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Flow simulations in porous media with immersed intersecting fractures
No full textA novel approach for fully 3D flow simulations in porous media with immersed networks of fractures is presented. The method is based on the discrete fracture and matrix model, in which fractures are represented as two-dimensional objects in a three-dimensional porous matrix. The problem, written in primal formulation on both the fractures and the porous matrix, is solved resorting to the constrained minimization of a properly designed cost functional that expresses the matching conditions at fracture-fracture and fracture-matrix interfaces. The method, originally conceived for intricate fracture networks in impervious rock matrices, is here extended to fractures in a porous permeable rock matrix. The purpose of the optimization approach is to allow for an easy meshing process, independent of the geometrical complexity of the domain, and for a robust and efficient resolution tool, relying on a strong parallelism. The present work is devoted to the presentation of the new method and of its applicability to flow simulations in poro-fractured domains
Topology optimization for heat transfer enhancement in Latent Heat Thermal Energy Storage
No full textPerformance of a Latent Heat Thermal Energy Storage depends strongly on the spatial layout of high conductive material and phase change material. Previous design studies have explored a limited design space and have rarely taken advantage of any formal optimization approach. This paper presents a topology optimization framework of a Thermal Energy Storage system involving phase change. We solve the Stefan problem for solidification with a fixed grid finite element method based on the apparent heat capacity technique, while the topology optimization problem is formulated using a density-based method. This approach allows to identify design trends that have been rarely investigated in the past. Firstly, we explore the inherent trade-off between discharged energy and required time for complete discharge. We obtain very different designs and highly varying performances at selected Pareto points. Secondly, by comparing results obtained in two and three dimensions we observe that 3D designs allow superior performances by presenting features that are not apparent in 2D. Thirdly, we propose a formulation of the design problem that yields a nearly constant thermal power output during the entire discharge process. If the maximum discharge time is sufficiently large, the optimized design presents fins that are disconnected from the internal tube
Reversed Brayton cycle for food freezing at very low temperatures: Energy performance and optimisation
No full textFreezing is a valuable method to increase food shelf life and to ensure high quality standards during long-term storage. Additional benefits to frozen food quality can be achieved by freezing at very low temperatures (< −50 °C): small ice crystal formation during fast freezing reduces food cell wall rupture, preventing water and texture loss during thawing. This paper presents the design of an innovative food freezing system operating at very low temperatures, based on a modified reversed Brayton cycle (rB cycle). The plant is composed of two interconnected sub-systems: a primary thermodynamic closed loop, operated by an rB cycle, and a secondary airflow loop which is devoted to food freezing by batch process. Relevant features of the designed rB cycle rely on the adopted environmentally safe working fluid, the optimised thermodynamics working conditions and the innovative cycle layout. A modelling framework for the system was developed to identify and design efficient operative settings for the plant components (turbo-machineries, heat exchangers, etc.) and to assess, via sensitivity analysis, the influence of the main design parameters on the global energy performance. The proposed system configuration, designed to maximise the coefficient of performance (CoP) value of the plant, was determined by means of nonlinear multivariable optimisation. In addition, the energy performance of the system can be increased by recovering waste heat available from the rB cycle
Analog-Digital System Modeling for Electromagnetic Susceptibility Prediction
No full textThe thesis is focused on the noise susceptibility of communication networks. These analog-mixed signal systems operate in an electrically noisy environment, in presence of multiple equipments connected by means of long wiring. Every module communicates using a transceiver as an interface between the local digital signaling and the data transmission through the network. Hence, the performance of the IC transceiver when affected by disturbances is one of the main factors that guarantees the EM immunity of the whole equipment. The susceptibility to RF and transient disturbances is addressed at component level on a CAN transceiver as a test case, highlighting the IC features critical for noise immunity. A novel procedure is proposed for the IC modeling for mixed-signal immunity simulations of communication networks. The procedure is based on a gray-box approach, modeling IC ports with a physical circuit and the internal links with a behavioural block. The parameters are estimated from time and frequency domain measurements, allowing accurate and efficient reproduction of non-linear device switching behaviours. The effectiveness of the modeling process is verified by applying the proposed technique to a CAN transceiver, involved in a real immunity test on a data communication link. The obtained model is successfully implemented in a commercial solver to predict both the functional signals and the RF noise immunity at component level. The noise immunity at system level is then evaluated on a complete communication network, analyzing the results of several tests on a realistic CAN bus. After developing models for wires and injection probes, a noise immunity test in avionic environment is carried out in a simulation environment, observing good overall accuracy and efficienc
Ordinal data: a new model with applications
No full textIn this work we propose a new discrete probability distribution useful when we work with ordered categorical data. The discrete-beta distribution has a highly flexible shape and it can be either over-dispersed or under-dispersed with respect to the binomial distribution. It has only two parameters, which have a very clear interpretation: the mean and the precision parameters of the beta latent variable. Adding directly covariates on parameters (according to CUB model framework), it is very suitable to regressio
Multi-Objective Reconfiguration of Radial Distribution Systems using Reliability Indices
No full textThis paper deals with the distribution network recon- figuration problem in a multi-objective scope, aiming to determine the optimal radial configuration by means of minimizing the active power losses and a set of commonly used reliability indices for- mulated with reference to the number of customers. The indices are developed in a way consistent with a mixed-integer linear pro- gramming (MILP) approach. A key contribution of the paper is the efficient implementation of the -constraint method using lexico- graphic optimization in order to solve the multi-objective optimiza- tion problem. After the Pareto efficient solution set is generated, the resulting configurations are evaluated using a backward/for- ward sweep load-flow algorithm to verify that the solutions ob- tained are both non-dominated and feasible. Since the -constraint method generates the Pareto front but does not incorporate deci- sion maker (DM) preferences, a multi-attribute decision making procedure, namely, the technique for order preference by simi- larity to ideal solution (TOPSIS) method, is used in order to rank the obtained solutions according to the DM preferences, facilitating the final selection. The applicability of the proposed method is as- sessed on a classical test system and on a practical distribution syste
Municipal Emergency Plans in Italy: requirements and drawbacks
No full textIn order to highlight the strengths and weaknesses of land use planning and emergency panning policies and strategies in EU, the Italian regulations and guidelines are used as example to discuss the distance between the European and national regulation and the disaster management and post-disaster procedures, that together with the land use planning are often conceived for a Municipal scale. Both anthropic and natural risks are dealt with in the emergency planning, but risk information derive from very diverse sources, with different levels of detail - from the risk assessment of major risk plants to the representation of risks provided by sectorial plans, each one focused on a single type of risk (i.e. flood, seismic, fire…). Emergency plans should aim at correlating the various risk evaluations, thus being able to provide a comprehensive emergency programme, both for people and territorial safety, but indeed the land use is often regulated by a completely different legislation and designing system. This lack of linkage between the procedures for Emergency and for Land Use Planning makes the emergency management less effective towards the achievement of a real safety of territories, as proved by recent disastrous events in the European territory. In order to solve these critical issues, the paper aims at providing hints on how to achieve a different approach both in land use and emergency planning, conceiving risk assessment as part of an integrated process composed by many important and interrelated phases, not only postdisaster emergency, but also structural interventions for the long- term prevention
Carbon recovery and re-utilization (CRR) from the exhaust of a solid oxide fuel cell (SOFC): Analysis through a proof-of-concept
No full textIn the context of the paradigm of Carbon Recovery and Re-utilization (or CRR), this work investigates the role of electrochemical generators (such as high-temperature fuel cells) to perform CRR as a practical secondary effect. In fact, the solid oxide fuel cell (SOFC) operating principle is inherently beneficial toward CO2 separation from the exhaust gas since the fuel is electrochemically oxidized resulting in no N2 mixing at the anode (fuel) electrode. An oxy-combustor downstream the fuel cell will complete the residual fuel (mostly H2 and CO) oxidation to yield a stream that contains only H2O and CO2. After water condensation and further drying, the captured CO2 is fed to a photobioreactor that can fix carbon into microalgae. In this work, results of the first SOFC-based poly-generation system with complete CO2 recovery in the form of fast-growing biomass (micro-algae) are presented, as developed in the EU-funded project SOFCOM (GA 278798, www.sofcom.eu). The overall plant layout is described, and results on the performance of the proof-of-concept plant units are provided