Browsing by Issue Date, starting with "2022-09-06"
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- Influence of Dimensionless Temperature on Droplet Impact onto Heated Liquid Films for Subcooled Boiling RegimesPublication . Vasconcelos, Daniel; Silva, A. R. R.; Barata, Jorge M MHeat and mass transfer mechanisms related to multiphase flows occur in several applications such as spray cooling, quenching, internal combustion engines and plasma spraying. These mechanisms have become increasingly important due to the need of achieving higher heat rate coefficients associated with phase-change processes, such as evaporation and condensation. Specifically, the phenomenon of droplet impact onto non-heated liquid films has been extensively researched, both experimentally and numerically. However, the influence of temperature on droplet impact and liquid film stability has been overlooked in the literature, which is a focal point in understanding interfacial phenomena. The main objective of this work is to experimentally study droplet impact onto heated liquid films. Therefore, an experimental facility was designed for this purpose. A borosilicate glass surface is used to contain the liquid film. This surface is placed above an aluminium block with four embedded cartridge heaters of 250W each, heating the liquid film by conduction. Immersion type-k thermocouples are employed for liquid film temperature measurements. Liquid film evaporation rates are calculated in order to ensure the liquid film thickness prior to the droplet impact. Water and n-decane are the fluids adopted due to their differences in thermophysical properties and saturation temperature. The impact conditions are 100 < W e < 300, 0.5 < h* < 1.5, and a dimensionless temperature of θ < 0.6. Qualitative analysis is performed regarding crater and central jet evolution, and quantitative data regarding evaporation rate and central jet height are measured. The dimensionless temperature affects the droplet impact phenomena, creating recirculation zones near the crater and the impact surface, and affecting the crater formation. The central jet height increases with increasing values of θ for h* = 1.0 and h* = 1.5, whereas for h* = 0.5, the measurements do not follow a similar tendency. The emerging time of the central jet is delayed for higher values of θ, meaning that future studies regarding crater evolution should be considered. The dimensionless temperature also promotes central jet breakup, as well increasing the number of secondary droplets originated from the breakup.
- Influence of Vapour Bubbles Size and Spacing on Droplet Impact Outcomes under Subcooled Boiling RegimesPublication . Vasconcelos, Daniel; Silva, A. R. R.; Barata, Jorge M MMultiphase flows are characterised as heterogeneous mixtures of two or more phases, such as gas-liquid or solid-liquid. These are extremely complex due to the underlying dynamics that may occur, which include interfacial phenomena, such as boiling and evaporation, and interactions between phases. Specifically, droplet impact onto heated wetted surfaces has been overlooked in the literature, which involves heat and mass transfer mechanisms related to sensible heating, condensation and evaporation. The droplet impact phenomenon occurs in several industrial applications, such as internal combustion engines, electronics cooling devices, heat exchangers, among others. In numerical terms, improving efficiency and reliability of simulations is of interest to understand the behaviour of mathematical models for complex physical systems. The main objective of this work is to numerically simulate droplet impact onto a heated liquid film in the presence of vapour bubbles in a 2D-axisymmetric assumption. The numerical model solves the incompressible mass, momentum and energy equations coupled with the VOF method and height functions for accurately capturing the interface. Phase-change processes such as evaporation and condensation are neglected for the current simulations. Water and n-decane are the fluids adopted for the simulation. The impact conditions are D0 = 3.0 mm, h* = 0.5, and U0 = 3.0 m/s. Different vapour bubble sizes, (Dv), and spacings, (xv), are studied to evaluate the vapour bubble phenomena and crown geometrical parameters. An initial vapour bubble is positioned on the axis of symmetry, xs = 0, and, therefore, in the droplet trajectory. Results indicate that vapour bubbles have a lower influence on crown diameter, and a higher influence on crown height. Increasing the size of vapour bubbles leads to a decrease in both crown diameter and height, whereas an increase in the vapour bubbles spacing causes an increase in the crown height and diameter. The initial vapour bubble positioning in relation to the axis of symmetry is a factor that should also be considered in future studies.
- The Impact of High Particles Concentration in a Biofuel Droplet CombustionPublication . Mendes, Tomás S. M.; Ferrão, Inês; Mendes, Miguel; Moita, A. S.; Silva, A. R. R.Aviation is one of the largest transportation sectors and is operated on fossil fuels, being responsible for about 2% of global CO2 emissions. In order to reduce the environmental impact, biofuels emerged as a promising solution. Additionally, a possible approach to improve the performance of biofuels is to add nanoparticles, leading to the concept of nanofuel. The present work evaluates the nanofuel droplet combustion of a biofuel containing high aluminum particle concentrations. To enhance the nanofuel stability, a preliminary study focusing on the addition of a surfactant was mandatory. Particle size of 40 nm and three particle concentrations from 1.0 to 4.0 wt.% were considered. The results show that the oleic acid effectively improves the stability, and no visible oxidation of the nanoparticles was reported. Regarding the single droplet combustion, the observations show that the addition of nanoparticles promotes micro-explosions, contrary to the combustion of pure biofuel, and increases the overall droplet burning rate.
- Non-Isothermal sloshing for space applications: from a ground-based experimental characterisation to microgravity conditionsPublication . Monteiro, F; Marques, Pedro; Simonini, A; Silva, A. R. R.; Mendez, M ALiquid cryogenic propellants are at the forefront of space propulsion due to their optimal trade-off between performance and weight (Sutton et al. 2017). As a result, investigations on the sloshing dynamics of such fuels have been carried out since the early 1960s. Sloshing, defined as the movement of the free liquid surface within a reservoir (Abramson, 1966), induces two types of undesirable effects: (a) displacement of fuel tank’s centre of mass, which disturbs the stability and manoeuvrability of the spacecraft; (b) thermal mixing between the pressurised ullage and subcooled liquid, which can generate large fluctuations in the tank pressure, leading to structural instabilities and thrust oscillations in the propulsive system. [...]
- Experimental Study of a Single Droplet Impinging Upon a Heated Dry Surface Using Jet Fuel and Biofuel MixturesPublication . Ribeiro, Daniela; Barata, Jorge M M; Silva, A. R. R.The aeronautical sector is dependent on fossil fuels which contribute to a considerable amount of pollutant emissions to the atmosphere. In an attempt to reduce these pollutant emissions in a short period of time and without several changes to the aircraft and their engines, this study focuses on the physical phenomena that happen inside the combustion chamber of an internal combustion engine during fuel injection. However, instead of considering only a conventional jet fuel, an alternative fuel is also investigated. The mixture is composed of at least 50% in volume of conventional jet fuel, as demanded by the current legislation, and a biofuel to decrease the consumption of petroleum-based fuels and to reduce pollutant emissions. The main goal of this study is to experimentally observe droplet impact on a heated dry stainless-steel surface for different fuel mixtures, impact energies (Weber number between 174 and 955, and Reynolds number between 1099 and 12365), and wall temperatures (Tw = 20 − 300 ºC) to identify the different heat regimes and the physical differences between the distinct sets of impact conditions. To achieve that purpose an experimental setup was designed and built including the acquisition of high-speed images, a droplet dispensing system, a heating device to accurately control the temperature of the target surface and the illumination of the impact site. To enhance the knowledge about the impacts, three different perspectives of the phenomena were captured. The typical front perspective, a second perspective where the camera has a ≈ 20◦ angle to the impact surface to capture more details of the impact, and finally, at a lower frame rate, an isometric perspective to measure the evaporation time. From these data, the impact regimes were identified and the influence of surface temperature, fluid properties, and impact energy was analyzed.
- Evaluation of the In Vitro Wound-Healing Potential of AyahuascaPublication . Gonçalves, Joana; Luís, Ângelo; Gallardo, Eugenia; Duarte, Ana Paulayahuasca is an Amazonian drink, which contains β-carboline alkaloids and N,N- dimethyltryptamine. The aim of this study was to evaluate the healing potential of decoctions of a commercial mixture, four individual plants and four mixtures of two plants used in the ayahuasca preparation. Thus, the cytotoxic potential of the samples was evaluated and a wound-healing assay was performed with a NHDF cell line. Subsequently, a parallel artificial membrane permeability assay was also performed, to verify if any psychoactive compound could be absorbed by skin fibroblasts. The integrity and permeability of the cell layer were also evaluated, using the transepithelial electrical resistance assay and Lucifer yellow permeability assay, respectively. The compounds absorbed by the cell layer were quantified by high-performance liquid chromatography coupled to a diode array detector. The results showed that only one sample showed cytotoxicity and all the others promoted the migration of skin fibroblasts. Additionally, it was also verified that β-carbolynic alkaloids and N,N-dimethyltriptamine were not absorbed by the cell layer, and in general, did not interfere with its permeability and integrity. To the best of our knowledge, this is the first study where ayahuasca’s wound-healing potential was evaluated.