Browsing by Issue Date, starting with "2018-03-16"
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- Experimental Study of a Single Droplet Impinging on Dry Surface with and without a Crossflow: Jet Fuel and Biofuel MixturesPublication . Cunha, Nuno Filipe Costa; Silva, André Resende Rodrigues daThe impact of droplets on different surfaces is a very complex and important phenomenon and it has variety of applications such as fire suppression systems, spray painting, crop spraying and blending preparation in internal combustion engines. Although these phenomena are widely studied, the present work aims to study the influence in mixing a biofuel in a conventional jet fuel and characterize the impact result of the droplets in these mixtures. This study allows the development of numerical models to determine the limit of the different impact phenomena. ln this study, an experimental facility was designed to study the droplet impact behavior on a smooth, dry aluminum surface. Four substances were analyzed: two mixtures (75% JF - 25%HVO and 50% JF - 50% HVO) and two other substances (100% JF and H20). Jet fuel was used to compare the effect of biofuel on the impact result, and H20 was used as the control substance. It was observed that the blends exhibit a different splash / non splash threshold suggesting that the biofuel properties influence the impact result_ To study the effect of a crossflow in this phenomenon , a wind tunnel was also designed and a comparison between normal impact and impact with a crossflow was made. Three crossflow velocities were used and the velocity of the impact was divided into two velocity vectors. Different results were observed comparing crossflow impact with normal impact and the velocity components seem to have some significance in the transition of the impact phenomena. The results were compared with some transition criteria proposed in the literature and the data indicate that none of them presents a good correlation for the data. It was also verified that a study of the surface roughness of the used plate would allow drawing more conclusions.
- Experimental Study of a Single Droplet Impinging upon Liquid Films: Jet Fuel and Biofuel MixturesPublication . Ribeiro, Daniela Filipa Santo; Silva, André Resende Rodrigues daThe present work is focused on a single droplet impinging upon a liquid film of the same fluid. This particular study is a matter of interest for several research areas and has a wide variety of applications such as fuel injection in internal combustion engines and processes involving spray paints, coatings and systems cooling. The human being started searching for new alternatives to reduce pollution, and since transports contribute with a significant portion, it is extremely necessary to bet on alternatives to fossil fuels. The introduction of biofuels in the aviation sector could be an example. The huge challenge is to modify and optimize piston engines to operate efficiently with alternative fuels. In order to achieve that, in these experiments, Jet Fuel and Biofuel mixtures were used. The main goal of this dissertation is to visualize the dynamic behavior of single droplets impinging upon liquid films with different relative thicknesses, several outcomes are possible. To accomplish that, four fluids were used: water (as reference), 100% Jet A-1, 75%/25% and 50%/50% mixtures of Jet A-1 and NEXBTL, respectively, since civil aviation only accept mixtures with at least 50% Jet Fuel in volume. To assure the accuracy of the calculations, the fluids physical properties were measured. An experimental facility was designed and built, and the setup includes a high-speed digital camera that was manually triggered with a specific exposure time. The impact site was illuminated by a led lamp through a diffusion glass to provide uniform back lighting. A syringe pump connected to the needle released the droplets with a specific pumping rate. The liquid film is held by a perspex container. Five needles were used with different inner diameters to yield five distinct droplet sizes for each fluid. Additionally, three impact heights were established to provide three impact velocities and Weber numbers for each needle. The liquid films depths considered were 10%, 50% and 100% of the droplet diameter. The existence of splash was reported as well as its characteristics. Some conclusions about the influence of the impact conditions and the fluids physical properties were indicated. Using the obtained data comparisons were made with some splashing thresholds available in the literature.