1,721,065 research outputs found
Storie di Marmi e di Famiglie: I Ritratti di Giulio e Francesco Maria Spinola
No full textIl saggio analizza i ritratti marmorei di Giulio e Francesco Maria Spinola, attribuiti a Francesco Baratta il Giovan
Experimental characterization of thermal and viscous powers in porous media under oscillating flow
No full textPorous materials are integrated components across various industries, offering unique properties such as high surface area, low density, and good permeability. They have a wide range of applications including energy conversion, with relevance in sound absorption and thermoacoustic phenomena. Understanding the intricate energy conversion mechanisms within the microstructure of porous materials under oscillating flows, such as sound waves, is paramount for optimizing their performance in these applications. The techniques currently used for testing porous materials enable the characterization of the behaviour of the porous matrix when subjected to an acoustic wave, without consideration to energetic quantities. Here, this paper presents two novel measurement techniques allowing for the experimental quantification of the power dissipated within the porous material, by making an explicit distinction between thermal relaxation and viscous dissipation effects. The study involves a model to quantify the viscous and thermal energetic behaviours from which analytical expressions guiding the elaboration of the proposed experimental techniques are derived, and finally validated through experimental data. Experimental tests have been carried out on three different samples (polyester fibers, wire mesh and triangular pores sample) largely used both in acoustic and thermoacoustic fields. The experimental data compared with the theoretical prediction for each sample allow to validate the measurement methodologies. By bridging theoretical modelling with experimental validation, this work contributes to the broader understanding and utilization of porous materials in energy conversion applications
Transport parameters for sound propagation in air saturated motionless porous materials: A review
No full textTransport parameters play a key role in characterizing the thermo-viscous behaviour of the microgeometry. Semi-phenomenological models provide valuable tools to establish a connection between the dynamic behaviour of porous materials and these transport parameters. However, each model has its limitations in terms of the frequency range and material types it can accurately represent. One of the most used semi-phenomenological acoustic models in the literature is the Johnson-Champoux-Allard-Lafarge (JCAL) model [J. Fluid. Mech. 176 (1987) 379–402, J. App. Phy. 70 (1998) 1975, J. Ac. Soc. Am. 102 (1998) 1995]. This model requires the knowledge of six transport parameters, known as the porosity φ, airflow resistivity σ, thermal characteristic length Λ′, viscous characteristic length Λ, high-frequency limit of tortuosity α∞, and static thermal permeability k0′, which establish a connection between the micro-geometrical features of the porous material and its macroscopic behaviour when subjected to sound waves. The JCAL model is applicable to all types of porous materials, and the required transport parameters can be measured using suitable devices. With recent advancements in additive manufacturing, it is now possible to create porous materials with precise and controlled geometries. Therefore, understanding the relationships between microgeometry and transport parameters is crucial for designing porous materials with specific acoustic properties. This study provides a comprehensive overview of all the transport parameters involved in characterizing the JCAL model. It synthesizes various direct, indirect, and inverse measurement techniques used to assess these parameters. Additionally, computational approaches for evaluating the transport parameters from representative elementary volumes (REV) of materials are presented. Finally, the study compiles the existing correlations between transport parameters and the microgeometry of the unit cell from the available literature
Neoplasie del colon e del retto.
No full textMentre le neoplasie benigne e maligne dell'intestino tenue sono relativamente rare, quelle del colon e del retto sono molto frequenti. Il carcinoma del colon e del retto (CCR) è la più frequente neoplasia dell'apparato gastroenterico e in assoluto una delle principali cause di morte per cancro. Infatti rappresenta la seconda causa di morte per cancro al mondo sia nelle donne che negli uomini. L'incidenza è di 15.3-19.4 nuovi casi ogni 100.000 abitanti per anno, mentre la mortalità è di 8.6-10.7 ogni 100.000 abitanti. La sopravvivenza a 5 anni è del 49-59%. I nuovi casi diagnosticati ogni anno in Italia sono circa 27.000. La quasi totalità dei CCR si sviluppa in una lesione preesistente denominata polipo adenomatoso in un intervallo di tempo di circa 10 anni
A novel model for macroscopic simulation of oscillating heat and fluid flow in porous media
No full textIn 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
Criteria for the selection of porous cores for thermoacoustic applications
No full textThe criteria indicated in this paper stem from the analysis of the viscous-thermal behaviour of the three most common typologies of porous cores for standing-wave thermoacoustic engines, namely cores provided with circular pores, with rectangular slits and with array of parallel cylindrical pins. Analytic solutions of spatially averaged thermal and viscous functions are used to describe the sound propagation within the cores by means of the Rott's theory of thermoacoustics. The performances of these three typologies of cores have been computed and compared by modelling the behaviour of a standing-wave thermoacoustic engine operating with different fluids. In previous works the comparison among different cores has been based on the ratio between the acoustic power generated and the power dissipated by viscous losses. Aim of this work is to report the same comparisons but in terms of acoustic power and global efficiency. In this way it is possible to consider all the losses involved in the heat to acoustic power conversion occurring in the core, including the entropy flux and the heat conduction through the stack
Wire mesh stack and regenerator model for thermoacoustic devices
No full textThermoacoustic technology can play a significant role in the development of renewable energies. Thermoacoustic engines and heat pumps (or refrigerators) are however characterized by a low efficiency attributed to suboptimal components. The core of these devices is a porous material, named stack (or regenerator), in which thermoacoustic conversion takes place. The most frequently used stack in the literature remains the wire mesh, although there is still a lack of formulation for the corresponding thermoviscous response functions. Essentially, all the dynamic thermal and viscous behaviors of a porous structure can be derived thanks to the Johnson-Champoux-Allard-Lafarge (JCAL) semi-phenomenological model, where transport parameters provide input information on the macroscopic level to the model. Here, we report a set of structure–property correlations between the transport parameters of the stack and the geometrical features of the wire mesh obtained from first-principles calculations. Validation of the model is carried out by means of experimental measurements performed on three different specimens. Our results show that the knowledge of the termoviscous functions for the wire mesh allows drawing preliminary considerations on the thermoacoustic efficiency of the stack, without needing to consider a full numerical simulation of the entire device
Analysis of heat capacity ratio on porous media in oscillating flow
No full textThe paper analyzes heat transfer between an oscillating compressible fluid and solid to evaluate the influence of finite thermal capacity on the heat exchanged between these media. This type of problem is often studied in porous media applications by assuming the solid to have its surface at a constant temperature. When this hypothesis fails, in the frequency domain, the analytical solutions of the thermal field involve a complex dimensionless parameter known in the literature as εs. It was introduced in thermo-acoustic applications to model the solid properties of the porous core. In this work, the parameter is revisited by deriving an analytical formulation valid for arbitrarily shaped channels of porous materials, and a physical interpretation of the parameter is proposed in terms of fluid and solid entropy oscillation. CFD-based simulations of the thermal coupling between solid and fluid on three different simplified geometries of porous material have been conducted to confirm this physical interpretation, by comparing numerical and analytical data. Furthermore, from a numerical point of view, the parameter εs has been written as the inverse of a dimensionless Robin boundary condition. In this work, it is verified that the latter can be adopted at the fluid-solid interfaces to reproduce the thermal effects of the solid material, without numerically simulating it, both in frequency and time domain
Porous cores in small thermoacoustic devices for building applications
No full textThe thermoacoustic behavior of different typologies of porous cores is studied in this paper with the goal of finding the most suitable solution for small thermoacoustic devices, including solar driven air coolers and generators, which can be used in future buildings. Cores provided with circular pores, with rectangular slits and with arrays of parallel cylindrical pins are investigated. For the type of applications in focus, the main design constraints are represented by the reduced amount of the input heat power and the size limitations of the device. In this paper, a numerical procedure has been implemented to assess the behavior of the different core typologies. For a fixed input heat power, the maximum acoustic power delivered by each core is computed and the corresponding engine configuration (length of the resonator and position of the core) is provided. It has been found that cores with parallel pins provide the largest amount of acoustic power with the smallest resonator length. This conclusion has been confirmed by experiments where additive manufactured cores have been tested in a small, light-driven, thermoacoustic prime mover
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