Transport Limitations in Zeolites and Biomass Pyrolysis

Biomass pyrolysis has been widely explored for its potential to generate a sustainable chemical source capable of producing synthetic fuels and chemicals. Lignocellulosic biomass is the carbon rich, inedible fraction of wood that is comprised of long oxygenated biopolymers, primarily cellulose, hemicellulose and the highly aromatic lignin. High temperature thermal conversion of biomass to bio-oil (pyrolysis oil) occurs on the order of milliseconds and converts long chain biopolymers to a carbon-rich liquid crude. The chemistry of biomass pyrolysis is greatly complicated by significant heat and mass transport challenges. The complex fluid dynamics of the reactive liquid intermediate are examined in situ with high temperature spatiotemporally resolved techniques (T = 450 – 1000 °C, 1 µm, 1 ms), chemical analyses and computational fluid dynamics. Discoveries include the mechanism for aerosol generation during pyrolysis, existence and control of the Leidenfrost effect, and understanding bio-oil microexplosions. Zeolites are widely utilized to catalytically upgrade fuels by cracking, hydrogenation and hydrodeoxegenation as well as having applications in separations, sorption, ion-exchange. While new hierarchical materials are synthesized with transport lengthscales that are increasingly smaller, substantial diffusional transport limitations persist in small particles, often dominating the observed rates. The presence of these transport limitations remains a significant technical challenge. In this work, a mechanistic understanding is developed to describe such limitations. The potential for surface barriers to diffusion are experimentally assessed by Zero Length Chromatography and frequency response methods, and further confirmed by dynamic Monte Carlo simulations

Numerical study of the spray impingement onto a solid wall

The modelling of turbulent multiphase flows has been gathering high interest in the last decades in the scientific community due to its relevance in several applications, such as in industrial and environmental processes or for chemical and biomedical purposes. In fact, regarding the industrial applications, the impingement of liquid fuel sprays onto engine surfaces has become a subject of interest due to its influence on the mixture preparation prior to combustion and, consequently, engine performance and pollutants emission (Barata and Silva, 2005). However, there is still a lack of knowledge concerning the spray-wall interaction but also concerning the exact phenomenon occurring during the process. These gaps do not allow defining the most favourable conditions for the optimal engine performance. Hence, the main challenge for the investigators lies in attaining a much deeper understanding of the phenomena involved in the spray impingement process, through either theoretical analysis or experimental investigation. Meanwhile, the splash phenomenon has been the focus of many researchers due to its relevance in the combustion process of small-bore, direct-injected gasoline and diesel engines, as well as in a variety of other industrial devices in which sprays impinge on solid surfaces. Bai and Gosman (1995) developed a model to predict the outcomes of spray droplets impacting on a wall with temperatures below the fuel boiling point. This model, which has been formulated using a combination of simple theoretical analysis and experimental data from a wide variety of sources, was later improved (Bai et al., 2002) by refining the dissipation energy term and by enhancing the post-splashing characteristics. In fact, recently, significant attention has been given to this regime either through the definition of transition criteria that better fit specific conditions of the experimental configuration under study or by characterizing the behaviour of the drop during all stages of the regime (expansion of the lamella, crown formation and propagation, etc.) through both theoretical analyses and experimental data. Beyond the transition criteria, another aspect that controls the characteristics of the secondary droplets after the impacts is the energy dissipation term and thus, it is essential its proper definition for adequately modelling these multi-phase flows. However, contrary to spreading, there is little literature available related to this particular parameter and, more important than that is the fact that there is a certain ambiguity even for what it represents exactly. In addition, the majority of the dissipative energy loss relationships have been deduced for the spread regime, i.e., from the beginning of the expansion of the lamella until the drops reaches its maximum extent (without splashing). This situation can be overcome through some simplifying assumptions, which obviously carries inaccuracy. The present work is dedicated to the study of the sprays impingement onto a solid wall through a crossflow. The major purpose of the thesis is to improve the accuracy of the base model, which is the model of Bai et al. (2002), through the employment of both new correlations for the deposition/splash transition criteria and energy dissipation loss relationships available in the literature. The numerical predictions are then compared with the experimental data of Arcoumanis et al. (1997) for two crossflow rates ( and ). From the results, it can be concluded that the employment of different transition criteria can bring better results (see also Silva et al., 2011). On the other hand, no improvements were seen by the employment of the new energy dissipative loss relationships in the base models, which calls for further research in this particular matter.A modelação de escoamentos turbulentos multifásicos tem vindo a gerar grande interesse nas últimas décadas na comunidade científica devido à sua importância em diversas aplicações, como por exemplo em sistemas industriais e ambientais, ou em processos químicos e biomédicos. De facto, no que diz respeito às aplicações industriais, o impacto do spray de combustível nas superfícies dos motores tornou-se um assunto de elevado interesse devido à sua influência na preparação da mistura antes da combustão e, consequentemente, no desempenho do motor e emissão de poluentes (Barata e Silva, 2005). Contudo, continua a ser necessária bastante investigação no que toca à interacção spray-parede mas também relativamente aos fenómenos específicos que ocorrem durante todo o processo. Estas lacunas não permitem ainda definir quais as condições óptimas no cilindro para o melhor desempenho do motor. Assim, o principal desafio para os investigadores prende-se com o estudo aprofundado dos fenómenos envolvidos no processo de impacto de sprays tanto através de análises teóricas como de investigações experimentais. Entretanto, o fenómeno de splash tem vindo a ser objecto de estudo de muitos investigadores devido à sua relevância no processo de combustão em motores de injecção directa a gasolina e gasóleo, mas também numa grande variedade de outros dispositivos industriais nos quais ocorre impacto de sprays em superfícies sólidas. Bai and Gosman (1995) desenvolveu um modelo para prever os resultados do impacto de gotas de sprays em paredes com temperatura abaixo do ponto de ebulição do combustível. Este modelo – formulado usando uma combinação de análises teóricas e dados experimentais de uma grande variedade de fontes – foi mais tarde melhorado (Bai et al., 2002) refinando o termo da energia de dissipação e melhorando as características de pós-impacto. De facto, recentemente tem sido dada uma grande atenção a este regime quer através da definição de critérios de transição que melhor se adequam às condições da configuração experimental em estudo, quer através da caracterização do comportamento das gotas durante todos os estágios do regime (expansão da ―lamela‖, formação da coroa e sua propagação, etc.) através de análises teóricas e de dados experimentais. Para além dos critérios de transição, outro dos aspectos que controlam as características das gotas secundárias após impacto é a energia de dissipação viscosa, sendo assim essencial a sua correcta definição para a modelação destes escoamentos. Contudo, ao contrário do spreading, existe pouco literatura disponível relacionada com este parâmetro em específico e, mais importante ainda, existe alguma ambiguidade sobre aquilo que este parâmetro representa exactamente. Além disso, a maioria das relações da energia de dissipação foram deduzidas para o regime de spread, i.e., desde o início da expansão da lamela até que a gota atinja a sua extensão máxima, ou seja, sem ocorrer splash. Esta situação pode ser superada através de algumas hipóteses assumidas mas que, obviamente acarretam erros. O presente trabalho é dedicado ao estudo de impacto de sprays em paredes sólidas com a presença de um escoamento cruzado. O principal objectivo da tese é melhorar a qualidade do modelo de atomização de base utilizado (modelo do Bai et al., 2002) através da utilização de novas correlações – para os critérios de transição entre deposition e splash –, e novas relações – para a energia de dissipação – disponíveis na literatura. Os resultados numéricos são então comparados com os dados experimentais do estudo do Arcoumanis et al. (1997) para escoamentos cruzado com duas velocidades diferentes (5 e 15 m/s). Dos resultados apresentados, conclui-se que a utilização de diferentes critérios de transição pode trazer melhores resultados mas apenas em alguns parâmetros estudados (ver também Silva et al., 2011). Por outro lado, não foram encontradas melhorias quando se introduziram no modelo de base as novas equações para a energia de dissipação, deixando claro a necessidade premente de maior investigação nesta área em particular.Fundação para a Ciência e a Tecnologia (FCT

Collision Dynamics of a Single Droplet onto a Heated Dry Surface: Jet Fuel and HVO Mixtures

The concern with the environment led the human being to develop new alternative fuels to reduce pollution and mitigate the emission of greenhouse gases. The air transport sector and the burning of fossil fuels are responsible for a huge portion of the pollution. Therefore, introducing new sustainable ways to provide energy, such as biofuel, is of major importance. However, in order to make these new energy sources more efficient and safer, it is necessary to carry out studies related to the injection of fuel into the combustion chambers, and the impact of droplets. This study focuses on an experimental investigation of a single droplet impact onto a heated solid surface. The main purpose of this work is to analyse the influence of wall temperature on the impact morphology of a single droplet and observe the possible outcomes. To do so, in these experimental tests, Jet Fuel and HVO (Hydroprocessed Vegetable Oil) mixtures were used. The fluids tested were: water (as a control group), 100% Jet A-1, 75% Jet A-1 and 25% NExBTL, 50% Jet A-1 and 50% NExBTL, and 100% NExBTL. The present work studies the impact outcomes depending on the working fluids and the wall temperature. The impact energy was kept constant. Therefore, the Weber number in this experiment was set to W e = 320 by varying the droplet diameter or the impact velocity. Furthermore, different wall temperatures were chosen, that vary from Tw = 25ºC to Tw = 330ºC, to seek for every possible impact phenomenon and characterise the impact morphology. The impact dynamics were captured using a high-speed digital camera and the images were digitally processed. It was possible to observe the heat regimes for all fluids, as well as two additional regimes for the mixtures of 75% jet fuel - 25% HVO and 50% jet fuel - 50% HVO.A preocupação com o ambiente levou o ser humano a desenvolver novos combustíveis alternativos para reduzir a poluição e mitigar a emissão de gases de efeito de estufa. O setor de transporte aéreo e a queima de combustíveis fósseis é responsável por grande parte da poluição. Por conseguinte, introduzir novas formas sustentáveis de fornecer energia, como os biocombustíveis, é de elevada importância. Contudo, de modo a tornar estes novos meios de energia mais eficientes e seguros, é necessário realizar estudos relativos à injecção de combustíveis nas câmaras de combustão e ao impacto de gotas. Este estudo é focado numa investigação experimental sobre o impacto de gotas numa superfície sólida quente. O principal objectivo deste trabalho é analisar a influência da temperatura da superfície na morfologia do impacto de uma única gota e observar os possíveis resultados. Para isso, nestes ensaios experimentais foram utilizadas misturas de Jet Fuel e HVO (Óleo Vegetal Hidroprocessado). Os fluidos utilizados foram: água (como grupo de controlo), 100% Jet A-1, 75% Jet A-1 e 25% NExBTL, 50% Jet A-1 e 50% de NExBTL, e 100% NExBTL. Estas misturas seguem os requisitos da aviação civil, no qual têm que conter um mínimo de 50% de jet fuel. O presente trabalho estuda os efeitos de impacto de uma gota em função da temperatura da superfície para diferentes fluidos. A energia de impacto foi mantida constante. Portanto, o número de Weber nesta experiência foi fixado em W e = 320, tendo variado ou o diâmetro da gota ou a velocidade de impacto. Além disso, foram escolhidas diferentes temperaturas da superfície, que variam entre Tw = 25ºC e Tw = 330ºC, para procurar obter cada fenómeno de impacto e caracterizar a morfologia do mesmo. As dinâmicas de impacto foram capturadas utilizando uma câmara digital de alta velocidade e as imagens foram processadas digitalmente. Foi possível observar os regimes de calor para todos os fluidos, bem como alguns adicionais para as misturas de 75% jet fuel - 25% HVO e 50% jet fuel - 50% HVO

Self‑propelled droplets on heated surfaces with angled self‑assembled micro/nanostructures

Directional and ratchet-like functionalized surfaces can induce liquid transport without the use of an external force. In this paper, we investigate the motion of liquid droplets near the Leidenfrost temperature on functionalized self-assembled asymmetric microstructured surfaces. The surfaces, which have angled microstructures, display unidirectional properties. The surfaces are fabricated on stainless steel through the use of a femtosecond laser-assisted process. Through this process, mound-like microstructures are formed through a combination of material ablation, fluid flow, and material redeposition. In order to achieve the asymmetry of the microstructures, the femtosecond laser is directed at an angle with respect to the sample surface. Two surfaces with microstructures angled at 45° and 10° with respect to the surface normal were fabricated. Droplet experiments were carried out with deionized water and a leveled hot plate to characterize the directional and self-propelling properties of the surfaces. It was found that the droplet motion direction is opposite of that for a surface with conventional ratchet microstructures reported in the literature. The new finding could not be explained by the widely accepted mechanism of asymmetric vapor flow. A new mechanism for a self-propelled droplet on asymmetric three-dimensional self-assembled microstructured surfaces is proposed

Impingement characteristics of an early injection gasoline direct injection engine: A numerical study

This paper describes the use of a Lagrangian discrete droplet model to evaluate the liquid fuel impingement characteristics on the internal surfaces of an early injection gasoline direct injection (GDI) engine. The study focuses on fuel impingement on the intake valve and cylinder liner between start of injection (SOI) and 20° after SOI using both a single- and multi-component fuel. The single-component fuel used was iso-octane and the multi-component fuel contained fractions of iso-pentane, iso-octane and n-decane to represent the light, medium and heavy fuel fractions of gasoline, respectively. A detailed description of the impingement and liquid film modelling approach is also provided Fuel properties, wall surface temperature and droplet Weber number and Laplace number were used to quantify the impingement regime for different fuel fractions and correlated well with the predicted onset of liquid film formation. Evidence of film stripping was seen from the liquid film formed on the side of the intake valve head with subsequent ejected droplets being a likely source of unburned hydrocarbons and particulate matter emissions. Differences in impingement location and subsequent location of liquid film formation were also observed between single- and multi-component fuels. A qualitative comparison with experimental cylinder liner impingement data showed the model to well predict the timing and positioning of the liner fuel impingement

Predicción del desgaste de moldes de inyección de plástico y aluminio

219 p.En esta tesis doctoral se han abordado los principales mecanismos de desgaste que aparecen en los moldes de inyección de plástico y aluminio. Ambos consisten en inyectar a alta temperatura, presión y velocidad un fluido (plástico y aluminio respectivamente), en un molde cuya cavidad da forma a la pieza final tras el proceso de solidificación. Este molde tiene que aguantar cientos de miles de ciclos en este entorno agresivo, lo cual lleva a limitar la vida del molde debido al desgaste que sufren estos, requiriendo reparaciones y paradas de producción inesperadas. La formación del desgaste de los moldes se genera debido a distintos mecanismos de desgaste. Algunos de estos mecanismos son comunes para ambos casos de procesos de producción estudiados, como la erosión y la corrosión. Mientras, otros son específicos, como la abrasión en la inyección de plástico y la adhesión del aluminio (die soldering) y la fatiga térmica en la inyección de aluminio. A lo largo de esta Tesis doctoral se describe la metodología seguida para generar unos modelos de predicción de desgaste de estos mecanismos de desgaste de moldes a partir de la experimentación de laboratorio realizada

Leidenfrost heat engine: Sustained rotation of levitating rotors on turbine-inspired substrates

The prospect of thermal energy harvesting in extreme environments, such as in space or at microscales, offers unique opportunities and challenges for the development of alternate energy conversion technologies. At microscales mechanical friction presents a challenge in the form of energy losses and wear, while presence of high temperature differences and locally available resources inspire the development of new types of heat engines for space and planetary exploration. Recently, levitation using thin-film boiling, via the Leidenfrost effect, has been explored to convert thermal energy to mechanical motion, establishing the basis for novel reduced-friction heat engines. In the Leidenfrost effect, instantaneous thin-film boiling occurs between a droplet and a heated surface, thereby levitating the droplet on its own vapor. This droplet state provides virtually frictionless motion and self-propulsion, whose direction can be designed into the system by asymmetrically texturing the substrate. However, sustaining such thermal to mechanical energy conversion is challenging because the Leidenfrost transition temperature for water on a smooth metal surface is 220°C and, despite the low thermal conductivity of the vapor layer, the droplet continuously evaporates. Further challenges include effective transfer of thermal energy into rotational, rather than linear motion, and driving solid components and not simply droplets. Here we present a Leidenfrost rotor, where a solid component is coupled to a rotating liquid volume using surface tension and levitated in continuous operation over a turbine-inspired substrate. We address two key challenges: we show how the liquid can be replenished to achieve the continuous operation of the device; and we show how a superhydrophobic coating to the substrate can broaden the temperature range of operation and the stability of the rotor. Because the liquid acts as a working substance by extracting heat from the substrate to produce useful work in the form of rotation of the coupled solid component, our results demonstrate that a Leidenfrost engine operating in a closed thermodynamic cycle is possible