Reaction zones and their structure in MILD combustion

Three-dimensional direct numerical simulation (DNS) of turbulent combustion under moderate and intense low-oxygen dilution (MILD) conditions has been carried out inside a cuboid with inflow and outflow boundaries on the upstream and downstreamfaces respectively. The initial and inflowing mixture and turbulence fields are constructed carefully to be representative of MILD conditions involving partially mixed pockets of unburnt and burnt gases. The combustion kinetics is modelled using a skeletal mechanism for methane-air combustion, including non-unity Lewis numbers for species and temperature dependent transport properties. The DNS data is analysed to study theMILD reaction zone structure and its behaviour. The results show that the instantaneous reaction zones are convoluted and the degree of convolution increases with dilution and turbulence levels. Interactions of reaction zones occur frequently and are spread out in a large portion of the computational domain due to the mixture non-uniformity and high turbulence level. These interactions lead to local thickening of reaction zones yielding an appearance of distributed combustion despite the presence of local thin reaction zones. A canonical MILD flame element, called as MIFE, is proposed which represents the averaged mass fraction variation for major species reasonably well, although a fully representative canonical element needs to include the effect of reaction zone interactions and associated thickening effects on the mean reaction rate.YM acknowledges the financial support of Nippon Keidanren and Cambridge Overseas Trust. EPSRC support is acknowledged by NS. The support of Natural Sciences and Engineering Research Council of Canada is acknowledged by TL. This work made use of the facilities of HECToR, the UK’s national high-performance computing service, which is provided by UoE HPCx Ltd at the University of Edinburgh, Cray Inc and NAG Ltd, and funded by the Office of Science and Technology through EPSRCs High End Computing Programme.This is an Accepted Manuscript of an article published by Taylor & Francis in Combustion Science and Technology on 26 Jun 2014, available online: http://wwww.tandfonline.com/10.1080/00102202.2014.902814

On the validity of Damköhler's first hypothesis in turbulent Bunsen burner flames: A computational analysis

The validity of Damköhler's first hypothesis, which relates the turbulent flame speed to turbulent flame surface area under the condition where the integral length scale of turbulence is greater than the flame thickness, has been assessed using three-dimensional Direct Numerical Simulations (DNS) of turbulent premixed Bunsen burner flames over a range of values of Reynolds number, pressure and turbulence intensity. It has been found for the Bunsen configuration that the proportionality between volume-integrated burning rate and the overall flame surface area is not strictly maintained according to Damköhler's first hypothesis. The discrepancy is found to originate physically from the local stretch rate dependence of displacement speed, and this helps to explain differences observed previously between flames with and without mean curvature. Approximating the local density-weighted flame propagation speed with the unstrained laminar burning velocity is shown to be inaccurate, and can have a significant influence on the prediction of the overall burning rate for flames with non-zero mean curvature. Using a two-dimensional projection of the actual scalar gradient for flame area evaluation is shown to exacerbate the loss of proportionality between volume-integrated burning rate and the overall flame surface area. The current analysis identifies the conditions under which Damköhler's hypothesis remains valid and the necessary correction for non-zero mean flame curvature. Further, it has been demonstrated that surface-weighted stretch effects on displacement speed need to be accounted for in order to ensure the validity of Damköhler's hypothesis under all circumstances. Finally, it has been found that the volume-integrated density-weighted scalar dissipation rate remains proportional to the overall burning rate for all flames considered here irrespective of the value of Reynolds number, pressure and turbulence intensity. However, this proportionality is lost when the scalar dissipation rate is evaluated using the two-dimensional projection of the actual scalar gradient

First-principles, atomistic thermodynamics for oxidation catalysis

Present knowledge of the function of materials is largely based on studies (experimental and theoretical) that are performed at low temperatures and ultra-low pressures. However, the majority of everyday applications, like e.g. catalysis, operate at atmospheric pressures and temperatures at or higher than 300 K. Here we employ ab initio, atomistic thermodynamics to construct a phase diagram of surface structures in the (T,p)-space from ultra-high vacuum to technically-relevant pressures and temperatures. We emphasize the value of such phase diagrams as well as the importance of the reaction kinetics that may be crucial e.g. close to phase boundaries.Comment: 4 pages including 2 figure files. Submitted to Phys. Rev. Lett. Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

Morphological and statistical features of reaction zones in MILD and premixed combustion

Direct numerical simulation (DNS) results of turbulent MILD premixed and conventional (undiluted) premixed combustion have been investigated to shed light on the physical aspects of reaction zones and their morphology inMILD combustion. Results of a premixed case are used for comparative analyses. The analyses show that the regions with strong chemical activity in MILD combustion are distributed over a substantial portion of the computational domain unlike in the premixed case where these regions are confined to a small portion of the domain. Also, interactions of reaction zones are observed in MILD combustion with their spatial extent increasing with dilution level. These interactions give an appearance of distributed combustion for MILD conditions. The morphology of these reaction zones is investigated using the Minkowski functionals and shapefinders commonly employed in cosmology. Predominant sheet-like structures are observed for the premixed combustion case whereas a pancake-like structure is observed as the most probable shape for the MILD cases. Spatial and statistical analyses of various fluxes involved in a progress variable transport equation are conducted to study autoignitive or propagative characteristics of MILD reaction zones. The results suggest that there are local regions with autoignition, propagating-flames, and their coexistence for the conditions considered in this study. Typically, reaction dominated or ignition front and propagating-flame dominated regions are entangled for high dilution cases. Scalar gradient plays a strong role on whether reaction or propagating-flame dominated activities are favoured locally.YM acknowledges the financial support of Nippon Keidanren and Cambridge Overseas Trust. EPSRC support is acknowledged by NS. This work made use of the facilities of HECToR, the UK’s national high-performance computing service, which is provided by UoE HPCx Ltd at the University of Edinburgh, Cray Inc and NAG Ltd, and funded by the Office of Science and Technology through EPSRCs High End Computing Programme.This is the accepted manuscript. The final version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S001021801400128X

Effects of initial radius on the propagation of premixed flame kernals in a turbulent environment

The effects of mean curvature on the propagation of turbulent premixed flames have been investigated using three-dimensional direct numerical simulations (DNS) with single step Arrhenius-type chemistry in the thin reaction zones regime. A number of spherical flame kernels with different initial radius have been studied under identical conditions of turbulence and thermochemistry. A statistically planar turbulent back-to-back flame has been simulated as a special case of a spherical kernel in the limit of infinite kernel radius. Statistical analysis in terms of standard and joint probability density functions (pdfs) clearly indicates that the mean curvature of the flame kernel configuration has a major influence on the propagation behavior of the flame. For the planar flame configuration the density-weighted displacement speed is found to be fairly constant throughout the flame brush, in good agreement with previous DNS results. By contrast, for the flame kernel configuration the density-weighted displacement speed is found to vary strongly through the flame brush, changing from values on the order of the corresponding laminar flame speed near the fresh gas side to considerably smaller values near the burned gas side. The joint pdfs of displacement speed and its components with curvature are extensively studied, allowing for an explanation of the observed phenomena in terms of local flame geometry and its interaction with the turbulent flow fie

Reflecting On The Modification Of A Summative Assessment Instrument To Redress Its Linguistic Complexity For Second-Language Learners In An ODL Context

This study attempted to determine whether simplifying the linguistic complexity of a written summative assessment instrument would improve the examination results of second-language learners; the academic literature suggests that linguistic complexity impacts negatively on the performance of second-language learners. The module chosen for the study was the capstone module of the marketing department at the University of South Africa, an open and distance learning institution. A 2007 examination paper was modified in an attempt to make it linguistically simpler and was used again in 2009. The results of first-language learners were compared with those of second language learners, who represent a significant component of the university’s student population, across the two periods in question. The results were analysed using an independent two-sample t-test. The findings do not support the premise that simplifying the linguistic complexity of an assessment instrument would have a positive impact on the examination performance of second-language learners. The article highlights a number of issues and questions that require further research

Suppressing subordinate reproduction provides benefits to dominants in cooperative societies of meerkats.

In many animal societies, a small proportion of dominant females monopolize reproduction by actively suppressing subordinates. Theory assumes that this is because subordinate reproduction depresses the fitness of dominants, yet the effect of subordinate reproduction on dominant behaviour and reproductive success has never been directly assessed. Here, we describe the consequences of experimentally preventing subordinate breeding in 12 groups of wild meerkats (Suricata suricatta) for three breeding attempts, using contraceptive injections. When subordinates are prevented from breeding, dominants are less aggressive towards subordinates and evict them less often, leading to a higher ratio of helpers to dependent pups, and increased provisioning of the dominant's pups by subordinate females. When subordinate breeding is suppressed, dominants also show improved foraging efficiency, gain more weight during pregnancy and produce heavier pups, which grow faster. These results confirm the benefits of suppression to dominants, and help explain the evolution of singular breeding in vertebrate societies

Building a biomimetic membrane for neutron reflectivity investigation : complexity, asymmetry and contrast

The preparation and investigation of model membranes is deserving growing interest both for the physics of complex systems, and for biology. The need of simplified models should preserve mimicking the qualifying characteristics of biological membranes, and keep non-invasive and detailed description. As a main feature, biological membranes are non-homogeneous in the disposition of components, both in the lateral and in the transverse direction. We prepared asymmetric supported membranes containing GM1 ganglioside in biomimetic proportion according to different protocols. Then, we studied their internal structure by neutron reflectometry, providing few-Angstrom sensitivity in the cross direction meanwhile avoiding radiation damage. This technique can also be profitably applied to study interactions at the membrane surface. The best protocol has proven to be the Langmuir-Blodgett/Langmuir-Schaefer depositions. Notably, also the simpler and most accessible protocol of vesicle fusion was found to be suitable for straightforward and good quality deposition of compositionally asymmetric membranes

Effects of turbulent reynolds number on the performance of algebraic flame surface density models for large eddy simulation in the thin reaction zones regime: A direct numerical simulation analysis

A direct numerical simulation (DNS) database of freely propagating statistically planar turbulent premixed flames with a range of different turbulent Reynolds numbers has been used to assess the performance of algebraic flame surface density (FSD) models based on a fractal representation of the flame wrinkling factor. The turbulent Reynolds number Rethas been varied by modifying the Karlovitz number Ka and the Damköhler number Da independently of each other in such a way that the flames remain within the thin reaction zones regime. It has been found that the turbulent Reynolds number and the Karlovitz number both have a significant influence on the fractal dimension, which is found to increase with increasing Retand Ka before reaching an asymptotic value for large values of Retand Ka. A parameterisation of the fractal dimension is presented in which the effects of the Reynolds and the Karlovitz numbers are explicitly taken into account. By contrast, the inner cut-off scale normalised by the Zel’dovich flame thicknessηi/δzdoes not exhibit any significant dependence on Retfor the cases considered here. The performance of several algebraic FSD models has been assessed based on various criteria. Most of the algebraic models show a deterioration in performance with increasing the LES filter width.Peer Reviewe