124 research outputs found
Emission of ions and charged soot particles by aircraft engines
In this article, a model which examines the formation and evolution of chemiions in an aircraft engine is proposed. This model which includes chemiionisation, electron thermo-emission, electron attachment to soot particles and to neutral molecules, electron-ion and ion-ion recombination, ion-soot interaction, allows the determination of the ion concentration at the exit of the combustor and at the nozzle exit of the engine. It also allows the determination of the charge of the soot particles. For the engine considered, the upper limit for the ion emission index EI<sub>i</sub> is of the order of (2-5) x10<sup>16</sup> ions/kg-fuel if ion-soot interactions are ignored and the introduction of ion-soot interactions lead about to a 50% reduction. The results also show that most of the soot particles are either positively or negatively charged, the remaining neutral particles representing approximately 20% of the total particles. A comparison of the model results with the available ground-based experimental data obtained on the ATTAS research aircraft engines during the SULFUR experiments (Schumann, 2002) shows an excellent agreement
Volatile particles formation during PartEmis: a modelling study
A modelling study of the formation of volatile particles in a combustor exhaust has been carried out in the frame of the PartEmis European project. A kinetic model has been used in order to investigate nucleation efficiency of the H<sub>2</sub>O-H<sub>2</sub>SO<sub>4</sub> binary mixture in the sampling system. A value for the fraction <IMG WIDTH='10' HEIGHT='13' ALIGN='BOTTOM' BORDER='0' src='http://www.atmos-chem-phys.net/4/439/2004/acp-4-439-img1.gif' ALT=''> of the fuel sulphur S(IV) converted into S(VI) has been indirectly deduced from comparisons between model results and measurements. In the present study, <IMG WIDTH='10' HEIGHT='13' ALIGN='BOTTOM' BORDER='0' src='http://www.atmos-chem-phys.net/4/439/2004/acp-4-439-img1.gif' ALT=''> ranges between roughly 2.5% and 6%, depending on the combustor settings and on the value assumed for the parameter describing sulphuric acid wall losses. Soot particles hygroscopicity has also been investigated as their activation is a key parameter for contrail formation. Growth factors of monodisperse particles exposed to high relative humidity (95%) have been calculated and compared with experimental results. The modelling study confirms that the growth factor increases as the soot particle size decreases
Variable-Frequency QPOs from the Galactic Microquasar GRS 1915+105
We show that the galactic microquasar GRS 1915+105 exhibits quasi-periodic
oscillations (QPOs) whose frequency varies continuously from 1-15 Hz, during
spectrally hard dips when the source is in a flaring state. We report here
analyses of simultaneous energy spectra and power density spectra at 4 s
intervals. The energy spectrum is well fit at each time step by an optically
thick accretion disk plus power law model, while the power density spectrum
consists of a varying red noise component plus the variable frequency QPO. The
features of both spectra are strongly correlated with one another. The 1-15 Hz
QPOs appear when the power law component becomes hard and intense, and
themselves have an energy spectrum consistent with the power law component
(with root mean square amplitudes as high as 10%). The frequency of the
oscillations, however, is most strikingly correlated with the parameters of the
thermal disk component. The tightest correlation is between QPO frequency and
the disk X-ray flux. This fact indicates that the properties of the QPO are not
determined by solely a disk or solely a corona.Comment: Accepted to ApJ Letters, 12 pages, 3 figures, AASTEX forma
Fast Transition between High-soft and Low-soft States in GRS 1915+105: Evidence for a Critically Viscous Accretion Flow
We present the results of a detailed analysis of RXTE observations of class
which show an unusual state transition between high-soft and low-soft
states in the microquasar GRS 1915+105. Out of about 600 pointed RXTE
observations, the source was found to exhibit such state transition only on 16
occasions. An examination of the RXTE/ASM data in conjunction with the pointed
observations reveals that these events appeared as a series of quasi-regular
dips in two stretches of long duration (about 20 days during each occasions)
when hard X-ray and radio flux were very low. The X-ray light curve and
color-color diagram of the source during these observations are found to be
different from any reported so far. The duration of these dips is found to be
of the order of a few tens of seconds with a repetition time of a few hundred
seconds. The transition between these dips and non-dips which differ in
intensity by a factor of ~ 3.5, is observed to be very fast (~ a few seconds).
It is observed that the low-frequency narrow QPOs are absent in the power
density spectrum (PDS) of the dip and non-dip regions of class and the
PDS is a power law in 0.1 - 10 Hz frequency range. There is a remarkable
similarity in the spectral and timing properties of the source during the dip
and non-dip regions in these set of observations. These properties of the
source are distinctly different from those seen in the observations of other
classes. This indicates that the basic accretion disk structure during both dip
and non-dip regions of class is similar, but differ only in intensity.
To explain these observations, we invoke a model in which the viscosity is very
close to critical viscosity and the shock wave is weak or absent.Comment: Replaced with correct figures, Jour. of Astrophysics and Astronomy
(accepted
Dust in an extremely metal-poor galaxy: mid-infrared observations of SBS 0335-052
The metal deficient (Z = Z_sun/41) Blue Compact Dwarf Galaxy (BCD) SBS
0335-052 was observed with ISOCAM between 5 and 17 mic. With a L_12mic/L_B
ratio of 2.15, the galaxy is unexpectedly bright in the mid-infrared for such a
low-metallicity object. The mid-infrared spectrum shows no sign of the
Unidentified Infrared Bands, which we interpret as an effect of the destruction
of their carriers by the very high UV energy density in SBS 0335-052. The
spectral energy distribution (SED) is dominated by a very strong continuum
which makes the ionic lines of [SIV] and [NeIII] very weak. From 5 to 17 mic,
the SED can be fitted with a grey-body spectrum, modified by an extinction law
similar to that observed toward the Galactic Center, with an optical depth of
A_V~19-21 mag. Such a large optical depth implies that a large fraction (as
much as ~ 75%) of the current star-formation activity in SBS 0335-052 is hidden
by dust with a mass between 3x10^3 M_sun and 5x10^5 M_sun. Silicate grains are
present as silicate extinction bands at 9.7 and 18 mic can account for the
unusual shape of the MIR spectrum of SBS 0335-052. It is remarkable that such a
nearly primordial environment contains as much dust as galaxies which are 10
times more metal-rich. If the hidden star formation in SBS 0335-052 is typical
of young galaxies at high redshifts, then the cosmic star formation rate
derived from UV/optical fluxes would be underestimated.Comment: 13 pages, 4 figures, requires aaspp4.sty, accepted in Ap
A smoother end to the dark ages
Independent lines of evidence suggest that the first stars, which ended the
cosmic dark ages, came in pairs, rather than singly. This could change the
prevailing view that the early Universe had a Swiss-cheese-like appearance.Comment: Nature News and Views, April 7, 201
On the effects of organic matter and sulphur-containing compounds on the CCN activation of combustion particles
The European PartEmis project (Measurement and prediction of emissions of aerosols and gaseous precursors from gas turbine engines) was focussed on the characterisation and quantification of exhaust emissions from a gas turbine engine. The combustion aerosol characterisation included on-line measurements of mass and number concentration, size distribution, mixing state, thermal stability of internally mixed particles, hygroscopicity, cloud condensation nuclei (CCN) activation potential, and off-line analysis of chemical composition. Based on this extensive data set, the role of sulphuric acid coating and of the organic fraction of the combustion particles for the CCN activation was investigated. Modelling of CCN activation was conducted using microphysical and chemical properties obtained from the measurements as input data. Coating the combustion particles with water-soluble sulphuric acid, increases the potential CCN activation, or lowers the activation diameter, respectively. The adaptation of a Köhler model to the experimental data yielded coatings from 0.1 to 3 vol-% of water-soluble matter, which corresponds to an increase in the fraction of CCN-activated combustion particles from ≤10<sup>−4</sup> to ≌10<sup>−2</sup> at a water vapour saturation ratio S<sub>w</sub>=1.006. Additional particle coating by coagulation of combustion particles and aqueous sulphuric acid particles formed by nucleation further reduces the CCN activation diameter. In contrast, particles containing a large fraction of non-volatile organic compounds grow significantly less at high relative humidity than particles with a lower content of non-volatile OC. The resulting reduction in the potential CCN activation with an increasing fraction of non-volatile OC becomes visible as a trend in the experimental data. While a coating of water-soluble sulphuric acid increases the potential CCN activation, or lowers the activation diameter, respectively, the non-volatile organic compounds, mainly found at lower combustion temperatures, can partially compensate this sulphuric acid-related enhancement of CCN activation of carbonaceous combustion aerosol particles
On the effects of hydrocarbon and sulphur-containing compounds on the CCN activation of combustion particles
International audienceThe European PartEmis project (''Measurement and prediction of emissions of aerosols and gaseous precursors from gas turbine engines'') was focussed on the characterisation and quantification of exhaust emissions from a gas turbine engine. A comprehensive suite of aerosol, gas and chemi-ion measurements were conducted under different combustor operating conditions and fuel sulphur concentrations. Combustion aerosol characterisation included on-line measurements of mass and number concentration, size distribution, mixing state, thermal stability of internally mixed particles, hygroscopicity, cloud condensation nuclei (CCN) activation potential, and off-line analysis of chemical composition. Modelling of CCN activation of combustion particles was conducted using microphysical and chemical properties obtained from the measurements as input data. Based on this unique data set, the role of sulphuric acid coatings on the combustion particles, formed in the cooling exhaust plume through either direct condensation of gaseous sulphuric acid or coagulation with volatile condensation particles nucleating from gaseous sulphuric acid, and the role of the organic fraction for the CCN activation of combustion particles was investigated. It was found that particles containing a large fraction of non-volatile organic compounds grow significantly less at high relative humidity than particles with a lower content of non-volatile OC. Also the effect of the non-volatile OC fraction on the potential CCN activation is significant. While a coating of water-soluble sulphuric acid increases the potential CCN activation, or lowers the activation diameter, respectively, the non-volatile organic compounds, mainly found at lower combustion temperatures, can partially compensate this sulphuric acid-related enhancement of CCN activation of carbonaceous combustion aerosol particles
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