Plastic is one of the most widely used materials worldwide. The problem with plastic arises when it becomes waste, which needs to be treated. One option is to transform plastic waste into synthetic fuels, which can be used as replacements or additives for conventional fossil fuels and can contribute to more sustainable plastic waste treatment compared with landfilling and other traditional waste management processes. Thermal and catalytic pyrolysis are common processes in which synthetic fuels can be produced from plastic waste. The properties of pyrolytic oil are similar to those of fossil fuels, but different additives and plastic stabilizers can affect the quality of these synthetic fuels. The quality of fuels and the permissible particle sizes and number density are regulated by fuel standards. Particle size in fuels is also regulated by fuel filters in vehicles, which are usually designed to capture particles larger than 4 μm. Problems can arise with the number density (quantity) of particles in synthetic fuels compared to that in fossil fuels. The present work is a numerical study of how particle size and number density (quantity) influence cavitation phenomena and cavitation erosion (abrasion) in common-rail diesel injectors. The results provide more information on whether pyrolysis oil (synthetic fuel) from plastic waste can be used as a substitute for fossil fuels and whether their use can contribute to more sustainable plastic waste treatments. The results indicate that the particle size and number density slightly influence cavitation phenomena in diesel injectors and significantly influence abrasion.

The presented paper addresses two significant issues of the present time. In general, the studies of the effect of synthetic fuels on cavitation formation and cavitation erosion prediction in the nozzle tip of common-rail diesel injectors were addressed. The first problem is plastic waste, which can have a significant negative environmental impact if not treated properly. Most plastic waste has high energy value, so it represents valuable material that can be used in resource recovery to produce various materials. One possible product is synthetic fuel, which can be produced using thermal and catalytic pyrolysis processes. The first issue addressed in the presented paper is the determination of fuel properties since they highly influence the fuel injection process, spray development, combustion, etc. The second is the prediction of cavitation development and cavitation erosion in a common-rail diesel injector when using pyrolytic oils from waste plastic. At first, pyrolytic oils from waste high- and low-density polyethylene were obtained using thermal and catalytic pyrolysis processes. Then, the obtained oils were further characterised. Finally, the properties of the obtained oils were implemented in the ANSYS FLUENT computational program and used in the study of the cavitation phenomena inside an injection nozzle hole. The cavitating flow in FLUENT was calculated using the Mixture Model and Zwart-Gerber-Belamri cavitation model. For the modelling of turbulence, a realisable k–ε model with Enhanced Wall Treatment was used, and an erosion risk indicator was chosen to compare predicted locations of cavitation erosion. The results indicate that the properties of the obtained pyrolytic oils have slightly lower density, surface tension and kinematic viscosity compared to conventional diesel fuel, but these minor differences influence the cavitation phenomenon inside the injection hole. The occurrence of cavitation is advanced when pyrolytic oils are used, and the length of cavitation structures is greater. This further influences the shift of the area of cavitation erosion prediction closer to the nozzle exit and increases its magnitude up to 26% compared to diesel fuel. All these differences have the potential to further influence the spray break-up process, combustion process and emission formation inside the combustion chamber.

Pollution of water by plastic contaminants has received increasing attention, owing to its negative effects on ecosystems. Small plastic particles propagate in water and can travel long distances from the source of pollution. In order to research the settling motion of particles in water flow, a small-scale experiment was conducted, whereby spherical plastic particles of varying diameters were released in an open-channel flow. Three approaches were investigated to numerically simulate the motion of particles. The numerical simulation results were compared and validated with experimental data. The presented methods allow for deeper insight into particle motion in fluid flow and could be extended to a larger scale to predict the propagation of mesoplastics in natural environments.

Erosion of Pelton turbine components due to cavitation and particle-laden flow is a major challenge in hydropower applications, particularly in sediment-rich river environments. This study presents a numerical investigation on how solid particles contribute to the erosion of a Pelton runner. Computational fluid dynamics (CFD) simulations were conducted using ANSYS CFX 2023 R2, incorporating a Lagrangian particle tracking approach and the Finnie abrasion model to predict erosion patterns under varying sediment concentrations. The results indicate that, under normal sediment conditions, particle erosion does not significantly contribute to blade tip damage. However, under extreme sediment loading, the predicted erosion patterns closely match real-world observations, particularly at the blade tip.

Solid waste presents a very large problem in the developed world. Waste plastics, which make up a large part of solid waste, have high energy value, which is discarded if they are not treated properly. Most of the plastic found in solid waste is produced from petrochemical material, so it can be used in resource recovery processes to produce various materials. One promising resource recovery process is the pyrolysis process, from which pyrolytic oil, gas, and solid residue are obtained. Pyrolytic oils have properties that are similar to conventional fossil fuels, and are promising fuels for use in heat engines or heating applications. In the present work, HDPE plastic in the form of plastic bottles caps was collected from solid waste and used in a thermal pyrolysis process for the production of pyrolytic oil. The obtained oil was characterised, and the obtained results were compared to conventional fuels. The obtained oil was used further in an oil burner fuel injection application, in which the spray breakup characteristics were monitored and analysed using VisiSize particle characterisation systems. The obtained results were compared to those of conventional fuel. The results indicate that the difference in fuel properties influences the spray breakup process slightly, but the differences are rather small. This indicates that from a spray development perspective, pyrolytic oil can be used as a substitute for conventional fuels in oil burners.

Flow in fluid machinery can include multiphase phenomena such as cavitation due to energy exchange between the machinery and flow of a liquid, which, in turn, produces local drops in pressure and, therefore, evaporation. When vapor structures are subjected to increased pressure, they collapse during the process of condensation. These collapses are driven by high ambient pressure and can, over time, induce damage to the fluid machinery solid walls—a process termed cavitation erosion. In addition, fluid machinery such as water turbines and pumps often operate in water that is not pure. In this water, solid particles can be present, for example, river sediment, which can also produce erosion. In the past, these causes of erosion were mostly investigated separately; however, as they sometimes occur simultaneously, studies that replicate these conditions are necessary. In the present work, we investigated the erosion of a Venturi channel surface with several different divergent angles using CFD simulations, which were validated experimentally using the soft paint method. Results showing the cavitation and particle erosion were obtained for different flow conditions (specifically with different vortical structures) that are expected to occur in fluid machinery. The results suggest that cavitation modifies the flow field in a manner that affects particle-induced erosion, leading to different macroscopic erosion patterns.