Phase appearance or disappearance in two-phase flows

This paper is devoted to the treatment of specific numerical problems which appear when phase appearance or disappearance occurs in models of two-phase flows. Such models have crucial importance in many industrial areas such as nuclear power plant safety studies. In this paper, two outstanding problems are identified: first, the loss of hyperbolicity of the system when a phase appears or disappears and second, the lack of positivity of standard shock capturing schemes such as the Roe scheme. After an asymptotic study of the model, this paper proposes accurate and robust numerical methods adapted to the simulation of phase appearance or disappearance. Polynomial solvers are developed to avoid the use of eigenvectors which are needed in usual shock capturing schemes, and a method based on an adaptive numerical diffusion is designed to treat the positivity problems. An alternate method, based on the use of the hyperbolic tangent function instead of a polynomial, is also considered. Numerical results are presented which demonstrate the efficiency of the proposed solutions

Design, development and testing of SOEC-based Power-to-Gas systems for conversion and storage of RES into synthetic methane

International and national initiatives are promoting the worldwide transition of energy systems towards power production mixes increasingly based on Renewable Energy Sources (RES). The integration of large shares of RES into the actual electricity infrastructure is representing a challenge for the power grids due to the fluctuating characteristics of RES. The adoption of long-term, large-scale Electric Energy Storage (EES) is envisaged as the key-option for promoting the integration of RES in the electricity sector by overcoming the issue of temporal and spatial decoupling of electricity supply and demand. Among the several EES options, one of the most promising is the conversion of energy from the electrical into the chemical form through the synthesis of H2 and synthetic natural gas (SNG) in Power-to-Gas (P2G) systems based on the electrolysis of water (and also CO2) in Solid Oxide Cells (SOCs). The application of SOC technology in P2G solutions shows attractiveness for the high efficiency of high-temperature electrolysis and the flexibility of SOCs that can operate reversibly as electrolyzers or fuel cells (rSOC) and can directly perform the electrochemical conversion of CO2 and H2O to syngas by co-electrolysis. The capability of reversible operation also allows the application of SOC-based systems to Power-to-Power (P2P) concepts designed for deferred electricity production. This dissertation is focused on the investigation of electricity storage using Power-to-Gas/Power systems based on SOCs. The aim of this Thesis has been the investigation of the thermo-electrochemical behavior of SOCs integrated P2G/P2P systems, with the purpose to identify the system configuration and the operating conditions that ensure the most efficient electricity-to-SNG (P2G) or electricity-to-electricity (P2P) conversion within the thermal limits imposed by state-of-the art SOC materials. To this purpose, a detailed thermo-electrochemical model of an SOC has been developed at cell level, validated on experimental data, extended at stack level and coupled with models of the main P2G/P2P components for the system analysis. Model validation was performed through the characterization of planar commercial SOCs in the reversible operation as electrolyzers (SOEC) and fuel cells (SOFC) with H2/H2O and CO/CO2 fuel mixtures at different reactant fractions and temperatures. The physical consistency of electrode kinetic parameters evaluated from the model was verified with the support of literature studies. The investigation of SOC-based P2P and P2G solutions was performed using the models developed. Three different configurations were analyzed and simulated: 1) hydrogen-based P2P with rSOC, 2) SOEC-based electricity storage into hydrogen with subsequent SNG production by methanation with CO2 and 3) electricity storage by co-electrolysis of water and carbon dioxide with SOEC for syngas production and subsequent upgrading to SNG by methanation. The performance of the P2P system was thoroughly assessed by analyzing the effects of rSOC stack operating parameters (inlet gas temperature, oxidant-to-fuel ratio, oxidant recirculation rate, cell current) and system configurations (pressurized/ambient rSOC operation, air/oxygen as oxidant/sweep fluid) on stack and system efficiency. The analysis allowed to identify the most efficient configuration of the P2P system, and to select the feasible operating currents (i.e., the currents included within the limits given by the physical thermal constraints of SOC materials) for which the highest roundtrip efficiency is achieved. Pressurized rSOC operation (10 bar) with pure oxygen as oxidant/sweep gas and full recirculation of the oxidant flow ensured the highest charging and discharging effectiveness, with a system roundtrip efficiency of 72% when the stack is operating at the maximum efficiency currents (-1.3 A/cm2 in SOEC and 0.3 A/cm2 in SOFC). A dynamic analysis was performed on the rSOC to determine the characteristic times of the thermal response of an SRU coupled with variable loads. The analysis showed that the SOEC is intrinsically more suitable to work with variable loads thanks to the balance between reaction endothermicity and losses exothermicity that reduces the magnitude and the rate of temperature fluctuations originated by current variations. A case study was presented to show the application of P2P with fluctuating RES. In the case study, the sizing of an rSOC-based P2P system designed for the minimization of the imbalance (i.e., the difference between effective and forecasted electricity production) of a 1 MW grid-connected wind farm was performed. An optimal number of cells was found, for which the imbalance is reduced by 77 %. The estimated roundtrip efficiency of the optimal-size P2P system coupled with the wind farm was 54 %. The P2G systems analyzed are composed by three main sections: a hydrogen/syngas production and storage section based on an SOEC stack; a methanation section based on chemical reactors; and an SNG conditioning section for the upgrading of the produced SNG to grid-injection quality. The design and operating conditions of the SOEC section were selected following the results of the analysis performed on the P2P system, and the SNG production section was designed on the basis of a commercial methanation process based on catalytic reactors. The plant efficiency evaluated by simulations was 65.4% for the H2-based P2G and 65.5% for the co-electrolysis based P2G without considering cogeneration or thermal integration between plant sections. Even if the efficiencies were similar for the two P2G configurations, the storage capacity of the H2-based P2G plant was higher, because of the higher operating current achieved by the SOEC stack. The results suggested that even if the co-electrolysis based P2G system presents a slightly higher efficiency, the choice of a H2-based P2G option can ensure a better exploitation of the installed capacity, and also eliminates the risks of carbon-deposition in the stack related to the use of carbon containing mixtures and of stack poisoning related to contaminants potentially present in CO2 streams (e.g., hydrogen sulphide). A case study assessing the effect of H2S poisoning of the SOEC stack on the P2G system performance was also presented. The results presented in this Thesis demonstrated that hydrogen-based P2P with rSOCs is the most efficient solution for local RES storage among the different SOC-based EES options investigated. The high values of roundtrip efficiency achieved demonstrated the competitiveness of rSOC-based P2P also with other large-scale EES options (PHS, CAES). The hydrogen-based P2P is however constrained to on-site applications due to the lack of a hydrogen transport infrastructure, while P2G solutions offer the possibility of transferring the electricity stored in the SNG form through the existing natural gas infrastructure, and also allow the direct use of SNG in already existing technologies (i.e., for mobility, heating, etc.), providing the technological bridge for transferring RES power to other markets different from the electrical one

Nanostructuring silicon probes via electrodeposition: characterization of electrode coatings for acute in vivo neural recordings

Understanding how the brain works will require tools capable of measuring neuron elec-trical activity at a network scale. However, considerable progress is still necessary to reliably increase the number of neurons that are recorded and identified simultaneously with existing mi-croelectrode arrays. This project aims to evaluate how different materials can modify the effi-ciency of signal transfer from the neural tissue to the electrode. Therefore, various coating materials (gold, PEDOT, tungsten oxide and carbon nano-tubes) are characterized in terms of their underlying electrochemical processes and recording ef-ficacy. Iridium electrodes (177-706 μm2) are coated using galvanostatic deposition under different charge densities. By performing electrochemical impedance spectroscopy in phosphate buffered saline it is determined that the impedance modulus at 1 kHz depends on the coating material and decreased up to a maximum of two orders of magnitude for PEDOT (from 1 MΩ to 25 kΩ). The electrodes are furthermore characterized by cyclic voltammetry showing that charge storage capacity is im-proved by one order of magnitude reaching a maximum of 84.1 mC/cm2 for the PEDOT: gold nanoparticles composite (38 times the capacity of the pristine). Neural recording of spontaneous activity within the cortex was performed in anesthetized rodents to evaluate electrode coating performance

Numerical study of EGR mixing and distribution in a piston engine intake line

[ES] Teniendo en cuenta la cantidad de motores de combustión interna que se encuentran en activo actualmente, y sus potenciales emisiones de contaminación si se realizaran de forma incontrolada por el parque automovilístico, las normativas internacionales son cada vez más estrictas en cuanto a la cantidad de gases perjudiciales para el medio ambiente que pueden emitir dichos motores de manera unitaria. Debido a ello, se han ido desarrollando e implantando técnicas de reducción de contaminantes como el downsizing en el cual se reduce el tamaño del motor para reducir el consumo, la implantación de motores híbridos y la Recirculación de Gases de Escape. Esta técnica de recirculación puede abordarse de dos maneras alternativas: la Recirculación de Gases de Escape de Ruta Larga inyecta dichos gases antes del compresor, mientras que la Recirculación de Gases de Escape de Ruta Corta (o alta presión) los reinyecta después del compresor, en el mismo colector de admisión del motor. Dado que en ambas configuraciones se produce una inyección directa del flujo recirculado en la corriente principal, en el presente trabajo se propone un estudio numérico de la mezcla entre las corrientes de aire y gases recirculados usando un software comercial de mecánica de fluidos computacional (STAR-CCM+). En la configuración de Ruta Larga se ha propuesto en primer lugar estudiar el efecto en los parámetros globales del compresor de una entrada heterogénea compuesta por aire y gases de escape. Para ello, se han analizado 9 puntos de funcionamiento distintos, tratando de abarcar el mapa completo del compresor centrífugo con una tasa de inyección constante. Se ha demostrado, por un lado, la necesidad de un esquema transitorio de cálculo para la obtención de resultados confiables en todo el dominio del compresor. Por otro lado, se ha demostrado que, con tasas de penetración de flujo estándar, la inyección de gases recirculados no tiene un impacto reseñable en las prestaciones del compresor, con excepción de la zona de bombeo. En segundo lugar, se ha desarrollado un diseño numérico de experimentos en configuración de Ruta Larga con el objetivo de encontrar correlaciones entre la condensación generada en dichas uniones (la cual puede aparecer bajo ciertas condiciones de operación del motor) y la mezcla entre las corrientes de aire y gases de escape. Se ha demostrado que la penetración de los gases en la corriente principal es un factor clave en la condensación generada, aumentando la cantidad de mezcla entre ambas corrientes. En la configuración de Ruta Corta se han realizado estudios de configuración numérica tratando de estudiar la influencia de factores como malla, tamaño del paso temporal y modelos de turbulencia en la distribución final de los gases de escape entre los diferentes cilindros del motor. Se ha demostrado que los submodelos RANS pueden predecir la mayor parte de puntos de operación tanto en variables medias como instantáneas comparando resultados numéricos con mediciones experimentales. Fijando una configuración numérica, posteriormente se han analizado diferentes mezcladores en colectores de motores de 4 y 6 cilindros, demostrando la aplicabilidad de los índices de mezclado desarrollados y cuantificando la influencia de los diferentes efectos físicos que influyen en la distribución y mezcla de los gases de escape en la corriente principal.[CA] Tenint en compte la quantitat de motors de combustió interna que es troben en actiu actualment, i les seues potencials emissions de contaminació si es realitzaren de manera incontrolada pel parc automobilístic, les normatives internacionals són cada vegada més estrictes quant a la quantitat de gasos perjudicials per al medi ambient que poden emetre aquests motors de manera unitària. A causa d'això, s'han anat desenvolupant i implantant tècniques de reducció de contaminants com el downsizing en el qual es redueix la grandària del motor per a reduir el consum, la implantació de motors híbrids i la Recirculació de Gasos de Fuita. Aquesta tècnica de recirculació pot abordar-se de dues maneres alternatives: la Recirculació de Gasos de Fuita de Ruta Llarga injecta aquests gasos abans del compressor, mentre que la Recirculació de Gasos de Fuita de Ruta Curta (o alta pressió) els reinjecta després del compressor, en el mateix collector d'admissió del motor. Atés que en totes dues configuracions es produeix una injecció directa del flux recirculat en el corrent principal, en el present treball es proposa un estudi numèric de la mescla entre els corrents d'aire i gasos recirculats usant un programari comercial de mecànica de fluids computacional (STAR-CCM+). En la configuració de Ruta Llarga s'ha proposat en primer lloc estudiar l'efecte en els paràmetres globals del compressor d'una entrada heterogènia composta per aire i gasos de fuita. Per a això, s'han analitzat 9 punts de funcionament diferents, tractant d'abastar el mapa complet del compressor centrífug amb una taxa d'injecció constant. S'ha demostrat, d'una banda, la necessitat d'un esquema transitori de càlcul per a l'obtenció de resultats de confiança en tot el domini del compressor. D'altra banda, s'ha demostrat que, amb taxes de penetració de flux estàndard, la injecció de gasos recirculats no té un impacte ressenyable en les prestacions del compressor, amb excepció de la zona de bombament. En segon lloc, s'ha desenvolupat un disseny numèric d'experiments en configuració de Ruta Llarga amb l'objectiu de trobar correlacions entre la condensació generada en aquestes unions (la qual pot aparéixer sota unes certes condicions d'operació del motor) i la mescla entre els corrents d'aire i gasos de fuita. S'ha demostrat que la penetració dels gasos en el corrent principal és un factor clau en la condensació generada, augmentant la quantitat de mescla entre tots dos corrents. En la configuració de Ruta Curta s'han realitzat estudis de configuració numèrica tractant d'estudiar la influència de factors com a malla, grandària del pas temporal i models de turbulència en la distribució final dels gasos de fuita entre els diferents cilindres del motor. S'ha demostrat que els submodelos RANS poden predir la major part de punts d'operació tant en variables mitjanes com instantànies comparant resultats numèrics amb mesuraments experimentals. Fixant una configuració numèrica, posteriorment s'han analitzat diferents mescladors en col·lectors de motors de 4 i 6 cilindres, demostrant l'aplicabilitat dels índexs de barrejat desenvolupats i quantificant la influència dels diferents efectes físics que influeixen en la distribució i mescla dels gasos de fuita en el corrent principal.[EN] Considering the amount of internal combustion engines (ICEs) existing nowadays, and the pollutants that they could potentially emit, it is no surprise that international standards are getting increasingly severe regarding the allowed limits of pollutants that can be released by such engines. For this reason, different techniques have been developed in order to diminish pollutants, as downsizing in which the engine size is reduced to decrease the consumption, the hybridation of engines and the exhaust gases recirculation (EGR). This recirculation technique can be addressed by 2 different paths: Low Pressure EGR (LP-EGR) which reintroduce the exhaust gases before the compressor, while High Pressure EGR (HP-EGR) injects exhaust gases after the compressor in the intake manifold. Since both configurations deal with a direct injection of the recirculated flow in the main stream, in the present work a numerical study of the mixing between air and EGR flows is proposed, using a commercial code of computational fluid dynamics (STAR-CCM+). In LP-EGR configuration has been proposed the study of the influence of a heterogeneous inlet (composed by air and exhaust gases) on the main performance of a centrifugal compressor. To do that, 9 different operating points have been analyzed, trying to cover the whole map of the compressor with a constant injection rate. It has been demonstrated the necessity of a transient scheme for obtaining reliable results in the complete domain of the compressor. On the other hand, it has been proved that, with standard penetration rates of the flow, EGR do not have a remarkable impact in the performance of the compressor, besides the surge zone. In LP-EGR scheme, a numerical design of experiments has been developed with the aim to find correlations between the generated volume condensation (which can appear under some operating points of the engine) and the mixing between air and exhaust gases. It has been proved that the penetration of the EGR in the main stream is a key factor in volume condensation, increasing the amount of mixing between the streams. In HP-EGR configuration, different studies of numerical configuration have been conducted trying to find the influence of factors like mesh, time-step size, and turbulence models in the final distribution of exhaust gases between the cylinders of the engine. RANS submodels have demonstrated that can predict most of the operating points both average and instantaneous variables in comparison with experimental measurements. After that, fixing a numerical setup, different mixers in 4 and 6 cylinder manifolds have been calculated, showing the applicability of the developed mixing indexes, and quantifying the influence of the different physical effects that can influence in the mixing and distribution between air and exhaust gases streams.García Olivas, G. (2021). Numerical study of EGR mixing and distribution in a piston engine intake line [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/179406TESI