Vincenzo Naso

Exergy use in the Italian society

The exergy concept is reviewed as a tool for resource accounting. Conversions of energy and material resources in Italian society are described in terms of exergy. Necessary concepts and conventions are introduced. Exergy losses in transformations of material resources and in the conversion of various forms of energy are described in some detail.

Experimental analysis of asymmetrical isoflux channel-chimney systems

Abstract Air natural convection in an asymmetrically heated channel with unheated extensions has been investigated experimentally. Local and maximum wall temperatures and heat transfer coefficients are presented, for different values of the process parameters. Optimal configurations in terms of the minimum values of maximum wall temperatures are obtained. Average Nusselt numbers and maximum dimensionless wall temperatures are correlated to the Rayleigh number and to the geometrical dimensionless parameters in the 10–1.5×10 5 range of the Rayleigh number times the expansion ratio. The addition of downstream unheated extensions improves the thermal performance of the channel for some configurations, the longer the extension and the lower the aspect ratio the lower the wall temperature in the channel.

Natural convection in vertical channels with an auxiliary plate

Research on natural convection in open channels is very extensive due to its role in many engineering applications such as thermal control of electronic systems. In this paper, a parametric analysis is carried out in order to add knowledge of heat transfer in air natural convection for a symmetrically heated vertical parallel plate channel with a central auxiliary heated or adiabatic plate. The two‐dimensional steady‐state problem is solved by means of the stream function–vorticity approach and the numerical solution is carried out by means of the control volume method. Results are obtained for both a heated and unheated auxiliary plate, for a Rayleigh number in the range 103–106, for a ratio of the auxiliary plate height to the channel plate height equal to 0, 0.5 and 1 and for a ratio of the channel length to the channel gap in the range 5–15. Correlations for maximum wall temperatures and average channel Nusselt numbers are proposed.

Numerical Analysis of Radiative Effects on Natural Convection in Vertical Convergent and Symmetrically Heated Channels

ABSTRACT Natural convection in a convergent channel with finite-thickness principal walls at uniform heat flux is investigated numerically. Heat conduction in the walls and the effects of walls emissivity are taken into account. Laminar two-dimensional steady-state conditions are assumed. Results in terms of wall dimensionless temperature profiles as a function of convergence angle, channel spacing, and heat flux are given for various values of wall emissivity. Wall temperatures decrease significantly, passing from δ = 0° to 2° at the lowest channel spacing. Streamlines and temperature fields show a recirculating region in the channel for large values of L w/b min and for δ equal to 5° and 10°. Correlations between Nusselt numbers and Rayleigh numbers are proposed. Dimensionless parameters are in the following ranges: 4.4 ≤ Ra′bmin ≤ 1.5 × 105, 10 ≤ L w /b min ≤ 58, 0.0 ≤ ϵ ≤ 1.0, and 0° ≤ δ ≤ 10°. Numerical predictions are in very good agreement with experimental data.

Effect on Natural Convection of the Distance Between an Inclined Discretely Heated Plate and a Parallel Shroud Below

An experimental study on air natural convection on an inclined discretely heated plate with a parallel shroud below was carried out. Three heated strips were located in different positions on the upper wall. The distance between the walls, b, was changed in the range 7.0-40.0 mm and two values of the heat flux dissipated by the heaters were taken into account. Several inclination angles between the vertical and the horizontal were tested. The wall temperature distribution as a function of the channel spacing and the inclination angle, the source heat flux, the number and the arrangement of the heat sources are presented

Combined thermal and optical analysis of laser back-scribing for amorphous-silicon photovoltaic cells processing

Abstract The numerical and experimental analysis of laser back-scribing fabrication of a-Si photovoltaic cells, made out of a multilayer thin film on a glass substrate, is carried out. The numerical study is performed by means of a rather simple combined optical and thermal model. Experiments are carried out throughout the three phases of the manufacturing process. The successive targets of the selective cut are a transparent conductive oxide thin film (TCO), a TCO⧹a-Si double layer and a TCO⧹a-Si⧹Al multilayer. Experimental results and predictions from the numerical model are compared in terms of the cut energy flux values. In the numerical study the cut energy flux is assumed to be the one which determines the melting of the material. The predicted cut energy fluxes are in good agreement with experimental results.

Experimental Investigation of Natural Convection in an Asymmetrically Heated Vertical Channel with an Asymmetric Chimney

An experimental investigation on air natural convection, in a vertical channel asymmetrically heated at uniform heat flux, with downstream unheated parallel extensions, is carried out. One extension is coplanar to the unheated channel wall and the distance between the extensions is equal to or greater than the channel gap (geometrically asymmetric chimney). Experiments are performed with different values of the wall heat flux, aspect ratio (L h /b), extension ratio (L/L h ) and expansion ratio (B/b). For the largest value of the aspect ratio (L h /b =10), the adiabatic extensions improve the thermal performance in terms of lower maximum wall temperature of the channel. Optimal configurations of the system with asymmetrical chimney are detected

Evaluation of the overall efficiency of a low pressure proton exchange membrane fuel cell power unit

Polymer electrolyte membrane fuel cells (PEMFC) at present can be considered the best choices to set up the auxiliary power unit of an hybrid electric vehicle. This paper describes how it is possible and convenient to manage a PEMFC at low pressure, using a blower instead of a compressor/expander. The polarization curve obtained from experimental tests on the stack confirms a typical behaviour at different values of pressure; although high pressure brings best performance, in the balance of the plant the air compressor generally absorbs more then 40% of the power generated from the stack. As a matter of fact, the overall efficiency of the whole FC apparatus can be improved by managing it at low pressure. In an FC/batteries hybrid electric vehicle, the FC is asked to operate not at partial load, but only at full load by using an ON-OFF control. Consequently it is possible to arrange a very compact and reliable apparatus in which the dynamic mass flow and pressure controls are not required and the mechanical and electric hardware is very simple.

Effects of ligaments shape on radiative heat transfer in metal foams

Purpose – The purpose of this paper is to investigate the impact of ligament shape on radiative behavior, with a specific focus on the inter-dependence among porosity, ligament shape and radiative characteristics. Design/methodology/approach – Using Surface Evolver to generate a base structure and then coherently modifying it, the model presented in this paper aims to tackle these challenges in an improved fashion, all the while making it possible to systematically assess the influence of ligament shape on radiation heat transfer in foams, focussing on the porosity-dependence of ligament shape. Findings – It is found that the prediction of numerical models, at constant size and specific surface of the cells, is strongly affected by the dependence of ligament shape on the porosity. Research limitations/implications – The above said dependence has, therefore, to be accounted for in robust modeling of radiation in foams with a wider range of porosities. Originality/value – The radiative behavior of metal foa...

Transient conductive–radiative numerical analysis of multilayer thin films heated by different laser pulses

Abstract A thermal and optical one-dimensional numerical analysis of semi-transparent single and multilayer thin films on a transparent thermically semi-infinite glass substrate, irradiated by a laser source, is presented, in classical conductive Fourier hypothesis. The absorption is evaluated by means of the classical optical matrix method. Both in thermal and optical models the effects of temperature on the properties are taken into account. A pulsed laser source impinges on the glass side of the structure. Four different typical pulse shapes are compared at the same energy amount: a rectangular on–off shape, a symmetric triangular shape, a Gaussian shape, an asymmetric triangular Weibull profile. Numerical calculations are performed, with reference to a Nd-YAG pulsed laser and to the three structures by means of which an amorphous silicon photovoltaic cell is progressively manufactured: a single transparent conductive oxide SnO 2 (TCO) layer, a double layer of amorphous silicon (a-Si) and TCO, an Al/a-Si/TCO multilayer thin film. In each case the single or composite thin films are on a glass substrate. Results are reported in terms of depth and time profiles of radiative coefficients and temperature.