Heat release rate as represented by [OH] × [CH2O] and its role in autoignition

Data from a recent instantaneous, simultaneous, high-resolution imaging experiment of Rayleigh temperature and laser induced fluorescence (LIF) of OH and CH₂O at the base of a turbulent lifted methane flame issuing into a hot vitiated coflow are analysed and contrasted to reference flames to further investigate the stabilization mechanisms involved. The use of the product of the quantified OH and semi-quantified CH₂O images as a marker for heat release rate is validated for transient autoigniting laminar flames. This is combined with temperature gradient information to investigate the flame structure. Super-equilibrium OH, the nature of the profiles of heat release rate with respect to OH mole fraction, and comparatively high peak heat release rates at low temperature gradients is found in the kernel structures at the flame base, and found to be indicative of autoignition stabilization.     

On the role of subthreshold currents in the Huber-Braun cold receptor model

We study the role of the strength of subthreshold currents in a four-dimensional Hodgkin-Huxley-type model of mammalian cold receptors. Since a total diminution of subthreshold activity corresponds to a decomposition of the model into a slow, subthreshold, and a fast, spiking subsystem, we first elucidate their respective dynamics separately and draw conclusions about their role for the generation of different spiking patterns. These results motivate a numerical bifurcation analysis of the effect of varying the strength of subthreshold currents, which is done by varying a suitable control parameter. We work out the key mechanisms which can be attributed to subthreshold activity and furthermore elucidate the dynamical backbone of different activity patterns generated by this model.     

LES/CMC modelling of ignition and flame propagation in a non-premixed methane jet

The Large Eddy Simulation (LES) / Conditional Moment Closure (CMC) model with detailed chemistry is used for modelling spark ignition and flame propagation in a turbulent methane jet in ambient air. Two centerline and one off-axis ignition locations are simulated. We focus on predicting the flame kernel formation, flame edge propagation and stabilization. The current LES/CMC computations capture the three stages reasonably well compared to available experimental data. Regarding the formation of flame kernel, it is found that the convection dominates the propagation of its downstream edge. The simulated initial downstream and radial flame propagation compare well with OH-PLIF images from the experiment. Additionally, when the spark is deposited at off-centerline locations, the flame first propagates downstream and then back upstream from the other side of the stoichiometric iso-surface. At the leading edge location, the chemical source term is larger than others in magnitude, indicating its role in the flame propagation. The time evolution of flame edge position and the final lift-off height are compared with measurements and generally good agreement is observed. The conditional quantities at the stabilization point reflect a balance between chemistry and micro-mixing. This investigation, which focused on model validation for various stages of spark ignition of a turbulent lifted jet flame through comparison with measurements, demonstrates that turbulent edge flame propagation in non-premixed systems can be reasonably well captured by LES/CMC.     

Ethanolic extracts of Moringa oleifera Lam.

This paper reports the evaluation of the antioxidant potential of the ethanolic extracts of the leaves (EL), flowers (EF), seed pods (EP) and seeds (ES) from Moringa oleifera Lam. The antioxidant potential was assessed, upon the addition of the extracts to fish oil, by means of the total extractable phenol content (TEP), the DPPH^· free radical scavenging efficiency and using pressurized differential scanning calorimetry (P-DSC). The results of TEP and DPPH^· showed that the ES extract does not present a potential to be used as an antioxidant additive, and it was thus discarded from the remaining analyses. Thus, the following treatments were prepared: pure fish oil, fish oil with BHT (100 mg kg^?1), fish oil with TBHQ (100 mg kg^?1) and fish oil with EL, EF and EP—all at the concentration of 100 mg kg^?1, in relation to the total extractable phenolics contained in each one of the extracts. The leaf and flower extracts displayed a protecting effect, with an increase in about 20 and 11 % of the OIT values, respectively. However, such protection was smaller than that conferred by the synthetic antioxidants utilized. As for the thermal analysis results, it was noticed that EL presented the highest thermal stability among the extracts.     

Synthesis and Characterization of Mixed-Ligand Nickel(II) Chelates of β-Diones and α-Diimines. Crystal and Molecular Structure of [Ni(ncup)2Etacet]ClO4

A series of new nickel(II) perchlorate complexes containing an α-diimine(enR) and the anion of a β-dione (1,3-ketoenol or 1,3-ketoester, βH) was prepared and characterized. The composition and the overall structure of the new chelates depend on ligand concentration, on steric and electronic effects induced by substituents within the ligands and the ability of the perchlorate group to coordinate. The IR and electronic excitation spectra of [Ni(enR)₂β]ClO₄ and [Ni(enR)β(O₂ClO₂)] indicate, in conjunction with other physicochemical measurements, bidentate coordination of the ligands and replacement of the (O,O′) perchlorato group by basic solvents. The structure of the new chelates was further supported by an X-ray structure analysis of [Ni(ncup)₂Etacet]ClO₄, where ncup denotes neocuproine and Etacet the anion of the ethyl acetoacetate (orthorhombic, space group Pc2₁n, a = 14.087(5), b = 14.713(5) and c = 15.952(5) Å, Z = 4). The coordination sphere of nickel is a distorted octahedron, arised from the chromophore NiN₄O₂, in which the base is favored by three neocuproine nitrogens and one Etacet oxygen. The apical sites are occupied by the remaining oxygen and nitrogen atoms, one from Etacet and one from neocuproine respectively.     

Soot particle size distribution measurements in a turbulent ethylene swirl flame

There is a need to better understand particle size distributions (PSDs) from turbulent flames from a theoretical, practical and even regulatory perspective. Experiments were conducted on a sooting turbulent non-premixed swirled ethylene flame with secondary (dilution) air injection to investigate exhaust and in-burner PSDs measured with a Scanning Mobility Particle Sizer (SMPS) and soot volume fractions (f_v) using extinction measurements. The focus was to understand the effect of systematically changing the amount and location of dilution air injection on the PSDs and f_v inside the burner and at the exhaust. The PSDs were also compared with planar Laser Induced Incandescence (LII) calibrated against the average f_v. LII provides some supplemental information on the relative soot amounts and spatial distribution among the various flow conditions that helps interpret the results. For the flame with no air dilution, f_v drops gradually along the centreline of the burner towards the exhaust and the PSD shows a shift from larger particles to smaller. However, with dilution air f_v reduces sharply where the dilution jets meet the burner axis. Downstream of the dilution jets f_v reduces gradually and the PSDs remain unchanged until the exhaust. At the exhaust, the flame with no air dilution shows significantly more particles with an f_v one to two orders of magnitude greater compared to the Cases with dilution. This dataset provides insights into soot spatial and particle size distributions within turbulent flames of relevance to gas turbine combustion with differing dilution parameters and the effect dilution has on the particle size. Additionally, this work measures f_v using both ex situ and in situ techniques, and highlights the difficulties associated with comparing results across the two. The results are useful for validating advanced models for turbulent combustion.     

Simulation of Hydrogen Auto-Ignition in a Turbulent Co-flow of Heated Air with LES and CMC Approach

Large-Eddy Simulations (LES) with the first order Conditional Moment Closure (CMC) approach of a nitrogen-diluted hydrogen jet, igniting in a turbulent co-flowing hot air stream, are discussed. A detailed mechanism (nine species, 19 reactions) is used to represent the chemistry. Our study covers the following aspects: CFD mesh resolution; CMC mesh resolution; inlet boundary conditions and conditional scalar dissipation rate modelling. The Amplitude Mapping Closure for the conditional scalar dissipation rate produces acceptable results. We also compare different options to calculate conditional quantities in CMC resolution. The trends in the experimental observations are in general well reproduced. The auto-ignition length decreases with an increase in co-flow temperature and increases with increase in co-flow velocity. The phenomena are not purely chemically controlled: the turbulence and mixing play also affect the location of auto-ignition. In order to explore the effect of turbulence, two options were applied: random noise and turbulence generator based on digital filter. It was found that stronger turbulence promotes ignition.     

LES/CMC of Blow-off in a Liquid Fueled Swirl Burner

Large Eddy Simulations of two-phase flames with the Conditional Moment Closure combustion model have been performed for flow conditions corresponding to stable and blow-off regimes in a swirl n-heptane spray burner. In the case of stable flame (i.e. low air velocity), the predicted mean and r.m.s. velocities and the location and shape of the flame agree reasonably well with experiment. In particular, the presence of localised extinctions is captured in agreement with experiment. Using model constants previously calibrated against piloted jet methane flames (Sandia F) with localised extinction, we obtain that at the experimentally determined blow-off velocity of the swirling spray flame, the predicted flame also blows off, demonstrating that the LES-CMC approach can capture the global extinction point in a realistic configuration.