Showing papers on "Thermal radiation published in 2005"

Near-field heat transfer in a scanning thermal microscope.

Abstract: We present measurements of the near-field heat transfer between the tip of a thermal profiler and planar material surfaces under ultrahigh vacuum conditions. For tip-sample distances below 10-8 m, our results differ markedly from the prediction of fluctuating electrodynamics. We argue that these differences are due to the existence of a material-dependent small length scale below which the macroscopic description of the dielectric properties fails, and discuss a heuristic model which yields fair agreement with the available data. These results are of importance for the quantitative interpretation of signals obtained by scanning thermal microscopes capable of detecting local temperature variations on surfaces.

Resonant-cavity enhanced thermal emission

Abstract: In this paper we present a vertical-cavity enhanced resonant thermal emitter---a highly directional, narrow-band, tunable, partially coherent thermal source. This device enhances thermal emittance of a metallic or any other highly reflective structure to unity near a cavity resonant frequency. The structure consists of a planar metallic surface (e.g., silver, tungsten), a dielectric layer on top of the metal that forms a vertical cavity, followed by a multilayer dielectric stack acting as a partially transparent cavity mirror. The resonant frequency can easily be tuned by changing the cavity thickness (thus shifting resonant emission peak), while the angle at which the maximum emittance appears can be tuned as well by changing the number of dielectric stack layers. The thermal emission exhibits an extremely narrow angular emission lobe, suggesting increased spatial coherence. Furthermore, we show that we can enhance the thermal emission of an arbitrarily low-emittance material, choosing a properly designed thermal cavity, to near unity.

Theory and performance of an infrared heater for ecosystem warming

Abstract: In order to study the likely effects of global warming on future ecosystems, a method for applying a heating treatment to open-field plant canopies (i.e. a temperature free-air controlled enhancement (T-FACE) system) is needed which will warm vegetation as expected by the future climate. One method which shows promise is infrared heating, but a theory of operation is needed for predicting the performance of infrared heaters. Therefore, a theoretical equation was derived to predict the thermal radiation power required to warm a plant canopy per degree rise in temperature per unit of heated land area. Another equation was derived to predict the thermal radiation efficiency of an incoloy rod infrared heater as a function of wind speed. An actual infrared heater system was also assembled which utilized two infrared thermometers to measure the temperature of a heated plot and that of an adjacent reference plot and which used proportional-integrative-derivative control of the heater to maintain a constant temperature difference between the two plots. Provided that it was not operated too high above the canopy, the heater system was able to maintain a constant set-point difference very well. Furthermore, there was good agreement between the measured and theoretical unit thermal radiation power requirements when tested on a Sudan grass (Sorghum vulgare) canopy. One problem that has been identified for infrared heating of experimental plots is that the vapor pressure gradients (VPGs) from inside the leaves to the air outside would not be the same as would be expected if the warming were performed by heating the air everywhere (i.e. by global warming). Therefore, a theoretical equation was derived to compute how much water an infrared-warmed plant would lose in normal air compared with what it would have lost in air which had been warmed at constant relative humidity, as is predicted with global warming. On an hourly or daily basis, it proposed that this amount of water could be added back to plants using a drip irrigation system as a first-order correction to this VPG problem.

Coherent thermal emission from one-dimensional photonic crystals

Abstract: Coherent thermal emission from surface relief gratings holds promise for spectral and directional control of thermal radiation but is limited to transverse magnetic waves, which can excite surface plasmon or phonon polaritons in the grating structure. We show in this letter that a coherent thermal source can be constructed with a thin polar material coated on a one-dimensional photonic crystal. The excitation of surface waves at the interface of the coated layer and the photonic crystal results in highly spectral and directional emission in the infrared for both the transverse electric wave and the transverse magnetic wave.

Highly directional radiation generated by a tungsten thermal source

Abstract: We report the design of a tungsten thermal source with extraordinarily high directivity in the near infrared, comparable to the directivity of a CO2 laser. This high directivity is the signature of the long-range correlation of the electromagnetic field in the source plane. This phenomenon is due to the resonant thermal excitation of surface-plasmon polaritons.

Deceleration of a Relativistic, Photon-Rich Shell: End of Preacceleration, Damping of MHD Turbulence, and the Emission Mechanism of Gamma-Ray Bursts

Abstract: (Abridged) We consider the interaction of a relativistically-moving shell, composed of thermal photons, a reversing magnetic field and a small admixture of charged particles, with a dense Wolf-Rayet wind. A thin outer layer of Wolf-Rayet material is entrained by the jet head; it cools and becomes Rayleigh-Taylor unstable, thereby providing an additional source of inertia and variability. Pair creation in the wind material, and the associated pre-acceleration, defines a characteristic radiative compactness at the point where the reverse shock has completed its passage back through the shell. We argue that the prompt gamma-ray emission is triggered by this external braking, at an optical depth ~1 to electron scattering. Torsional waves, excited by the forced reconnection of the reversing magnetic field, carry a fluctuating current, and are damped at high frequencies by the electrostatic acceleration of electrons and positrons. We show that inverse Compton radiation by the accelerated charges is stronger than their synchrotron emission, and is beamed along the magnetic field. Thermal radiation that is advected out from the base of the jet cools the particles. The observed relation between peak energy and isotropic luminosity is reproduced if the blackbody seeds are generated in a relativistic jet core that is subject to Kelvin-Helmholtz instabilities with the Wolf-Rayet envelope. This relation is predicted to soften to E_peak ~ L_iso^{1/4} below an isotropic luminosity L_iso ~ 3x10^{50} ergs/s. The duration of spikes in the inverse-Compton emission is narrower at higher frequencies, in agreement with the observed relation. The transition from prompt gamma-ray emission to afterglow can be explained by the termination of the thermal X-ray seed and the onset of synchrotron-self-Compton emission.

Impact of Flowfield-Radiation Coupling on Aeroheating for Titan Aerocapture

Abstract: A methodology is developed that enables fully coupled computation of three-dimensional flow fields including radiation, assuming an optically thin shock layer. The method can easily be incorporated into existing computational fluid dynamics codes and does not appreciably increase the cost or affect the robustness of the resulting simulations. Further improvements in the accuracy of radiative heating predictions in an optically thin gas can be achieved by using a view-factor method rather than the standard tangent slab approach. These techniques are applied to the Titan aerocapture aeroheating problem, which is dominated by strong radiative heating. For this application, neglecting the nonadiabatic effects caused by radiation coupling results in an overprediction of radiative heating levels by about a factor of 2. Radiative coupling effects also significantly lower predicted convective heating by reducing boundary-layer edge temperatures. In addition, it is shown that the tangent slab approximation overpredicts radiative heating levels by a minimum of 20% in the stagnation region for this application. Over an entire design trajectory, correctly modeling radiative heat transfer results in a more than a factor of 2 reduction in total stagnation-region heat load over an uncoupled analysis.

Tuning of the thermal radiation spectrum in the near-infrared region by metallic surface microstructures

Abstract: This paper reviews our recent research on thermal radiation from metallic surface microstructures to develop selective radiators for thermophotovoltaic generation. Numerical simulation showed that two different peaks appeared on the emissivity spectra of metallic gratings. One was originated from surface plasmon polaritons, which was dependent on the grating period and angle. The other peak was explained by the microcavity effect which arose from each microcavity on a grating surface. The microcavity effect became dominant with deepening gratings, and showed suitable properties for selective radiators in thermophotovoltaic generation: a high emissivity within the visible and near-infrared regions, and angle independence. We developed two kinds of two-dimensional metallic gratings based on Si and W with the period of 1.0–2.0 µm. The W gratings composed of rectangular microcavities displayed a high emissivity in the near-infrared region as expected from the calculation results. It was confirmed experimentally that the W selective radiators have advantages for high-power and high-efficiency thermophotovoltaic systems.

Entropy generation through radiative transfer in participating media: analysis and numerical computation

Abstract: Thermodynamics’ second law analysis is the gateway for optimization in thermal equipments and systems. Through entropy minimization techniques it is possible to increase the efficiency and overall performance of all kinds of thermal systems. This approach is becoming common practice in the analysis and/or design of thermal equipments. However, evaluation of entropy generation due to radiative transfer in participating media seems to be lacking. Since radiation is the dominant mechanism of heat transfer in high-temperature systems, such omission seems quite unjustifiable. Although the subject of entropy production through radiative transfer has been dealt with for quite some time, notably by Max Planck himself, it has not been approached in the perspective of its numerical calculation in a way that is compatible and coherent with the standard heat transfer approach. In the present work, the issue of entropy generation by radiative transfer in participating media is approached from the view-points of its mathematical modeling and numerical calculation using standard radiative heat transfer techniques, namely the discrete ordinates method. Effects from emission, absorption and scattering are isolated and considered independently.

Thermal radiation from magnetic neutron star surfaces

Abstract: We investigate the thermal emission from magnetic neutron star surfaces in which the cohesive effects of the magnetic field have produced the condensation of the atmosphere and the external layers. This may happen for sufficiently cool (T ≤ 10 6 ) atmospheres with moderately intense magnetic fields (about 10 13 G for Fe atmospheres). The thermal emission from an isothermal bare surface of a neutron star shows no remarkable spectral features, but it is significantly depressed at energies below some threshold energy. However, since the thermal conductivity is very different in the normal and parallel directions to the magnetic field lines, the presence of the magnetic field is expected to produce a highly anisotropic temperature distribution, depending on the magnetic field geometry. In this case the observed flux of such an object looks very similar to a BB spectrum, but depressed by a nearly constant factor at all energies. This results in a systematic underestimation of the area of the emitter (and therefore its size) by a factor 5-10 (2-3).

Coupled fire dynamics and thermal response of complex building structures

Abstract: Simulation of the effects of severe fires on the structural integrity of buildings requires a close coupling between the gas phase energy release and transport phenomena, and the stress analysis in the load-bearing materials. The connection between the two is established primarily through the interaction of the radiative heat transfer between the solid and gas phases with the conduction of heat through the structural elements. This process is made difficult in large, geometrically complex buildings by the wide disparity in length and time scales that must be accounted for in the simulations. A procedure for overcoming these difficulties used in the analysis of the collapse of the World Trade Center towers is presented. The large scale temperature and other thermophysical properties in the gas phase are predicted using the NIST Fire Dynamics Simulator. Heat transfer to subgrid scale structural elements is calculated using a simple radiative transport model that assumes the compartment is locally divided into a hot, soot laden upper layer and a cool relatively clear lower layer. The properties of the two layers are extracted from temporal averages of the results obtained from the Fire Dynamics Simulator. Explicit formulae for the heat flux are obtained as a function of temperature, hot layer depth, soot concentration, and orientation of each structural element. These formulae are used to generate realistic thermal boundary conditions for a coupled transient three-dimensional finite element code. This code is used to generate solutions for the heating of complex structural assemblies.

Transient radiative heat transfer within a suspension of coal particles undergoing steam gasification

Abstract: Transient radiative heat transfer in chemical reacting media is examined for a non-isothermal, non-gray, absorbing, emitting, and Mie-scattering suspension of coal particles, whose radiative properties vary with time as the particles undergo shrinking by endothermic gasification. A numerical model that incorporates parallel filtered collision-based Monte Carlo ray tracing, finite volume method, and explicit Euler time integration scheme is formulated for solving the unsteady energy equation that couples the radiative heat flux with the chemical kinetics. Variation of radiative properties, attenuation characteristics, temperature profiles, and extent of the chemical reaction are reported as a function of time. It is found that radiation in the visible and near IR spectrum incident on a cloud of coal particles greater than 2.5μm is more likely to be forward scattered than absorbed, but the opposite is true as the particles shrink below 1.3μm. The medium becomes optically thinner as the particles shrink and this effect is more pronounced for smaller initial coal particles because these offer higher volume fraction to particle diameter ratio and, consequently, attain higher temperatures, reaction rates, and shrinking rates.

Planar heterogeneous structures for coherent emission of radiation.

Abstract: The coupling of excited surface polaritons with thermal radiation through diffraction by gratings results in coherent thermal emission in a certain frequency range toward well-defined directions for p polarization. A planar coherent source is proposed that uses multilayers of negative permittivity (epsilon) and negative permeability (mu) materials. Owing to the excitation of surface polaritons at the interface between the negative-epsilon and negative-mu layers, coherent emission can be achieved for both s and p polarization. Moreover, one can control the emission frequency and direction by adjusting the layer thicknesses.

Single-defect Bragg stacks for high-power narrow-band thermal emission

Abstract: The radiative properties of single-defect Bragg stacks are investigated in the midinfrared region with the transfer-matrix theory. For a sufficiently small number of layers we find a regime where the structure emits radiation within a narrow range of wavelengths (δλ∕λ⩽0.01) and with an emissivity close to 1 in all directions. A description of the electric-field distribution inside the structure allows us to interpret this behavior in terms of coupling between the localized defect states and surface waves. This result should find broad applications in infrared spectroscopy, chemical sensing, and thermophotovoltaic conversion.

Transient radiation heat transfer within a nongray nonisothermal absorbing-emitting-scattering suspension of reacting particles undergoing shrinkage

Abstract: A nonisothermal, nongray, absorbing, emitting, and anisotropically scattering suspension of reacting particles exposed to concentrated thermal radiation is considered. The steam gasification of coal is selected as the model thermochemical reaction. The unsteady energy equation that couples the radiative heat flux with the chemical kinetics is solved by means of a numerical model that incorporates Monte Carlo ray tracing, the finite-volume method, and an explicit Euler time integration scheme. Two modeling approaches are applied: (1) a quasi-continuous model that assumes a homogeneous medium and utilizes its macroscopic radiative properties (absorption and scattering efficiencies and scattering phase function), and (2) a particle-discrete model that assumes an ensemble of randomly positioned particles and traces the interaction of radiation with each particle by geometric optics. Temperature profiles and reaction extent are computed using both approaches. The quasi-continuous approach is superior ...

On the determination of surface emissivity from Satellite observations

Abstract: Terrestrial brightness temperatures measured from satellites have been used to determine the surface emissivity. The results not only depend on surface temperature and on atmospheric properties, but also on the type of surface scattering. For otherwise identical conditions (same emissivity, same nonscattering atmosphere), the radiation above the Lambertian surface is larger than for a specular surface if the incidence angle is smaller than about 55/spl deg/. The opposite is true for larger angles. The effect leads to overestimates of emissivity for observations especially near nadir with the use of algorithms assuming specular reflection. The problem may be solved by the introduction of a specularity parameter to characterize realistic surfaces by a combination of specular and Lambert scattering. A simple solution lies in the use of conically scanning radiometers at a constant incidence angle near 55/spl deg/. Although the topic applies to all ranges of thermal radiation, the present discussion concentrates on the microwave spectrum in the Rayleigh-Jeans approximation.

Predicted Light Curves for a Model of Solar Eruptions

Abstract: We determine the thermal radiation generated by a loss-of-equilibrium model for CMEs and eruptive solar flares. The magnetic configuration of the model consists of an outward-moving flux rope with a vertical current sheet below it. Reconnection at the sheet releases magnetic energy, some of which is converted into thermal energy that drives chromospheric evaporation along the newly connected field lines exiting the current sheet. The thermal energy release is calculated by assuming that all of the Poynting flux flowing into the reconnection region is eventually thermalized. We find that the fraction of the released magnetic energy that goes into thermal energy depends on the inflow Alfven Mach number. The evolution of the temperatures and densities resulting from chromospheric evaporation is calculated using a simple evaporative cooling model. Using these temperatures and densities, we calculate simulated flare light curves for TRACE, the SXT on Yohkoh, and GOES. We find that when the background magnetic field strength is weak, the radiation emitted by the reconnected X-ray loops beneath a CME is faint. Additionally, it is possible to have two CMEs with nearly the same trajectories and speeds that have a significant difference in the peak intensities of their light curves. We also examine the relationship between the thermal energy release rate and the derivative of the soft X-ray light curve and discuss the implications for the Neupert effect.

Increase in radiation intensity in a quasi-spherical ``double liner''/``dynamic hohlraum'' system

Abstract: A concept of the magnetic implosion of quasi-spherical liners, concentration of their kinetic energies, conversion of energy into thermal radiation, confinement of its energy in the cavity of an emitting plasma shell in the “double liner”/“dynamic hohlraum” system, and the irradiation of a spherical target is proposed for the physics of high energy densities and inertial confinement fusion. The radiation intensity on the target was shown to increase considerably due to capture of radiation in the process of converting the kinetic energy of the liner into radiation. The dynamics of the liners and the generation of radiation are simulated by the ZETA code using a physical model developed for a nonequilibrium plasma in a cylindrical geometry. The effect of the instability and inhomogeneity of the liners on confinement of radiation energy is estimated.

Thermal radiation from nanoparticles

Abstract: A simple and universal criterion of the efficiency of energy loss by thermal radiation is obtained for the class of small conducting particles, including, in addition to metals and graphite, the majority of carbides of metals important for practical applications, such as tungsten and titanium carbides.

Thermal radiation of conducting nanoparticles

Abstract: The thermal radiation of small conducting particles was investigated in the region where the Stephan-Boltzmann law is not valid and strongly overestimates radiation losses. The new criterion for the particle size, at which black body radiation law fails, was formulated. The approach is based on the magnetic particle polarization, which is valid until very small sizes (cluster size) where due to drop of particle conductivity the electric polarization prevails over the magnetic one. It was also shown that the radiation power of clusters, estimated on the basis of the experimental data, is lower than that given by the Stephan-Boltzmann law.

Thermal radiation in 1D photonic crystals

Abstract: Optical properties of periodic structures, or photonic crystals, have been studied extensively over the last two decades While thermal radiation from surface grating structures has also been investigated, radiation inside a photonic crystal has not been investigated completely We have developed a method to analyze thermal radiation in 1D periodic layered media using a modified dyadic Green's function method and the fluctuation–dissipation theorem Using the method, thermal radiation between layers in periodic structures made of ultra-thin metallic films is analyzed The unusual features of thermal radiation in such structures are explained

Application of a New Method for Radiative Heat Transfer to Flat Glass Tempering

Abstract: Temperature distributions within semi-transparent materials like glass strongly determine their behavior. It is fundamental to take radiation into account to determine exact temperature distributions. The Rosseland Approximation for radiation is usually an appropriate method, but is only valid with an optically thick glass. In the present paper, we propose an improved approximation that is both efficient and sufficiently accurate even for the semi-transparent region. This new radiation model is used to determine the temperature along the flat glass thickness during tempering. The impact of temperature evaluation on residual stresses is discussed.