This work is devoted to the numerical study of the influence of heterogeneous kinetics on the oxidation rates of a single carbon particle in quiescent and non-quiescent environments. The coal ...particle is represented by moisture- and ash-free nonporous carbon while the coal rank is implemented using several kinetic rate expressions. The model includes six gaseous chemical species (O2, CO2, CO, H2O, H2, N2). Three heterogeneous reactions are employed (C+O2, C+CO2 and C+H2O), along with two homogeneous semi-global reactions, namely carbon monoxide oxidation and the water–gas shift reaction. Several semi-global reaction rate expressions taken from the literature are utilized. The Navier–Stokes equations coupled with the energy and species conservation equations are used to solve the problem by means of the pseudo-steady state approach. At the surface of the particle, the balance of mass, energy and species concentration is applied including the effect of the Stefan flow and heat loss due to radiation at the surface of the particle. The model and the code used are validated against an analytic two-film model. Good agreement is observed. The numerical simulations performed reveal that the origin of heterogeneous kinetics has a significant effect on the carbon consumption rates of the particle. In particular, the maximal discrepancy between results is achieved in a kinetically controlled regime and is proportional by a factor of 10 in respect to carbon mass flux on the particle surface. Additionally, the influence of the particle Reynolds number referring to the laminar flow regime, the ambient O2 mass fraction and the temperature on the regimes of combustion and gasification is discussed.
This work is devoted to the development and validation of a sub-model for the partial oxidation of a spherical char particle moving in an air/steam atmosphere. The particle diameter is 2 mm. The coal ...particle is represented by moisture- and ash-free nonporous carbon while the coal rank is implemented using semi-global reaction rate expressions taken from the literature. The submodel includes six gaseous chemical species (O2, CO2, CO, H2O, H2, N2). Three heterogeneous reactions are employed, along with two homogeneous semi-global reactions, namely carbon monoxide oxidation and the water-gas-shift reaction. The distinguishing feature of the subgrid model is that it takes into account the influence of homogeneous reactions on integral characteristics such as carbon combustion rates and particle temperature. The sub-model was validated by comparing its results with a comprehensive CFD-based model resolving the issues of bulk flow and boundary layer around the particle. In this model, the Navier-Stokes equations coupled with the energy and species conservation equations were used to solve the problem by means of the pseudo-steady state approach. At the surface of the particle, the balance of mass, energy and species concentration was applied including the effect of the Stefan flow and heat loss due to radiation at the surface of the particle. Good agreement was achieved between the sub-model and the CFD-based model. Additionally, the CFD-based model was verified against experimental data published in the literature (Makino et al. (2003) Combust. Flame 132, 743-753). Good agreement was achieved between numerically predicted and experimentally obtained data for input conditions corresponding to the kinetically controlled regime. The maximal discrepancy (10%) between the experiments and the numerical results was observed in the diffusion-controlled regime. Finally, we discuss the influence of the Reynolds number, the ambient O2 mass fraction and the ambient temperature on the char particle behaviour.
Cette étude est consacrée au développement et à la validation d’un sous-modèle pour l’oxydation partielle d’une particule de charbon sphérique se déplaçant dans une atmosphère air/vapeur. Le diamètre de la particule est de 2 mm. La particule de charbon est représentée par du carbone non poreux exempt d’humidité et de cendres, alors que la qualité du charbon est établie en utilisant des expressions du taux de réaction semi-globale extraites de la littérature. Le sous-modèle inclut six espèces chimiques gazeuses (O2, CO2, CO, H2O, H2, N2). Trois réactions hétérogènes sont utilisées, ainsi que deux réactions semi-globales homogènes, à savoir l’oxydation du monoxyde de carbone et la réaction du gaz à l’eau. Les particularités distinctives du modèle de sous-maille se trouvent dans la prise en compte de l’influence des réactions homogènes sur les caractéristiques intégrales telles que les taux de combustion du carbone et la température de la particule. Le sous-modèle a été validé en comparant ses résultats avec un modèle complet basé sur la CFD résolvant les questions de flux volumique et de couche limite autour de la particule. Dans ce modèle, les équations de Navier-Stokes couplées aux équations de conservation de l’énergie et des espèces ont été utilisées pour résoudre le problème au moyen de l’approche en état pseudo-stationnaire. À la surface de la particule, l’équilibre de la masse, de l’énergie et de la concentration des espèces a été appliqué, y compris l’effet de l’écoulement de Stefan et l’effet de la perte de chaleur due aux rayonnements à la surface de la particule. Une bonne adéquation a été atteinte entre le sous-modèle et le modèle basé sur la CFD. En outre, le modèle basé sur la CFD a été comparé aux données expérimentales publiées dans la littérature (Makino et al. (2003) Combust. Flame 132, 743-753). Une bonne concordance a été atteinte entre les données prédites numériquement et celles obtenues expérimentalement pour les conditions d’entrée correspondant au régime contrôlé par la cinétique. La divergence maximale (10 %) entre les expériences et les résultats numériques a été observée dans le régime contrôlé par la diffusion. Enfin, nous discutons de l’influence du nombre de Reynolds, de la fraction massique d’O2 ambiant et de la température ambiante sur le comportement de la particule de charbon.
This work is devoted to a numerical study of the impact of the ambient mass fraction of H2O and the Reynolds number on the partial oxidation of a single coal particle. The range of parameters ...considered in this study are 10 − 5 ≤ Y
H2O ≤ 0.5 and 0 ≤ Re ≤ 50. The model includes six gaseous chemical species (O2, CO2, CO, H2O, H2, N2). Three heterogeneous reactions (C + O 2, C + CO2 and C + H2O) and two homogeneous semi-global reactions, namely carbon monoxide oxidation and water gas shift reaction, are employed. Semi-global reaction rate expressions were utilized. The Navier-Stokes equations coupled with the energy and species conservation equations were used to study the sensitivity of the mathematical model in dependence on the water content. Based on the results of the numerical simulations carried out for a coal particle with a diameter of 2 mm, we found out that the carbon consumption and surface-averaged mass fractions of CO, CO2 and H2O are identical and are unchanged at higher temperatures of the ambient gas, T
in > 2500 K and Y
H2O ≤ 10 − 2. The increase in H2O concentration in the ambient atmosphere leads to the splitting of CO2 and T-flame sheets surrounding the particle. This regime exists at T
in ≥ 2500 K and Y
H2O > 0.1.
For the first time an excess of photons above an energy threshold of 730 GeV from the giant radio galaxy M 87 has been measured at a significance level above 4 σ. The data have been taken during the ...years 1998 and 1999 with the HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes. The excess of $107.4\,\pm\,26.8$ events above 730 GeV corresponds to an integral flux of 3.3% of the Crab flux or $N_\gamma(E > 730\,\mbox{GeV}) = (0.96 \pm 0.23) \times 10^{-12}$ phot cm-2 s-1. M 87 is located at the center of the Virgo cluster of galaxies at a relatively small redshift of $z = 0.00436$ and is a promising candidate among the class of giant radio galaxies for the emission of TeV γ-radiation. The detection of TeV γ-rays from M 87 – if confirmed – would establish a new class of extragalactic source in this energy regime since all other AGN detected to date at TeV energies are BL Lac type objects.
TeV γ-rays from the BL Lac object 1ES 1959+650 have been measured during the years 2000 and 2001 with a significance of 5.2 σ at a value of 5.3% of the Crab flux and in May 2002 during strong ...outbursts with >$23\,\sigma$ at a flux level of up to 2.2 Crab, making 1ES 1959+650 the TeV Blazar with the third best event statistics. The deep observation of 197.4 h has been performed with the HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes (IACT system). 1ES 1959+650 is located at a redshift of $z = 0.047$, providing an intermediate distance between the nearby Blazars Mkn 421 and Mkn 501, and the much more distant object H1426+428. This makes 1ES 1959+650 an important member of the class of TeV Blazars in view of the absorption of TeV photons by the diffuse extragalactic background radiation (DEBRA). The differential energy spectrum of 1ES 1959+650 during the flares can be fitted by a power law with a spectral index of $2.83 \pm 0.14_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$ or by a power law with an exponential cut-off at $(4.2^{+0.8}_{-0.6~{\mbox{\tiny stat}}} \pm 0.9_{\mbox{\tiny sys}})$ TeV and a spectral index of $1.83 \pm 0.15_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$. The low state differential energy spectrum obtained with lower statistics can be described by a pure power law with a spectral index of $3.18 \pm 0.17_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$.
The detection of TeV γ-rays from the blazar H 1426+428 at an integral flux level of $(4 \pm 2_{\mathrm{stat}} \pm 1_{\mathrm{syst}}) \times 10^{-12}~\mathrm{erg}~\mathrm{cm}^{-2}~\mathrm{s}^{-1}$ ...above 1 TeV with the HEGRA imaging atmospheric Cherenkov telescope system is reported. H 1426+428 is located at a redshift of $z =$ 0.129, which makes it the most distant source detected in TeV γ-rays so far. The TeV radiation is expected to be strongly absorbed by the diffuse extragalactic background radiation (DEBRA). The observed energy spectrum of TeV photons is in good agreement with an intrinsic power law spectrum of the source $\propto E^{-1.9}$ corrected for DEBRA absorption. Statistical errors as well as uncertainties about the intrinsic source spectrum, however, do not permit strong statements about the density of the DEBRA infrared photon field.
Between early 1997 and late 2002, the HEGRA collaboration operated a stereoscopic system of four (later five) imaging atmospheric Cherenkov telescopes. In this paper we present the calibration ...schemes which were developed for the system, and report on the performance of the detector over the years. In general, the telescope system was very well understood, regarding both the absolute calibration and the slight changes in performance over the years. The system had an energy threshold of 500 GeV for observations at zenith and under optimum detector conditions. With the corresponding calibration schemes, a systematic accuracy of 15% on the absolute energy scale has been achieved. The continuous sensitivity monitoring provided a relative accuracy of a few percent, and showed that the threshold did not exceed 600 GeV throughout the entire operation time. The readout electronics and the imaging quality of the dishes were well monitored and stable. The absolute pointing had an accuracy of at least 25
″; this number was guaranteed throughout the whole lifetime of the experiment.
Data taken with the HEGRA system of imaging atmospheric Cherenkov telescopes during the 1997 flares of Markarian 501 (Mkn 501) are reanalyzed using an algorithm providing improved energy resolution. ...A resolution of 10% to 12% is obtained by accounting for the variation of the Cherenkov light yield with the height of the shower maximum in the atmosphere. The improved energy resolution is particularly relevant for the study of the high-energy cutoff in the spectrum, which might be caused by interactions with the intergalactic infrared background radiation. The reanalysis presented here confirms the results obtained in the previous analysis, but hints a steeper slope of the spectrum in the region around 20 TeV.
Context.A sample of 14 young open star clusters has been observed in the TeV energy regime with the stereoscopic system of the HEGRA (High Energy Gamma-Ray Astronomy) Cherenkov telescopes from 1997 ...to 2002, resulting in more than 300 h of observation time. Aims.Young open star clusters may contribute to the acceleration of cosmic rays. The detection of γ-rays (from decaying $\pi^0$s produced in hadronic interactions) from these objects could be evidence for such a contribution. The results of our observations are compared to available γ-ray data and to a simple hadronic model in the framework of shock front acceleration of cosmic rays in the stellar winds of the cluster members to test the potential of the presently available data on young open star clusters to constrain this type of model. Methods.The stereoscopic system of HEGRA Cherenkov telescopes makes use of the atmospheric imaging technique. Air showers initiated by primary Gamma-Rays are recorded as elliptical images in the telescope cameras. The images from the different telescopes are then superimposed to reconstruct the parameters of the primary particle. This technique (stereoscopy) was pioneered by the HEGRA experiment. Results.No significant excess has been found in the analysed data set of young open star clusters. The derived upper limit on the TeV gamma-ray flux from Berkeley 87 and the available EGRET data from the same direction do not allow us to fully constrain the simple hadronic model used here. The comparison of the upper limits derived for all 14 objects with the flux detected from TeV J2032+4130 (under the assumption of an association of the TeV-signal with the compact stellar association Cyg OB2) suggests that γ-ray emission from young open star clusters as an object class cannot be ruled out.
Design and status of IceCube KESTEL, M
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
12/2004, Letnik:
535, Številka:
1-2
Journal Article
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