Showing papers on "Thermal radiation published in 2006"

The studies of thermal conductivity in GdVO 4 , YVO 4 , and Y 3 Al 5 O 12 measured by quasi-one-dimensional flash method

Abstract: We have measured thermal conductivity of GdVO4, YVO4, and Y3Al5O12. In order to avoid the miss leading from three-dimensional (3D) thermal diffusion, we developed the quasi-one-dimensional (q1D) flash method. By taking in account the heat radiation effect in transparent materials for this measurement, YVO4 was found to have larger thermal conductivity than GdVO4. The measured thermal conductivities were 12.1, 10.5, 10.1, 8.9, and 8.5 W/mK for c-cut YVO4, c-cut GdVO4, YAG, a-cut YVO4, and a-cut GdVO4, respectively. The dependence of Nd-conductivity coefficient (dκ/dCNd) for convenient evaluation of the doping effect in thermal conductivity is also discussed.

Thermal radiation of various gravitational backgrounds

Abstract: We present a simple and general procedure for calculating the thermal radiation coming from any stationary metric. The physical picture is that the radiation arises as the quasi--classical tunneling of particles through a gravitational barrier. We show that our procedure can reproduce the results of Hawking and Unruh radiation. We also show that under certain kinds of coordinate transformations the temperature of the thermal radiation will change in the case of the Schwarzschild black holes. In addition we apply our procedure to a rotating/orbiting system and show that in this case there is no radiation, which has experimental implications for the polarization of particles in circular accelerators.

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

Abstract: 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 layer of Wolf-Rayet material is entrained at the head of this outflow; it cools and becomes Rayleigh-Taylor unstable, thereby providing an additional source of inertia and variability. The gamma rays streaming across the forward shock load the wind material with electron-positron pairs and push it to relativistic speeds close to the engine. This 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 MHD waves, excited by the forced reconnection of the reversing magnetic field, carry a fluctuating current and are Landau damped at high frequencies on the parallel motion of the light charges. We show that the heated charges cool primilarly by inverse Compton radiation, which 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—both its amplitude and scaling—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 below an isotropic luminosity Liso ~ 3 × 1050 ergs s-1. Spectrally harder bursts will arise in outflows which encounter no dense stellar envelope. The duration of spikes in the inverse-Compton emission is narrower at higher frequencies, as observed. 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.

High performance midinfrared narrow-band plasmonic thermal emitter

Abstract: The blackbody radiation spectrum is fundamental to any thermal emitter. However, by properly designing the emitter structure, a narrow bandwidth and high power infrared source can be achieved. This invention consists of a triple layer structure by sandwiching a dielectric SiO2 layer between two Ag metal films on the Si substrate. The top Ag layer is perforated by periodic holes. When the device was heated, the background thermal radiation was suppressed by the bottom Ag whose emissivity is very low. The thermal radiation generated in the SiO2 layer resonant between two metal films and the Ag∕SiO2 and the Ag/air surface plasmon polaritons are induced and converted to light radiation. Strong resonance at Ag∕SiO2 (1,0) degenerate modes results in the coherent light radiation at the wavelength associated with the dielectric constant of SiO2 and the lattice constant of the perforated hole array. The ratio of the full width at half maximum to the peak wavelength is 0.114. This narrow bandwidth and high power infrared light source can be used to explore the biological response of cells and plants.

Coherent thermal antenna using a photonic crystal slab.

Abstract: We show that a photonic crystal film can emit coherent thermal radiation. We demonstrate the key role of leaky waves existing at the air-photonic crystal interface. The frequency and direction of emission depend on the lattice parameters. This paves the way towards the design of coherent infrared antennas.

Anisotropic thermal emission from magnetized neutron stars

Abstract: Context. The thermal emission from isolated neutron stars is not well understood. The X-ray spectrum is very close to a blackbody but there is a systematic optical excess flux with respect to the extrapolation to low energy of the best blackbody fit. This fact, in combination with the observed pulsations in the X-ray flux, can be explained by anisotropies in the surface temperature distribution. Aims. We study the thermal emission from neutron stars with strong magnetic fields B > 10 13 G in order to explain the origin of the anisotropy. Methods. We find (numerically) stationary solutions in axial symmetry of the heat transport equations in the neutron star crust and the condensed envelope. The anisotropy in the conductivity tensor is included consistently. Results. The presence of magnetic fields of the expected strength leads to anisotropy in the surface temperature. Models with toroidal components similar to or larger than the poloidal field reproduce qualitatively the observed spectral properties and variability of isolated neutron stars. Our models also predict spectral features at energies between 0.2 and 0.6 keV for B = 10 13 -10 14 .

Modelling soil dust aerosol in the Bodélé depression during the BoDEx campaign

Abstract: We present regional model simulations of the dust emission events during the Bod´ elDust Experiment (BoDEx) that was carried out in February and March 2005 in Chad. A box model version of the dust emission model is used to test different input parameters for the emission model, and to compare the dust emissions computed with observed wind speeds to those calculated with wind speeds from the regional model simulation. While field observa- tions indicate that dust production occurs via self-abrasion of saltating diatomite flakes in the Bod ´ el´ e, the emission model based on the assumption of dust production by saltation and using observed surface wind speeds as input parameters re- produces observed dust optical thicknesses well. Although the peak wind speeds in the regional model underestimate the highest wind speeds occurring on 10-12 March 2005, the spatio-temporal evolution of the dust cloud can be reasonably well reproduced by this model. Dust aerosol interacts with solar and thermal radiation in the regional model; it is re- sponsible for a decrease in maximum daytime temperatures by about 5 K at the beginning the dust storm on 10 March 2005. This direct radiative effect of dust aerosol accounts for about half of the measured temperature decrease compared to conditions on 8 March. Results from a global dust model suggest that the dust from the Bod´ el´ e is an important contrib- utor to dust crossing the African Savannah region towards the Gulf of Guinea and the equatorial Atlantic, where it can con- tribute up to 40% to the dust optical thickness.

MHD and radiation effects on moving isothermal vertical plate with variable mass diffusion

Abstract: An analysis is performed to study the efiects of thermal radiation on unsteady free convective ∞ow over a moving vertical plate with mass transfer in the presence of magnetic fleld. The ∞uid considered here is a gray, absorbing-emitting radiation but a nonscattering medium. The plate temperature is raised to T 0 w and the concentration level near the plate is also raised linearly with time. The dimensionless governing equations are solved using the Laplace transform technique. The velocity, temperature and concentration are studied for difierent parameters like the magnetic fleld parameter, radiation parameter, thermal Grashof number, mass Grashof number and time. It is observed that the velocity decreases with increasing magnetic fleld parameter or radiation parameter.

Effects of surface radiation on natural convection in a rayleigh-benard square enclosure: steady and unsteady conditions

Abstract: Coupled laminar natural convection with radiation in air-filled square enclosure heated from below and cooled from above is studied numerically for a wide variety of radiative boundary conditions at the sidewalls. A numerical model based on the finite difference method was used for the solution of mass, momentum and energy equations. The surface-to-surface method was used to calculate the radiative heat transfer. Simulations were performed for two values of the emissivities of the active and insulated walls (ɛ1=0.05 or 0.85, ɛ2=0.05 or 0.85) and Rayleigh numbers ranging from 103 to 2.3×106 . The influence of those parameters on the flow and temperature patterns and heat transfer rates are analyzed and discussed for different steady-state solutions. The existing ranges of these solutions are reported for the four different cases considered. It is founded that, for a fixed Ra, the global heat transfer across the enclosure depends only on the magnitude of the emissivity of the active walls. The oscillatory behavior, characterizing the unsteady-state solutions during the transitions from bicellular flows to the unicellular flow are observed and discussed.

Direct calculation of thermal emission for three-dimensionally periodic photonic crystal slabs.

Abstract: We perform direct thermal emission calculations for three-dimensionally periodic photonic crystal slabs using stochastic electrodynamics following the Langevin approach, implemented via a finite-difference time-domain algorithm. We demonstrate that emissivity and absorptivity are equal, by showing that such photonic crystal systems emit as much radiation as they absorb, for every frequency, up to statistical fluctuations. We also study the effect of surface termination on absorption and emission spectra from these systems.

Bulb with sensing function

Abstract: The utility model discloses a bulb with a sensing function. The structure combines an infrared sensor and a bulb to form the bulb with a sensing function. The application of the fact that the infrared sensor has a thermal radiation sensing function and the use of the induction of thermal radiation emitted by a human body are used as a power switch which starts lamp light of the bulb, and the bulb uses an LED luminous component as a luminous body. When the infrared sensor can not sense thermal radiation, the bulb can automatically cut off a power supply which supplies the LED luminous component to achieve the purposes of effective energy saving and the bulb which has high practicability.

Modeling radiation heat transfer with participating media in solid oxide fuel cells

Abstract: Solid oxide fuel cell (SOFC) technology has been shown to be viable, but its profitability has not yet been seen. To achieve a high net efficiency at a low net cost, a detailed understanding of the transport processes both inside and outside of the SOFC stack is required. Of particular significance is an accurate determination of the temperature distribution because material properties, chemical kinetics, and transport properties depend heavily on the temperature. Effective utilization of the heat can lead to a substantial increase in overall system efficiency and decrease in operating cost. Despite the extreme importance in accurately predicting temperature, the SOFC modeling community appears to be uncertain about the importance of incorporating radiation into their models. Although some models have included it, the majority of models ignore radiative heat transfer. SOFCs operate at temperatures around or above 1200 K, where radiation effects can be significant. In order to correctly predict the radiation heat transfer, participating gases must also be included. Water vapor and carbon dioxide can absorb, emit, and scatter radiation, and are present at the anode in high concentrations. This paper presents a simple thermal transport model for analyzing heat transfer and improving thermal management within planar SOFCs. The model was implemented using a commercial computational fluid dynamic code and includes conduction, convection, and radiation in a participating media. It is clear from this study that radiation must be considered when modeling solid oxide fuel cells. The effect of participating media radiation was shown to be minimal in this geometry, but it is likely to be more important in tubular geometries.

On the Entropy Generation Formula of Radiation Heat Transfer Processes

Abstract: Because thermal radiation is a long-range phenomenon, the local radiative heat flux is dependent on the temperature distribution of the entire enclosure under consideration and is not determined by the local temperature gradient. In the community of heat transfer, traditionally, the conduction-type formula of entropy generation rate is used to calculate the entropy generation rate of radiation heat transfer. In the present study, three counterexamples are considered. The discrete ordinates method is employed to solve the radiative transfer equation and then solve the radiative entropy generation rate. The results show that the traditional formulas of entropy generation rate for heat transfer generally cannot be used to calculate the local entropy generation rate of radiation heat transfer. Only in optically extremely thick situations, the traditional formula of entropy generation rate for heat transfer can be approximately used to calculate the local entropy generation rate of radiation heat transfer.

The effect of radiation shields around the air condenser and compressor of a refrigerator on the temperature distribution inside it

Abstract: In almost all domestic refrigerators–freezers all components are assembled in the same relative position since several years ago. It is also known that the condenser releases heat at high temperatures (first law of thermodynamics) as well as the compressor. This heat is rejected to the environment in almost all practical situations partially by natural air convection. However, part of it is due to thermal radiation that causes an overheating of the refrigerator–freezer surfaces adjacent to those equipments. As a consequence there are more heat gains to the refrigerator–freezer through these surfaces and hence higher air temperatures inside. This paper describes how a simple technique can be very useful in order to minimize that part of heat transfer by radiation. The improvement is achieved by placing a radiation shield – a sheet of aluminium foil – over the surfaces close to the condenser and the compressor. For validating this technique a refrigerator–freezer was monitored with thermocouples for the measurements of the inside air temperatures in two situations: with and without the radiation shield. Results show that with this practice the average inside air temperatures in the refrigerator–freezer could decrease to about 2 K. An available commercial code was used in order to simulate the air temperature distribution and air velocities inside the refrigerator cabinet in both situations. Results from the experimental apparatus and from simulations show that there is a good agreement between them which validates the experiments carried out. Also an available commercial code, the Fluent, was used to simulate the internal air temperature in both situations.

Mathematical models of dependence of surface temperatures of exposed metal plates on environmental parameters

Abstract: The present paper sets out the mathematical models that control the surface temperature of metal plates during atmospheric exposure. Heat transfer can be modelled by a series of heat transfer coefficients for different thermal processes, i.e. conduction, natural convection, forced convection, radiative heat losses to the sky, radiative heat transfer to or from the ground, incoming solar radiation, and evaporation and condensation. The model is applied to predict the surface temperature of near horizontal galvanised steel plates. Undercooling of a galvanised plate relative to the ambient air, i.e.the extent to which the temperature of the plate falls below that of its surroundings, is calculated for both clear and cloudy nights, with and without a slight breeze. It is found that breezes and clouds significantly decrease the extent of undercooling. The temperature differences between the plate and the ambient air after sunrise are also calculated. It is found that the prime factors controlling the p...