux vector and the temperature gradient is proposed to cover the fundamental behaviors of diffusion (macroscopic in both space and time), wave (macroscopic in space but microscopic in time), phonon-electron interactions (microscopic in both space and time), and pure phonon scattering The model is generalized from the dual-phase-lag concept accounting for the laging behavior in the high-rate response While the phase lag of the heat flux captures the small-scale response in time, the phase lag of the temperature gradient captures the small-scale response in space The universal form of the energy equation facilitates identifications of the physical parameters governing the transition from one mechanism (such as diffusion or wave) to another (the phonon-electron interaction)

Hydrogen Sensors - A review

Abstract: Hydrogen sensors are of increasing importance in connection with the development and expanded use of hydrogen gas as an energy carrier and as a chemical reactant. There are an immense number of sensors reported in the literature for hydrogen detection and in this work these sensors are classified into eight different operating principles. Characteristic performance parameters of these sensor types, such as measuring range, sensitivity, selectivity and response time are reviewed and the latest technology developments are reported. Testing and validation of sensor performance are described in relation to standardisation and use in potentially explosive atmospheres so as to identify the requirements on hydrogen sensors for practical applications.

Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium

Abstract: The dependence of the strength of the electron-phonon coupling and the electron heat capacity on the electron temperature is investigated for eight representative metals, Al, Cu, Ag, Au, Ni, Pt, W, and Ti, for the conditions of strong electron-phonon nonequilibrium. These conditions are characteristic of metal targets subjected to energetic ion bombardment or short-pulse laser irradiation. Computational analysis based on first-principles electronic structure calculations of the electron density of states predicts large deviations (up to an order of magnitude) from the commonly used approximations of linear temperature dependence of the electron heat capacity and a constant electron-phonon coupling. These thermophysical properties are found to be very sensitive to details of the electronic structure of the material. The strength of the electron-phonon coupling can either increase (Al, Au, Ag, Cu, and W), decrease (Ni and Pt), or exhibit nonmonotonic changes (Ti) with increasing electron temperature. The electron heat capacity can exhibit either positive (Au, Ag, Cu, and W) or negative (Ni and Pt) deviations from the linear temperature dependence. The large variations of the thermophysical properties, revealed in this work for the range of electron temperatures typically realized in femtosecond laser material processing applications, have important implications for quantitative computational analysis of ultrafast processes associated with laser interaction with metals.