Over the last years, there have been many investigations into new ways to obtain clean and efficient energy production. In this spirit, this study aims to successfully adapt low emission aero-engines combustors to burn biofuels. Biofuels are the immediate alternative to fossil-fuel powered aero-engines, given that by regulation is possible to apply today, a biofuel into the aeronautical industry. There are some regulations to have into account, being the biggest one the fact that the new blend must be constituted of at least 50% of conventional jet fuel (JF). In this work were selected four blends to study: 100% JF; 75% JF and 25% NEXBTL (biofuel); 50% JF and 50% NEXBTL and H2O. A smooth, dry aluminium plate was used as the impact surface and the objective was to observe the splash-deposition limits and characterize the dynamic behaviour of the droplets with and without crossflow. Different velocities of the crossflow were chosen and the impact angle of the droplet was also analysed. The splash-deposition threshold was also compared with those proposed by other authors.

The main breakup regimes are visualized and studied with the help of a high-speed camera and a continuous jet of air, provided by a wind tunnel, is used to transition the droplet into the various stages of secondary atomization. The use of Jet Fuel and Biofuel mixtures is the focal point of this study. All the known regimes are encountered and each transitional We is calculated. The variation of We due to Oh is described, in addition to the comparison between We number thresholds and correlations existent in the literature for each regime transition. The influence of these numbers on the dimensionless characterization of time and later the maximum cross stream diameter is shown. Finally, a correlation for the temporal evolution of the drops’ cross stream diameter for all regimes, assuming its deformation is not a linear function of time, is proposed.

The 3D incompressible Navier-Stokes equations are coupled with the CLSVOF method and employed to numerically simulate the phenomena of single droplet impact onto liquid films. A solution-adaptive mesh refinement tool, based on the gradient of the volume fraction scalar, is adopted in order to reduce computational cost. Three different fluids are taken into account: 100% jet fuel and 75%/25% and 50%/50% of jet fuel and biofuel, respectively. Quantitative analysis of the crown height and outer diameter is performed for different impact conditions, such as the influence of the impact velocity and dimensionless thickness, between experimental and numerical results, and the qualitative analysis includes the occurrence of splashing and overall crown evolution. Numerical results show that the crown outer diameter measurements are in good agreement with the experimental cases, presenting a slight discrepancy for the lower liquid film thickness of h*= 0.2. The crown height measurements are under-predicted for the current model, maintaining a similar trend for dimensionless thicknesses of h*= 0.5 and h*= 1 while, for the lower thickness, the crown disintegrates at earlier stages. The crown curvature and rim instabilities exhibit significant differences, and the splashing phenomenon occurs for both the experimental and numerical outcomes.

The demand for aviation fuel is constantly increasing and has become apprehensive due to the depletion of fossil fuels. The use of petroleum sourced fuels as an energy source in the air transportation industry consists of an unsustainable alternative. Due to this, the introduction of alternative jet fuel is required to mitigate these issues. Thus, the present work simulates single droplet combustion numerically in a drop tube furnace. In this context, jet-A1 and n-Hexadecane were investigated. Additionally, it is studied the validation of approximating the combustion of alternative jet fuel to n-hexadecane. The employed model consists of a two-way coupling approach between the fuel droplets and the carrier fluid following an Eulerian-Lagragangian schematic. The continuous phase is primarily modeled and further coupled with the fuel droplets known as the discrete phase. A combustion model is employed in order to simulate the natural phenomenon of single droplet combustion to the intended utilized fuels. The results reveal that the droplet diameter reduces as the time evolves under the representation of the d2 law enabling the computation of different combustion characteristics being affected by the fuel composition.

The phenomenon of bubble encapsulation results from droplet impact on a liquid film for specific impact conditions, but there is no established criterion for predicting its onset. Phenomenon visualization from two perspectives, the common lateral perspective and a bottom perspective, provided insights into the dynamics and formation mechanisms. Namely, the bottom shadowgraphs show capillary wavy patterns and perturbations imposed on the steady liquid film, which suggests a greater role of the liquid film in the onset of bubble encapsulation. Also, some considerations about the cavity development underneath the bubble limited by the solid wall allow concluding that the cavity shape is independent of the bubble encapsulation phenomenon. Additionally, using the bottom shadowgraphs, the crown closure time shows a systematic decrease in the dimensionless film thickness of 0.5<<0.6, which will be subject of future work. Finally, while most drop impact correlations focus on using the droplets' characteristics and thermophysical properties, the experimental results point in a different direction. Considering correlations relating the Ohnesorge and Reynolds numbers, the new criterion for the onset of bubble encapsulation uses drop characteristics and properties in the Reynolds number, while the liquid film thickness and thermophysical properties are used in the Ohnesorge number because most of the crown material comes from the liquid film. Therefore, the criterion based on 100% occurrence of bubble encapsulation is not a threshold, but a range: =(34.5/)/(ℎ) , with ∈[1.022,1.142]. Other authors observed this phenomenon and despite being outside the validation range of this correlation, the values are close to their boundaries.

The aeronautical sector has been, in the last decade, one of those that most invested in more efficient and ecological solutions in order to reduce significantly greenhouse gas and pollutant emissions. The introduction of biofuels in fuel mixtures for aircraft engines is a promising alternative in this sector. The main objective of this experimental study is understanding the influence of crossflow variation on droplet deformation and consequent impact outcomes. An experimental facility was developed and validated to study the impact of single droplets onto a dry, smooth aluminium impact surface under the influence of several crossflow velocities. Different crossflow velocities of 4,5,6,7 and 8m/s were tested. A combination of conventional jet fuel and a biofuel was considered to understand the behavior of jet fuel and biofuel mixtures, and three fluids were used: 100% jet fuel, 75% jet fuel/25% biofuel and 50% jet fuel/50% biofuel. Several parameters, including velocity components, impact angle and eccentricity, were analysed for the different crossflow velocities, and the spread and splash regimes were also defined for the different fluids. The results display that, for each crossflow velocity, an increase in the droplet impact velocity causes a shift from the spread to the splash regime. The presence of a crossflow induces deformation on the droplet, altering its outcome. Ellipsoidal droplets promote the occurrence of spreading, whereas splashing tends to occur for spherical forms, corresponding to higher and lower eccentricity values, respectively. A substantial increase in the crossflow velocity leads to aerodynamic breakup of the droplet.

The environmental impact and the dependence upon fossil fuels in the aeronautical sector have promoted the demand for alternative and greener fuels. The implementation of alternative fuels is one of the main challenges for this sector in the near future. A possible short-term solution might be the blending of biofuels with jet fuel, which would allow for the use of greener fuels and a reduction in the greenhouse gases and pollutant emissions without significant changes in the existing fleets of the companies, with the purpose to develop a “drop-in” fuel. In this context, this work examines the combustion characteristics of single droplets of Jet A-1 (JF), hydroprocessed vegetable oil (NExBTL), and their mixtures in a drop-tube furnace (DTF). The objective of this work is to evaluate the influence of the mixture composition on the fuel characteristics. Droplets with diameters of 155 ± 5 μm, produced by a commercial droplet generator, were injected into the DTF, whose wall temperature and oxygen concentration were controlled. Experiments were conducted for three temperatures (900, 1000, and 1100 °C). The combustion of droplets was evaluated through the images obtained with a high-speed camera coupled with a high magnification lens and an edge detection algorithm. From the images allowed for the analysis of droplet combustion, data are reported for the temporal evolution of droplet sizes and burning rates. The results revealed that the mixtures followed the D2 law, except the mixture with 75% JF for a DTF wall temperature of 1100 °C. The 75% JF mixture did not follow the D2 law as a result of the occurrence of puffing and microexplosions, which enhanced the burning rates. Additionally, it was observed that the mixtures with a higher content of JF present brighter flames and higher burning rates.

Within the wide possibilities for the resultant outcome of a single droplet impinging upon a liquid film, there is an unstudied phenomenon that seems to be underestimated or forgotten in time. The formation of a bubble by the crown sheet closure at the top after crown splash is the focus of this paper. The phenomenon was deeply studied as well as its occurrence, main features, and physical significance. It is important to understand completely the phenomenon to asses if it should be encouraged or avoided inside the combustion chamber of an IC engine, since it has higher production of secondary atomization due to the larger lifetime, and on the other hand, the bubble bursting increase the production of airborne particles that could affectthe pollutant emissions. Several impacts conditions were tested and it was found three different types of bubble formation depending on the jets formed at the closure point. The event was observed both from below and from the side, enhancing the knowledge about the droplet-film impact itself. The main features were studied including, the bubbling height and diameter, and the propagation velocity of the first perturbation imposed on the liquid film.

A Diretiva 2009/28/CE veio incentivar a procura significativa de biocombustíveis na UE. Ao longo dos últimos anos, a preocupação sobre a mudança indireta do uso da terra, em simultâneo com as preocupações ambientais, no que diz respeito aos impactos e emissões de gases de efeito de estufa (GEE), gerou o debate do tema da utilização dos biocombustíveis. No seguimento dessas preocupações, a atenção virou-se para uma utilização dos resíduos agrícolas, como fonte de biomassa para a produção de bioenergia, como meio de atenuar impactos sobre a terra e outros recursos naturais, assim como a diminuição considerável da produção de GEE em comparação com os combustíveis fósseis para transportes. Neste trabalho, pretendeu-se avaliar o potencial de produção da palha de milho para bioenergia nas zonas ecológicas, do Campo e Charneca, na região de Lisboa e Vale do Tejo. Foram considerados 3 itinerários para o aproveitamento das várias componentes da palha: aproveitamento total da palha; aproveitamento dos resíduos da debulha da maçaroca (camisas e carolo); aproveitamento apenas do carolo da espiga de milho. Foram amostradas aleatoriamente plantas em 6 campos e quantificada a população potencial, em explorações representativas do sistema de produção. Determinou-se a matéria seca total por planta e dos seus componentes (folhas, caules, grão, camisas e carolo) bem como o teor de nutrientes em cada um dos componentes. A produção de palha de milho por unidade de superfície estimada, nos sistemas de produção da região, foi de 12,4 t MS ha-1; para uma superfície média de 20 000 ha o potencial de produção será de 248 000 t de matéria seca de palha, o que significa um potencial de 4 092 000 GJ, por ano. Em média, a matéria seca da palha representa 45% do total da matéria seca da planta; representando as folhas 9,1%; os caules 22,4%; as camisas 6%; o carolo 7,7%. Dos itinerários tecnológicos de recolha e transporte da palha de milho considerados, a recolha total de palha apresenta custos avultados, em termos de exportação de nutrientes e de matéria orgânica. O aproveitamento do carolo representa o itinerário com menor impacto nos valores de exportação dos principais nutrientes.