Direct numerical simulations and models for hot burnt gases jet ignition

This work uses multiple three-dimensional Direct Numerical Simulations (DNSs) to i) investigate the ig- nition process of a cold lean premixed mixture at atmospheric conditions by a jet of hot burnt gases that may be cooled before injection ii) evaluate models able to predict the outcome of such a scenario in terms of ignition. Understanding and being able to model ignition of cold premixed mixtures by hot burnt gases is essential to design systems like engines (to ensure ignition) and flameproof enclosures (to prevent ignition). Limited work has focused on the combined effects of the jet injection speed and temperature on ignition. This is difficult to do by using experiments only and DNS is a natural approach to gain knowledge on that point. By varying the hot jet injection speed and temperature, the three- dimensional, kinetically detailed, DNSs allow a parametric study of the impact of these parameters on the ignition process and provide data to build and test models. Simulations prove that jet injection speed and temperature (usually less than the adiabatic flame temperature because of cooling effects through the injection hole) directly govern ignition. Chemical Explosive Mode Analysis (CEMA) is used to char- acterize the reacting flow structure which is strongly impacted by the jet injection speed. Based on the DNSs conclusions, a zero-dimensional Lagrangian model where a small element of the jet burnt gases mixes at a certain rate with the fresh gases while it potentially ignites is found to be a good candidate to predict the outcome of an ignition sequence (success or failure)

Les Ferritines chez A. thaliana (fonction et régulation)

Fe is essential for all cells because it is the cofactor of numerous proteins, however, excess free Fe is potentially deleterious for the cell. Ferritins are multimeric proteins, present in all the kingdoms of life that can store iron in a safe and bioavailable form. In mammals, ferritins are the main Fe store. They have been predicted to fulfil the same function in plants, but direct evidences are lacking. In plants, ferritin synthesis in response to iron overload is mainly regulated at the transcriptional level, whereas, in animals, it is mainly regulated at the post-transcriptional level by the aconitase dependent IRP/IRE system. The aims of my PhD project were: (i) to elucidate ferritin function in plant physiology and (ii) to decipher the signaling pathway leading to ferritin accumulation in response to iron overload. (i) To directly study ferritin function in plants, a loss-of-function approach was developed in Arabidopsis. We present evidence that ferritins do not constitute the major iron pool either in seeds for seedling development or in leaves for proper functioning of the photosynthetic apparatus. The loss of ferritins in vegetative and reproductive organs resulted in sensitivity to excess iron. Furthermore, the absence of ferritin led to a strong deregulation of expression of several metal transporter genes in the stalk, over-accumulation of iron in reproductive organs, and a decrease in fertility. Finally, I showed that in the absence of ferritin, plants had higher levels of ROS, and increased activity of enzymes involved in their detoxification. Ferritins are also involved in iron-detoxification during senescence to avoid ROS accumulation. Seeds ferritins are also involved in the protection against oxidative stress during germination and appear to take part in the integrated iron homeostasis establishment. Taken together, my work showed that Arabidopsis ferritins are essential factors that integrate iron and redox homeostasis, while they do not constitute a major iron source for development. (ii) To study ferritin regulation in A. thaliana, the characterization of mutants in the three genes encoding aconitase permitted us to demonstrate that the IRP/IRE system does not occur in the regulation of iron metabolism in plants. Nevertheless, AtFer1 mRNA stability studies have revealed that iron treatment leads to the destabilization of the AtFer1 mRNA. We identified the presence of DST sequences, characterized as mRNA stability determinant, in the 3'-UTR of AtFer1 mRNA. Using chimeric constructs in which the AtFer1 3'-UTR or the AtFer1 3'-UTR with a mutated DST sequence were fused downstream of reporter genes, we have shown that the DST sequence in the 3'-UTR of AtFer1 is functional and sufficient for the iron-dependent mRNA degradation. Using dst1 and dst2 mutants, which are unable to destabilize transcript via DST sequences, we have shown that the DST1 and DST2 gene products, acting in trans in the DST-dependent degradation pathway, are involved in the degradation of AtFer1. Therefore, in addition to the transcriptional regulation described so far, iron is also involved in DST-dependent post-transcriptional regulation of AtFer1 expression. In conclusion, my work has shown that Arabidopsis ferritins are essential elements, which prevent iron toxicity by maintaining a proper labile iron level into the cell, and that sophisticated mechanisms, involving transcriptional and post-transcriptional regulations, permit the tight adjustment of the ferritin accumulation required for the optimal effectiveness of this systemMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Plasmids carrying DHA-1 β-lactamases

International audienceThe aim of this review is to provide an update on the plasmids mediating DHA-1 cephalosporinase in Klebsiella pneumoniae. These plasmids have been mainly found in this bacterium but not only. The first was isolated from Salmonella sp. in France in the early 1990s. They are currently reported worldwide. Bla (DHA-1) beta-lactamase gene is usually co-expressed with many other antibiotic resistance genes such as extended-spectrum beta-lactamases (bla (CTX-M) -, bla (SHV) -types), oxacillinases (bla (OXA-1,) bla (OXA-30) ), penicillinases (bla (TEM) -type), carbapenemases (bla (OXA48) , bla (KPC-2) ), aminoglycosides (aacA, aadA, armA), fluoroquinolones (qnrB4, aac6'-1b-cr), and sulfonamide (sul1) resistance genes. Plasmids carrying DHA-1 cephalosporinase have different sizes (22 to 313 kb), belong to diverse groups of incompatibility (R, L/M, FII(k), FIB, A/C2, HI2, HIB), and are self-transferable or not. The multidrug resistance region consists of a mosaic structure composed of resistance genes, insertion sequences, composite transposon, and integrons

Jet ignition prediction in a zero-dimensional pre-chamber engine model

This paper presents a multi-chamber, multi-zone engine model to predict the ignition of a lean main chamber by a pre- chamber. The two chambers are connected by small cylindrical holes: the flame is ignited in the pre-chamber, hot gases propagate through the holes and ignite the main chamber through Turbulent Jet Ignition (TJI). The model original fea- tures are: (i) separate balance equations for the pre- and main chambers, (ii) a specific model for temperature and com- position evolution in the holes and (iii) a DNS-based model to predict the ignition of the main chamber fresh gases by the burnt gases turbulent jets exiting the holes. Chemical reactions during TJI are the result of two competing mixing processes: (1) the hot jet gases mix with the fresh main chamber to produce heated zones and (2) at the same time, these hot gases cool down. (1) increases combustion and leads to ignition while (2) decreases it and can prevent ignition. The overall outcome (ignition or failure) is too complex to be modelled simply and the present model relies on recent DNSs of TJI which provided a method to predict the occurrence of ignition. Incorporating this DNS information into the engine model allows to predicts whether ignition will occur or not, an information which is not accessible otherwise using simple models. The resulting approach is tested on multiple cases to predict ignition limits for very lean cases, effects of H2 injection into the pre-chamber and optimum size for the holes connecting the two chambers as a function of equivalence ratio

Ferritins and iron storage in plants.

International audienceIron is essential for both plant productivity and nutritional quality. Improving plant iron content was attempted through genetic engineering of plants overexpressing ferritins. However, both the roles of these proteins in the plant physiology, and the mechanisms involved in the regulation of their expression are largely unknown. Although the structure of ferritins is highly conserved between plants and animals, their cellular localization differ. Furthermore, regulation of ferritin gene expression in response to iron excess occurs at the transcriptional level in plants, in contrast to animals which regulate ferritin expression at the translational level. In this review, our knowledge of the specific features of plant ferritins is presented, at the level of their (i) structure/function relationships, (ii) cellular localization, and (iii) synthesis regulation during development and in response to various environmental cues. A special emphasis is given to their function in plant physiology, in particular concerning their respective roles in iron storage and in protection against oxidative stress. Indeed, the use of reverse genetics in Arabidopsis recently enabled to produce various knock-out ferritin mutants, revealing strong links between these proteins and protection against oxidative stress. In contrast, their putative iron storage function to furnish iron during various development processes is unlikely to be essential. Ferritins, by buffering iron, exert a fine tuning of the quantity of metal required for metabolic purposes, and help plants to cope with adverse situations, the deleterious effects of which would be amplified if no system had evolved to take care of free reactive iron

Self-ignition scenarios after rapid compression of a turbulent mixture weakly-stratified in temperature

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Investigation of flame kernel expansion in a stratified mixture using DNS and LES

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Large Eddy Simulation of Pre-Chamber Ignition in an Internal Combustion Engine

International audienceUsing homogeneous lean mixtures is an efficient way to reduce fuel consumption and pollutant emissions in internal combustion engines. However, lean combustion requires breakthrough technologies to induce reliable ignition and fast combustion. One of these technologies uses pre-chamber to create multiple hot turbulent jets and provide ignition sites for the lean mixture. In this paper, the behaviour of a pre-chamber ignition system used to ignite the main chamber of a real engine is studied using large eddy simulation with direct integration of analytically reduced chemistry using the dynamic thickened flame model. The large eddy simulation allows to analyze the flow entering and leaving the pre-chamber, to measure the cooling and quenching effects introduced by the hot gas passages through the ducts connecting pre-and main chambers and to analyze the ignition and combustion sequences. For the case studied here, small amount of flame kernels are exhausted from the pre-chamber. Hot products penetrate the main chamber, disperse and mix with the fresh reactants and lead to ignition. The combustion in the main chamber starts in a distributed reaction mode before reaching a flamelet propagation mode

CNG direct injection spark-ignition engine with high turbulence and high compression ratio: numerical and experimental investigations

International audienceThis study carried on within the H2020 GasOn project scope of work showed the interest to improve the turbulence of a CNG direct injection spark ignited engine with high compression by a dedicated cylinder head design. The in-cylinder air motion was optimally configured through 3D-CFD simulation in order to cope with the high performance targets (~240 N.m/L at 1500 rpm, ~80kW/L at 4500 rpm) within the cylinder pressure limit of 160 bar (Pcylmax+2s = 180 bar). To sustain such level of pressure a Diesel engine basis was used. The compression ratio was set to 13.4:1 in line with the knock resistance of the natural gas. This newly design engine was compared to a reference engine developed in a previous study. The flow test bench measurements confirmed the high tumble level of the optimized cylinder head. This new cylinder head was next tested on a single cylinder engine equipped with high mass flow rate gas injector prototype developed by Continental. The results, integrating the boundary conditions of the air path system including LP-EGR, validated the good effect of faster combustion revealed by the lower fuel consumption and the high tolerance to EGR dilution in stoichiometric conditions. Consequently, the results showed a significant improvement compared to the previous engine and a performance attaining Diesel-like break thermal engine efficiencies (~41%)