Theoretical and numerical analysis of the evaporation of mono and multicomponent single fuel droplets

Single fuel droplet vaporization, with special attention to the case of ethanol, is considered in this study. First, we showed, using an order-of-magnitude analysis and detailed unsteady simulations, that the commonly used quasi-steady assumption is not suitable for an accurate description of the liquid phase during the evaporation process. Second, we demonstrated that an increase in the relative importance of radiation explains the departures of the evaporation rate from the d2-law observed experimentally when sufficiently large droplets – initial radius above 0.25 mm – evaporated in ambient temperatures around 800 K. The multicomponent formulation included here, in which the physical properties of both liquid and gas phases depend on the concentration of the different species involved, was validated by comparing our numerical results with experimental data of ethanol, n-heptane, ethanol–water and n-dodecane–n-hexadecane droplets available in the literature. Because of its technological relevance, we dedicated special attention to the effect of the droplet water content and ambient humidity on the evaporation time of ethanol droplets. Our computations showed higher vaporization rates with increasing ambient humidity as a consequence of the extra heat generated during the condensation of moisture on the droplet surface.The authors express their gratitude to Professor F. Williams in the conception and guidance of this work, in particular, and all the ongoing work on ethanol droplet vaporization and combustion. This work was supported by the project ENE2015-65852-C2-1-R (MINECO/FEDER,UE). The authors are grateful for the comments and suggestions offered by an anonymous referee during the revision of the paper

A multipurpose reduced mechanism for ethanol combustion

New multipurpose skeletal and reduced chemical-kinetic mechanisms for ethanol combustion are developed, along the same philosophical lines followed in our previous work on methanol. The resulting skeletal mechanism contains 66 reactions, only 19 of which are reversible, among 31 species, and the associated reduced mechanism contains 14 overall reactions among 16 species, obtained from the skeletal mechanism by placing CH3CHOH, CH2CH2OH, CH3CO, CH2CHO, CH2CO, C2H3, C2H5, C2H6, S - CH2, T - CH2, CH4, CH2OH, CH3O, HCO, and O in steady state. For the reduced mechanism, the steady-state relations and rate expressions are arranged so that computations can be made sequentially without iteration. Comparison with experimental results for autoignition, laminar burning velocities, and counterflow flame structure and extinction, including comparisons with the 268-step, 54-species detailed San Diego Mechanism and five other mechanisms in the literature, support the utility of the skeletal and reduced mechanisms, showing, for example, that, in comparison with the San Diego mechanism, they reduced the computational time by a factor of 4 (71 % faster) and 12 (93 % faster), respectively. Measures of computation times and of extents of departures from experimental values are defined and employed in evaluating results. Besides contributing to improvements in understanding of the mechanisms, the derived simplifications may prove useful in a variety of computational studies.This work was supported by projects ENE2015-65852-C2-1-R (MINECO/FEDER, UE) and BYNV-ua37crdy (Fundacion Iberdrola Espaha)

Modified multipurpose reduced chemistry for ethanol combustion

We present in this short communication a modification to our previous ethanol reduced combustion chemistry (Millán-Merino, 2018) that eliminates nonphysical values of the species concentrations which we discovered in applying the mechanism to the combustion of an isolated ethanol droplet. This unsteady test is reported here to check the multipurpose character of the reduced mechanism for a problem that combines non-homogeneous autoignition, rich and lean premixed-flame propagation, and the development of a diffusion flame, as well as a the presence of a cold moving boundary at the droplet surface. During the computations, production and consumption rates of the alfa-hydroxyethyl (CH3CHOH) intermediary radical became unbalanced, invalidating its steady-state hypothesis, which was used during the derivation of the reduced scheme. This difficulty is removed here by taking CH3CHOH out of steady state, thereby augmenting slightly the reduced mechanism.This work was supported by the project ENE2015-65852-C2-1-R (MINECO/FEDER,UE)

Gene signatures of early response to anti-TNF drugs in pediatric inflammatory bowel disease

T. Around a 20–30% of inflammatory bowel disease (IBD) patients are diagnosed before they are 18 years old. Anti-TNF drugs can induce and maintain remission in IBD, however, up to 30% of patients do not respond. The aim of the work was to identify markers that would predict an early response to anti-TNF drugs in pediatric patients with IBD. The study population included 43 patients aged <18 years with IBD who started treatment with infliximab or adalimumab. Patients were classified into primary responders (n = 27) and non-responders to anti-TNF therapy (n = 6). Response to treatment could not be analyzed in 10 patients. Response was defined as a decrease in over 15 points in the disease activity indexes from week 0 to week 10 of infliximab treatment or from week 0 to week 26 of adalimumab treatment. The expression profiles of nine genes in total RNA isolated from the whole-blood of pediatric IBD patients taken before biologic administration and after 2 weeks were analyzed using qPCR and the 2−∆∆Ct method. Before initiation and after 2 weeks of treatment the expression of SMAD7 was decreased in patients who were considered as non-responders (p value < 0.05). Changes in expression were also observed for TLR2 at T0 and T2, although that did not reach the level of statistical significance. In addition, the expression of DEFA5 decreased 1.75-fold during the first 2 weeks of anti-TNF treatment in responders, whereas no changes were observed in non-responders. Expression of the SMAD7 gene is a pharmacogenomic biomarker of early response to anti-TNF agents in pediatric IBD. TLR2 and DEFA5 need to be validated in larger studies.This work was funded by Instituto de Salud Carlos III (grants numbers PI16/00559 and PI19/00792), Consejería de Educación y Deporte de la Comunidad de Madrid (grant number PEJ16/MED/AI-1260), and by the Gregorio Marañón Health Research Institute (grant number PRE-2018-2), The study was cofunded by ERDF Funds (FEDER) from the European Commission, “A way of making Europe

EducaFarma 9.0

Memoria ID-020 Ayudas de la Universidad de Salamanca para la innovación docente, curso 2020-2021

EducaFarma 11.0

Memoria ID2022-036. Ayudas de la Universidad de Salamanca para la innovación docente, curso 2022-2023

Educafarma 10.0

Memoria ID-030. Ayudas de la Universidad de Salamanca para la innovación docente, curso 2021-2022

EDUCACIÓN AMBIENTAL Y SOCIEDAD. SABERES LOCALES PARA EL DESARROLLO Y LA SUSTENTABILIDAD

Este texto contribuye al análisis científico de varias áreas del conocimiento como la filosofía social, la patología, la educación para el cuidado del medio ambiente y la sustentabilidad que inciden en diversas unidades de aprendizaje de la Licenciatura en Educación para la Salud y de la Maestría en Sociología de la SaludLas comunidades indígenas de la sierra norte de Oaxaca México, habitan un territorio extenso de biodiversidad. Sin que sea una área protegida y sustentable, la propia naturaleza de la región ofrece a sus visitantes la riqueza de la vegetación caracterizada por sus especies endémicas que componen un paisaje de suma belleza

Theoretical and numerical analysis of isolated ethanol droplets: evaporation and combustion

Mención Internacional en el título de doctorOn Thursday 28 November 2019, the European Union (EU) Parliament declared a global climate and environmental emergency, with 429 out of 654 votes in favour of immediately taking action against global warming by implementing ambitious actions to limit the average temperature rise to 1.5°C. The commitment of the EU implies a reduction of 55% in the CO₂ emissions by 2030 to become climate neutral by 2050. As indicated by the BP Energy Outlook 2019, the world final energy demand by end users is expected to increase in the next decades, with around 80% of this energy coming from carbon-containing fuels: Oil, Natural Gas and Coal. The data published by the EU revealed that the share of renewable energy in 2017 was 17.5%, with the total energy consumption growing for third year in a row to reach 1561 Mtoe, in line with a worldwide trend that augmented CO₂ emissions in a 2.7% in 2018 with respect to the values of 2017. Even if the plan of the EU is achieved, by 2030 only 27% of all the energy consumed will come from Renewable Energy Sources (RES). The remaining energy will necessarily come from the increasingly large range of greener fuels (hydrogen, ammonia, biofuels, synthetic natural gas and other synthetic fuels as DME), with a much better carbon footprint than heavy hydrocarbons (gasoline, diesel and coal), and they should be used as efficiently as possible to achieve the target of reducing by 55% the emissions of greenhouse gases by 2030. In this thesis we focus in ethanol C₂H₅OH, byproduct of plant fermentation that is been extensively used in engines to lower Green House Gas (GHG) emissions. In most practical applications, the fuel or mix of fuels are injected as a spray inside the combustion chamber, forming a cloud of small droplets that evaporates, first, mixing with the surrounding ambient air before the flame is formed either by autoignition or with the help of an external source of heat. In this thesis we will study in detail the transient one-dimensional evaporation and combustion of an isolated ethanol droplet immersed in a hot and humid atmosphere. In Chapter 2 of the thesis we formulate the problem, writing the system of equations that describes the whole evaporation and combustion process, using first principles only and, intentionally, avoiding the utilization of semi-empirical correlations or ad-hoc fitting parameters often used in the literature to improve the matching between numerical simulations and experimental measurements. Chapter 3 deals with pure evaporation of spherically-symmetric multicomponent ethanol droplets in an inert ambient. After performing a exhaustive validation of both the model and the numerical code used to integrate the system of equations, we theoretically determined the different characteristic times of the evaporation process setting the limits of validation of the classical d²-law, according to which the square of the droplet diameter decays linearly with time. This theory is valid in the limit in which the heat conductive time in both the liquid and gas phases is much shorter than the droplet lifetime. Even though this limit is valid in small droplets, we demonstrate in this chapter, using both theoretical and numerical arguments, that this hypothesis is only valid for sufficiently small droplets or ambient temperatures close to the boiling temperature of the liquid phase, when the radiation from the gas surrounding the droplet can be neglected. As the droplet radius and/or the ambient temperature are increased, radiative heating gains relevance, deviating the droplet evaporation rate from the predictions of the d²-law. Moreover, the characteristic radiation time, defined as the time needed to evaporate the droplet using only the heat radiated from the ambient, becomes comparable to the characteristic heat conductive time of the liquid phase, suggesting that the heating of the liquid fuel is not quasi-steady. This chapter also analyses the effect of water content and ambient moisture in the evaporation rate of ethanol droplets. Their effect is opposed regarding the vaporization rate of ethanol, with moisture reducing the time needed to vaporize the liquid fuel. On the other hand, both the presence of ambient humidity and water in the droplet content would increase the droplet lifetime, mainly as a consequence of a lower vaporization rate induced by the large concentration of water in the droplet during the last stages of the vaporization process. In Chapters 4 and 5 we turned our attention to droplet autoignition and combustion. Droplet combustion is a complex unsteady phenomena that involves autoignition and rich and lean unsteady premixed and diffusion flames. The description of such complicate phenomena requires the utilization of detailed combustion chemical kinetics, hundreds of elementary reaction steps and radical species to make the calculations accurate but, also, computationally very expensive. To cut the cost of the calculations, it is normal to use skeletal or reduced chemical schemes. Nevertheless, they are usually developed to describe some specific combustion configurations that are not capable of describing the complex dynamics undergone by the flame after the sudden autoignition event. For that reason, Chapter 4 is dedicated to develop a multipurpose reduced chemical scheme, that involves only 14 overall steps among 16 reactive chemical species, capable of describing autoignition, premixed and diffusion flames and extinction. The new mechanism is derived by introducing chemical-kinetic steady-state approximations for the relevant intermediate species, using, as starting point, a skeletal mechanism of 66 steps and 31 chemical species. The reduced chemistry description here described achieved computation time savings of 93 % when compared with the computational time of the detailed mechanism. Finally, in Chapter 5 the reduced mechanism is used to study the conditions that would lead to droplet autoignition. During the simulations it was found that the reduced mechanisms developed in Chapter 4 predicted accurately autoiginition times but, after ignition, during the transition to the quasi-steady droplet combustion, the simulations with the reduced mechanism stalled, giving nonphysical values of species concentrations. Detailed analysis of the flame structure revealed that the quasi-steady hypothesis fails for the intermediate radical α-hydroxyethyl. After taking this radical out of the steady state, a modified and more robust reduced mechanism of 15 steps and 17 reactive chemical species was capable of completely describing the unsteady combustion of ethanol droplets. Our numerical results depicted a map that defines the critical ambient temperature Tᶜ[subíndice ∞] below which autoignition can not take place. In ambient temperatures below T¹[subíndice ∞], the droplet evaporates completely before ignition. For sufficiently large ambient temperatures T[subíndice ∞] > Tᶜ[subíndice ∞], the autoignition event suddenly raised the temperature to form a lean premixed flame that propagates towards the droplet surface consuming all the available oxygen before bouncing back as a diffusion flame, that evolves to reach a quasi-steady state. Additionally, we analyzed the effect of the droplet water content and of the ambient moisture and their effect on the autoignition time, to discover that ambient humidity accelerates droplet autoignition while the initial droplet water content delays the beginning of combustion. The analysis extends to describe the structure of the quasi-steady flame structures that emerge during the last stage of the combustion process. The flame is located at a distance that ranges between three and seven times the initial droplet radius, comparable to the inter-droplet distance in sprays. Surprisingly, the reaction region turns out to be much thicker than expected, with a chemically active region that spans distances of the order of several droplet diameters. This result contrasts with previous results and questions a great number of theoretical and asymptotic studies that assumed infinitely thin reaction regions.El jueves 28 de noviembre de 2019, el Parlamento Europeo declaró le emergencia climática y medioambiental con 429 votos de 654 a favor de tomar medidas inmediatas contra el calentamiento global mediante la ejecución de acciones ambiciosas para limitar el aumento de la temperatura media a 1.5°C. El compromiso de la Unión Europea (UE) implica una reducción del nivel de las emisiones de CO₂ del 55% para el año 2030 con el objetivo de llegar a la neutralidad climática en 2050. Como se indica en el informe BP Energy Outlook 2019, se espera que la demanda de energía por parte de los usuarios finales aumente en las próximas décadas, con un 80% de esta energía adicional proveniente de combustibles con carbono: petróleo, gas natural y carbón. Los datos publicados por la UE revelan que las energías renovables aportaron en 2017 un 17.5% del total de una energía consumida que creció por tercer año consecutivo alcanzando 1561 Mtoe, en línea con la tendencia mundial que provocó un aumento de emisiones de CO2 de un 2.7% en 2018 con respecto a 2017. Incluso si las previsiones de la UE son correctas, en 2030 sólo el 27% de la energía consumida provendrá de Fuentes de Energía Renovables (FER). El resto de la energía provendrá necesariamente del cada vez más amplio catálogo de combustibles verdes (hidrógeno, amoníaco, biocombustibles, gas natural sintético y otros combustibles sintéticos como el DME), con una huella ambiental mucho menor que los hidrocarburos pesados (gasolina, gas-oil y carbón), que deberían ser utilizados con la mayor eficiencia posible para lograr el objetivo de reducir las emisiones de gases de efecto invernadero (GEI) en un 55% en 2030. En esta tesis centraremos nuestra atención en el etanol C₂H₅OH, obtenido en la fermentación de plantas, y que es ampliamente utilizado en motores para reducir las emisiones de GEI. En la mayoría de las aplicaciones prácticas, el combustible, o la mezcla de combustibles, es inyectado en forma de spray en la cámara de combustión, formando una una nube de pequeñas gotas que primero se deben evaporar, mezclándose a continuación con el aire antes de que se genere la llama, bien por autoignición o mediante ignición forzada. En esta tesis estudiaremos en detalle la evaporación y combustión no estacionaria de una gota esférica de etanol en una atmósfera caliente y húmeda. En el capítulo 2 de la tesis formularemos el problema general, escribiendo el sistema de ecuaciones que describe el proceso completo de evaporación y combustión, utilizando sólo primeros principios y evitando, intencionadamente, la utilización de correlaciones semi-empíricas o parámetros de ajuste ad-hoc utilizados a menudo en la literatura para mejorar la concordancia entre los resultados de las simulaciones numéricas y las medidas experimentales. El capítulo 3 está dedicado al análisis, en condiciones de simetría esférica, de la evaporación pura de una gota de etanol con varios componentes en una atmósfera inerte. Se realiza una validación exhaustiva de los modelos y del código numérico utillizado para integrar el sistema de ecuaciones y se determinan los diferentes tiempos característicos presentes en el proceso de evaporación, indicando los límites de validez de la clásica ley d2, de acuerdo con la cual el cuadrado del diámetro de la gota decae linealmente con el tiempo. Esta teoría es válida en el límite en que los tiempos de conducción de calor en las fases líquida y gaseosa es mucho menor que el tiempo de vida de la gota. Utilizando argumentos teóricos y numéricos, en este capítulo demostramos que este límite es válido cuando la radiación incidente en la gota es despreciable, lo que ocurre solo para gotas pequeñas y cuando la temperatura ambiente es cercana a la temperatura de ebullición de la fase líquida. A medida que el tamaño inicial de la gota y/o la temperatura ambiente aumentan, el calentamiento por radiación se hace más importante y la velocidad de vaporización difiere de las predicciones de la ley d2. El tiempo característico de evaporación por radiación, definido como el tiempo necesario para evaporar la gota considerando únicamente el efecto de la radiación, se hace comparable con el tiempo característico de conducción de calor en la fase líquida, sugiriendo la posible no estacionariedad del calentamiento de la fase líquida dentro de la gota. En este capítulo también se analizan los efectos del contenido inicial de agua en la gota y de la humedad ambiente sobre la velocidad de vaporización de las gotas de etanol. El efecto de estas dos variables en la velocidad de vaporización es opuesto, con la humedad ambiente reduciendo el tiempo necesario para vaporizar el combustible líquido. Por otro lado, tanto la presencia de agua en la gota como la humedad ambiente aumenta el tiempo de vida de la gota a causa de una menor velocidad de vaporización, asociada a la alta concentración de agua en las últimas etapas del proceso de evaporación. En los capítulos 4 y 5 centraremos la atención en la autoignición y combustión de gotas. La combustión de una gota es un complejo fenómeno no estacionario que implica ignición (autoinducida o forzada), llamas ricas y pobres de premezcla y llamas de difusión no estacionarias. La descripción de un fenómeno tan complicado requiere la utilización de una química detallada, que implica cientos de reacciones elementales y especies, para lograr una descripción precisa que lleva asociada un elevado coste computacional. Con el fin de aligerar el consumo de recursos de cálculo, se suele utilizar cinéticas mínimas, en las que se elimina de la cinética detallada las reacciones y especies que no juegan un papel importante, o bien cinéticas reducidas en las que el coste es incluso inferior al introducir la hipótesis de estado estacionario para ciertos radicales minoritarios. Normalmente, estas cinéticas se desarrollan para la descripción de configuraciones específicas de combustión y no son capaces de describir la compleja dinámica de la llama tras la súbita autoignición. Por esta razón, el capítulo 4 está dedicado al desarrollo de un mecanismo reducido multipropósito formado por tan sólo 14 reacciones globales y 16 especies reactivas, capaz de describir adecuadamente la autoignición y las llamas premezcladas y de difusión, así como su extinción. El mecanismo reducido aquí desarrollado se obtiene mediante la introducción de la hipotesis de estado estacionario para las especies intermedias apropiadas, utilizando como punto de partida un mecanismo mínimo formado por 66 reacciones elementales y 31 especies químicas. La cinética reducida obtenida consigue reducciones de tiempo de cálculo del 93% respecto al mecanismo detallado. Finalmente, en el capítulo 5, el mecanismo reducido desarrollado se utiliza para estudiar las condiciones que producen autoignición. Al realizar las simulaciones numéricas, se encontró que el mecanismo reducido desarrollado en el capítulo 4 predecía satisfactoriamente los tiempos de autoignición pero, después de la misma, durante la transición hacia la combustión casi-estacionaria, las simulaciones fallaban y se obtenían valores no físicos para las concentraciones de las especies. Un análisis detallado de la estructura de la llama mostró que la hipótesis de estado estacionario falla, en estas condiciones, para el radical intermedio CH2CHOH. Eliminando la hipótesis de estado estacionario para esta especie se obtiene un mecanismo reducido modificado más robusto, con 15 pasos y 17 especies reactivas, capaz de describir la combustión no estacionaria de una gota de etanol. Mediante simulación numérica hemos obtenido un mapa de autoignición que define la temperatura ambiente crítica Tᶜ[subíndice ∞] por debajo de la cual la autoginición no tiene lugar. A temperaturas ambiente por debajo de Tᶜ[subíndice ∞], la gota se evapora completamente antes de la autoignición, mientras que para temperatura ambiente suficientemente alta T[subíndice ∞] > Tᶜ[subíndice ∞], la autoignición genera un frente de llama pobre que se propaga hacia la gota consumiendo todo el oxígeno disponible antes de rebotar como una llama de difusión que evoluciona hacia un estado casi-estacionario. Además, se ha analizado el efecto del contenido inicial de agua en la gota y de la humedad en el ambiente sobre el tiempo de autoignición, hallándose que la humedad ambiente acelera la autoignición mientras que, por el contrario, el agua contenida inicialmente en la gota retarda el inicio de la combustión. Tras la autoignición, el análisis se extiende para describir la estructura casi-estacionaria que surge durante la última etapa de la combustión de la gota. La llama se sitúa a una distancia entre tres y siete veces el radio inicial de la gota, comparable a la distancia entre gotas en sprays. Sorprendentemente, la zona de reacción resulta ser mucho más gruesa de lo esperado y la zona químicamente activa se extiende varios diámetros de gota. Este resultado contrasta con resultados previos y cuestiona un gran número de estudios teóricos y asintóticos que consideran el límite de llama infinitamente delgada.Programa de Doctorado en Mecánica de Fluidos por la Universidad Carlos III de Madrid; la Universidad de Jaén; la Universidad de Zaragoza; la Universidad Nacional de Educación a Distancia; la Universidad Politécnica de Madrid y la Universidad Rovira i VirgiliPresidente: Javier Ballester Castañer.- Secretario: Pierre Boivin.- Vocal: Francesco Saverio Marr

Single-step mechanisms for the autoignition of fuel-air mixtures in constant volume batch reactors

International audienceA new methodology to build simple and irreversible Arrhenius single-step mechanisms has been developed to accurately reproduce autoignition properties of fuel-air mixtures in constant volume applications. The main idea of the method is to define virtual chemical species, designed such as the global heat release is properly recovered, bypassing the need to account for the minor species in recovering the adiabatic temperature. A classical asymptotic expression, relating the ignition delay time with the reaction rate constant, is then used to fit the Arrhenius coefficients accordingly with the computations made with the complete mechanism .The main difference with previous procedures, is that a single parameter is required to recover ignition properties for a wide range of initial conditions (temperature, pressure, and equivalence ratio). The minimum amount of species were used to match the equilibrium properties of the completed mechanism and were tabulated as a polynomial function for the initial conditions, which speed up the procedure of finding the proper correlation between the initial-final compositions. This methodology also allows recovering the original species by simply substituting the virtual species mass fraction in the post processing stage. The following procedure has been extended to develop a two-step mechanism where a new step for the fuel degradation has been added and coupled with the combustion step. This extension points out the robustness of the approximation done in this work and their flexibility to be adapted to different situations