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Zimmer L.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Zimmer L.,Ecole 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.

Jansky J.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Bourdon A.,Ecole 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.

Cuquel A.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Cuquel A.,Ecole 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.

Renaud A.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Ducruix S.,Ecole Centrale Paris | Scouflaire P.,Ecole Centrale Paris | Zimmer L.,Ecole 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.

Riviere P.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Riviere P.,Ecole Centrale Paris | Soufiani A.,CNRS Molecular and Macroscopic Energy & Combustion Laboratory | Soufiani A.,Ecole 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.

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