Showing papers on "Thermal published in 2009"

Reduced density matrices and entanglement entropy in free lattice models

Abstract: We review the properties of reduced density matrices for free fermionic or bosonic many-particle systems in their ground state. Their basic feature is that they have a thermal form and thus lead to a quasi-thermodynamic problem with a certain free-particle Hamiltonian. We discuss the derivation of this result, the character of the Hamiltonian and its eigenstates, the single-particle spectra and the full spectra, the resulting entanglement and in particular the entanglement entropy. This is done for various one- and two-dimensional situations, including also the evolution after global or local quenches.

Thermal Convection: Patterns, Evolution and Stability

Abstract: Preface . Acknowledgements . 1 Equations, General Concepts and Methods of Analysis. 1.1 Pattern Formation and Nonlinear Dynamics. 1.2 The Navier-Stokes Equations. 1.3 Energy Equality and Dissipative Structures. 1.4 Flow Stability, Bifurcations and Transition to Chaos. 1.5 Linear Stability Analysis: Principles and Methods. 1.6 Energy Stability Theory. 1.7 Numerical Integration of the Navier-Stokes Equations. 1.8 Some Universal Properties of Chaotic States. 1.9 The Maxwell Equations. 2 Classical Models, Characteristic Numbers and Scaling Arguments. 2.1 Buoyancy Convection and the Boussinesq Model. 2.2 Convection in Space. 2.3 Marangoni Flow. 2.4 Exact Solutions of the Navier-Stokes Equations for Thermal Problems. 2.5 Conductive, Transition and Boundary-layer Regimes. 3 Examples of Thermal Fluid Convection and Pattern Formation in Nature and Technology. 3.1 Technological Processes: Small-scale Laboratory and Industrial Setups. 3.2 Examples of Thermal Fluid Convection and Pattern Formation at the Mesoscale. 3.3 Planetary Structure and Dynamics: Convective Phenomena. 3.4 Atmospheric and Oceanic Phenomena. 4 Thermogravitational Convection: The Rayleigh-Benard Problem. 4.1 Nonconfined Fluid Layers and Ideal Straight Rolls. 4.2 The Busse Balloon. 4.3 Some Considerations About the Role of Dislocation Dynamics. 4.4 Tertiary and Quaternary Modes of Convection. 4.5 Spoke Pattern Convection. 4.6 Spiral Defect Chaos, Hexagons and Squares. 4.7 Convection with Lateral Walls. 4.8 Two-dimensional Models. 4.9 Three-dimensional Parallelepipedic Enclosures: Classification of Solutions and Possible Symmetries. 4.10 The Circular Cylindrical Problem. 4.11 Spirals: Genesis, Properties and Dynamics. 4.12 From Spirals to SDC: The Extensive Chaos Problem. 4.13 Three-dimensional Convection in a Spherical Shell. 5 The Dynamics of Thermal Plumes and Related Regimes of Motion. 5.1 Introduction. 5.2 Free Plume Regimes. 5.3 The Flywheel Mechanism: The 'Wind' of Turbulence. 5.4 Multiplume Configurations Originated from Discrete Sources of Buoyancy. 6 Systems Heated from the Side: The Hadley Flow. 6.1 The Infinite Horizontal Layer. 6.2 Two-dimensional Horizontal Enclosures. 6.3 The Infinite Vertical Layer: Cats-eye Patterns and Temperature Waves. 6.4 Three-dimensional Parallelepipedic Enclosures. 6.5 Cylindrical Geometries under Various Heating Conditions. 7 Thermogravitational Convection in Inclined Systems. 7.1 Inclined Layer Convection. 7.2 Inclined Side-heated Slots. 8 Thermovibrational Convection. 8.1 Equations and Relevant Parameters. 8.2 Fields Decomposition. 8.3 The TFD Distortions. 8.4 High Frequencies and the Thermovibrational Theory. 8.5 States of Quasi-equilibrium and Related Stability. 8.6 Primary and Secondary Patterns of Symmetry. 8.7 Medium and Low Frequencies: Possible Regimes and Flow Patterns. 9 Marangoni-Benard Convection. 9.1 Introduction. 9.2 High Prandtl Number Liquids: Patterns with Hexagons, Squares and Triangles. 9.3 Liquid Metals: Inverted Hexagons and High-order Solutions. 9.4 Effects of Lateral Confinement. 9.5 Temperature Gradient Inclination. 10 Thermocapillary Convection. 10.1 Basic Features of Steady Marangoni Convection. 10.2 Stationary Multicellular Flow and Hydrothermal Waves. 10.3 Annular Configurations. 10.4 The Liquid Bridge. 11 Mixed Buoyancy-Marangoni Convection. 11.1 The Canonical Problem: The Infinite Horizontal Layer. 11.2 Finite-sized Systems Filled with Liquid Metals. 11.3 Typical Terrestrial Laboratory Experiments with Transparent Liquids. 11.4 The Rectangular Liquid Layer. 11.5 Effects Originating from the Walls. 11.6 The Open Vertical Cavity. 11.7 The Annular Pool. 11.8 The Liquid Bridge on the Ground. 12 Hybrid Regimes with Vibrations. 12.1 RB Convection with Vertical Shaking. 12.2 Complex Order, Quasi-periodic Crystals and Superlattices. 12.3 RB Convection with Horizontal or Oblique Shaking. 12.4 Laterally Heated Systems and Parametric Resonances. 12.5 Control of Thermogravitational Convection. 12.6 Mixed Marangoni-Thermovibrational Convection. 12.7 Modulation of Marangoni-Benard Convection. 13 Flow Control by Magnetic Fields. 13.1 Static and Uniform Magnetic Fields. 13.2 Historical Developments and Current Status. 13.3 Rotating Magnetic Fields. 13.4 Gradients of Magnetic Fields and Virtual Microgravity. References . Index .

Design of wide-angle solar-selective absorbers using aperiodic metal-dielectric stacks

Abstract: Spectral control of the emissivity of surfaces is essential in applications such as solar thermal and thermophotovoltaic energy conversion in order to achieve the highest conversion efficiencies possible. We investigated the spectral performance of planar aperiodic metal-dielectric multilayer coatings for these applications. The response of the coatings was optimized for a target operational temperature using needle-optimization based on a transfer matrix approach. Excellent spectral selectivity was achieved over a wide angular range. These aperiodic metal-dielectric stacks have the potential to significantly increase the efficiency of thermophotovoltaic and solar thermal conversion systems. Optimal coatings for concentrated solar thermal conversion were modeled to have a thermal emissivity 94% of the incident light. In addition, optimized coatings for solar thermophotovoltaic applications were modeled to have thermal emissivity 85% of the concentrated solar radiation.

Quantum thermal bath for molecular dynamics simulation.

Abstract: Molecular dynamics (MD) is a numerical simulation technique based on classical mechanics. It has been taken for granted that its use is limited to a large temperature regime where classical statistics is valid. To overcome this limitation, the authors introduce in a universal way a quantum thermal bath that accounts for quantum statistics while using standard MD. The efficiency of the new technique is illustrated by reproducing several experimental data at low temperatures in a regime where quantum statistical effects cannot be neglected.

Origin of the Temperature Oscillation in Turbulent Thermal Convection

Abstract: We report an experimental study of the three-dimensional spatial structure of the low-frequency temperature oscillations in a cylindrical Rayleigh-Benard convection cell. Through simultaneous multipoint temperature measurements it is found that, contrary to the popular scenario, thermal plumes are emitted neither periodically nor alternately, but randomly and continuously, from the top and bottom plates. We further identify a new flow mode-the sloshing mode of the large-scale circulation (LSC). This sloshing mode, together with the torsional mode of the LSC, are found to be the origin of the oscillation of the temperature field.

Solar-driven biochar gasification in a particle-flow reactor

Abstract: The steam-gasification of biochar with concentrated solar radiation is experimentally investigated using a 3 kW solar reactor prototype consisting of a cylindrical cavity-receiver containing an opaque tubular absorber. Particles of beech charcoal are used as the biomass feedstock in a continuous steam-particle flow through the tubular absorber. A reactor model that couples radiative, convective, and conductive heat transfer to the chemical kinetics is formulated and validated by comparing numerically computed and experimentally measured temperatures and carbon conversions. The simulation model is further applied to examine the thermal performance of 100 kW and 1 MW scaled-up solar reactor containing multiple tubular absorbers, yielding a theoretical maximum solar-to-chemical energy conversion efficiency of 39% and 50%, respectively. Major sources of irreversibility are associated with re-radiation losses through the cavity's aperture.

Role of Nanoscale Precipitates on the Enhanced Magnetostriction of Heat-Treated Galfenol (Fe1-xGax) Alloys

Abstract: We report neutron diffuse scattering measurements on highly magnetostrictive ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}$ alloys ($0.14lxl0.20$) with different thermal treatments. This diffuse scattering scales with magnetostriction and exhibits asymmetric peaks at the (100) and (300) reciprocal lattice positions that are consistent with the coexistence of short-range ordered, coherent nanometer-scale precipitates embedded in a long-range ordered, body-centered cubic matrix. A large peak splitting is observed at (300) for $x=0.19$, which indicates that the nanoprecipitates are not cubic and have a large elastic strain. This implies a structural origin for the enhanced magnetostriction.

Effect of the position of a circular cylinder in a square enclosure on natural convection at Rayleigh number of 107

Abstract: Numerical calculations are carried out for the natural convection induced by temperature difference between a cold outer square cylinder and a hot inner circular cylinder for Rayleigh number of Ra=107. This study investigates the effect of the inner cylinder location on the heat transfer and fluid flow. The location of inner circular cylinder (δ) is changed vertically along the centerline of square enclosure. The natural convection bifurcates from unsteady to steady state according to δ. Two critical positions of δC,L and δC,U as a lower bound and an upper bound are δC,L=0.05 and δC,U=0.18, respectively. Within the defined bounds, the thermal and flow fields are steady state. In the unsteady region, the natural convection shows a single frequency and multiple frequency periodic patterns alternately according to δ. When the inner cylinder locates at δ≥δC,U, the space between the upper surface of inner cylinder and the top surface of the enclosure forms a relatively shallow layer where the natural convectio...

Incorporating self‐consistently calculated mineral physics into thermochemical mantle convection simulations in a 3‐D spherical shell and its influence on seismic anomalies in Earth's mantle

Abstract: [1] Phase assemblages of mantle rocks calculated from the ratios of five oxides (CaO-FeO-MgO-Al2O3-SiO2) by free energy minimization were used to calculate the material properties density, thermal expansivity, specific heat capacity, and seismic velocity as a function of temperature, pressure, and composition, which were incorporated into a numerical thermochemical mantle convection model in a 3-D spherical shell. The advantage of using such an approach is that thermodynamic parameters are included implicitly and self-consistently, obviating the need for ad hoc parameterizations of phase transitions which can be complex in regions such as the transition zone particularly if compositional variations are taken into account. Convective planforms for isochemical and thermochemical cases are, however, not much different from those computed using our previous, simple parameterized reference state, which means that our previous results are robust in this respect. The spectrum and amplitude of seismic velocity anomalies obtained using the self-consistently calculated material properties are more “realistic” than those obtained when seismic velocity is linearly dependent on temperature and composition because elastic properties are dependent on phase relationship of mantle minerals, in other words, pressure and temperature. In all cases, the spectra are dominated by long wavelengths (spherical harmonic degree 1 to 2), similar or even longer wavelength than seismic tomographic models of Earth, which is probably due to self-consistent plate tectonics and depth-dependent viscosity. In conclusion, this combined approach of mantle convection and self-consistently calculated mineral physics is a powerful and useful technique for predicting thermal-chemical-phase structures in Earth's mantle. However, because of uncertainties in various parameters, there are still some shortcomings in the treatment of the postperovskite phase transition. Additionally, transport properties such as thermal conductivity and viscosity are not calculated by this treatment and are thus subject to the usual uncertainties.

Bionics in textiles: flexible and translucent thermal insulations for solar thermal applications.

Abstract: Solar thermal collectors used at present consist of rigid and heavy materials, which are the reasons for their immobility. Based on the solar function of polar bear fur and skin, new collector systems are in development, which are flexible and mobile. The developed transparent heat insulation material consists of a spacer textile based on translucent polymer fibres coated with transparent silicone rubber. For incident light of the visible spectrum the system is translucent, but impermeable for ultraviolet radiation. Owing to its structure it shows a reduced heat loss by convection. Heat loss by the emission of long-wave radiation can be prevented by a suitable low-emission coating. Suitable treatment of the silicone surface protects it against soiling. In combination with further insulation materials and flow systems, complete flexible solar collector systems are in development.

Near-field radiative heat transfer between two plane surfaces with one having a dielectric coating

Abstract: The effect of a dielectric coating on the near-field radiative heat transfer between two plane surfaces is numerically studied in the framework of the fluctuational electrodynamics. The dielectric coating is assumed to be a SiC or SiO2 film, which is on top of the emitter. The results show that the near-field radiative flux between the plane surfaces can be either diminished or enhanced by the dielectric coating, depending on the thermal radiative properties of the emitter and the receiver. Furthermore, the dielectric coating effect on the near-field radiative flux can be very different from that on the far-field radiative flux. Detailed analysis on the variations of the TE- and TM-wave components of the radiative flux by adding the dielectric coating is provided, along with the physical mechanisms that account for these changes. Dielectric coatings such as SiC and SiO2 films are widely seen in microelectronic structures and nanofabrication devices. The results obtained in this work should be valuable for further study and nanotechnological applications of near-field radiative heat transfer.

Dynamics of glass-forming liquids. XIII. Microwave heating in slow motion.

Abstract: Using time-resolved nonlinear dielectric relaxation measurements at fields as high as 450 kV/cm, the nonthermal effects of energy absorption are studied for simple and associating polar liquids in their supercooled state. The experiment is a low frequency analog of microwave heating and facilitates tracking the flow of energy in time, as it accumulates in slow degrees of freedom and transfers eventually to the vibrational heat bath of the liquid. Most findings agree with a phenomenological model of heterogeneous relaxation regarding structure and configurational temperature. The relevant thermal behavior of monohydroxy alcohols differs considerably from the cases of simple nonassociating liquids due to their distinct origins of the prominent dielectric absorption mode for the two classes of liquids. Nonthermal effects are observed as dynamics that are accelerated without increasing sample temperature, but for the present low frequencies the changes remain too small to explain the high efficiencies reporte...

Improving Heat Transfer Models of Air Gaps in Bench Top Tests of Thermal Protective Fabrics

Abstract: An improved model has been developed to simulate heat transfer in horizontal air spaces between thermal protective fabrics and test sensors in bench top tests, such as the thermal protective performance test. This model calculates the radiation and convection heat transfer from the test specimen to the test sensor. Radiation heat transfer is calculated by treating the bottom boundary of the enclosure as a series of isothermal rectangular pieces. Convection heat transfer is calculated using an empirical correlation and by assuming that convection only occurs over a portion of the cross-section of the enclosure. Predicted times required to exceed the Stoll second degree burn criterion were found to be within 3 % of those measured during actual bench top tests of steel shimstock using air gaps from 6.4 mm (1/4 in.) to 19.1 mm (3/4 in.).