Browsing by Issue Date, starting with "2021-08-30"
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- Collision Dynamics of a Single Droplet onto a Heated Dry Surface: Jet Fuel and HVO MixturesPublication . Pinto, Pedro Miguel Moreira; Vasconcelos, Daniel; Ribeiro, Daniela; Barata, Jorge M M; Silva, AndréUnderstanding the phenomena behind droplet impact is essential to improve the effectivenessin many industrial applications, such as fuel injection in an internal combustion engine andrapid spray cooling of hot surfaces. This study focuses on an experimental investigation ondroplet impact onto a heated wall. The purpose of this work is to analyse the influence of walltemperature on the morphology of a single droplet impact and observe the possible outcomes.In these experiments, the observation of all heat regimes was possible. Since there is an urgeto implement biofuels in general aviation, the fluids analysed are fuel based. The fluids usedwere distilled water, as a control group, 100% jet fuel, and a mixture of 50% biofuel with 50%jet fuel. This mixture corresponds to the maximum of renewable energy source fuel due to theminimum of 50% of jet fuel required by civil aviation. The impact energy was kept constant, andthe Weber number was set to 320. Furthermore, different wall temperatures were chosen (25◦C−320◦C) to seek every possible impact phenomenon and characterise the impact morphology.The impact dynamics were captured using a high-speed digital camera and the images weredigitally processed.
- Comparison of Commercial and Open-Source CFD Solvers on Surface Tension Dominated FlowsPublication . Vasconcelos, Daniel; Silva, André; Barata, Jorge M MProblems involving multiphase flows require a physical understanding of how the phenomenadevelop and the specific interactions they manifest. For surface tension relevant flows, suchas bubbles and droplets, the importance of modelling and predicting small-scale behaviouris crucial in accurately defining the liquid-gas interface and complex interactions that may takeplace. Axisymmetric numerical simulations of single droplets impacting onto thin liquid films areperformed using commercial and open-source CFD codes. ANSYS Fluent®is the commercialsoftware employed, whereas Basilisk is the open-source CFD solver adopted. The incompress-ible Navier-Stokes equations are coupled and handled differently throughout each software.A solution-adaptive mesh refinement tool is adopted to reduce computational cost. Softwarecomparison is based on quantitative and qualitative analysis, namely crown height and outerdiameter measurements, and the crown curvature and occurrence of splashing, respectively.Results show that Basilisk simulations are in good agreement with the experimental data. Fluent follows the tendency for the crown outer diameter however, in terms of height, the modelunder-predicts its growth and collapses at later stages of the impact for lower thicknesses
- The influence of Wettability on the Droplet Impact onto Micro-Structured SurfacesPublication . Ribeiro, Daniela; Foltyn, Patrick; Silva, André; Lamanna, Grazia; Weigand, BernhardThe flourishing of applications in need of self-cleaning mechanisms increased the search forwater repellent hydrophobic surfaces with induced roughness. Disclosing the small-scale inter-face phenomena on the wetting behavior is essential to design efficient hydrophobic materialswith defined topography. On the other hand, the spreading behavior concerning the forma-tion of thin films on a surface is required to assure the quality of spray cooling and coatings.The contact angle undoubtedly plays an important role in the droplet impact, providing differ-ent outcomes. Moreover, an open question is, how surface topography can affect the impactprocess. Therefore, to evaluate these matters, different surface patterns were manufacturedto assess the surface topography influence on the impact dynamic behavior. Additionally, thewettability of the micro-structured surfaces was flexibly influenced through plasma activationand plasma polymerization. The impact of distilled water and isopropanol droplets on the dif-ferent surface patterns was captured from three perspectives providing high-quality images ofthe phenomena. Different surface morphologies can be obtained depending on the surfacemicro-structures and wettability, affecting spreading shape and evolution. The fluid penetrationwithin the micro-structures is a key feature influencing not only the structures of the outcomesbut also the transition between regimes.
- Effect of Surface Wettability on the Droplet Impact Morphologies on Dry Smooth Polycarbonate SurfacesPublication . Foltyn, Patrick; Ribeiro, Daniela; Silva, André; Lamanna, Grazia; Weigand, BernhardThe surface wettability has a significant influence on the morphology and spreading behaviorduring droplet impacts on dry smooth walls. On the way for predicting spreading diameters independency of the wetting behavior, the experimental database was extended by an experi-mental study in which distilled water and isopropanol droplets have impacted onto dry smoothLexan®(Polycarbonate) surfaces at four different impact velocities. The range of Reynoldsnumbers was set between1,135and12,240and for the Weber number between80and1,165.The surface material, and thus also the characteristic surface roughness, were kept constant,while the wetting behavior was modified using plasma activation and plasma polymerizationprocesses. Different contact angles have been investigated in a range from full wetting to non-wetting for water and from full wetting to partial wetting for isopropanol. The experiments havebeen conducted on a newly designed test rig. High-speed diffuse backlight images at20 kHzfrom the top and the lateral perspective are acquired on a shared CMOS-sensor simultane-ously. A bottom perspective in a total internal reflection configuration is acquiring the footprintof the droplet impact. This enables to better define the maximum spreading diameter and todistinguish between wetted and non-wetted areas.
- Design and Fabrication of a Low Cost, Push Mode Piezoelectric Stream Droplet Generator with Interchangeable NozzlePublication . Cardoso, João; Moura, Cátia Isabel Neves; Ribeiro, Daniela; Vasconcelos, Daniel; Barata, Jorge M M; Silva, AndréThe interest in studying droplet related phenomena has been increasing over the last decades.In the fluid dispensing equipment industry, a major problem is to minimize droplet diameter andto eject droplets in a controlled manner with a low-cost device. Taking all this into account, anew low-cost droplet stream generator was designed and fabricated. The material used to man-ufacture the stream droplet generator structure was a 3D printable material, namely PLA, thatminimizes the device cost. This structure has three separate components: piezoelectric lid, fluidchamber, and pinhole holder. The disruptive waves of the disturbance mechanism were applieddirectly to the fluid instead of being applied to the precision pinhole. The interchangeable noz-zle used was a round stainless-steel high precision optical pinhole with three different sizes:100μm,150μm, and200μm. Jet attribute properties (droplet diameter, droplet velocity, anddistance between droplets) were measured as the different conditions changed (piezoelectriccell frequency, outlet pressure, and fluid). The present work studied the spray characteristicsand different monodisperse regimes. A full characterisation of them is presented and discussedin detail.
- The pros and cons of the implementation of a chronic care model in European rural primary care: the points of view of European rural general practitionersPublication . Kurpas, Donata; Petrazzuoli, Ferdinando; Szwamel, Katarzyna; Randall-Smith, Jane; Blahova, Beata; Dumitra, Gindrovel; Javorská, Kateřina; Mohos, András; Simões, José Augusto Rodrigues; Tkachenko, Victoria; Kern, Jean-Baptiste; Holland, Carol A; Gwyther, HollyIntroduction: This article describes the views of European rural general practitioners regarding the strengths, weaknesses, opportunities and threats (SWOT) of the implementation of a chronic care model (CCM) in European rural primary care. Methods: This was a mixed-methods online survey. Data were collected from 227 general practitioners between May and December 2017. Categorical data were analysed using descriptive methods while free-text responses were analysed using qualitative methods. The setting was rural primary care in nine European countries (including Central and Eastern Europe). Main outcomes measures were respondents’ evaluations of a chronic care model in their rural healthcare settings in terms of SWOT. Results: The SWOT analysis showed that the expertise of healthcare professionals and the strength of relationships and communications between professionals, caregivers and patients are positive components of the CCM system. However, ensuring adequate staffing levels and staff competency are issues that would need to be addressed. Opportunities included the need to enable patients to participate in decision making by ensuring adequate health literacy. Conclusion: The CCM could certainly have benefits for health care in rural settings but staffing levels and staff competency would need to be addressed before implementation of CCM in suchsettings. Improving health literacy among patients and their carers will be essential to ensure their full participation in the implementation of a successful CCM.
- Single droplet combustion of aluminum nanoparticles added to a biofuel: effect of particle concentration and ambient temperaturePublication . Ferrão, Inês; Mendes, Miguel; Moita, A. S.; Silva, AndréThe fast expansion and growth of the aviation sector, coupled with the greenhouse gas andpollutant emissions, requires urgent solutions. A starting point to change this sector paradigmcould be a new energy source for aviation gas turbines, focused on potential carbon neutral fu-els, e.g., biofuels. A possible solution is nanoparticles addition to improve the biofuel propertiesand mitigate the problems inherent to their use. The present work experimentally evaluates thecombustion characteristics of single droplets of HVO (NExBTL) with aluminum nanoparticles.The ambient temperature was varied from600°Cto1000°C. Three particle concentrations (0.2,0.5, and 1.0 wt.%) were investigated with a fixed particle size of 40nm. This study examinedthe combustion of droplets with an initial diameter of 250μmusing a falling droplet method.The results reveal that for nanofuels, the droplet size evolution curve is not in agreement withD2– law and display a unique disruptive burning phenomenon at the end of the droplet life-time. It was noticed that the burning rate of biofuel is considerably enhanced with the addition ofnanoparticles, being the highest value for the nanofuel with a particle concentration of 1.0 wt.%.