Auto-ignition of near-ambient temperature H2/air mixtures during flame-vortex interaction

International audienceThis paper demonstrates auto-ignition in reactants at approximately 350 K, upstream of curved H 2 /air flame surfaces during flame/vortex interaction. Temperature fields were measured using laser Rayleigh scattering during head-on interactions of toroidal-vortices with stagnation flames. Repeatable ignition occurred along the ring of the vortex-slightly towards the center-when it was approximately 1 mm upstream of the wrinkled flame surface. The resultant outwardly propagating toroidal flame led to approximately twice the volumetric heat release rate over the duration of the interaction. The ignition occurred in a region of low fluid dynamic strain rate that was farther from the flame than the region of maximum vorticity. Evidence of additional ignition pockets was found upstream of other flame wrinkles, preferentially near the highest magnitude flame curvatures. Different hypotheses for explaining this observation are discussed. The possibility of substantial heat release driven by auto-ignition and complicated diffusion has implications for reaction rate closure models and transport models used in turbulent combustion simulations

In vivo MRI assessment of permanent middle cerebral artery occlusion by electrocoagulation: pitfalls of procedure

Permanent middle cerebral artery (MCA) occlusion (pMCAO) by electrocoagulation is a commonly used model but with potential traumatic lesions. Early MRI monitoring may assess pMCAO for non-specific brain damage. The surgical steps of pMCAO were evaluated for traumatic cerebral injury in 22 Swiss mice using diffusion and T2-weighted MRI (7T) performed within 1 h and 24 h after surgery. Temporal muscle cauterization without MCA occlusion produced an early T2 hyperintensity mimicking an infarct. No lesion was visible after temporal muscle incision or craniotomy. Early MRI monitoring is useful to identify non-specific brain injury that could hamper neuroprotective drugs assessment

Experimental study on curvature effects and preferential diffusion for perturbed laminar premixed ammonia-air flames

Ammonia-air flames are known for low reactivity and have been posing as a huge hindrance in employing the chemical as a sustainable fuel of tomorrow. Curvature is a parameter that could influence the flame structure and so the position of the maximum heat release rate. Flame-acoustic interactions on a Bunsen burner are performed to study the local flame response to highly perturbed flows. NH2* chemiluminescence is used to study the reactivity of these flames. Non-perturbed flames are used as a reference to understand the inherent behaviour of Bunsen ammonia flames. A case study has been chosen for an equivalence ratio ranging between 1.0 and 1.4 at atmospheric conditions to study perturbed flames. The objective is to study the effect of curvature induced by the perturbations on the reactivity of the flame. It was seen that this given case study was quite complex as the flame response was to multiple factors like the effect of Lewis number, convective-diffusion velocities, decomposition of ammonia into hydrogen, thereby, promoting preferential diffusion of hydrogen in both large-scale and locally for certain cases apart from the generated acoustic perturbation which itself dictates the flow regime of the fresh gases, etc. Since the Damköhler number was around 1, the perturbation time scales and the reactivity time scales were comparable and so none of the effects could be ignored. It was concluded that for richer flames where Le>1, the negative curvature promoted the production of hydrogen leading to local enhancement in reactivity. A change in the local thickness due to the induced curvature was seen for all conditions

Isolating strain and curvature effects in premixed flame/vortex interactions

This study focuses on the response of premixed flames to a transient hydrodynamic perturbation in an intermediate situation between laminar stretched flames and turbulent flames: an axisymmetric vortex interacting with a flame. The reasons motivating this choice are discussed in the framework of turbulent combustion models and flame response to the stretch rate. We experimentally quantify the dependence of the flame kinematic properties (displacement and consumption speeds) to geometrical scalars (stretch rate and curvature) in flames characterized by different effective Lewis numbers. Whilst the displacement speed can be readily measured using particle image velocimetry and tomographic diagnostics, providing a reliable estimate of the consumption speed from experiments remains particularly challenging. In the present work, a method based on a budget of fuel on a well chosen domain is proposed and validated both experimentally and numerically using two-dimensional direct numerical simulations of flame/vortex interactions. It is demonstrated that the Lewis number impact neither the geometrical nor the kinematic features of the flames, these quantities being much more influenced by the vortex intensity. While interacting with the vortex, the flame displacement (at an isotherm close to the leading edge) and consumption speeds are found to increase almost independently of the type of fuel. We show that the total stretch rate is not the only scalar quantity impacting the flame displacement and consumption speeds and that curvature has a significant influence. Experimental data are interpreted in the light of asymptotic theories revealing the existence of two distinct Markstein numbers, one characterizing the dependence of flame speed to curvature, the other to the total stretch rate. This theory appears to be well suited for representing the evolution of the displacement speed with respect to either the total stretch rate, curvature or strain rate. It also explains the limited dependence of the flame displacement speed to Lewis number and the strong correlation with curvature observed in the experiments. An explicit relationship between displacement and consumption speeds is also given, indicating that the fuel consumption rate is likely to be altered by both the total stretch rate and curvature

Clinical Imaging of Choroid Plexus in Health and in Brain Disorders: A Mini-Review

The choroid plexuses (ChPs) perform indispensable functions for the development, maintenance and functioning of the brain. Although they have gained considerable interest in the last years, their involvement in brain disorders is still largely unknown, notably because their deep location inside the brain hampers non-invasive investigations. Imaging tools have become instrumental to the diagnosis and pathophysiological study of neurological and neuropsychiatric diseases. This review summarizes the knowledge that has been gathered from the clinical imaging of ChPs in health and brain disorders not related to ChP pathologies. Results are discussed in the light of pre-clinical imaging studies. As seen in this review, to date, most clinical imaging studies of ChPs have used disease-free human subjects to demonstrate the value of different imaging biomarkers (ChP size, perfusion/permeability, glucose metabolism, inflammation), sometimes combined with the study of normal aging. Although very few studies have actually tested the value of ChP imaging biomarkers in patients with brain disorders, these pioneer studies identified ChP changes that are promising data for a better understanding and follow-up of diseases such as schizophrenia, epilepsy and Alzheimer’s disease. Imaging of immune cell trafficking at the ChPs has remained limited to pre-clinical studies so far but has the potential to be translated in patients for example using MRI coupled with the injection of iron oxide nanoparticles. Future investigations should aim at confirming and extending these findings and at developing translational molecular imaging tools for bridging the gap between basic molecular and cellular neuroscience and clinical research

In vivo evaluation of a hybrid nanoparticle for molecular imaging of amyloid aggregation

International audienceAmyloid-β (Aβ) fibrillization is described as a central event in the pathogenesis of Alzheimer’s disease (AD). Amyloid imaging is expected to play a pivotal role in early and differential diagnosis of dementias, and in the evaluation of anti-Aβ treatments. Luminescent conjugated oligothiophenes (LCO) have been proposed as optical biomarkers of protein fibrillation [1]. In this paper, we evaluated a fluorescent magnetic hybrid nanoprobe (HNP5011), based on gadolinium fluoride nanoparticles functionalized with luminescent conjugated polythiophenes moieties (Fig. 1). The aim of this study was to investigate its potential for molecular imaging in a rat model bearing intracerebral pre-aggregated Aβ peptides

Développement de radiotraceurs de la neuroinflammation pour l'imagerie des pathologies neurodégénératives

PhD performed in Service Hospitalier Frédéric Joliot (Gene expression imaging unit, CEA/INSERM U803), Orsay, France.Inflammatory processes such as microglial or endothelial activation are involved in many neurodegenerative conditions. Neuroinflammation imaging is considered an attractive tool for fondamental research, diagnosis and therapeutic evaluation in neuropathologies. First, an aptamer was selected against a recombinant fragment of the endothelial target VCAM-1, but proved unable to bind the target protein in native conformation, as expressed by a cell line. Second, five radioligands of the peripheral benzodiazepine receptor (PBR), a marker of microglial activation, were evaluated in vivo using PET (Positron Emission Tomography) imaging in a rat model of neuroinflammation, and were compared to [11C]PK11195. Four radiotracers displayed a better contrast than [11C]PK11195. In a competitive field of research, this work demonstrates the efficiency of in vivo screening of radiotracers for fast selection of clinically relevant molecules.Thèse de doctorat réalisée au Service Hospitalier Frédéric Joliot (Laboratoire d’Imagerie Moléculaire Expérimentale, CEA/INSERM U803), Orsay, France.Des processus inflammatoires tels que l’activation des cellules microgliales et de l’endothélium vasculaire sont impliqués dans de nombreuses pathologies neurodégénératives. L’imagerie de la neuroinflammation offre des perspectives d’étude, de diagnostic précoce, et d’évaluation thérapeutique pour un large spectre de neuropathologies. Premièrement, un aptamère a été sélectionné contre un fragment recombinant de la cible endothéliale VCAM-1, mais perd sa spécificité de reconnaissance de la protéine lorsqu’elle est exprimée en conformation native par la cellule. Deuxièmement, cinq radioligands du récepteur périphérique aux benzodiazépines, marqueur de l’activation microgliale, ont été évalués in vivo en imagerie TEP et comparés au [11C]PK11195 en utilisant des rats lésés. Quatre ont montré des performances supérieures au [11C]PK11195 via un contraste amélioré. Dans un domaine en pleine expansion, ces travaux définissent un crible efficace pour la sélection d’un radiotraceur d’intérêt