Subgrid scale modelling for MILD combustion

A simple closure for filtered reaction of a reaction progress variable is analysed in this study using explicitly filtered DNS data of turbulent MILD combustion of methane for Large Eddy Simulation (LES). The conditional averages of major and minor species mass fractions, and reaction rate constructed from the DNS data along with those obtained using flamelet and Perfectly Stirred Reactor (PSR) models suggest that the PSR can serve as a good canonical reactor for MILD combustion modelling. The flamelet predictions of reaction rate are observed to be poor because it does not include effects of flame interactions, which are abundant in the MILD combustion. The PSR solution obtained over a wide range of residence time along with presumed beta sub-grid PDF seems a reasonable closure for the filtered reaction rate for the LES filter size greater than three flame thermal thicknesses. Both spatial variations and joint PDF of modelled and DNS values of filtered reaction rates are analysed.Y. M. acknowledges the financial support of Nippon Keidanren. EPSRC support is acknowledged. 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 EPSRC’s High End Computing Programme.This is the final published version. It first appeared at: http://www.sciencedirect.com/science/article/pii/S1540748914003356

Scalar gradient behaviour in MILD combustion

The results of three-dimensional Direct Numerical Simulation (DNS) of Moderate, Intense Low-oxygen Dilution (MILD) and conventional premixed turbulent combustion conducted using a skeletal mechanism including the effects of nonunity Lewis numbers and temperature dependent transport properties are analysed to investigate combustion characteristics using scalar gradient information. The DNS data is also used to synthesise laser induced fluorescence (LIF) signals of OH, CH2O, and CHO. These signals are analysed to verify if they can be used to study turbulent MILD combustion and it has been observed that at least two (OH and CH2O) LIF signals are required since the OH increase across the reaction zone is smaller inMILD combustion compared to premixed combustion. The scalar gradient PDFs conditioned on the reaction rate obtained from the DNS data and synthesised LIF signals suggests a strong gradient in the direction normal to the MILD reaction zone with moderate reaction rate implying flamelet combustion. However, the PDF of the normal gradient is as broad as for the tangential gradient when the reaction rate is high. This suggests a non-flamelet behaviour, which is due to interaction of reaction zones. The analysis of the conditional PDFs for the premixed case confirms the expected behaviour of scalar gradient in flamelet combustion. It has been shown that the LIF signals synthesised using 2D slices of DNS data also provide very similar insights. These results demonstrate that the so-called flameless combustion is not an idealised homogeneous reactive mixture but has common features of conventional combustion while containing distinctive characteristics.The financial supports of Nippon Keidanren, Cambridge Overseas Trust and EPSRC are acknowledged. The direct simulations were made using 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 published version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S0010218013003799

Modelling paradigms for MILD combustion

Three-dimensional Direct Numerical Simulation (DNS) data of methane-air MILD combustion is analysed to study the behaviour of MILD reaction zones and to identify a suitable modelling paradigm for MILD combustion. The combustion kinetics in the DNS was modelled using a skeletal mechanism including non-unity Lewis number effects. The reaction zones under MILD conditions are highly convoluted and contorted resulting in their frequent interactions. This leads to combustion occurring over a large portion of the computational volume and giving an appearance of distributed combustion. Three paradigms, standard flamelets, mild flame elements (MIFEs) and PSR, along with a presumed PDF model are explored to estimate the mean and filtered reaction rate in MILD combustion. A beta function is used to estimate the presumed PDF shape. The variations of species mass fractions and reaction rate with temperature computed using thesemodels are compared to the DNS results. The PSR-based model is found to be appropriate, since the conditional averages obtained from the DNS agree well with those obtained using the PSR model. The flamelets model with MIFEs gives only a qualitative agreement because it does not include the effects of reaction zone interactions.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 Springer at http://link.springer.com/article/10.1007%2Fs12572-014-0106-x

DNS of MILD combustion with mixture fraction variations

Direct numerical simulations of Moderate or Intense Low-oxygen Dilution combustion inside a cubical domain are performed. The computational do- main is specified with inflow and outflow boundary conditions in one direction and periodic conditions in the other two directions. The inflowing mixture is constructed carefully in a preprocessing step and has spatially varying mixture fraction and reaction progress variable field. Thus, this mixture in- cludes a range of thermo-chemical state for a given mixture fraction value. The combustion kinetics is modelled using a 58-step skeletal mechanism in- cluding a chemiluminescent species, OH∗, for methane-air combustion. The study of reaction zone structures in the physical and mixture fraction spaces shows the presence of ignition fronts, lean and rich premixed flames and non-premixed combustion. These three modes of combustion are observed without the typical triple-flame structure and this results from the spatio-temporally varying mixture fraction field undergoing turbulent mixing and reaction. The flame index and its pdf are analysed to estimate the fractional contributions from these combustion modes to the total heat release rate. The lean premixed mode is observed to be quite dominant and contribution of non-premixed mode increased from about 11% to 20% when the mean oxygen mole fraction in the inflowing mixture is reduced from about 2.7% to 1.6%. Also, the non-premixed contribution increases if one decreases the integral length scale of the mixture fraction field. All of these results and observations are explained on physical basis.N.A.K.D. acknowledges the financial support of the Qualcomm European Research Studentship Fund in Technology. This work used the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk) using computing time provided by EPSRC under the RAP project number e419 and the UKCTRF (e305). NS acknowledges the support of EPSRC. Y. M. acknowledges the support of JSPS Grant-in-Aid for Young Scientists (B) Grant Number 16K18026

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

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

Reciprocal regulation of airway rejection by the inducible gas-forming enzymes heme oxygenase and nitric oxide synthase

Obliterative bronchiolitis (OB) develops insidiously in nearly half of all lung transplant recipients. Although typically preceded by a CD8+ T cell–rich lymphocytic bronchitis, it remains unresponsive to conventional immunosuppression. Using an airflow permissive model to study the role of gases flowing over the transplanted airway, it is shown that prolonged inhalation of sublethal doses of carbon monoxide (CO), but not nitric oxide (NO), obliterate the appearance of the obstructive airway lesion. Induction of the enzyme responsible for the synthesis of CO, heme oxygenase (Hmox) 1, increased carboxyhemoglobin levels and suppressed lymphocytic bronchitis and airway luminal occlusion after transplantation. In contrast, zinc protoporphyrin IX, a competitive inhibitor of Hmox, increased airway luminal occlusion. Compared with wild-type allografts, expression of inducible NO synthase (iNOS), which promotes the influx of cytoeffector leukocytes and airway graft rejection, was strikingly reduced by either enhanced expression of Hmox-1 or exogenous CO. Hmox-1/CO decreased nuclear factor (NF)-κB binding activity to the iNOS promoter region and iNOS expression. Inhibition of soluble guanylate cyclase did not interfere with the ability of CO to suppress OB, implicating a cyclic guanosine 3′,5′-monophosphate–independent mechanism through which CO suppresses NF-κB, iNOS transcription, and OB. Prolonged CO inhalation represents a new immunosuppresive strategy to prevent OB

Assessment of SGS closure for isochoric combustion of hydrogen-air mixture

Direct numerical simulation (DNS) data of freely propagating turbulent premixed flame of stoichiometric hydrogen air mixture inside a closed vessel is analysed to study a sub-grid combustion closure based on unstrained flamelet approach. This modelling framework needs closures for the sub-grid scale (SGS) reaction rate and scalar dissipation rate. The results show that the closure models for these two SGS quantities work quite well. The dissipation rate closure involves a scale dependent parameter, β c , which is related to the flame curvature induced effects. The reactivity of reactant mixture increases with time in isochoric combustion because the mixture temperature and pressure increases with time. This also influences the parameter β c and thus the dynamic evaluation of this parameter is investigated using the DNS data

Detection and Characterization of Oncogene Mutations in Preneoplastic and Early Neoplastic Lesions

While it has been nearly 30 years since its discovery, the ras family of genes has not yet lost its impact on basic and clinical oncology. These genes remain central to the field of molecular oncology as tools for investigating carcinogenesis and oncogenic signaling, as powerful biomarkers for the identification of those who have or are at high risk of developing cancer, and as oncogene targets for the design and development of new chemotherapeutic drugs. Mutational activation of the K-RAS proto-oncogene is an early event in the development and progression of the colorectal, pancreatic, and lung cancers that are the major causes of cancer death in the world. The presence of point mutational "hot spots" at sites necessary for the activation of this proto-oncogene has led to the development of a number of highly sensitive PCR-based methods that are feasible for the early detection of K-RAS oncogene mutations in the clinical setting. In light of these facts, mutation at the K-RAS oncogene has the potential to serve as a useful biomarker in the early diagnosis and risk assessment of cancers with oncogenic ras signaling. This chapter describes a highly sensitive method for detecting mutant K-RAS, enriched PCR, and its application to early detection of alterations in this oncogene in preneoplastic and early neoplastic lesions of the colon and rectum