This study examines the burning characteristics and disruptive burning phenomena of single droplets of aluminum nanoparticles (n-Al) stably suspended in a biofuel (HVO). The biofuel used in the present work is a promising alternative fuel already tested in the aviation sector to reduce greenhouse gas and pollutant emissions. Experiments were conducted with two particle sizes (40nm and 70nm) and two particle concentrations (0.5 wt.% and 1.0 wt.%) to study its influence when added to the biofuel. The effect of size and concentration of the aluminum nanoparticles was studied at 1100 °C in a drop tube furnace. This experimental facility allows the study of combustion characteristics of falling droplets, ensuring there is no influence of the supporting fiber on the burning rate and disruptive burning phenomena occurrence. A CMOS high - speed camera coupled with a high magnification lens was used to evaluate the droplet size, burning rate, and micro-explosions. Based on this procedure, pure biofuel droplets were compared with those of biofuel blended with nanoparticles. The results suggest that the combustion characteristics of pure HVO can be enhanced with the addition of aluminum nanoparticles. Furthermore, by decreasing the particle size, a slight increase in the burning rate of nanofuels was noticed. Additionally, an increase in the particle concentration leads to a pronounced increase in the burning rate. The particle concentration also influences the delay and intensity of micro-explosions, disruptive burning phenomena detected at the end of the droplet lifetime.

Air transport plays a crucial role in globalization, connecting people and businesses worldwide. Nonetheless, its reliance on fossil fuels significantly contributes to greenhouse gas emissions and pollution. The continuous growth of this sector, alongside the environmental issues and the depletion of fossil fuels, has promoted the investigation of alternative and sustainable energy sources that could supply the aviation industry. Biofuels, drop-in fuels, are considered a promising alternative since they can offer carbon neutrality and substantially further reduction of pollutant emissions, leading to an attractive replacement for conventional jet fuel. Besides reducing dependence on fossil fuel and offering cleaner combustion, many challenges remain regarding biomass availability, process cost, and fuel properties like energy content, viscosity, and volatility. One of the ways to improve the performance of biofuels could be the addition of nanoparticles. This new fuel generation is noted as nanofuels centered on dispersing nanoparticles stably suspended in conventional liquid fuels. In particular, metallic nanoparticles have shown increased fuel energy density, enhancements in combustion rates, reduced ignition delay and fuel consumption, and decreased emissions. According to the previous discussion, the present study investigates the influence of metallic particles on sustainable aviation fuel. This fundamental study initially focuses on nanofuel preparation and stability to address its potential use in real applications. Several preparation approaches are suggested to maintain the stability of nanofuels, particularly for high particle concentrations. Following this, both experimental and numerical investigations are conducted on the combustion of a single droplet, exploring several furnace temperatures, particle sizes, and concentrations. In light of this, the droplet size evolution, disruptive burning phenomena, and potential mechanisms that affect nanofuel combustion are discussed. The experiments conducted in a drop tube furnace suggest that nanoparticle addition promotes the appearance of disruptive burning phenomena regardless of the furnace temperature and particle and size concentrations. In addition, a departure from the D2 -law is noticed, affecting the droplet burning rate. Based on this, the addition of nanoparticles to a liquid fuel is numerically studied. Subsequently, an experimental analysis of the spray under non reacting conditions is performed. An experimental facility was developed using a commercial air-assisted atomizer with external mixing to ease the operation of nanofuels. Imaging and Phase-Doppler Interferometer techniques were used to understand the breakup length, spray cone angle and droplet size, and velocity distributions for several air-fuel ratios. The findings suggest that in terms of single droplet combustion, the addition of nanoparticles possesses a beneficial role, whereas, in atomization, an adverse impact is observed, particularly when the particle concentration is increased. Finally, the development of a laboratory combustion chamber for liquid fuels is discussed. An experimental setup was designed and constructed, which plays a significant role in researching alternative and sustainable fuels.

The internal flow in corrugated tubes of different helical pitch, covering from the laminar to turbulent regime, was studied in order to characterize the three-dimensional flow and the influence of corrugation geometry on pressure drop and convective heat transfer. With water as working fluid and an imposed wall heat flux, ranging from around 4 to 33 kW/m2, a numerical model was developed with a CFD commercial software, where k−ω SST was used to model turbulence. Experimental tests were performed covering Reynolds numbers in the range from around 300 up to 5000, which allowed to identify the transition region and validate the numerical model. The results show that due to the swirl induced by the corrugation, the critical Reynolds number for the start of transition to turbulent flow is reduced. The thermal performance factor, which quantifies the heat transfer enhancement at the expense of pressure drop, was used to compare the corrugated tubes against the reference case of smooth tube. Based on this, all investigated corrugated geometries performed better than the smooth tube, except for low Reynolds numbers (Re<500) in the laminar regime. Overall, the corrugated tube with the lowest pitch showed a clearly better performance for the intermediate range of Reynolds numbers (1000<Re<2300), being an optimal choice for a wider range of operating conditions in the transitional and turbulent flow regimes.