Molecular Dynamics Calculations of the Thermal Conductivity of Silica Based Crystals

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76214/1/AIAA-2002-3343-984.pd

Non-Darcian effects on natural convection in porous media confined between horizontal cylinders

The inertia, boundary and velocity-square terms, normally not included in the flow analysis, are included in the study of natural convection between isothermal, concentric cylinders (inner cylinder heated) filled with saturated, porous media. The results show that all of these effects reduce the heat transfer rate with the boundary term being the most significant. It is shown that since at high Rayleigh numbers the flow adjacent to the confining walls becomes of boundary-layer type, with a very thin sublayer over which the velocity reaches its maximum value, then as long as the contribution of the velocity-square term is small, Darcy's model holds for very large Rayleigh and Prandtl numbers. A flow regime diagram showing the pseudo-conduction, Darcy and non-Darcy regimes, is given.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26039/1/0000112.pd

Boundary-layer treatment of film condensation in the presence of a solid matrix

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26163/1/0000240.pd

Structural metrics of high-temperature lattice conductivity

An atomic structure-based model for high-temperature lattice conductivity is developed for both compact crystals and cage-bridge crystals. For compact crystals, where long-range acoustic phonons dominate, the Debye temperature TDTD and Grüneisen parameter γγ are estimated using interatomic potentials to arrive at the lattice conductivity relation. Under the assumption of homogeneous deformation, TDTD is estimated according to a simplified force constant matrix and a phenomenological combinative rule for force constants, which is applicable to an arbitrary pair of interacting atoms. Also, γγ is estimated from a general Lennard-Jones potential form and the combination of the bonds. The results predicted by the model are in close agreement with the experimental results. For cage-bridge crystals, where both short-range acoustic phonons and optical phonons may be important, a simple mean-free path model is proposed: The phonon mean-free path of such a crystal at high temperatures is essentially limited by its structure and is equal to the cage size. This model also shows good agreement with the results of experiments and molecular dynamics simulations. Based on this atomic-level model, the structural metrics of crystals with low or high lattice conductivity are discussed, and some strategies for thermal design and management are suggested.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87699/2/123507_1.pd

Some aspects of enhanced heat diffusion in fluids by oscillation

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25971/1/0000037.pd

Enhanced nonradiative relaxation and photoluminescence quenching in random, doped nanocrystalline powders

Nonradiative relaxation and photoluminescence quenching in nanocrystalline powders doped with rare-earth elements are of interest in optical bistability, random laser, and other optoelectronic applications. Here, the luminescence quenching of a one-dimensional random medium made of multilayer nanoparticles (Y2O3)(Y2O3) doped with rare-earth elements (Yb3+)(Yb3+) is analyzed by considering the transport, transition, and interaction of the fundamental energy carriers. The nonradiative decay and luminescence quenching in random media are enhanced compared to single crystals, due to multiple scattering, enhanced absorption, and low thermal conductivity. The coherent wave treatment is used to calculate the photon absorption, allowing for field enhancement and photon localization. The luminescent and thermal emission is considered as incoherent. The size-dependent absorption coefficient and penetration depth are observed. The nonradiative decay is identified as a multiphonon relaxation process, and is found to be enhanced compared to bulk materials. The luminescence quenching and nonlinear thermal emission, occurring with increasing irradiation intensity, are predicted.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87544/2/104331_1.pd

An improved analytic solution for analysis of particle trajectories in fibrous, two‐dimensional filters

The Kuwabara solution for creeping fluid flow through periodic arrangement of cylinders is widely used in analytic and numerical studies of fibrous filters. Numerical solutions have shown that the Kuwabara solution has systematic errors, and when used for the particle trajectories in filters it results in some error in the predicted filter efficiency. The numerical solutions, although accurate, preclude further analytic treatments, and are not as compact and convenient to use as the Kuwabara solution. By reexamining the outer boundary conditions of the Kuwabara solution, a correction term to the Kuwabara solution has been derived to obtain an extended solution that is more accurate and improves prediction of the filter efficiency. By comparison with the numerical solutions, it is shown that the Kuwabara solution is the high porosity asymptote, and that the extended solution has an improved porosity dependence. A rectification is explained that can make particle collection less efficient for periodic, in‐line arrangements of fibers with particle diffusion or body force. This rectification also results in the alignment of particles with inertia (i.e., high Stokes number particles).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70636/2/PHFLE6-6-2-507-1.pd

Funicular state in drying of a porous slab

Drying of a non-hygroscopic porous slab initially saturated with liquid, up to the time of the first appearance of dried patches (critical time), is studied experimentally using glass beads and convective heating, and analytically using the volume averaged conservation equations for capillary driven liquid flow and empirical constitutive relations. Good agreement has been found between the predicted and measured results and the results show that: 1. (a) The surface mass transfer rate depends on the surface saturation. This surface saturation coefficient, which is also a function of both surface geometry and the free stream velocity, is determined experimentally.2. (b) The critical time is a ratio of the internal liquid transport conductance (Peclet number) to the external vapor transport conductance (Biot number).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26671/1/0000215.pd

Adiabatic reverse combustion in a packed bed

Downward propagation of a combustion front in a packed bed of wood particles, with air supply from below, is examined theoretically and experimentally. Using a single-step reaction, with a kinetic model of char oxidation as the dominant mechanism, and assuming local thermal equilibrium, but allowing a local chemical nonequilibrium between the solid and gas phases, the front speed uF, the adiabatic temperature Tr, and the extent of solid consumption rs/ns are determined as functions of the entering air pole-velocity u>ng. Both oxygen-limited and fuel-limited regimes, with the boundary marked by the stoichiometric burning, are examined. In the oxygen-limited regime, Tr, uF, rs/ns, and the thickness of the front [delta]F all increase with u>ng. In the fuel-limited regime, the reverse occurs and the extinction at high u>ng is predicted. In the oxygen-limited regime, where the bed is not yet fluidized, the experimental results are in good agreement with the predictions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31263/1/0000169.pd

Thermoelectric performance of films in the bismuth-tellurium and antimony-tellurium systems

Coevaporated bismuth-tellurium and antimony-tellurium films were fabricated under various deposition conditions (controlled evaporation rates of individual species, substrate temperature, and substrate material), and their thermoelectric (TE) properties (Seebeck coefficient, electrical resistivity, and carrier concentration) were measured in search of optimal TE performance. The tellurium atomic concentration was varied from 48% to 74%, the substrate temperature ranged from 130 to 300 °C, and glass, mica, magnesium oxide, and sapphire substrates were used. The chemical composition and crystal structure of the films were recorded (using microprobe and x-ray diffractometer, respectively), analyzed, and compared with available standard Bi2Te3Bi2Te3 and Sb2Te3Sb2Te3 single-crystal samples. High-performance TE films had tellurium atomic concentration around 60% and were deposited at a substrate temperature between 260 and 270 °C.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87588/2/114903_1.pd