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This work aims to assess the effect of the operating parameters of the gasifying agent preheating temperature and equivalence ratio (ER) on the conversion of sewage sludge (SS) to syngas through gasification and combined heat and power (CHP) generation. A novel gasification model was simulated in Aspen Plus to represent a fixed-bed updraft gasifier to generate syngas from SS through an equilibrium approach restricted by temperature. The novelty of this work is that the model was developed by applying the gasifying agent preheating temperature as an operating variable instead of the gasification temperature. It was calibrated by using a set of experimental values and then validated by comparing the numerical results with the experimental outcomes related to nine different operating conditions of air preheating temperatures and ER. A good agreement between the simulation and experimental results was observed. The optimum gasification process parameters of the air preheating temperature and ER were predicted to be 150 °C and 0.2, respectively. The CHP generation potentiality of SS was assessed to be 2.54 kW/kg SS as dry solids (DS), of which 0.81 kW was electrical and the remainder was thermal power. The conversion of SS to CHP through the proposed treatment can reduce 0.59 kg CO₂/kg SS as DS emissions compared with that of natural gas combustion to generate a similar quantity of energy.

In this article, an application of the macroscopic porous media approach, suitable for CFD simulations in oscillating flows, is proposed for a simplified Standing Wave Thermo-Acoustic Engine (SWTAE), composed of a hot buffer tube, a stack - where the energy conversion takes place - and the rest of the resonator. While for a Travelling Wave Thermo-Acoustic Engine (TWTAE) a macroscopic model for porous media has been successfully applied to both regenerators (similar to the stack in a SWTA) and heat exchangers, for SWTAE this is not true. The results illustrate that a Local-Thermal Non-Equilibrium model is required to start up the SWTAE, otherwise (with the Local Thermal Equilibrium model) the thermoacoustic instability cannot arise. Furthermore, the comparison between the simulation conducted at the microscopic scale and that one at the macroscopic level, depicts that a purely Darcy-linear model employed for the macroscopic model, characterized for oscillating flows, overpredicts the pressure amplitude at periodic steady-state. For this reason, a Forchheimer-like coefficient needs to be implemented to fit the macroscopic solution with the microscopic one.

This paper presents the numerical solution of thermal field of a set of geothermal coaxial probes coupled with the surrounding soil. These probes, used for the excavation of Naples underground’s tunnels with the artificial ground freezing technique, are converted for the heat transfer fluid transport for a geothermal plant. One-dimensional energy equation has been used with a single unknown in order to extract the solution of the temperature inside the probe, the temperature of the fluid, which is a boundary condition for calculating the temperature range of the soil surrounding the probes. In turn, the latter is used as a source term for the solution of the energy equation for the fluid. This paper also presents a comparison between two different probe materials i) internal and external stainless-steel crown; ii) internal rigid PVC crown and external stainless-steel crown. Such a numerical scheme allows to take into account the 3D effects of the fluid dynamics in porous media as is the soil, and, at the same time, considering a “reduced order model” inside the geothermal probes, as is the 1D model, for considering the thermal coupling effects.

In thermoacoustics, stacks and regenerators are porous media where energy conversion takes place. Modelling full thermoacoustic devices with a CFD approach, in order to capture some nonlinearities, can be extremely expensive from a computational perspective compared to a standard linear approach used in the frequency domain. At the same time, macroscopic models for porous media developed for steady-state flows cannot be directly applied in oscillating flow conditions. Moreover, macroscopic models in the available literature for oscillating flows are inaccurate at high frequencies or require a closure coefficient to be determined numerically (with Direct Numerical Simulations) or experimentally. In this article, a time domain macroscopic model for heat and fluid flow is proposed based on the concepts of complex Darcy and Nusselt numbers in the linear regime. Such coefficients, introduced in the past to describe the oscillatory phenomena, have been used for the first time to build a CFD macroscopic model in terms of their real and imaginary parts. For two different porous media, a parallel plate and a transversal pin array, the developed macroscopic model is verified with the microscopic solution. Furthermore, for a transversal pin array stack, the proposed model is validated against experimental data from the available literature, showing a very good agreement. The findings of this paper can help to strongly reduce the computational costs of oscillatory flow simulations without prior direct numerical simulations of the porous core.

Hospital Operating Rooms (ORs) are working spaces which demand an utmost clean air environment for enhancing both the safety and the comfort conditions of medical staff and patients, in order to reduce the risk of post-operative complications. In this context, an integrated experimental-numerical thermo-fluid dynamic study has been carried out in the present work, to investigate air contamination due to UltraFine Particles (UFPs) inside an actual, but unoccupied, OR, equipped with a laminar air flow system, located near Milan in Northern Italy. The UFPs considered in the present study are geometrically representative of surgical smoke and SARS-CoV, SARS-CoV-2 (COVID-19) virus particles. The flow and thermal fields in the OR have been calculated by using the Realizable k-e turbulence model. A transient passive scalar species transport equation, based on the drift flux model, has been implemented along with the particle deposition boundary conditions at the OR walls, to analyze the UFPs concentration within the OR. The numerical model has been used, after validation against experimental data, to reproduce the quantities of interest in the OR. Moreover, type A and type B uncertainties have been associated to the experimental measurements. The velocity and temperature fields in the OR, obtained from both the numerical and experimental analysis, are compliant to the technical standards. A good agreement is observed between experimental and numerical results, in terms of velocity, temperature and UFPs concentration. A time of 19 min to completely evacuate UFPs from the OR has been obtained from both experimental and numerical analysis, with a deviation smaller than 2% between the results.

In this paper a macroscopic model for porous media in oscillating flow is derived for CFD applications. In non -linear modelling of thermoacoustic devices, their cores, useful for the energy conversion, cannot be, in general, modelled like a traditional porous media through friction factor and Nusselt number correlations, recovered from steady state. In this work the analytical solutions in the frequency domain of the linearized Navier-Stokes equations are spatially averaged for an oscillating flow between parallel plates case to build a macroscopic model of an equivalent porous media through a complex permeability and Nusselt number. The results, obtained by assuming a zero mean temperature gradient, show that there is a good agreement between macroscopic solution of a porous box (representing a parallel plates stack) and the average of the solutions of oscillating flow between parallel plates, both in terms of temperature and velocity fields.

In this study, the sustainability of low-temperature geothermal field exploitation in a carbonate reservoir near Mondragone (CE), Southern Italy, is analyzed. The Mondragone geothermal field has been extensively studied through the research project VIGOR (Valutazione del potenzIale Geotermico delle RegiOni della convergenza). From seismic, geo-electric, hydro-chemical and groundwater data, obtained through the experimental campaigns carried out, physiochemical features of the aquifers and characteristics of the reservoir have been determined. Within this project, a well-doublet open-loop district heating plant has been designed to feed two public schools in Mondragone town. The sustainability of this geothermal application is analyzed in this study. A new exploration well (about 300 m deep) is considered to obtain further stratigraphic and structural information about the reservoir. Using the derived hydrogeological model of the area, a numerical analysis of geothermal exploitation was carried out to assess the thermal perturbation of the reservoir and the sustainability of its exploitation. The effect of extraction and reinjection of fluids on the reservoir was evaluated for 60 years of the plant activity. The results are fundamental to develop a sustainable geothermal heat plant and represent a real case study for the exploitation of similar carbonate reservoir geothermal resources.

Domestic Vertical Axis Wind Turbines (VAWT) are able to operate also when the wind speed is low and can be installed in the open spaces of housing areas. The Darrieus turbine is the most promising since it is characterized by higher efficiency, even if for applications at higher wind speeds with respect to Savonius VAWTs. Several researchers try to optimize the aerodynamics of the Darrieus rotors, allowing wind turbines to start rotation even in the presence of low wind speeds. In this paper the authors propose a numerical investigation of the flow field around a 1:4 scaled model of a pair of blades installed on an innovative Darrieus-type VAWT. A Reynolds Averaged Navier-Stokes-based CFD model was validated on the basis of the wind tunnel data, and the scaled turbine performance was analysed by numerically evaluating the effect of different geometric configurations and rotation angles on the turbine torque coefficients. The results of the simulations confirmed the capabilities of the proposed configuration to give valuable performance even for low wind speeds.

The valorization of residual biomass plays today a decisive role in the concept of &ldquo, circular economy&rdquo, 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 &ldquo, INNOVARE&mdash, Innovative plants for distributed poly-generation by residual biomass&rdquo, 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.

The new standards on energy saving for new and existing buildings have animated both researchers and technicians in recent years, aiming at reducing the dependence on fossil fuels, improving indoor comfort, and systems efficiency. In this scenario, special attention must be paid to historical buildings that need to preserve their key testimonial heritage within the society. This paper describes the design and realization stages of a pilot system based on a ground-coupled heat pump, operating both in heating and cooling modes, installed in the monumental site of Saints Marcellino and Festo (SM&F), in Naples, Southern Italy. This study aims to demonstrate that low-enthalpy geothermal systems can be employed as energy retrofit applications in buildings of historical, artistic, and cultural interest and, at the same time, to prove that the use of this technology allows achieving the objectives, set at global level by the current regulations, and requiring a reduction of carbon dioxide emissions (tCO2) of 53% compared to technology using fossil fuels.

Artificial Ground Freezing technique is often used to build underground structures, such as urban tunnels. modeling activities are necessary to optimize the design of the freezing process. The paper introduces a numerical solution of the phenomenon of water freezing for various soil types, using the finite element method combined with the model order reduction technique. In particular, the characteristic based split method is used as a base to build a model reduction scheme by means of Sparse Subspace Learning. The aim of this study is to develop a mathematical model able to carry out numerical results with negligible CPU time, by varying three parameters reproducing the characteristics of the soil, i.e. the porosity, the Darcy number and the Rayleigh number. The obtained results show how the soil parameters influence the freezing process and the dimension of ice wall.

In Operating Rooms (OR), the transport of Colony-Forming Units (CFU) can generate surgical site infections in patients and, as a consequence, CFU must be properly evacuated from the surgical site. Moreover, Fine Particles (FP) and UltraFine Particles (UFP) can be generated during surgeries by cutting and cauterizing surgical tools, with consequent effects on the health of patients and medical staff. Therefore, indoor air quality, comfort and safety conditions in OR must take into account CFU, FP and UFP concentrations. Due to the importance of the topic, the authors have carried out a review of the experimental, numerical and experimental-numerical combined works available in the literature, that analyse generation, diffusion and deposition of CFU, FP and UFP in ORs. Numerical studies have been majorly focused on the simulation of CFU using computational fluid dynamics, in order to model the motion inside ORs and the evaluation of factors that influence it. Experimental studies mainly deal with the analysis of particles and CFU motion and deposition. Based on the present analysis, it appears that more research effort is needed to analyse FP and UFP in ORs. Technical standards and national laws have been reviewed in order to analyse the constraints commonly used in the design and operation of ORs and ventilation systems, such as to find the technical aspects that are not yet properly considered.

A numerical investigation of heat and fluid flow in electro-osmotically driven systems is presented, by considering plain channels and channels packed with charged solid particles. The results show that the introduction of charged solid particles a_ects the internal potential distribution, fluid flow and temperature distribution in the channel. Under the analysed conditions, the effect on heat transfer is confined to the centre of the channel. This topic needs to be further investigated since it is of interest in practical applications.

Objective of the work is to investigate stress and deformation that conrneal tissue and donor graft undergo during endothelial keratoplasty. In order to attach the donor graft to the cornea, different air bubble pressure profiles acting on the graft are considered. This study is carried out by employing a three-dimensional nonlinear finite element methodology, combined with a contact algorithm. The ocular tissues are treated as isotropic, hyper-elastic and nearly-incompressible materials. The contact algorithm, based on the penalty-based node-to-surface approach, is used to model the donor graft-corneal interface region. First, the proposed computational methodology is tested against benchmark data for bending of the plates over a cylinder. Then, the influence of geometrical and material parameters of the graft on the corneal contact-structural response is investigated. The results are presented in terms of Von Mises stress intensity, displacement and mean contact force. Results clearly indicate that the air bubble pressure plays a key role in the corneal stress and strain, as well as graft stiffness and thickness.

Small and medium size district heating networks have seen some spread over the last two decades, especially in cities which are not supplied by natural gas grids. In this paper, the authors investigate the energetic feasibility of a small district heating network feeding two schools in a city of Southern Italy (Mondragone). In particular, the district heating is powered by a low-temperature geothermal fluid pumped from a well located 2.1 km away from the city center. Geothermal fluid has a temperature of 34 degrees C and a flow rate of 6 l/s. The buildings have been rehabilitated by an energy point of view and are equipped of low-temperature fan-coils in order to ensure their compatibility with the 4th generation district heating system. Energy analysis were conducted through the implementation of transient simulation model in TRNSYS environment. The feasibility of two configurations of the system is compared: the first, with one heat pump installed nearby the geothermal well feeding the grid, and the second one with the heat pumps installed in each building substation. Transient simulations results show that the first configuration is energetically less efficient than the second one due to the high thermal losses (55% of the total consumption) of the district supply pipes.

Purpose This paper aims to provide a comprehensive literature review on modelling electro-osmotic flow in porous media. Design/methodology/approach Modelling electro-osmosis in fluid systems without solid particles has been first introduced. Then, after a brief description of the existing approaches for porous media modelling, electro-osmotic flow in porous media has been considered by analysing the main contributions to the development of this topic. Findings The analysis of literature has highlighted the absence of a universal model to analyse electro-osmosis in porous media, whereas many different methods and assumptions are used. Originality/value For the first time, the existing approaches for modelling electro-osmotic flow in porous have been collected and analysed to provide detailed indications for future works concerning this topic.

The present work describes the application of the generalised porous medium model to study heat and fluid flow in healthy and glaucomatous eyes of different subject specimens, considering the presence of ocular cavities and porous tissues. The 2D computational model, implemented into the open-source software OpenFOAM, has been verified against benchmark data for mixed convection in domains partially filled with a porous medium. The verified model has been employed to simulate the thermo-fluid dynamic phenomena occurring in the anterior section of four patient-specific human eyes, considering the presence of anterior chamber (AC), trabecular meshwork (TM), Schlemm's canal (SC), and collector channels (CC). The computational domains of the eye are extracted from tomographic images. The dependence of TM porosity and permeability on intraocular pressure (IOP) has been analysed in detail, and the differences between healthy and glaucomatous eye conditions have been highlighted, proving that the different physiological conditions of patients have a significant influence on the thermo-fluid dynamic phenomena. The influence of different eye positions (supine and standing) on thermo-fluid dynamic variables has been also investigated: results are presented in terms of velocity, pressure, temperature, friction coefficient and local Nusselt number. The results clearly indicate that porosity and permeability of TM are two important parameters that affect eye pressure distribution. Graphical abstract Velocity contours and vectors for healthy eyes (top) and glaucomatous eyes (bottom) for standing position.

The interest of calculating the effects of thermal bridges in buildings energy consumption is growing, due to recent energy saving regulations applied in different countries. The widespread use of insulating materials to reduce energy requirements of buildings, often employed for intermediate insulation of the building envelope, makes thermal bridges a crucial point in the energy analysis of building envelopes. Furthermore, heat losses through thermal bridges often lead to building pathologies due to moisture condensation. Therefore, thermal bridges need to be correctly characterized in the building design stage in order to reduce heat losses and avoid materials degradation. The authors numerically simulate, by using finite elements, the steady-state and dynamic three-dimensional (3D) heat and vapor transport in inhomogeneous thermal bridges and building envelopes. The aim of the present work is to show the importance of taking into account the presence of inhomogeneities (i.e., metal stud) in building materials for the calculation of actual heat losses and water condensation in 3D thermal bridges. The obtained heat transfer results are verified against the reference data of the technical standard UNI EN ISO 10211. The proposed microscopic approach is essential to calculate the actual heat losses of three-dimensional thermal bridges and building envelopes and to overcome condensation problems.

Thermal contact conductance is an important parameter in a wide range of thermal phenomena, and consequently a large number of experimental, numerical and statistical investigations have been presented in literature. An analysis of thermal contact resistance is carried out to predict heat transfer between nominally flat, parallel rough surfaces in contact, in absence of interstitial conductive media (i.e. in vacuum), by means of a statistical approach. The rough surface is described through a probabilistic model based on the peak height variability and invariant asperity curvature radius

PurposeThe purpose of the paper is to numerically simulate steady‐state thermo‐solutal convection in rectangular cavities with different aspect ratios, subject to horizontal temperature and concentration gradients, and validate the results against numerical and experimental data available from literature.Design/methodology/approachThe fully explicit Artificial Compressibility (AC) version of the Characteristic Based Split (CBS) scheme is adopted to solve double diffusion (DD) problems. A stabilization analysis is carried out to efficiently solve the problems considered in the present work. The thermal and solutal buoyancy forces acting on the fluid have been taken into account in case of aiding and opposing flow conditions.FindingsThe stability limits derived by the authors for the thermo‐solutal convection assume a fundamental role to efficiently solve the DD problems considered. In the cases characterized by higher Rayleigh number the convergent solution is obtained only by employing the new stability conditions. The efficient matrix free procedure employed is a powerful tool to study complex DD problems.Originality/valueIn this paper, the authors extend the stabilization analysis for the AC‐CBS scheme to the solution of DD, fundamental to efficiently solve the present problems, and apply the present fully explicit matrix free scheme, based on finite elements, to the solution of DD natural convection in cavities.

In this work, the authors present an artificial compressibility (AC) version of the characteristic-based split (CBS) algorithm for the numerical solution of flow through a fluid-saturated porous medium. In this study, a fully matrix-inversion-free version of the CBS algorithm is used for the first time to solve the generalized porous medium flow equations for forced-, free-, and mixed-convection problems. The efficiency, the accuracy, and the steady-state convergence of the AC-CBS algorithm are verified through comparison with numerical and analytical solutions available in the literature. The advantages of the proposed AC-CBS scheme include easy parallelization and implementation procedure.

PurposeThe purpose of this paper is to introduce a robust mathematical model and finite element‐based numerical approach to solve solid oxide fuel cell (SOFC) problems.Design/methodology/approachA robust mathematical model is constructed by studying pros and cons of different SOFC and other fuel cell models. The finite element‐based numerical approach presented is a unified approach to solve multi‐disciplinary aspects arising from SOFC problems. The characteristic‐based split approach employed here is an efficient way of solving various flow, heat and mass transfer regimes in SOFCs.FindingsThe results presented show that both the model and numerical algorithm proposed are robust. Furthermore, the approaches proposed are general and can be easily extended to other similar problems of practical interest.Originality/valueThe model proposed is the first of this kind and the unified approach for solving flow, heat and mass transfer within a fuel cell is also novel.

In this paper the influence of obstructions on microchannel electro-osmotic flow is investigated for the first time. To carry out such a study, regular obstructions are introduced into microchannels and flow rates are numerically calculated. The effect of channel width on flow rates is analysed on both free and obstructed channels. The solid material considered for channel walls and obstructions is silicon, and the electrolyte is deionised water. The parameters studied include channel width, obstruction size and effective porosity of the channel. The effective porosity is varied between 0.4 and 0.8 depending on other chosen parameters. The results clearly demonstrate that, under the analysed conditions, introduction of obstructions into channels wider than $$100\,\upmu \hbox {m}$$ enhances the flow rate induced by electro-osmosis.