Numerical and asymptotic analyses of lean hydrogen-air deflagrations

Numerical and asymptotic methods are used to address the structure and burning rate of lean hydrogen-air deflagrations with detailed account of the underlying chemical reactions involved. Although most computations are performed for atmospheric normal values of the pressure and initial temperature, subatmospheric and elevated pressures as well as cryogenic and preheated mixtures are also considered. The whole range of compositions ranging from the lean flammability limit to stoichiometric mixtures has been investigated, with particular attention given to near-limit lean flames. The chemistry description is investigated first. A short mechanism consisting of seven elementary reactions, of which only three are reversible, is shown to provide good predictions of hydrogen-air lean-flame burning velocities. It is also demonstrated that adding only two irreversible direct recombination steps to the seven-step mechanism accurately reproduces burning velocities of the full detailed mechanism for all equivalence ratios at normal atmospheric conditions and that an eight-step detailed mechanism, constructed from the seven-step mechanism by adding to it the fourth reversible shuffle reaction, improves predictions of O and OH profiles. For conditions near the lean flammability limit all reaction intermediaries have small concentrations in the important thin reaction zone that controls the hydrogen-air laminar burning velocity and therefore follow a steady state approximation, while the main species react according to the global irreversible reaction 2H2 + O2 !2H2O. An explicit expression for the non-Arrhenius rate of this one-step overall reaction for hydrogen oxidation is derived from the seven-step detailed mechanism, for application near the flammability limit. The one-step results are used to calculate flammability limits and burning velocities of planar deflagrations. Furthermore, implications concerning radical profiles in the deflagration and reasons for the success of the approximations are clarified. The new reduced-chemistry descriptions can be useful for both analytical and computational studies of lean hydrogen-air flames, decreasing required computation times. The inner structure of the thin reactive layer of hydrogen-air fuel-lean deflagrations close to the flammability limit is investigated next. The analysis, which employes seven elementary reactions for the chemistry description, uses the ratio of the characteristic radical and fuel concentrations as a small asymptotic parameter, enabling an accurate analytic expression for the resulting burning rate to be derived. The analysis reveals that the steady-state assumption for chemical intermediaries, applicable on the hot side of the reactive layer, fails, however, as the crossover temperature is approached, providing a nonnegligible higher-order correction to the burning rate. The results can be useful, for instance, in future investigations of hydrogen deflagration instabilities near the lean flammability limit. Finally, conditions away from the flammability limit, including moderately lean and stoichiometric flames, are explored. Under these conditions, the steady-state assumption for H atoms is seen to fail, and the one-step mechanism for hydrogen oxidation must be replaced with a two-step reduced mechanism comprising a thermally sensitive branching reaction 3H2 + O2 ­2H2O + 2H and an exothermic recombination reaction H + H ! H2. It is seen that the activation temperature of the branching step is sufficiently large that branching occurs in a relatively thin layer at a temperature slightly above the crossover value, whereas radical recombination occurs in a distributed manner both upstream and downstream from this layer, yielding a flame structure that in many aspects resembles that found by Zel’dovich in his analysis of branched-chain flames with model chemistry. The leading-order solution of the resulting problem determines the flame propagation velocity. The solution involves the numerical integration of the conservation equations in the recombination regions with appropriate jump conditions imposed at the branching sheet, whose temperature is obtained by the analysis of the branching layer. The results compare reasonably well with those of detailed-chemistry computations for varying conditions of composition, pressure and initial temperature. __________________________________________________En esta tesis se aborda el estudio teórico de deflagraciones pobres en mezclas de hidrógeno y aire mediante el uso combinado de métodos numéricos y asintóticos, que, a partir de una descripción detallada de las reacciones químicas implicadas, permiten clarificar la estructura interna de las llamas y proporcionan expresiones simplificadas para el ritmo de reacción. Aunque la mayoría de los cálculos se han realizado con valores atmosféricos normales de presión y temperatura inicial, se han considerado otros valores, incluyendo mezclas precalentadas y precomprimidas, así como condiciones de presión subatmosférica y mezclas criogénicas. El rango de dosados investigado va desde el límite pobre de inflamabilidad hasta condiciones estequiométricas, aunque se ha profundizado especialmente en el estudio de llamas cerca del límite de inflamabilidad. En primer lugar, se considera la descripción de la química. Partiendo de un mecanismo detallado de veintiuna reacciones elementales reversibles se busca el mínimo conjunto de reacciones elementales capaces de describir con precisión las llamas premezcladas de hidrógeno. Se demuestra que un mecanismo corto de siete reacciones elementales, de las que sólo tres de ellas son reversibles, proporciona una buena predicción para la velocidad de propagación de las llamas cuando el dosado es suficientemente pequeño. Se demuestra, además, que añadiendo dos reacciones de recombinación irreversibles al mecanismo de siete pasos se consigue extender la precisión del mecanismo para cubrir el rango completo de condiciones de inflamabilidad en condiciones atmosféricas normales. Los cálculos indican también que un mecanismo corto de ocho pasos, construido a partir del mecanismo corto de siete pasos mediante la adición de la cuarta reacción de intercambio de radicales, mejora las predicciones para los perfiles de O y OH. Seguidamente, se estudia la propagación de deflagraciones de hidrógeno en condiciones cercanas al límite de inflamabilidad partiendo del mecanismo de siete reacciones elementales. La capa de reacción que controla la velocidad de propagación laminar resulta ser muy delgada y contiene concentraciones muy pequeñas de todas las especies intermedias, de forma que todas ellas se encuentran en estado estacionario, mientras que las especies principales reaccionan según la reacción global irreversible 2H2 + O2 !2H2O. El análisis proporciona una expresión explícita de tipo no-Arrhenius para el ritmo de esta reacción global. Este mecanismo reducido de un paso permite en particular el cálculo de los límites de inflamabilidad y de la velocidad de propagación para deflagraciones pobres planas. El estudio incluye un repaso de las implicaciones que los perfiles de radicales tienen en la deflagración, junto con las razones que permiten que las aproximaciones funcionen. El nuevo mecanismo reducido desarrollado en este capítulo de la tesis puede ser de utilidad en posteriores estudios analíticos y permite su fácil implementación en códigos computacionales para el cálculo de llamas pobres de hidrógeno, disminuyendo los costes de cálculo. Se estudia a continuación la estructura interna de la capa delgada de reacción de las deflagraciones pobres en mezclas de hidrógeno y aire próximas al límite de inflamabilidad. En el análisis, que emplea siete reacciones elementales para la descripción de la química, se usa el cociente entre las concentraciones de H, que es el radical dominante, y del combustible como parámetro asintótico pequeño, lo que permite obtener una descripción analítica precisa del ritmo de combustión. El análisis revela que la hipótesis de estado estacionario para las especies intermedias, que resulta apropiada en el lado caliente de la capa reactiva, falla, sin embargo, conforme nos acercamos a la temperatura de cruce, donde existe una capa interna delgada en la que el término de transporte difusivo de los radicales es comparable a los de producción y consumo asociados a la química. El análisis de esta región proporciona una corrección relativamente importante al ritmo de combustión. Los resultados obtenidos son útiles, por ejemplo, para la futura investigación de inestabilidades de llamas en mezclas pobres de hidrógeno. Finalmente, el estudio se centra en llamas pobres relativamente lejos del límite de inflamabilidad. Bajo estas condiciones, la hipótesis de estado estacionario para el radical H falla y el mecanismo de un paso para la oxidación del hidrógeno debe ser sustituido por un mecanismo reducido de dos pasos, que incluye una reacción de ramificación con una fuerte dependencia con la temperatura 3H2 + O2 ­ 2H2O + 2H y una reacción exotérmica de recombinación H + H ! H2. Se observa que la temperatura de activación de la reacción de ramificación es lo suficientemente grande como para considerar que la producción de radicales ocurre en una capa relativamente delgada a una temperatura ligeramente por encima de la temperatura de cruce (definida como la temperatura a la que el ritmo de ambas reacciones se iguala), mientras que la recombinación de radicales ocurre de forma distribuida aguas arriba y aguas abajo de esta capa delgada en regiones de espesor comparable al de la llama. La estructura resultante se parece en muchos aspectos a la que encontró Zel’dovich en su análisis de llamas dominadas por reacciones de ramificación, basado en una descripción modelo de dos etapas para la química. El problema que determina la velocidad de propagación de la llama se reduce en primera aproximación a la integración numérica de las ecuaciones de conservación en la regiones exteriores de recombinación, con condiciones de contorno que incluyen condiciones de salto a través de la capa interna de ramificación, que se se encuentra a una temperatura que se determina a partir del análisis de su estructura interna. La solución que se obtiene de este análisis tipo Zeldovich se compara con cálculos numéricos del problema inicial completo, dando resultados satisfactorios en un amplio rango de condiciones de composición, presión y temperatura inicial

The differential diffusion effect of the intermediate species on the stability of premixed flames propagating in microchannels

The propagation of premixed flames in adiabatic and non-catalytic planar microchannels subject to an assisted or opposed Poiseuille flow is considered. The diffusive-thermal model and the well-known two-step chain-branching kinetics are used in order to investigate the role of the differential diffusion of the intermediate species on the spatial and temporal flame stability. This numerical study successfully compares steady-state and time-dependent computations to the linear stability analysis of the problem. Results show that for fuel Lewis numbers less than unity, LeF 1, flames propagating in adiabatic channels suffer from oscillatory instabilities. The Poiseuille flow stabilises the flame and the effect of LeZ is opposite to that found for LeF < 1. Small values of LeZ further destabilise the flame to oscillating or pulsating instabilities.This research was supported by the Spanish Ministerio de Ciencia e Innovación (MICINN) [Projects #ENE2011-27686-C02-01, #ENE2012-33213]; the Comunidad de Madrid [Project #S2009/ENE-1597, CONSOLIDER CSD2010-00011].Publicad

One-step reduced kinetics for lean hydrogen–air deflagration

A short mechanism consisting of seven elementary reactions, of which only three are reversible, is shown to provide good predictions of hydrogen–air lean-flame burning velocities. This mechanism is further simplified by noting that over a range of conditions of practical interest, near the lean flammability limit all reaction intermediaries have small concentrations in the important thin reaction zone that controls the hydrogen–air laminar burning velocity and therefore follow a steady state approximation, while the main species react according to the global irreversible reaction 2H2 + O2 → 2H2O. An explicit expression for the non-Arrhenius rate of this one-step overall reaction for hydrogen oxidation is derived from the seven-step detailed mechanism, for application near the flammability limit. The one-step results are used to calculate flammability limits and burning velocities of planar deflagrations. Furthermore, implications concerning radical profiles in the deflagration and reasons for the success of the approximations are clarified. It is also demonstrated that adding only two irreversible direct recombination steps to the seven-step mechanism accurately reproduces burning velocities of the full detailed mechanism for all equivalence ratios at normal atmospheric conditions and that an eight-step detailed mechanism, constructed from the seven-step mechanism by adding to it the fourth reversible shuffle reaction, improves predictions of O and OH profiles. The new reduced-chemistry descriptions can be useful for both analytical and computational studies of lean hydrogen–air flames, decreasing required computation times

The role of conductive heat losses on the formation of isolated flame cells in Hele-Shaw chambers

The propagation of low-Lewis-number premixed flames is analyzed in a partially confined Hele-Shaw chamber formed by two parallel plates separated a distance h apart. An asymptotic-numerical study can be performed for small gaps compared to the flame thickness deltaT . In this narrow-channel limit, the prob- lem formulation simplifies to a quasi-2D description in which the velocity field is controlled by domi- nant viscous effects. After accounting for conductive heat losses through the plates in our formulation, we found that the reaction front breaks into one or several isolated flame cells where the temperature is large enough to sustain the reaction, both in absence and in presence of buoyancy effects. Under these near-limit conditions, the isolated flame cells either travel steadily or undergo a slow random walk over the chamber in which the reacting front splits successively to form a tree-like pathway, burning only a small fraction of the fuel before reaching the end of the chamber. The production of quasi-2D circular or comet-like flames under specific favorable conditions is demonstrated in this paper, with convection, conductive heat losses and differential diffusion playing an essential role in the formation of the isolated one and two-headed flame cells.This work was supported by the project ENE2015-65852-C2-1-R (FV,MSS,DMR) and ENE2015-65852-C2-2-R (DFG,VK) (MINECO/FEDER, UE). Daniel Martínez-Ruiz would like to thank Amable Liñán for fruitful discussions

Enhancing Energy Production with Exascale HPC Methods

High Performance Computing (HPC) resources have become the key actor for achieving more ambitious challenges in many disciplines. In this step beyond, an explosion on the available parallelism and the use of special purpose processors are crucial. With such a goal, the HPC4E project applies new exascale HPC techniques to energy industry simulations, customizing them if necessary, and going beyond the state-of-the-art in the required HPC exascale simulations for different energy sources. In this paper, a general overview of these methods is presented as well as some specific preliminary results.The research leading to these results has received funding from the European Union's Horizon 2020 Programme (2014-2020) under the HPC4E Project (www.hpc4e.eu), grant agreement n° 689772, the Spanish Ministry of Economy and Competitiveness under the CODEC2 project (TIN2015-63562-R), and from the Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Pesquisa (RNP). Computer time on Endeavour cluster is provided by the Intel Corporation, which enabled us to obtain the presented experimental results in uncertainty quantification in seismic imagingPostprint (author's final draft

Applying future Exascale HPC methodologies in the energy sector

The appliance of new exascale HPC techniques to energy industry simulations is absolutely needed nowadays. In this sense, the common procedure is to customize these techniques to the specific energy sector they are of interest in order to go beyond the state-of-the-art in the required HPC exascale simulations. With this aim, the HPC4E project is developing new exascale methodologies to three different energy sources that are the present and the future of energy: wind energy production and design, efficient combustion systems for biomass-derived fuels (biogas), and exploration geophysics for hydrocarbon reservoirs. In this work, the general exascale advances proposed as part of HPC4E and its outcome to specific results in different domains are presented.The research leading to these results has received funding from the European Union's Horizon 2020 Programme (2014-2020) under the HPC4E Project (www.hpc4e.eu), grant agreement n° 689772, the Spanish Ministry of Economy and Competitiveness under the CODEC2 project (TIN2015-63562-R), and from the Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Pesquisa (RNP). Computer time on Endeavour cluster is provided by the Intel Corporation, which enabled us to obtain the presented experimental results in uncertainty quantification in seismic imaging.Postprint (author's final draft

Running title: Non-toxic broad anti-tumor activity of an EGFR×4-1BB bispecific trimerbod

32 p.-4 fig.Purpose: The induction of 4-1BB signaling by agonistic antibodies can drive the activation and proliferation of effector T cells and thereby enhance a T-cell–mediated antitumor response. Systemic administration of anti-4-1BB–agonistic IgGs, although effective preclinically, has not advanced in clinical development due to their severe hepatotoxicity.Experimental Design: Here, we generated a humanized EGFR-specific 4-1BB-agonistic trimerbody, which replaces the IgG Fc region with a human collagen homotrimerization domain. It was characterized by structural analysis and in vitro functional studies. We also assessed pharmacokinetics, antitumor efficacy, and toxicity in vivo.Results: In the presence of a T-cell receptor signal, the trimerbody provided potent T-cell costimulation that was strictly dependent on 4-1BB hyperclustering at the point of contact with a tumor antigen-displaying cell surface. It exhibits significant antitumor activity in vivo, without hepatotoxicity, in a wide range of human tumors including colorectal and breast cancer cell-derived xenografts, and non–small cell lung cancer patient-derived xenografts associated with increased tumor-infiltrating CD8+ T cells. The combination of the trimerbody with a PD-L1 blocker led to increased IFNγ secretion in vitro and resulted in tumor regression in humanized mice bearing aggressive triple-negative breast cancer.Conclusions: These results demonstrate the nontoxic broad antitumor activity of humanized Fc-free tumor-specific 4-1BB-agonistic trimerbodies and their synergy with checkpoint blockers, which may provide a way to elicit responses in most patients with cancer while avoiding Fc-mediated adverse reactions.This work was supported by grants from the European Union [IACT Project (602262), H2020-iNEXT (1676)]; the Spanish Ministry of Science, Innovation and Universities and the Spanish Ministry of Economy and Competitiveness (SAF2017-89437-P, CTQ2017-83810-R, RTC-2016-5118-1, RTC-2017-5944-1), partially supported by the European Regional Development Fund; the Carlos III Health Institute (PI16/00357), co-founded by the Plan Nacional de Investigación and the European Union; the CRIS Cancer Foundation (FCRIS-IFI-2018); and the Spanish Association Against Cancer (AECC, 19084). C. Domínguez-Alonso was supported by a predoctoral fellowship from the Spanish Ministry of Science, Innovation and Universities (PRE2018-083445). M. Zonca was supported by the Torres Quevedo Program from the Spanish Ministry of Economy and Competitiveness, co-founded by the European Social Fund (PTQ-16-08340).Peer reviewe

Analysis of an idealized counter-current microchannel-based reactor to produce hydrogen-rich syngas from methanol

In an effort to investigate the suitability of the concept of portable hydrogen production, we examine numerically the combustion of a very rich methanol-air mixture in a micro-gap assembly consisting of multiple counter-current channels of finite length separated by thin solid conducting walls. Within the mathematical framework of the narrow-channel approximation, the problem can be formulated as a one-dimensional model for a single channel with an extra term representing heat transfer from the hot stream products to the fresh reactants in adjacent channels. We show that the heat recirculation enables superadiabatic temperatures inside the reactor and promotes the oxidation of methanol far beyond the conventional rich limit of flammability. The result is a feasible thermal partial oxidation that produces hydrogen without the need for a catalyst. The paper presents an analysis of the model burner performance with detailed gas-phase kinetics in stationary regimes in terms of operating variables such as the equivalence ratio and the gas inflow velocity, and in terms of physical parameters such as the length of the reformer and the conductivity of the wall material. The idealized microreactor predicts maximum hydrogen yield of the order of 60% at equivalence ratios between 3 and 6.This research was supported by projects ENE2015-65852-C2-1-R and ENE2015-65852-C2-2-R (MINECO/FEDER, UE)

BioAnnote: A software platform for annotating biomedical documents with application in medical learning environments

Automatic term annotation from biomedical documents and external information linking are becoming a necessary prerequisite in modern computer-aided medical learning systems. In this context, this paper presents BioAnnote, a flexible and extensible open-source platform for automatically annotating biomedical resources. Apart from other valuable features, the software platform includes (i) a rich client enabling users to annotate multiple documents in a user friendly environment, (ii) an extensible and embeddable annotation meta-server allowing for the annotation of documents with local or remote vocabularies and (iii) a simple client/server protocol which facilitates the use of our meta-server from any other third-party application. In addition, BioAnnote implements a powerful scripting engine able to perform advanced batch annotations.1.093 JCR (2013) Q2, 32/102 Computer science, theory & methods; Q3, 68/102 Computer science, interdisciplinary applications, 54/77 Engineering, biomedical, 16/25 Medical informaticsUE