• Energy Research

Energy generation from waste renewable sources represent an efficient way to provide green power with the highest environmental benefits, tackling problems related to the high costs for their disposal through the conversion of these wastes in biofuels. However, several challenges hinder their intensified use, as the huge variability in the amount and composition of these sources forces authors to enlarge their studies on the entire biomass-to-energy chain sustainability where the power technology can be installed with the highest profits. In present work, the technical, environmental and economic impact of the entire biomass-to-energy supply chain is assessed with reference to a real commercially available Combined Heat and Power (CHP) system, the CMD ECO20X based on biomass gasification, installed as operational demonstration in the Municipality of Laurino in the National Park of Cilento, Vallo di Diano, and Alburni (PNCVD) in Southern part of Italy. Several calculation tools previously developed by authors for the analysis of the performance of the various components of the ECO20X system are here employed to define the mass and energy fluxes that characterize its operations in a local supply chain where forest management residues (oak and beech trees) and olive pomace from oil mills in the area are exploited. The analysis aims to quantify the energy absorption necessary for the pretreatment operations of the organic residue (shredding, briquetting, drying) which are essential for gasification, and how much they affect the production deriving from the biomass cogeneration process itself. Then, measurements in terms of pollutants related to the energy production at the municipality and of the air quality in the area, help in the evaluation of the plant environmental impact from a global perspective, by virtue of data obtained from an LCA analysis conducted considering one year functioning of the CHP plant.

The valorization of residual biomass plays today a decisive role in the concept of “circular economy”, according to which each waste material must be reused to its maximum extent. The collection and energy valorization at the local level of biomass from forest management practices and wildfire prevention cutting can be settled in protected areas to contribute to local decarbonization, by removing power generation from fossil fuels. Despite the evident advantages of bioenergy systems, several problems still hinder their diffusion, such as the need to assure their reliability by extending the operating range with materials of different origin. The Italian project “INNOVARE—Innovative plants for distributed poly-generation by residual biomass”, funded by the Italian Ministry of Economic Development (MISE), has the main scope of improving micro-cogeneration technologies fueled by biomass. A micro-combined heat and power (mCHP) unit was chosen as a case study to discuss pros and cons of biomass-powered cogeneration within a national park, especially due to its flexibility of use. The availability of local biomasses (woodchips, olive milling residuals) was established by studying the agro-industrial production and by identifying forest areas to be properly managed through an approach using a satellite location system based on the microwave technology. A detailed synergic numerical and experimental characterization of the selected cogeneration system was performed in order to identify its main inefficiencies. Improvements of its operation were optimized by acting on the engine control strategy and by also adding a post-treatment system on the engine exhaust gas line. Overall, the electrical output was increased by up to 6% using the correct spark timing, and pollutant emissions were reduced well below the limits allowed by legislation by working with a lean mixture and by adopting an oxidizing catalyst. Finally, the global efficiency of the system increased from 45.8% to 63.2%. The right blending of different biomasses led to an important improvement of the reliability of the entire plant despite using an agrifood residual, such as olive pomace. It was demonstrated that the use of this biomass is feasible if its maximum mass percentage in a wood matrix mixture does not exceed 25%. The project was concluded with a real operation demonstration within a national park in Southern Italy by replacing a diesel genset with the analyzed and improved biomass-powered plant and by proving a decisive improvement of air quality in the real environment during exercise.

Among the most relevant fields of research recently investigated for improving the performance of gasoline direct injection (GDI) engines, there are ultrahigh injection pressures and the flash-boiling phenomenon. Both perform relevant roles in improving the air/fuel mixing process, reducing tailpipe emissions and implementing new combustion methods. When a high-temperature fuel is released into an environment with a pressure lower than the fuel’s saturation pressure, flash boiling occurs. Due to complex two-phase flow dynamics and quick droplet vaporization, flash boiling can significantly modify spray formation. Specifically, if properly controlled, flash boiling produces important benefits for the fuel–air mixture formation, the combustion quality and, in general, for overall engine operation. Flash boiling was broadly investigated for classical injection pressure, but few works concern ultrahigh injection pressure. Here, the investigation of the spray produced by a multihole injector was performed using both experimental imaging techniques and CFD simulations aiming to highlight the combined impact of the injection pressure and the flash boiling occurrence on the spray morphology. The shadowgraph method was employed to observe the spray experimentally. The information gathered allows for assessing the performances of an Eulerian–Lagrangian algorithm purposely developed. Breakup and evaporation models, appropriate for flashing sprays, were implemented in a CFD (Computational Fluid Dynamics) code. The experimental results and the CFD simulations demonstrate a good agreement, demonstrating that through adoption of a flash-boiling breakup model, it is possible to reproduce non-evaporating and superheated sprays while changing few simulation parameters. Finally, the results also show the significance of injection pressure in preventing spray collapse.

<div class="section abstract"><div class="htmlview paragraph">Ultra-high injection pressures, as well as flash-boiling occurrence, are among the most important research fields recently explored for improving Gasoline Direct Injection (GDI) engine performance. Both of them play a key role in the enhancement of the air/fuel mixing process, in the reduction of tailpipe pollutant emissions, as well as in the investigation of new combustion concepts. Injector manufacturers are even more producing devices with ultra-high injection pressures capable of working with flashing sprays. Flash-boiling of fuel sprays occurs when a super-heated fuel is discharged into an environment whose pressure is lower than the saturation pressure of the fuel and can dramatically alter spray formation due to complex two-phase flow effects and rapid droplet vaporization. In GDI engines, typically, it occurs during the injection process when high fuel temperatures make its saturation pressures higher than the in-cylinder one. Flash boiling significantly affects the spray structure and fuel-air mixture formation with, potentially, if spray collapse is avoided (with the consequent risk of spray-wall impingement), positive consequences for the engine performance and pollutant emissions. So, this work proposes a combined experimental and numerical characterization of the spray issued by a multi-hole device to highlight the combined role of the injection pressure (up to 700 bar) and the flash boiling occurrence on the spray morphology. Experimental observations of the spray were performed using the Mie-scattering technique. Collected data allowed to evaluate the capabilities of an Eulerian-Lagrangian code in reproducing the injection processes. CFD models for flashing and non-flashing conditions were developed featuring specific breakup and vaporization models suitable for flashing sprays. The numerical model achieves a pretty good level of agreement with the experimental data, and, in particular, it highlights the importance of injection pressure in avoiding spray collapse.</div></div>

Abstract Flash boiling is the sudden change from liquid to vapour phase and in IC engines it typically occurs during the injection process with a gasoline direct injection (GDI) setup and when high fuel temperatures result in saturation pressures higher than the in-cylinder ones. The flash boiling, due to the complex evolution of a multi-phase flow and a rapid droplet vaporisation, promotes spray atomisation and affects significantly the spray structure and fuel–air mixture formation, with consequences for the engine performance and pollutant emissions. Hence, a deep knowledge of such phenomenon is required to improve the combustion process and reduce the pollutant production. The breakup mechanism of a superheated spray is quite different from a regular one, because it features the nucleation and growth of bubbles within liquid droplets. In this paper, an innovative flash-boiling breakup model was embedded in a Eulerian–Lagrangian two-phase solver using OpenFOAM library. The model assigns to the droplets a radial velocity component due to the bubble explosion, making it capable of reconstructing the spray expansion. The code was tested using experimental data of the ECN Spray G injector covering various thermodynamic conditions. The numerical model achieves a good level of agreement with the experimental data and, in particular, it is able to reconstruct the spray collapse. Besides, it provides useful information regarding parameters that are intrinsically difficult to measure experimentally.

The current ongoing rise in environmental pollution is leading research efforts toward the adoption of propulsion systems powered by gaseous fuels like hydrogen, methane, e-fuels, etc. Although gaseous fuels have been used in several types of propulsion systems, there are still many aspects that can be improved and require further study. For this reason, we considered it important to provide a review of the latest research topics, with a particular focus on the injection process. In advanced engine systems, fuel supply is achieved via enhanced direct injection into the combustion chamber. The latter involves the presence of under-expanded jets. Under-expanded jets are a particular kind of compressible flow. For this reason, the review initially provides a brief physical explanation of them. Next, experimental and numerical CFD investigation techniques are discussed. The last section of this manuscript presents an analysis of the jet’s structure. The injection parameters commonly used are examined; next, the characteristics of the near-nozzle field are reviewed and finally, the far-field turbulent mixing, which strongly affects the air–fuel mixture formation process, is discussed.

The reuse of architectural heritage is a topic of great interest for scientific research, involving aspects ranging from the architectural compatibility of the interventions to the performance updating of the artefacts, from the point of view of both energy consumption and internal comfort suitable for the new use. Compatible technological solutions exploit the passive cooling activating latent physical mechanisms of the building, of the envelope or its parts, such as openings and disused shafts. This work concerns the conversion of an old chimney, completely integrated into the historical envelope, into a ventilation duct for the air exchange and the internal comfort improvement of an old factory, proposing an adaptive retrofit solution during adaptive reuse intervention. Thermo-fluid dynamics analyses, performed with an ad hoc CFD solver for flows with flotation effects, verified the effective functionality of the device in summer and winter conditions. The results show that, in summer, the activation of passive ventilation improves the indoor comfort of the environment, while, in winter, it worsens them. This study demonstrates the usefulness of activating passive cooling phenomena in preserving historical architecture. Finally, the future potential of the application is presented by integrating the ventilation chimney with a mechanical control system to optimize its operation even in winter conditions.