C.L. Tien

Short-pulse laser heating on metals

Abstract This work analyzes microscopic radiation-metal interactions in short-pulse laser heating processes and their effects on the material thermal response. The radiation-metal interactions are treated as a coupled two-step process : (1) the absorption of photon energy by electrons and (2) the subsequent heating of the metal lattice through electron-phonon collisions. Key parameters, pulse duration to relaxation, relaxation to diffusion, and heating intensity, are introduced to classify this two-step heating regime and the conventional one-step heating regime. In the two-step heating regime, the microscopic energy transfer among photons, electrons and phonons enlarges the size of the heat-affected region and lowers the peak metal lattice temperature rise significantly. The predicted transient reflectivity changes agree with subpicosecond laser heating experiments.

Annual review of heat transfer

The volumes of Annual Review of Heat Transfer published up to 2005 were edited by Professor Chang-Lin Tien. Chang-Lin had a long-lasting impact on the heat transfer community through his pioneering research. The current editors decided to use Volume XIV as a bridge between the past and the future by summarizing Chang-Lins contributions and reviewing current and future research directions in areas in which Professor Tien made a significant impact. In this volume, his contributions are divided into six topical areas: radiation and combustion, micro/nanoscale heat transfer, phase change and heat pipes, porous media, materials processing and laser materials interactions, and energy systems. Previous volumes of Annual Review in Heat Transfer all aspects of heat transfer and fluid flow are examined by an array of the top international specialists in the field. Future volumes are being planned to include contemporary achievements in the thermal and fluids sciences.

Fully developed natural counterflow in a long horizontal pipe with different end temperatures

Abstract The temperature difference applied across the two ends of a horizontal duct generates a natural counterflow in which colder fluid flows along the bottom of the duct towards the warm end while a warmer stream flows in the opposite direction along the top. The paper presents an asymptotic solution for the velocity and temperature distributions in the middle portion of a long horizontal pipe with conducting wall. The influence of circumferential heat conduction through the pipe wall on the temperature distribution in the fluid has been investigated. An analytical expression has been established for the net axial heat-transfer rate by counterflow natural convection. The present results for the temperature distribution in the pipe wall are compared with numerical results reported recently.

Thermal conductivities of quantum well structures

This work analyzes the size and the boundary effects of a gallium arsenide- (GaAs) based quantum well (QW) structure on the thermal conductivity of the well material. Calculations show that the order of phonon mean free path (MFP) is equal to or even longer than the typical dimension of the well (-200 A or less). Hollands model is applied to match the thermal conductivity data of bulk GaAs from 2 to above 600 K. The equation of phonon radiative transfer (EPRT) developed from the Boltzmann transport equation is then introduced for the heat transport in the QW structure. Boundary conditions are built from the diffuse phonon mismatch theory, and approximate solutions are obtained for the cases of heat flow perpendicular and parallel to the well. Results show that the thermal conductivity of the quantum well can be one order-of-magnitude lower than that of its corresponding bulk form at room temperature. The size and boundary effects also cause anisotropy of the thermal conductivity, even though the unit cell of GaAs is cubic.

Natural Convection in a Horizontal Porous Medium Subjected to an End-to-End Temperature Difference

Natural convection in a porous medium filling a slender horizontal space with an end-to-end temperature difference is studied analytically. The end-to-end temperature difference gives rise to a horizontal counterflow pattern augmenting the heat transfer rate through the porous medium. Two basic geometries are considered: horizontal layer confined between 2 adiabatic and impermeable parallel plates, and horizontal cylinder surrounded by an adiabatic and impermeable cylindrical surface. Nusselt number relations are derived in terms of the Rayleigh number and the cavity aspect ratio. The end-wall permeability is shown to affect the heat transfer rate through the medium. (12 refs.)

Facet heating of quantum well lasers

This work investigates the temperature rise and heating mechanisms at the facets of quantum well lasers. An analytical solution of the heat conduction equation yields the temperature distribution in the laser and the temperature rise at the laser facets. The heat generation mechanisms are discussed and modeled through a one‐dimensional carrier diffusion equation. The normalized results from the models agree well with available experimental data but the absolute value of the maximum temperature rise is about 5 times lower than that of the measurement. This discrepancy is explained by the reduction of thermal conductivity caused by phonon reflection and transmission at the GaAs/AlGaAs interfaces. Averaging the calculated results over a probe diameter around 1.5 μm, as is often used in the microprobe Raman spectroscopy measurement of the facet‐temperature rise, reveals that the actual peak temperature at the facet is only 2–5 times higher than the measured value. This is a surprising result considering that ...

Effect of axial conduction and metal-helium heat transfer on the local stability of superconducting composite media

Abstract The growth or collapse of a local normal zone in a superconducting winding structure saturated with single phase liquid helium (composite superconductor) is studied analytically. The history of a given temperature disturbance is derived from the solution to the transient heat conduction equation in a one-dimensional infinite solid with temperature dependent rate of internal heat generation, communicating laterally with a channel filled with stagnant helium. The combined diffusion by axial heat conduction and lateral heat transfer to the helium channel and its effect on the collapse or growth behaviour of a local disturbance is presented analytically. The paper develops a theoretical criterion for local stability (recovery) expressed in terms of dimensionless groups accounting for heat generation in the normal zone, metal axial conduction cooling, lateral cooling provided by the helium channel and, most importantly, the amount and spatial extent of the sudden release of energy responsible for the local disturbance.

Mechanisms of local thermal stability in high temperature superconductors

Abstract The impact of high temperature ceramic superconductors on the thermal stability of superconducting devices is two-fold. Firstly, nitrogen becomes the probable cryogenic coolant, and secondly, the new materials exhibit thermophysical properties different from conventional superconductors. This work analyses transient heat transfer to subcooled and supercritical nitrogen and compares the results to those for helium. An extensive review of the thermophysical properties of YBa2Cu3O7 assesses their effect on thermal stability. Anisotropic thermal conductivity is shown to strongly influence the intrinsic thermal stability of a composite strip superconductor.

Glass microsphere cryogenic insulation

Abstract The present paper discusses the new concept of glass microsphere cryogenic insulation and describes significant advances made in the past few years. Recent progress in microsphere characterization, basic heat transfer mechanisms, existing experimental data, and potential applications is presented.

Recent advances in high-performance cryogenic thermal insulation

Abstract The present paper describes significant advances made in the past few years in high-performance cryogenic thermal insulation. Evacuated multilayer insulation has become widely accepted for high-performance insulation applications. An improved understanding of the fundamental heat transfer characteristics in multilayer insulation has increased considerably the predictability of insulation performance. The major drawback of performance degradation due to compressive loads and penetrations still remain in the application of multilayer insulation. To remedy this drawback, a new type of high-performance cryogenic insulation consisting of metallized microspheres has been proposed and has demonstrated in recent theoretical and experimental investigations its tremendous potential for applications.