155 research outputs found
Detector Assessment for 1D Single-Shot Spontaneous Raman Scattering for Temperature and Multi-Species Measurements in Flames
International audienceThe critical aspect of 1D single-shot Spontaneous Raman Scattering (SRS) experiments in flames is the requirement of high efficiency of the detection system associated with a fast temporal gating. Single-shot SRS measurements in flames are performed either with ICCD or with back-illuminated CCDs associated with a fast shutter. Two types of back-illuminated CDD detectors are used: a back-illuminated CCD (BI-CCD) and electron multiplying CCD (BI-EMCCD). The purpose of the present paper is to compare the three detectors: the ICCD with its intensifier gating and the back-illuminated CCDs with a Pockels cell shutter developed in a previous work (Ajrouche et al, 2015). The accuracy and uncertainty of 1D single-shot SRS measurements of temperature and density are quantified in near-adiabatic CH4/air flames. This is performed for N 2 number density (high signal levels), and CO number density (low signal levels) corresponding to signal close to the detectability limit. The temperature is determined by modeling the vibration-rotation spectra of N 2 offering advantage of not requiring reference temperature and the modeled spectra are used as smoothed spectra to determine the instantaneous number densities. On one hand, the BI-CCD with the Pockels cell shutter is the most efficient detection systems in extreme low light situations for single-shot temperature measurements, and on the other hand the BI-EMCCD is the most powerful tool for best detectability of low density species
Model Equation for the Dynamics of Wrinkled Shockwaves: Comparison with DNS and Experiments
International audienceA model equation for the dynamics and the geometry of the wrinkled front of shock waves, obtained for strong shocks in the Newtonian limit, is tested by comparison with direct numerical simulations and a shock tube experiment
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Alkali metal emissions in early stage of a pulverized-coal flame: DNS analysis of reacting layers and chemistry tabulation
The intricate coupling between coal pyrolysis, gas phase combustion and the emissions
of alkali metal, such as sodium, is studied in early stage of a temporally
evolving three-dimensional planar turbulent jet carrying pulverized-coal particles.
Complex chemistry is used to account for both the combustion of volatile hydrocarbons
and the sodium containing species. The response of the sodium chemistry
is analyzed in the mixture fraction space, along with the topology of the reactions
zones. Combustion is found to start preferentially in partially premixed flames,
which then evolve toward di usion-like reactive layers and reach chemical equilibrium.
From the direct numerical simulation (DNS) database, the possibility
of modeling the dynamics of sodium species using one-dimensional premixed
flamelet generated manifolds (FGM) is investigated. A chemical lookup table
is constructed for the combustion of the partially premixed volatiles and an additional
three-dimensional simulation is performed to compare the tabulated sodium
species against their reference counterparts with complex chemistry. Quantitative
analysis of the performance of the developed chemistry tabulation confirms the
validity of the approach. Perspectives for the modeling of sodium emissions in
pulverized-coal furnaces and boilers are finally drawn.National Natural Science Foundation of China and China Postdoctoral Science Foundatio
Étude de l'interaction d'une décharge électrique avec un écoulement laminaire par diagnostics optiques
International audienceLes technologies récentes mises au point en moteur à allumage commandé et injection directe essence sont tournées vers le downsizing et la combustion diluée par EGR. Ces modes de combustion sont prometteurs pour la réduction des émissions CO2 mais restreignent fortement la stabilité de l'allumage, particulièrement pour les mélanges pauvres. L'allumage par décharge électrique présente un fort potentiel pour répondre à ces contraintes, à condition de maîtriser les couplages existants entre la décharge plasma et les mécanismes d'initiation de la combustion. Une caractérisation des propriétés physico-chimiques de la décharge ainsi que l'étude de la topologie de l'arc et de l'écoulement environnant sont réalisées par diagnostics optiques. La décharge électrique est générée par une bobine crayon (Marque Audi) entre deux électrodes placées en configuration pointes-pointes et écartées de 3 mm dans l'air à pression atmosphérique. Les évolutions temporelles du courant et de la tension délivrés aux électrodes sont mesurées pour évaluer l'énergie maximum qui peut être déposée dans le gaz. Les résultats obtenus apportent des éléments de compréhension des mécanismes de couplage entre une décharge électrique et l'écoulement mais constituent également une base de données pour la validation de simulation numérique
Numerical study of HCl and SO2 impact on potassium emissions in pulverized-biomass combustion
National Natural Science Foundation of China (51706200), the China Postdoctoral Science Foundation (2018M632460), the Fundamental Research Funds for the Central Universities (2018FZA4012), the Engineering and Physical Sciences Research Council (EPSRC) and the Royal Society of the UK
Acoustical Excitation for Damping Estimation in Rotating Machinery
In experimental modal analysis a structure is excited with a force in order to estimate the frequency response function. Typically, this force is generated by a shaker or a hammer impact. Both methods have proven their usefulness, but have some well-known disadvantages. A main disadvantage of the shaker is that it has to be fixed to the structure whereas with a hammer it is not possible to excite a specific frequency. To overcome these disadvantages, alternative non-contact methods can be used. There are several non-contact techniques, i.e. pressurized air, laser, acoustics, etc. By using acoustics as an excitation technique it is easy to select an excitation signal going from random noise to a simple sine. Also the equipment to produce the acoustic excitation is rather cheap. However, the state of the art does not offer a straightforward technique to estimate the excitation force, making it difficult for applications such as experimental modal analysis. In this research, acoustic excitation is compared with hammer excitation to estimate the frequency response function of two shafts. Especially a method to validate the force induced by the acoustics is derived. The final purpose of this research is to estimate the damping properties of rotating machinery
Automated EEG background analysis to identify neonates with hypoxic-ischemic encephalopathy treated with hypothermia at risk for adverse outcome: A pilot study
Background: To improve the objective assessment of continuous video-EEG (cEEG) monitoring of neonatal brain function, the aim was to relate automated derived amplitude and duration parameters of the suppressed periods in the EEG background (dynamic Interburst Interval= dIBIs) after neonatal hypoxic-ischemic encephalopathy (HIE) to favourable or adverse neurodevelopmental outcome. Methods: Nineteen neonates (gestational age 36-41 weeks) with HIE underwent therapeutic hypothermia and had cEEG-monitoring. EEGs were retrospectively analyzed with a previously developed algorithm to detect the dynamic Interburst Intervals. Median duration and amplitude of the dIBIs were calculated at 1h-intervals. Sensitivity and specificity of automated EEG background grading for favorable and adverse outcomes were assessed at 6h-intervals. Results: Dynamic IBI values reached the best prognostic value between 18 and 24h (AUC of 0.93). EEGs with dIBI amplitude ≥15 μV and duration 10s were specific for adverse outcome (89-100%) at 18-24h (n = 10). Extremely low voltage and invariant EEG patterns were indicative of adverse outcome at all time points. Conclusions: Automated analysis of the suppressed periods in EEG of neonates with HIE undergoing TH provides objective and early prognostic information. This objective tool can be used in a multimodal strategy for outcome assessment. Implementation of this method can facilitate clinical practice, improve risk stratification and aid therapeutic decision-making. A multicenter trial with a quantifiable outcome measure is warranted to confirm the predictive value of this method in a more heterogeneous dataset
A Priori Testing of Flamelet Generated Manifolds for Turbulent Partially Premixed Methane/Air Flames
Regional Sanctions Against Burundi: A Powerful Campaign and Its Unintended Consequences
This paper examines the impact of regionally imposed sanctions on the trajectory of the Burundian regime and its involvement in the peace process following the 1996 coup in the country. Despite the country's socioeconomic and geopolitical vulnerability, the Buyoya government withstood the pressure from the sanctions. Through a vocal campaign against these sanctions, the new government mitigated the embargo's economic consequences and partially reestablished its international reputation. Paradoxically, this campaign planted the seed for comprehensive political concessions in the long term. While previous literature has attributed the sanctions' success in pressuring the government into negotiations to their economic impact, the government actually responded to the sanction senders' key demand to engage in unconditional, inclusive peace talks under the auspices of the regional mediator once the economy had already started to recover. The regime's anti-sanctions campaign, with its emphasis on the government's willingness to engage in peace talks, backfired, with Buyoya forced to negotiate after having become entrapped in his own rhetoric
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