A new highly sensitive cavity ring-down spectrometer (CRDS) system was designed and assembled to determine the aerosol extinction coefficient (b(ext)) at 532 nm. The performance of the CRDS was tested by the monodisperse polystyrene latex spheres (PSL) particles with diameters between 200 and 500 nm. By comparing the tested results with Mie theory curve, the uncertainty of the newly developed CRDS system was determined to be < 3%. Simultaneous measurements of the size distribution and extinction coefficient of ambient aerosols were conducted in Orleans, France, from 26th October to 21st December 2012 by using a scanning mobility particle sizer (SMPS) coupled to the CRDS system. For the non-dehydrated aerosols measured from 26th October to 4th November, the average b(ext) has been found to be 41 +/- 35 Mm(-1). For the dehydrated aerosols measured from 7th November to 21st December, b(ext) expresses a good agreement with the particle number (N) and volume (V) concentration, the average values of b(ext), N and V are 36 +/- 31 Mm(-1), 3300 +/- 2700 cm(-3) and 3.1 +/- 2.8 x 10(9) nm(3)/cm(3), respectively. Further analysis of the particle size distribution reveals that car and boiler emissions maybe the main aerosol sources in Orleans. In addition, back trajectory results indicate that the air parcel transported from Atlantic Ocean may play a role in cleaning up the ambient air in Orleans.

Flickering is an oscillating instability of a diffusion flame. An experimental study has been performed on two different diffusion flame of methane and ethylene to elucidate flickering instability and controlling it by the mean of a mechanical stabilizer. Flame imaging and temporal evolution of transmitted signal methods have been employed to monitor the flame behavior and measure flickering frequency. Three different regimes of stability are identified: stable, transient, and full unstable. The measured flickering frequency of ethylene flame is 11 Hz and it is demonstrated to be independent of fuel jet velocity. The calculated vortex mean velocity is 500 mm/s and 800 mm/s without and with "pinch-off" formation respectively and the "pinch-off" velocity is around 1450 mm/s. Moreover, the ability of a mechanical actuator to stabilize the flame has been demonstrated for methane and ethylene. The experiment reveals the existence of a limit of stabilizing height at each flowrate. However, positioning stabilizer below the burner shows a limited effect. Numerical simulation shows a smoother effect of stabilizer on air streamlines around the flame limiting the action of vortex on the flame. These results prove that flickering instability could be suppressed by only controlling the air surrounded the flame.

Electric fields were proved to affect combustion stability, flame form, and pollutant emissions. The influence of the electric field on soot formation and growth is still an open topic. In this context, the effect of an external electric field on a 67 mm laminar diffusion flame was studied and ethylene was used as well-suited fuel to investigate soot formation and growth. The flame was placed under a uniform high voltage difference and laser Extinction/Scattering techniques were employed to monitor soot volume fraction, particles? diameter and particles? number concentration at different locations in the flame body. Interestingly, when a potential of 8 kV was applied, the flame appeared to be shorter and larger, soot volume fraction diminished whilst the particles? diameter increased under the effect of the electric field. The ionic wind is considered the main cause of the geometrical modifications of the flame due to the increase in the burning rate and the acceleration of charged species. It was, besides the action of the electric field on the growth mechanism, the plausible source for the soot formation variation in the flame.

An innovative technique was developed for the direct measurement of the absolute radiant flux emitted from transient flames. The design of the experimental device, called FAIRS (Fast Absolute Infra-Red Sensor), is detailed in this work. The main concept of FAIRS is based on the combination of a carbon nano-tube-based black body as a sensitive element, coupled to a fast IR HgCdTe detector via an achromatic optical setup. A specific calibration protocol based on a laboratory blackbody (ambient to 850 °C) allows the qualification of the FAIRS for absolute radiative heat-flux measurements, with a response time less than 1 µs that was checked, thanks to pulsed laser irradiation. It is thus demonstrated that FAIRS is a good candidate for transient measurements, with a simplified calibration procedure. FAIRS was coupled with ultra-fast schlieren imaging on spherical expanding CH4/air and C3H8/air flames. In this condition, it is possible to correlate the real time flame diameter to its absolute radiative heat losses.

Abstract. Rate coefficients for the reaction of NO3 radicals with a series of aromatic aldehydes were measured in a 7300 liter simulation chamber at ambient temperature and pressure by relative and absolute methods. The rate coefficients for benzaldehyde (BA), ortho-tolualdehyde (O-TA), meta-tolualdehyde (M-TA), para-tolualdehyde (P-TA), 2,4-dimethyl benzaldehyde (2,4-DMBA), 2,5-dimethyl benzaldehyde (2,5-DMBA) and 3,5-dimethyl benzaldehyde (3,5-DMBA) were: k1 = 2.6 ± 0.3, k2 = 8.8 ± 0.8, k3 = 4.8 ± 0.5, k4 = 4.9 ± 0.5, k5 = 15.1 ± 1.4, k6 = 12.7 ± 1.2 and k7 = 6.2 ± 0.6, respectively, in the units of 10−15 cm3 molecule−1 s−1 at 298 ± 2 K. The rate coefficient k13 for the reaction of the NO3 radical with deuterated benzaldehyde (benzaldehyde-d1) was found to be half that of k1. The end product of the reaction with an excess of NOx was measured to be C6H5C(O)O2NO2. Theoretical calculations of aldehydic bond energies and reaction pathways indicate that NO3 radical reacts with aromatic aldehydes through the abstraction of aldehydic hydrogen atom. The atmospheric implications of the measured rate coefficients are briefly discussed.

An applied DC electric field was experimentally demonstrated to modify the flame structure and gas dynamic in an ethylene diffusion flame. The aim of this paper is to investigate the influence of the electric field on the flow field and its impacts on the flame behavior. A numerical study has been performed to elucidate the experimental observations and to monitor the effect of electric body force on the flame. The numerical model was validated by comparing the computed results to experimental measurements from the literature. The resulting computed flame shape was compared to a visible image taken during the experiment. The simulated OH mole fraction, the burning rate and the computed velocity and temperature are presented. The developed model proved the ability to reproduce qualitatively the experimental flame behavior when submitted to the electric field. The electric field is shown to modify the flame shape (flame tip, flame shortness and flame deformation), to promote the burning process and to improve the ion production. Results show that the modifications are due to an air entrainment acting specifically near the burner zone enhancing the mixture and changing the fluid dynamic in this region. The ionic wind is demonstrated to increase the maximum burning rate and promoting ions’ formation mostly near the burner. A more detailed model (detailed ions’ chemistry and soot model with charged particles, detailed electric diffusion) is necessary to gain a better understanding of the influence of electric field on diffusion combustion and soot formation.

The effect of silane addition on the laminar flame speed (Su0) of flames propagating in hydrogen–nitrous oxide–argon mixtures has been investigated experimentally for the first time using the spherically expanding flame technique in a constant volume combustion chamber. Replacing hydrogen by silane and maintaining the equivalence ratio constant, much higher flame speeds, explosion peak pressures, and pressure rise coefficients were measured. A previously developed detailed reaction model has been updated based on ab initio thermodynamic properties calculations and collision limit violation analysis. The improved reaction model demonstrates encouraging performance in predicting the flame speed, with a mean absolute error below 11%. To explain the effect of silane addition on the flame dynamics, a number of parameters have been calculated including OH and H rate of production, heat release rate per reaction, and sensitivity coefficient on Su0. The dynamics of freely propagating flames in SiH4–H2–N2O–Ar mixtures is essentially controlled by reactions of the H–O–N chemical system: N2O+H=N2+OH, OH+H2=H2O+H, and N2O(+M)=N2+O(+M). Whereas silane addition does not influence much the rate of production of OH, it significantly modifies that of H with a number of pyrolytic chemical pathways of silicon hydrides, such as SiH+H2=SiH2+H and Si+H2=SiH+H, which act as sink of H atom as they proceed in the backward direction. The reactions forming SiO(s) and SiO2(s), such as SiO+OH=SiO2(s)+H and 2SiO=2SiO(s), are exothermic and significantly contribute to the temperature increase. The adiabatic, constant pressure flame temperature for mixture containing silane is significantly higher, up to several 100’s K. The increase of Su0 induced by silane addition seems to be mostly related to the large increase of the flame temperature which leads to higher energy release rate.

This paper presents an experimental and modelling study of NO formation in high pressure premixed flames. Experiments were performed in a high-pressure counterflow burner in which laminar premixed CH4/air flames were stabilised at equivalence ratios of E.R = 0.7, 1 and 1.2 and for pressures varying from 0.1 to 0.7 MPa. We report quantitative NO mole fraction profiles measured by Laser Induced Fluorescence. The effects of pressure and equivalence ratio on NO formation are discussed. These results are compared to the simulations using two reaction mechanisms: NOmecha2.0 associated to a detailed mechanism for methane oxidation: GDFkin (R) 3.0 and the mechanism from Klippenstein a al., which is the most recent high-pressure NOx formation mechanism available in the literature. In general, both mechanisms are able to predict NO correctly in lean and stoichio-metric high pressure flames; however, in rich flames, GDFkin (R) 3.0_NOmecha2.0 gives the best predictions. The performances of these mechanisms are also tested on NO measurements in high-pressure flames from the literature. A kinetic analysis is then presented to identify the main pathways that lead to the formation and consumption of NO and highlight the differences between the two mechanisms, as well as a sensitivity analysis to identify important reactions that influence the formation/consumption of NO in our high pressure flames.

Combustion-generated soot which have a significant impact on climate change and human health is dependent on its size and morphology [1,2]. That’s why a large part of research in the combustion domain has been dedicated to soot formation and reduction processes. Application of electric fields during combustion has shown a potential effect in reducing soot production [3]; it has been studied for many years and the ionic wind constitutes the main explanation of the observed effects on the flame structure and pollutant emissions. The interaction mechanisms of an electric field with a diffusion flame involve both ionic wind and direct action on charged particles. Here, the effects of a dc electric field on the soot characteristics of a laminar 65 mm-long ethylene diffusion flame burning in an air co-flow are investigated.

A laboratory-scale laminar counterflow burner was used to investigate NO formation in high pressure premixed CH4/H-2/air flames. New experimental results on NO measurements by LIF were obtained at high pressure in CH4/H-2/air flames with H-2 content fixed at 20% in the fuel at pressures ranging from 0.1 to 0.7 MPa and an equivalence ratio progressively decreased from 0.74 to 0.6. The effects of hydrogen addition, equivalence ratio and pressure are discussed. These results are satisfactorily compared to the simulations using two detailed mechanisms: GDFkin (R) 3.0_NOmecha2.0 and the mechanism from Klippenstein et al., which are the most recent high-pressure NOx formation mechanisms available in the literature. A kinetic analysis based on Rate of Production/Rate of Con- sumption and sensitivity analyses of NO is then presented to identify the main pathways that lead to the formation and consumption of NO. In addition, the effect of hydrogen addition on NO formation pathways is described and analysed. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Influence of electric fields on flames has been studied for many years and the ionic wind constitutes the main explanation of the observed effects on the flame structure and pollutant emissions. However, previous works have been limited to small flames. The interaction mechanisms of an electric field with longer flames, involving both ionic wind and buoyancy are not fully identified. In the present paper, the effects of a D.C. electric field on a laminar 88-mm-long ethylene diffusion flame burning in ambient air are investigated. Based on the calculated electric field configuration, the influence of both downward and upward electric field is compared via imaging, electrical diagnostic and soot measurements. The application of a negative (directed downstream) electric field triggers a flickering instability and an electric instability at higher field strength, in which self-sustained flame oscillations of flame length directly affect ion current. Conversely, the flame is stabilized by a positive electric field. In-situ soot volume fraction measurements show that the electric field decreases the average soot volume fraction measured on a stable flame axis, whereas flame oscillations lead to a sooting flame.

Atmospheric particle is one of the major air pollutants, and believed to be important for air quality, radiative forcing and climate. Measurements of aerosol optical properties, size distribution and PM10 concentration were conducted at Orleans, central France during spring (7 March to 25 April) and autumn (25 October to 5 December) 2013. The average values of aerosol scattering coefficient (b(sca)), absorption coefficient (b(abs)), single scattering albedo (SSA) at 532 nm and PM10 concentration are 54.9 +/- 58.2 Mm(-1), 10.6 +/- 10.9 Mm(-1), 0.81 +/- 0.10 and 30.6 +/- 21.6 mu g/m(3) for the spring campaign, and 35.4 +/- 36.7 Mm(-1), 3.9 +/- 4.4 Mm(-1), 0.83 +/- 0.13 and 17.4 +/- 11.8 mu g/m(3) for the autumn campaign, respectively. During the whole observation, the air parcel transported from Atlantic Ocean plays a role in cleaning up the ambient air in Orleans, while the air mass coining from the Eastern Europe induces the pollution events in Orleans. In this study, a simple approach, which based on the diurnal variation of PM10 concentration, Boundary layer depth (BLD) and the human activity factor derived from anthropogenic emission rate, was introduced to estimate the contribution of secondary aerosol to ambient aerosols. Our results show that secondary particles formation trigged by photochemical reactions and oxidations can contribute maximum of 64% and 32% for PM10 mass concentration during the spring and autumn time, respectively. These results highlight that photochemical reactions can enhance the atmospheric oxidation capacity and may faster the secondary particle formation and then play an important role in air quality. (C) 2018 Elsevier B.V. All rights reserved.

The rate coefficient for the possible reaction of OH radical with N2O was determined to be k1 < 1 × 10-17 cm3 molecule-1 s-1 between 253 and 372 K using pulsed laser photolysis to generate OH radicals and pulsed laser induced fluorescence to detect them. The rate coefficient for the reaction of NO3 radical with N2O was measured to be k2 < 5 × 10-20 cm3 molecule-1 s-1 at 298 K using a direct method that involves a large reaction chamber equipped with cavity ring down spectroscopic detection of NO3 and N2O5. Various tests were carried out ensure the accuracy of our measurements. Based on our measured upper limits, we suggest that these two reactions alter the atmospheric lifetime of N2O of ∼120 years by less than 4%.

The rate coefficients for the reactions of NO3 radicals with methane (CH4), ethane (C2H6), propane (C3H8), n-butane (n-C4H10), iso-butane (iso-C4H10), 2,3-dimethylbutane (C6H14), cyclopentane (C5H10) and cyclohexane (C6H12) at atmosphere pressure (1000 ± 5 hPa) and room temperature (298 ± 1.5 K) were measured using an absolute method. Careful attention was paid to the role of secondary reactions and impurities. The upper limits of rate coefficients for methane and ethane at 298 K are <4 × 10-20 and <5 × 10-19 cm3 molecule-1 s-1, respectively. The rate coefficients at 298 K for propane, n-butane, iso-butane, 2,3-dimethybutane, cyclopentane and cyclohexane are, (9.2 ± 2.9) × 10-18, (1.5 ± 0.4) × 10-17, (8.2 ± 2.2) × 10-17, (5.8 ± 2.4) × 10-16, (1.5 ± 0.6) × 10-16 and (1.3 ± 0.4) × 10-16 cm3 molecule-1 s-1, respectively. Rate coefficients for the reactions of NO3 radical with two deuterated n-butanes (butane-D10 and butane-1,1,1,4,4,4-D6) are also reported. We show that the rate coefficients for NO3 reactions correlate with the enthalpy change for the reaction, thereby suggesting that the mechanism for NO3 reactions with alkanes is through H atom abstraction. The measured rate coefficients are compared with available literature values. This study increases the number of available rate coefficients for the reactions of NO3 with alkanes and sets significantly lower upper limits for reaction of NO3 with ethane and methane. The atmospheric significance of our reported rate coefficients is briefly discussed.

This paper reports on the ignition and flame propagation characteristics of aluminum/copper oxide (Al/CuO) nanothermite at different packing density, manufactured from 40 nm commercial Al and CuO nanopowders. A 3.5 W continuous wave laser was used to ignite the samples in argon at atmospheric pressure, and a high speed camera captured the flame propagation. The high speed images revealed that the fast laser heating creates significant material ablation, followed by heat transfer along the heated surface. The bulk ignition occurs near the edge of the top surface, followed by the self-sustained burning. Lightly pressed powders (90% porosity) ignited in ~0.1 ms and the burning front propagated at around 200 m/s, while the dense pellets (40–60% porosity) ignited in ~1 ms and the burning front propagated at around 10 m/s. These results indicate that the reaction mechanism changes from mass convection to heat diffusion with increasing the packing density. The ignition and burn speeds of these Al/CuO nanothermites at different equivalence ratios (ERs), along with SEM images of pre- and post-combustion, illustrate that the homogeneity of the mixture is a critical parameter for optimizing the performance. The Al rich mixtures show significantly lower ignition delays and higher burn speeds.

Two different experimental methods (relative rate and absolute rate methods) were used to measure the rate coefficients for the reactions of NO3 radical with six methacrylate esters: methyl methacrylate (MMA, k1), ethyl methacrylate (EMA, k2), propyl methacrylate (PMA, k3), isopropyl methacrylate (IPMA, k4), butyl methacrylate (BMA, k5), and isobutyl methacrylate (IBMA, k6). In the relative rate method, the loss of the esters relative to that of a reference compound was followed in a 7300 L Teflon-walled chamber at 298 ± 2 K and 1000 ± 5 hpa. In the absolute method, the temporal profiles of NO3 and N2O5 were followed by using a dual channel cavity ring-down spectrometer in the presence of an excess of ester in the 7300 L chamber. The rate coefficients from these two methods (weighted averages) in the units of 10-15 cm3 molecule-1 s-1 at 298 K are k1 = 2.98 ± 0.35, k2 = 4.67 ± 0.49, k3 = 5.23 ± 0.60, k4 = 7.91 ± 1.00, k5 = 5.91 ± 0.58, and k6 = 6.24 ± 0.66. The quoted uncertainties are at the 2σ level and include estimated systematic errors. Unweighted averages are also reported. In addition, the rate coefficient k7 for the reaction of NO3 radical with deuterated methyl methacrylate (MMA-d8) was measured by using the relative rate method to be essentially the same as k1. The trends in the measured rate coefficient with the length and nature of the alkyl group, along with the equivalence of k1 and k7, strongly suggest that the reaction of NO3 with the methacrylate esters proceeds via addition to the double bond on the methacrylate group. The present results are compared with those from previous studies. Using the measured values of the rate coefficients, along with those for reactions of these esters with OH, O3, and chlorine atoms, we calculated the atmospheric lifetimes of methacrylate esters. We suggest that NO3 radicals do contribute to the atmospheric loss of these unsaturated esters, but to a lesser extent than their reactions with OH and O3.