Particle Emissivity Research Articles

Irregular Particles in Comet C/1995 O1 Hale‐Bopp Inferred from its Mid‐Infrared Spectrum

An analysis of the Infrared Space Observatory (ISO) infrared spectra of comet C/1995 O1 Hale-Bopp has been conducted. The particles in the coma are assumed to be irregular aggregates that are built by a diffusion-limited aggregation (DLA) procedure and are composed of olivine, both amorphous and crystalline, and glassy carbon. To simulate the morphological structure of the interstellar dust, the silicate component is placed in the inner layers of the aggregate, while the carbon dipoles occupy the outermost layers. The particle emissivities are calculated using the discrete dipole approximation (DDA) method of Draine & Flatau. The dust size distribution functions at different heliocentric distances (2.8, 2.9, and 3.9 AU) have been determined as the solution to three overdetermined systems of equations for the infrared flux at each wavelength in the 8-40 μm region. These size distributions can be well fitted by a power law, n(r) ∝ r-α, having power indexes that decrease with heliocentric distance as α = -3.6 ± 0.3 at 2.8 AU and -3.3 ± 0.3 at 3.9 AU.

Constraints on the Circumstellar Disk Masses in the IC 348 Cluster

A 52 × 52 region toward the young cluster IC 348 has been imaged in the millimeter continuum at 40 × 49 resolution with the Owens Valley Radio Observatory interferometer to a rms noise level of 0.75 mJy beam-1 at 98 GHz. The data are used to constrain the circumstellar disk masses in a cluster environment at an age of ~2 Myr. The mosaic encompasses 95 known members of the IC 348 cluster, with a stellar mass distribution that peaks at 0.2–0.5 M⊙. None of the stars are detected in the millimeter continuum at an intensity level of 3 σ or greater. The mean observed flux for the ensemble of 95 stars is 0.22 ± 0.08 mJy. Assuming a dust temperature of 20 K, a mass opacity coefficient of κo = 0.02 cm2 g-1 at 1300 μm, and a power-law index of β = 1 for the particle emissivity, these observations imply that the 3 σ upper limit to the disk mass around any individual star is 0.025 M⊙ and that the average disk mass is 0.002 ± 0.001 M⊙. The absence of disks with masses in excess of 0.025 M⊙ in IC 348 is different at the ~3 σ confidence level from Taurus, where ~14% of the stars in an optically selected sample have such disk masses. Compared with the minimum mass needed to form the planets in our solar system (~0.01 M⊙), the lack of massive disks and the low mean disk mass in IC 348 suggest either that planets more massive than a few Jupiter masses will form infrequently around 0.2–0.5 M⊙ stars in IC 348 or that the process to form such planets has significantly depleted the disk of small dust grains on timescales less than the cluster age of ~2 Myr.

AN UNSTEADY CONDUCTION AND TWO-PHASE RADIATION IN A 2-D RECTANGULAR ENCLOSURE WITH GAS AND PARTICLES

A problem of combined conductive and two-phase radiative heat transfer in a two-dimensional rectangular enclosure with two-phase (gas-particles) media is analyzed. A two-phase radiative transfer equation (RTE) considering radiation by both gas and particles is studied. Its nonlinear integrodifferential RTE is solved using the discrete ordinates method (DOM, or so-called S N method). To validate the program, we compare the solution in a two-dimensional rectangular black enclosure with others. The DOM is then applied to the unsteady thermal development in two-phase media contained in a rectangular enclosure. A parametric study is performed by changing the gas and particle absorption coefficients, particle number density, particle emissivity, wall emissivity, and aspect ratio of the enclosure. The results confirm a significant effect of the two-phase radiation on the thermal development in the geometry. However, it is found that the conduction is predominant near the hot wall.

Heat transfer by radiation from a hot particle to ambient water through the vapor layer

Heat transfer by thermal radiation from a hot spherical particle to ambient water through the concentric vapor shell is analyzed by the use of different theoretical models. In the case of the vapor layer thickness Δ comparable with the radiation wavelength, the wave effects are taken into account. For greater values of Δ, the problem solution is derived by combination of the Mie theory for the particle emissivity, the Fresnel’s equations for radiation reflection/refraction on the vapor–water interface, and the radiation transfer equation for calculation of heat absorption profiles in water. A justified approximate description of the radiation heat transfer between corium melt particles and boiling water, as applied to vapor explosion analysis in the case of nuclear reactor severe accident, is proposed. Numerical examples illustrate effects of the particle size and the vapor shell thickness.

ANALYSIS OF TWO-PHASE RADIATION IN THERMALLY DEVELOPING POISEUILLE FLOW

Combined conductive and radiative heat transfer in a thermally developing two-phase Poiseuille flow in a cylindrical duct is studied here. A two-phase radiative transfer equation (RTE) considering radiation by both gas and particles is taken into account. A complexform of nonlinear integrodifferential RTE is solved by the discrete ordinates method (DOM, or so called SN method) in axisymmetric geometry. After such validation, namely, the solution in a two-dimensional channel flow between two flat plates is compared with that solved by the zone method, the program is then applied to fully developed gas-particle two-phase flow in a cylindrical duct. A parametric study is performed for gas and particle absorption coefficients, particle number density, particle emissivity, and wall emissivity. The results show a significant effect of two-phase radiation on the thermal characteristics. However, in all cases, it was found that conduction is predominant near the wall.

CFD simulations on a coal-coal reburning test facility

Fuel staging or fuel reburning is a possible primary measure for the reduction of NOx emissions from fossil-fired steam generators. It is intended to investigate if this primary measure can be applied efficiently to coal-fired combustion systems where pulverized coal is also used as a reburning fuel: coal reburning over a coal fire. This technology has to be tested accordingly. The company ENEL SpA is at present evaluating, together with ANSALDO Energia SpA, the option of demonstrating and extending the use of this technology in its coal-fired utility steam generators. Both companies have decided to erect a boiler simulation facility (5 MWth) at ENEL's experimental area Santa Gilla (Cagliari). This facility is intended to replicate the time/temperature profiles of real boilers. One tool to support the design of firing systems is CFD simulation. Consequently, CFD simulation was applied to perform a ‘proof of concept’. Most important for the design of firing systems for the furnaces of utility steam generators or test facilities is the distribution of temperature inside the combustion chamber and the furnace exit temperature—especially the radiation transport responsible for the amount of energy transferred from the gaseous environment of the hot furnace to the furnace walls. Subsequently, this determines the mean furnace exit temperature. This is why the accuracy of temperature calculations depends directly on the radiation transport model, the gas emissivity approximations, the particle emissivity and the radiative properties of the furnace enclosure. The results of several combustion simulations of the coal reburning test facility are shown and discussed. Estimates of the possible effect of different wall temperatures on the combustion process itself and on the gas temperature inside the furnace are presented. © 1997 by John Wiley & Sons, Ltd.

Improved Temperature Measurements of Burning Char and Coal Particles Using an FT-IR Spectrometer

A novel method for temperature measurements on individual burning char and coal particles with an FT-IR spectrometer has been developed. The technique is demonstrated for monitoring emission spectra of individual moving particles that require a few milliseconds to pass the field of view of a conventional scanning FT-IR spectrometer. The accurate particle surface temperature is calculated from a best match of the measured emission spectrum to a detailed physical radiance model spectrum. The technique is applied to measure the surface temperature of 90−125 μm particles with temperatures from 1000 to 2200 K in an entrained flow reactor. A one-temperature calibration of the FT-IR spectrometer is sufficient for accurate measurements throughout a broad temperature range. Background radiation and a fluctuating particle feeding rate are handled by subtraction of two successive measurements. The single-particle emission spectra are useful for testing the assumptions about particle emissivity as a function of wavel...

Radiation Emitted from Fluidizing Particles adjacent to a Heated Surface in a Fluidized Bed.

We experimentally investigated the radiation heat exchange occurring in a gas-solid fluidized bed between the fluidizing particles and a heated heat transfer surface. To evaluate this heat exchange, radiation emitted from fluidizing particles located adjacent to the heated surface was measured using an infrared imager through the transparent MgF2 heated surface, which was remotely heated by a CO2 laser. Our results revealed that the fluidizing particles are indeed heated by the surface. In addition, we found that these particles frequently emit a significant amount of radiation energy against the surface, which suppresses the radiation heat exchange between it and the fluidizing particles, and that the radiation energy increases with decreasing particle diameter and increasing particle emissivity.

Alpha particle emission from platinum and its influence on DRAM operation

Ferroelectrics such as SrTiO3 have been studied as DRAM capacitor insulators. Platinum is commonly used as their electrode material. However, platinum contains the isotope Pt190, emitting alpha particles which cause soft errors. The authors measured the alpha particle emissivity of platinum coated silicon wafers. The resultant emissivity is consistent with the calculation. The impact on DRAM operation was estimated.

Particle emissivity in circumstellar disks

view Abstract Citations (479) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Particle Emissivity in Circumstellar Disks Beckwith, Steven V. W. ; Sargent, Anneila I. Abstract Submillimeter continuum observations of 29 pre-main-sequence objects in Taurus and Orion are used to study the wavelength dependence of particle emission. These objects are mostly T Tauri stars whose long-wavelength emission is thought to originate in circumstellar disks. The flux densities imply power-law frequency distributions with spectral indices between two and three in almost all cases. If the emission is optically thin, the particle emissivities have power-law indices between -1 and 1; otherwise, these values are lower limits. It is argued that in most cases the emission is optically thin at wavelengths near 1 mm, so the measured indices should be close to the true values. The indices derived for the circumstellar particles are substantially smaller than those thought to obtain in the diffuse interstellar medium and dense molecular clouds; they imply that the millimeter-wave opacities are larger near these stars than in the interstellar medium. Particle shape and composition changes can produce the observed behavior. The effect is to decrease the required amount of disk material needed to produce the observed millimeter-wave continuum emission. Publication: The Astrophysical Journal Pub Date: November 1991 DOI: 10.1086/170646 Bibcode: 1991ApJ...381..250B Keywords: Particle Emission; Shell Stars; Stellar Mass Ejection; Millimeter Waves; Pre-Main Sequence Stars; Radio Stars; T Tauri Stars; Astrophysics; STARS: CIRCUMSTELLAR SHELLS; STARS: MASS LOSS; STARS: PRE--MAIN-SEQUENCE full text sources ADS | data products SIMBAD (37)

The synchrotron and cyclo-synchrotron absorption cross-section

We emphasize the usefulness of the concept of a cross-section for the synchrotron and the cyclo-synchrotron absorption process. We show that the previously known relation between the cross-section and the single particle emissivity is most easily derived by applying the Einstein coefficients and their relations to a system with three energy levels. Using this relation and known emissivities we derive useful analytical expressions for the synchrotron and cyclotron absorption cross-section

Evaluation of radiative heat transfer properties in dense particulate media

Radiant properties of a two-dimensional layer of diffusely reflecting spherical particles have been evaluated without resorting to any simplification. Approximate expressions have been derived to correlate the results with particle emissivity and layer concentration of particles. The calculated layer properties are used in the multiple-layer model to compare and discuss the capability of both pseudo-homogeneous and heterogeneous models for predicting radiative heat transfer rates in dense particulate media.

Radiative heat-transfer model in the interior of a pulverized coal furnace

A practical mathematical model simulating radiative heat transfer in the furnace of a pulverized coal boiler is presented. The inclusion of this model in a pulverized coal combustion model allows for testing its validity and its sensitivity to furnace walls and particle emissivity values, by comparison with measurements in a 550 MW power plant boiler.

A survey for circumstellar disks around young stellar objects

view Abstract Citations (1591) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Survey for Circumstellar Disks around Young Stellar Objects Beckwith, Steven V. W. ; Sargent, Anneila I. ; Chini, Rolf S. ; Guesten, Rolf Abstract Continuum observations at 1.3 mm of 86 pre-main sequence stars in the Taurus-Auriga dark clouds show that 42% have detectable emission from small particles. The detected fraction is only slightly smaller for the weak-line and "naked" T Tauri stars than for classical T Tauris, indicating that the former stars often have circumstellar material. In both categories, the column densities of particles are too large to be compatible with spherical distributions of circumstellar matter -- the optical extinctions would be too large; the particles are almost certainly in spatially thin, circumstellar disks. Models of the spectral energy distributions from 10 to 1300 μm indicate that for the most part the disks are transparent at 1.3 mm, although the innermost (≪1 AU) regions are opaque even at millimeter wavelenths. The aggregate particle masses are between 10-5 and 10-2 M⊙. The disk mass does not decrease with increasing stellar age up to at least 107 years among the stars detected at 1.3 mm. There is some evidence for temperature evolution, in the sense that older disks are colder and less luminous. There is little correlation between disk mass and Hα equivalent width among the detected stars, suggesting that the Hα line is not by itself indicative of disk mass. Spectral indices for several sources between 1.3 and 2.7 mm suggest that the particle emissivities ɛ are weaker functions of frequency ν than is the usual case of interstellar grains. Particle growth via adhesion in the dense disks might explain this result. The typical disk has an angular momentum comparable to that generally accepted for the early solar nebula, but very little stored energy, almost five orders of magnitude smaller than that of the central star. Our results demonstrate that disks more massive than the minimum mass of the proto-solar system commonly accompany the birth of solar-mass stars and suggest that planetary systems are common in the Galaxy. Publication: The Astronomical Journal Pub Date: March 1990 DOI: 10.1086/115385 Bibcode: 1990AJ.....99..924B Keywords: Molecular Clouds; Pre-Main Sequence Stars; Sky Surveys (Astronomy); Stellar Envelopes; T Tauri Stars; Auriga Constellation; Computational Astrophysics; Emission Spectra; H Alpha Line; Milky Way Galaxy; Stellar Evolution; Taurus Constellation; Astrophysics; STARS: PRE-MAIN-SEQUENCE; STARS: CIRCUMSTELLAR SHELLS full text sources ADS | data products SIMBAD (90)

Optical emission studies of atomic, molecular, and particulate carbon produced from a laser vaporization cluster source

We perform optical emission studies on the species generated by the laser vaporization of graphite into a pulsed helium flow. With short flow channels prior to free-jet expansion the emission at short downstream distances is dominated atomic carbon lines and three band systems of C2: d 3∏g–a 3∏u (Swan), C 1 ∏g–A 1∏u (Deslandres–d’Azambuja), and D 1∑+u –X 1∑+g (Mulliken). Partially rotationally resolved spectra of the Δv=−1 Swan bands are analyzed using a band contour approach to obtain rotational temperatures and rough vibrational distributions for C2 d 3∏g. The direct emission spectrum from the short-channel source becomes a weaker, structureless continuum when viewed at long downstream distances. Similar long-lived continua are induced by a low-fluence laser at 266, 355, or 532 nm when it crosses the expansion emanating from a source with a long flow channel. We attribute these continua to the incandescence of hot carbon particles, T=2500–4000 K. The continua are fit to a model in which the Planck blackbody function is modified by the particle emissivity to determine particle temperatures and obtain crude estimates of particle sizes. These data provide the first direct evidence that carbon particles are produced in a laser-vaporization cluster source.

The influence of a thermal plasma on synchrotron radiation

The synchrotron emission from relativistic electrons in a thermal plasma with large-scale random magnetic fields is considered. In this case, the spectral synchrotron power of a single electron can be given in closed form allowing exact analytical expressions for the synchrotron emissivity, absorption coefficient, intensity and total energy loss of particles to be derived. The influence of various physical parameters such as gas density, magnetic field strength, particle's Lorentz factor on the resulting emissivities, intensities and energy loss is discussed in detail. Below the Razin– Tsytovich frequency vR = 20 Hz (ne/l cm−3) (B/l Gauss)−1, the spectral appearance of synchrotron radiation both in the optically thin and thick case is quite different than the vacuum behaviour. Since in the quasar broad line regions, vR is of the order 1011 Hz the suppression of synchrotron radiation may explain why most quasars are radio quiet. Likewise, the necessary physical conditions for the occurrence of synchrotron masering in the optically thick case are given. We obtain optical depth |τ|>1 for compact nonthermal sources. The total energy loss of a single particle is shown to be exponentially reduced at Lorentz factors less than γR = 2·1. 10−3 (ne/1 cm−3)½ (B/1 Gauss)−1.

Measurement Of The Emissivity Of Small Particles At Elevated Temperatures

A technique for measuring the emissivity of single, nominally 50-µm-sized tungsten particles at elevated temperatures is demonstrated. Single particles are electrodynamically suspended in the cavity of a cw Nd:YAG laser and subsequently heated by the laser beam. The light emitted by the heated particles is focused onto the slit of a spectrometer. The detector used is an intensified gatable diode array that allows for the collection of time-resolved spectral data. Spectral radiance is deduced by calibrating the system against a standard lamp. Assuming that the relationship between emissivity and wavelength is a smooth function, the particle temperature and emissivity are determined by a curve-fitting procedure involving Planck's spectral radiation law. Experimental demonstration and error estimates of the technique are presented.

Thermal radiation of gas/solid mixtures

AbstractThermal radiation of gas/solid mixtures in high‐temperature heat exchangers is of considerable importance for the design of heat exchange surfaces. Therefore, an experimental plant was constructed to measure solid particle emissivities, since there is a lack of relevant data in the existing literature. A theoretical equation was derived to describe the solid particle emissivity as a function of layer thickness, specific solid surface area, solid loading and absorption and scatter coefficients. Measurements were carried out for fluidized bed ash and quartz sand. The emissivities of these particle fractions increase with increasing layer thickness, slid loading, specific solid surface area and temperature. The description of these data by the derived model equation is very satisfactory. No dependence on wavelength was observed. The ash components do exert some influence which, for the present, cannot be described exactly.

Quantized synchrotron radiation in strong magnetic fields

Spectra from electrons making synchrotron transitions in high magnetic fields are calculated using the exact quantum transition rates for Landau states up to n = 500, and the results are compared with spectra calculated from the classical formula and the asymptotic quantum formula. The behavior of the transition rates are examined at low and high harmonics as a function of the spin state of the electron. The calculations confirm those of Herold, Ruder, and Wunner (1982) for low Landau states, extending them to higher states and individual spin state transitions. The results also confirm the dominance of ground-state transitions at high field strengths noted by White (1976). Single particle emissivities for electrons with both spin-up and spin-down in the initial state are calculated using these transition rates. Spectra for thermal electron distributions at transrelativistic temperatures and for steady state injection of monoenergetic electrons with isotropic pitch angles are also presented.

A two-particle model for rocket plume radiation

A radiation model is proposed for the plumes of solid rocket motors firing into what is essentially a vacuum. The plume contains cold scattering particles and hot emitting/absorbing particles in overlapping conical clouds; the half-angle of the hot-particle cloud may be smaller than that of the cold-particle cloud. The clouds have axially varying radiative properties, with the hot particles imagined to lose emissive power and particle emissivity with increasing axial distance. Parametric calculations are made using the hybrid Monte Carlo, radiosity/irradiation technique to show which parameters are critical and how they may be inferred from engineering tests. Nomenclature B = blackbody emissive power aT4, W/m2 7 = radiant intensity (radiance), W/m2 • sr / = radiosity, W/m2 K = extinction coefficient, m1; with subscript, absorption or scattering coefficient, m1 M = number of Monte Carlo rays n = power law exponent N = number of surfaces and volumes q = radiant flux, W/m2 R = radial location, m s = slant path, m / = optical depth T = temperature, K z = distance from the apex of the cone along its surface, m ft = angle from conical axis c = emissivity 0 = polar angle from surface normal TT =3.14159... a = Stefan-Boltzmann constant, W/m2 • K4 = azimuthal angle from cone generatrix ( = 0 is directed downstream) a) = albedo for single scatter 0 = solid angle, sr Subscripts