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Jansky J.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Bourdon A.,École Centrale Paris
Applied Physics Letters | Year: 2011

Simulations of the influence of electrode geometries on helium discharge ignition and dynamics in thin dielectric tubes are presented. In all studied cases, as observed in experiments, the discharge ignition occurs at the outer edges of the high voltage ring and the influence of the width of the grounded ring on the discharge dynamics is shown. Taking into account the change of permittivity encountered by the discharge as it exits from the tube, the velocity of the discharge front is shown to increase at the tube exit before decreasing downstream similarly to experimental observations. © 2011 American Institute of Physics.

Zimmer L.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Zimmer L.,École Centrale Paris | Yoshida S.,Japan Aerospace Exploration Agency
Experiments in Fluids | Year: 2012

In this paper, experimental results obtained with laser-induced plasma spectroscopy to retrieve local compositions are presented for an ambient pressure up to 5.0 MPa in a still cell. Well-controlled mixtures of gases are introduced and plasma is obtained with the fundamental emission of a pulsed Nd:YAG laser. Simultaneously, plasma shape and spectrally resolved data are taken with a temporal resolution down to 2 ns. First, the temporal evolutions of a high-pressure nitrogen plasma are analyzed as function of spark energy. It is shown that plasma changes orientation from an elongated shape parallel to the laser line to a perpendicular one in a very short time. Results are reported for both spatial and spectral variations. Afterward, the effects of increased carbon concentration are discussed in both shape and spectra. It is seen that strong intensity due to the atomic carbon emissions appear for the high-pressure case. From those experiments, calibration strategies are proposed to get equivalence ratio under high-pressure conditions with a ratio of carbon versus nitrogen and oxygen. The delay between plasma and measurements is set to 2,000 ns and the signal is integrated for 5,000 ns, so as to yield a good signal to noise ratio and a good sensitivity of the technique to changes in mixture fraction. Calibration curves are reported for equivalence ratio up to 1.00 and for pressure from 1.0 to 5.0 MPa. It is shown that typical uncertainties are limited to 7.5% regardless the equivalence ratio in a single shot approach using a spectral fit procedure, whereas it accounts to two times more in a more classical peak ratio approach. Increasing the pressure tends to increase the precision as lower pressure had higher uncertainties. © Springer-Verlag 2011.

Riviere P.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Riviere P.,École Centrale Paris | Soufiani A.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Soufiani A.,École Centrale Paris
International Journal of Heat and Mass Transfer | Year: 2012

Statistical narrow-band (SNB) model parameters for H 2O, CO 2, CH 4 and CO, and correlated-k (CK) parameters for H 2O and CO 2 are generated from line by line calculations and recently improved spectroscopic databases in wide temperature and spectral ranges. Results from the new parameters are compared to direct line by line calculations and to results from earlier model parameters [A. Soufiani, J. Taine, High temperature gas radiative property parameters of statistical narrow-band model for H 2O, CO 2 and CO and correlated-k (ck) model for H 2O and CO 2, Int. J. Heat Mass Transfer 40 (1997) 987-991] in terms of band averaged spectral transmissivities, Planck mean absorption coefficients, and total emissivities. The comparisons show first a good agreement between updated SNB, CK and LBL results. Significant improvements on earlier parameters are observed for H 2O and CO 2, especially at very high temperatures and path lengths. Model parameters and computer programs illustrating their implementation are provided as Supplementary data. © 2012 Elsevier Ltd. All rights reserved.

Jalabert L.,University of Tokyo | Sato T.,University of Tokyo | Ishida T.,University of Tokyo | Fujita H.,University of Tokyo | And 5 more authors.
Nano Letters | Year: 2012

The thermal conductance of a single silicon nanojunction was measured based on a Lab-in-a-TEM (microelectromechanical systems in a transmission electron microscope) technique and was found to be at least 2 orders of magnitude larger than the ones of long nanowires in the 380-460 K temperature range. The predominance of ballistic phonon transport appears as the best hypothesis to retrieve quantitative predictions despite the geometrical irregularity of the junction. The measurement is based on a MEMS structure including an electrostatic actuator that allows producing nanojunctions with the accuracy based on the resolution of a transmission electron microscope. The thermal conductance is measured by two integrated resistors that are simultaneously heating and measuring the local temperatures at the nearest of the nanojunction. The considerable thermal conductance of short nanojunctions constitutes a new key element in the design of nanosystems and in the understanding of the damaging of mechanical micronanocontacts. This conducting behavior is also paving the way for the development of nanoscale cooling devices as well as of the recent phononic information technology. © 2012 American Chemical Society.

Coussement A.,Roosevelt University | Coussement A.,École Centrale Paris | Coussement A.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Gicquel O.,École Centrale Paris | And 2 more authors.
Proceedings of the Combustion Institute | Year: 2013

Chemistry tabulation techniques such as flamelet models are a popular way to account for detailed chemistry effects in numerical simulation. These techniques are based on the identification of a low dimensional manifold in chemical space that accurately represents chemical evolutions associated to a specific combustion regime. During the last years, several authors used the Principal Component Analysis (PCA) to identify low dimensional manifold for combustion problems. However, full coupling between this manifold and flow solver has not yet been performed to the authors knowledge. The present paper introduces a new approach called Manifold Generated by a Local PCA or MG-L-PCA, which fully couple the manifold identified by a PCA and a DNS flow solver. The first part of the paper presents the PCA approach. Then, the coupling between this manifold and a DNS solver is presented. The MG-L-PCA approach is finally validated against a DNS simulation of flame vortex interaction using both detailed mechanism and a FPI manifold. Unlike FPI, the MG-L-PCA reproduces the dispersion in the chemical space induced by the flame-vortex interaction both for the species and the source terms. © 2012 The Combustion Institute.

Cuquel A.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Cuquel A.,École Centrale Paris | Durox D.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Schuller T.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory
Proceedings of the Combustion Institute | Year: 2013

The influence of confinement side walls on the response of premixed conical flames submitted to velocity disturbances is investigated experimentally and theoretically. When the flame is sufficiently confined, the burnt gases cannot fully expand at the nozzle outlet. In that case, the confinement ratio Cr = R0/R1 between the burner and flame tube radii and the burnt to unburnt gas volume expansion ratio E = u/b need to be taken into account in the description of the flame transfer function (FTF). The main effect of confinement is an acceleration of the fresh stream of reactants induced by the over-pressure of the confined burnt gases. Experiments on steady flames reveal that the flame height increases for increasing confinement ratios C r, leading to a gain shift and a phase drop for the FTF. A theoretical analysis is conducted to model this acceleration and examine its impact on the steady flame shape and flame response to flow disturbances. The change in the FTF phase is shown to be related to a reduction of the mean time lag between heat release rate perturbations and velocity modulations induced by both the velocity acceleration in the fresh reactants and a change in the location of the steady flame front. An expression to scale the FTF of confined flames is derived based on a modification of the classical reduced frequency x used to scale the response of unconfined flames. This expression includes explicitly the confinement ratio Cr and the volume expansion ratio E. This new dimensionless number enables to plot FTF measured for different confinements with reasonable collapse. It may also be used to transpose FTF gathered on single burners to multiple injection configurations when the burnt gases cannot fully expand due to the presence of neighboring flames. © 2012 The Combustion Institute.

Mirat C.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Durox D.,École Centrale Paris | Schuller T.,École Centrale Paris
Proceedings of the Combustion Institute | Year: 2015

Self-sustained combustion oscillations observed at limit cycles in a swirl combustor equipped with a series of steam assisted liquid fuel injectors are analyzed with the flame describing function (FDF) framework. It is first shown that for globally fuel lean injection conditions, the spray flames investigated burn in a non-premixed mode. A non-sooty and a sooty regime are explored. Their frequency response to acoustic forcing is characterized by exploiting the OH∗ chemiluminescence signal and a velocity signal measured by Laser Doppler velocimetry at the air injection unit outlet. It is found that the FDF of these non-premixed swirl spray flames features a distinct response compared to the FDF of lean premixed swirl flames. The gain and phase lag of these FDF are both a strong function of the perturbation amplitude. At a fixed forcing frequency, the FDF phase lag increases with the perturbation level for the non-sooty flame investigated and it decreases for the sooty flame case. The sooty flame has also a much lower cut-off frequency for the FDF gain than the non-sooty flame. It is then shown that these features are essential to reproduce the correct instability bands and oscillation frequencies observed at limit cycles in the experiments. For the sooty flame, combustion is always found stable in agreement with the low cut-off frequency found for the FDF. A linear analysis for the non-sooty flame fails to capture the low frequency instabilities observed in the experiments that have the largest oscillation levels. Only the use of the FDF yields the correct dynamical states observed in the combustor. It is further concluded that the OH∗ chemiluminescence signal may safely be used to infer the frequency response of these non-premixed swirl spray flames at globally fuel lean injection conditions. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Renaud A.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Ducruix S.,École Centrale Paris | Scouflaire P.,École Centrale Paris | Zimmer L.,École Centrale Paris
Proceedings of the Combustion Institute | Year: 2015

A laboratory-scale two-stage swirling burner fueled with dodecane is studied experimentally with the help of high speed (20 kHz) spray PIV and chemiluminescence imaging. For a lean operating point at 73 kW, fuel staging is changed and a hysteresis cycle is highlighted. The flame is first lifted from the injector exit plane and exhibits a strong thermo-acoustic instability. The transition to a more stable state is studied by changing the staging parameter while keeping the power constant. Thanks to a well-controlled environment, several runs are performed to ensure that the phenomena observed during these transitions are reproducible. In less than 10 ms, flashback happens in the inner recirculation zone and the flame attaches itself to the injector in a tulip-like shape. The spray then presents a wider angle. Thermo-acoustic instabilities are barely detected and the flame exhibits a strong response to the precessing vortex core. The flame finally widens into a more common V-shaped flame, either in less than 50 ms (for half of the experiments) or after more than 500 ms. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Xiong S.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Xiong S.,École Centrale Paris | Ma J.,University of Minnesota | Volz S.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | And 2 more authors.
Small | Year: 2014

Dislocations significantly influence the physical properties of nanomaterials. Nonequilibrium molecular dynamics simulations uncover significant reductions in thermal conductivity when <110> Si nanowires contain axial screw dislocations. The effect can act in combination with other known thermal conductivity limiting mechanisms, and thus can enable the further optimization of the figure of merit for a new family of complex thermoelectric nanomaterials. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Gomart H.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Taine J.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2011

For radiative heat transfer applications, in particular in homogenized phases of porous media, an exhaustive and accurate validity criterion of the radiative Fourier law, depending only on the logarithmic derivative of the temperature field and an effective absorption coefficient, accounting for possible multiple scattering phenomena, has been established for a semitransparent medium. This effective absorption coefficient is expressed as a function of the absorption coefficient, the albedo, and the scattering asymmetry parameter. The criterion can be applied to semitransparent media that do not follow Beer's laws related to extinction, absorption, and scattering. © 2011 American Physical Society.

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