Benjamin Legrand

Ignition and combustion of levitated magnesium and aluminum particles in carbon dioxide

Abstract This article considers ignition and combustion of single particles of magnesium and aluminum in carbon dioxide at pressures 0.1-2 MPa. An experimental setup with an electrodynamic levitator inside a high-pressure chamber was employed. The CO2-laser was used for heating to ignition of the particles. The results show that ignition mechanisms of Mg and Al in CO2 are different. Experiments with Mg indicate the existence of the critical partial pressure of CO2, whereas the ignition probability of Al particles in CO2 is low but independent on pressure. Analysis of flame images and combustion parameters shows that the mechanism of Mg particle burning in CO2 corresponds to conventional models of vapor-phase diffusion-controlled combustion, whereas in the case of Al exothermic processes on the particle surface or close to it play a leading part in the burning process.

Ignition and combustion of levitated magnesium particles in carbon dioxide

This paper considers ignition and combustion of small (50–100 μm) single particles of magnesium and 50-50 magnesium-aluminum alloy in the atmosphere of carbon dioxide or its mixtures with argon. This investigation is of interest for both basic combustion science and applications to rocket engines, including those using Martian CO2 as an oxidizer, An experimental setup with an electrodynamic levitator inside a high-pressure chamber was employed. A CO2 laser was used for heating to ignition of the particles. The laser was switched off after ignition. The experiments were conducted with the oxidizer at room temperature over the range of pressures from 0.1 to 2 MPa. Effects of the CO2 concentration and pressure on the critical ignition conditions, ignition delay times, and burning times have been determined for Mg particles. The results clearly indicate that ignition of Mg in CO2 is controlled by chemical kinetics and that its combustion is controlled by diffusion in gas phase. Quantitative disagreement of the observed critical ignition pressures with previous experimental data on ignition of Mg disks in CO2 is explained by the differences in heat-loss mechanisms. The measured values of the burning rate correlate well with previous experimental results on combustion of 2-mm particles and with a quasi-steady model of Mg particle burning in CO2. In contrast to pure Mg and Al, particles of Mg−Al alloy did not ignite in CO2 under the present experimental conditions.

Rate constants for the homogeneous gas-phase Al/HCl combustion chemistry

Experimental reaction rate data for the Al/HCl system are very scarce. Such data are needed for the comprehension and for the numerical simulation of the combustion of aluminum particles as encountered in solid segmented motors. Toward this end, we have examined the homogeneous chemistry of this system, computed rate parameters for important reactions using conventional Transition State Theory (TST) and RRKM/master equation simulations, and estimated rate parameters for reactions where rigorous computations are not presently feasible. The reaction mechanism presented in this study consists of 15 species participating in 39 reversible elementary reactions for which rate parameters have been estimated or computed.

Characterization of akinetes from cyanobacterial strains and lake sediment: A study of their resistance and toxic potential

Nostocalean cyanobacteria are known to proliferate abundantly in eutrophic aquatic ecosystems, and to produce several cyanotoxins, including anatoxin-a. In this study, we investigated both the resistance and toxic potential of the akinetes (resistant cells), using cyanobacterial cultures and akinetes extracted from the sediment of Lake Aydat (France) sampled in the winter and spring. Intact and lysed akinetes were differentiated using a double control based on the autofluorescence of akinetes and SYTOX-green staining. The percentage of resistant akinetes found in several different abiotic stress conditions was highly variable, depending on the species and also on the sampling season. Thus, the resistance of akinetes and their ability to germinate seems to follow a species-specific process, and akinetes can undergo physiologic changes during the sedimentary phase of the Nostocale life cycle. This study also revealed the first evidence of anatoxin-a genes in akinetes, with anaC and anaF genes detected in akinetes from all cyanobacterial producer cultures. The low number of anaC genes, almost exclusively detected using nested PCR, in the sediment at Lake Aydat suggests a limited but existent past population of toxic Nostocales in this lake. Given the key role of akinetes in the annual cycle and subsequent summer proliferation, it can be interesting to integrate the surveillance of akinetes in the management of lakes exposed to recurrent cyanobacterial blooms.

Molecular tools to detect anatoxin-a genes in aquatic ecosystems: Toward a new nested PCR-based method

Over the last few decades, cyanobacterial mass occurrence has become a recurrent feature of aquatic ecosystems. This has led to ecosystem exposure and health hazards associated with cyanotoxin production. The neurotoxin anatoxin-a and its homologs can be synthesized by benthic cyanobacterial species in lotic systems, but also by planktonic lacustrine species such as Dolichospermum (also known as Anabaena). However, only a few studies have focused on anatoxin-a occurrence and its biosynthesis genes in freshwater lakes. The initial aim of this study was to evaluate the molecular tools available in the literature to detect anatoxin-a biosynthesis genes in lacustrine environments. Having tested different sets of PCR primers, we found that that some sets of primers, such as anxC, were too specific and did not amplify anatoxin-a biosynthesis genes in all producing strains. On the other hand, some sets of primers, such as atxoa, seemed not to be specific enough, amplifying numerous non-specific bands in environmental samples, especially those from sediments. Furthermore, anaC and anaF amplification exhibited different band intensities during electrophoresis, suggesting a high variation in number of gene copies between samples. As a result, we proposed a new nested PCR-based method which considerably improved the amplification of the anaC gene in our environmental samples, eliminating non-specific bands and weak detections. Using this tool, our study also highlighted that anatoxin-a genes are widely distributed throughout freshwater lakes. This suggests the need for further ecological investigations into anatoxin-a in these ecosystems.

Akinetes May Be Representative of Past Nostocalean Blooms: a Case Study of Their Benthic Spatiotemporal Distribution and Potential for Germination in a Eutrophic Lake

ABSTRACT Monitoring of water and surface sediment in a French eutrophic lake (Lake Aydat) was carried out over a 2-year period in order to determine whether akinetes in sediment could be representative of the most recent bloom and to estimate their germination potential. Sediment analysis revealed two akinete species, Dolichospermum macrosporum and Dolichospermumflos-aquae, present in the same proportions as observed for the pelagic populations. Moreover, similar spatial patterns observed for vegetative cells in the water column and akinete distributions in the sediment suggest that akinetes in the sediment may be representative of the previous bloom. However, the relationship between akinetes in the sediment and vegetative cells in the water column was not linear, and other factors may interfere. For example, our results highlighted horizontal transport of akinetes during the winter. The benthic overwinter phase did not seem to influence the percentages of intact akinetes, which remained stable at approximately 7% and 60% for D. macrosporum and D. flos-aquae, respectively. These percentages may thus be the result of processes that occurred in the water column. The intact overwintering akinetes showed germination rates of up to 90% after 72 h for D. flos-aquae or 144 h for D. macrosporum. The difference in akinete germination rates between these two species demonstrates different ecological strategies, which serve to expand the window for germination in time and space and thus optimize colonization of the water column by nostocalean cyanobacteria. IMPORTANCE Cyanobacteria have the ability to proliferate and to form blooms. These blooms can then affect the local ecology, health, and economy. The akinete, a resistant cell type that persists in sediment, is an important intermediate phase between previous and future blooms. We monitored the water column and the surface sediment of a French eutrophic lake (Lake Aydat) to investigate the relationship between vegetative cells in the water column and akinetes in the sediment. This study focused on the characterization of spatiotemporal akinete distributions, cellular integrity, and germination potential. Species-specific ecological strategies were highlighted and may partly explain the temporal succession of species in the water column. Akinetes may also be used to understand past nostocalean blooms and to predict future ones.

Studies of the burning of levitated magnesium particles in CO2

An experimental setup with an electrodynamic levitator inside a high pressure chamber is employed. Heat-up to the ignition point of the particles is obtained with a COi-laser. In the present work, experiments were conducted at ambient temperature over the range of Copyright

Benthic Archives Reveal Recurrence and Dominance of Toxigenic Cyanobacteria in a Eutrophic Lake over the Last 220 Years

Akinetes are resistant cells which have the ability to persist in sediment for several decades. We have investigated the temporal distribution of akinetes of two species, Dolichospermum macrosporum and Dolichospermum flos-aquae, in a sediment core sampled in Lake Aydat (France), which covers 220 years. The upper part, from 1907 to 2016, the number of akinetes fluctuated but stayed at high concentrations, especially for D. macrosporum in surface sediment (with the maximal value close to 6.105 akinetes g DW−1 of sediment), suggesting a recurrence of blooms of this species which was probably closely related to anthropic eutrophication since the 1960s. Before 1907, the abundance of akinetes of both species was very low, suggesting only a modest presence of these cyanobacteria. In addition, the percentage of intact akinetes was different for each species, suggesting different ecological processes in the water column. This percentage also decreased with depth, revealing a reduction in germination potential over time. In addition, biosynthetic genes of anatoxin-a (anaC) and microcystin (mcyA) were detected. First results show a high occurrence of mcyA all down the core. In contrast, anaC gene was mostly detected in the surface sediment (since the 1980s), revealing a potentially more recent occurrence of this cyanotoxin in Lake Aydat which may be associated with the recurrence of blooms of D. macrosporum and thus with anthropic activities.

Preliminary analysis of a high pressure spray and cloud combustion module for the ISS

Combustion of droplet sprays and particle clouds is a very important area due to numerous applications to engines and power systems as well as to problems of industrial safety and clean environment. However, combustion of two-phase systems is not well understood yet. In the combustion of sprays or clouds gravitational effects add another difficulty due to the sedimentation of the particles and droplets. Indeed, stability of a spray or a cloud under normal gravity can only be achieved by stirring this two-phase mixture which produces a turbulent flow field. Therefore, under normal gravity conditions, the combustion characteristics of two-phase mixtures can only be obtained in turbulent flow. On the other hand, and namely for high-pressure conditions, these characteristics are in fact also strongly influenced by natural convection. It is proposed to develop a combustion facility for the International Space Station, which would make it possible to study high pressure combustion of spray and clouds as well as...