196,919 research outputs found
Instabilità, antipolitica e aspirazioni di cambiamento frustrate
No full textAnalisi delle strategie di politica e politica economica messe in atto dal governo italiano nell'anno 2007
Instability, Anti-politics, and Frustrated Aspirations for Change
No full textAnalysis of the policy strategy emerging from the behaviour of the Italian government in the the year 2007, with special attention to the economic policies enacted during that year
Pricing agricultural inputs from biodiversity-rich ecosystems and habitats without input markets
No full textPurpose: The purpose of this paper is to focus on the analysis of an understudied problem in the economic literature. It proposes a valuation methodology for inputs that come from biodiversity-rich ecosystems/habitats and are used in agro-food production at zero input cost because there is not a market for such inputs. Design/methodology/approach: Following Onofri et al. (2017), the authors computed the value of the marginal productivity of different inputs in three selected case studies (Angola, Mozambique and Brazil). Results are theory based and rigorous but show a strong contingency, case based, relative dimension that is captured, in the framework, by the “relativity ratio.” The ratio expresses the relative weight of the value generated by the input that comes from biodiversity-rich ecosystems/habitats in the per capita monthly available income of the farmer and aims at conveying additional insights to the economic valuation. Findings: In this paper, the assessment of agricultural inputs value (price) in the absence of inputs markets is done, with an application to three different case studies. The inputs are peculiar since they come from habitats and ecosystems that are very biodiversity-rich. Originality/value: The paper proposes a practical, though rigorous, methodology for the assessment of the value (price) of agricultural inputs in absence of inputs markets. Markets do not exist since the inputs come from biodiversity-rich habitats and ecosystems
Thermochemical erosion analysis for graphite/carbon-carbon rocket nozzles
No full textA study is conducted to predict graphite/carbon-carbon nozzle erosion behavior in solid rocket motors for wide variations of propellant formulations. The numerical model considers the solution of Reynolds-averaged Navier- Stokes equations in the nozzle, heterogeneous chemical reactions at the nozzle surface, variable transport and thermodynamic properties, and heat conduction in the nozzle material.Two different ablation models are considered and compared: a surface equilibrium approach and a finite-rate model. Results show that the erosion rate is diffusion limited for metallized propellants, ensuring sufficiently high wall temperatures, and it is kinetic limited for nonmetallized propellants. For low surface temperatures, the two models are consistent with each other and predict the same erosion rate, while the surface equilibrium model overpredicts the recession at low surface temperatures. The calculated results show an excellent agreement with the experimental data from the ballistic test and evaluation system motor firings, and the finite-rate model actually improves the predictions when the kinetic-limited regime is approached. 2010. Copyright © 2010 by D. Bianchi, F. Nasuti, M. Onofri, and E. Martelli
Numerical analysis of film cooling in advanced rocket nozzles
No full textThe key demand on future space transportation systems is the concurrent reduction of Earth-to-orbit launch costs and increase of launcher reliability and operational efficiency. A common way of slightly improving performance of gas-generator open-cycle engines is the injection of the turbine exhaust gas into the nozzle divergent section, which is also used for wall film cooling. The present study focuses on a numerical parametric analysis of the film-cooling efficiency in dual-bell nozzles. The secondary gas injection is made in the first bell, and it is found that the expansion fan originating from the inflection helps the film to better protect the wall. The results of fully-attached-flow simulations are also used to study the influence of film cooling on the expected behavior of nozzle side loads during operation with separated flow in the second bell. Copyright © 2009 by E. Martelli, F. Nasuti, and M. Onofri
Analysis on the effect of channel aspect ratio on rocket thermal behavior
No full textA trade-off analysis is performed on a test case representative of the cooling system of 1 MN thrust class oxygen/hydrogen liquid rocket engine. The aim of the analysis is to find the channel aspect ratio that maximizes the heat extracted from the hot-gas, for a given coolant pressure drop and hot-gas side wall temperature. The analysis requires many cooling channel flow calculations which are performed by means of a simplified model, referred to as quasi-2D, and an accurate conjugate heat transfer model based on numerical integration of the Navier-Stokes and Fourier's Equations. Both models are able to describe the whole cooling device composed by the coolant and the solid domain, which is exposed the hot-gas, with different computational time and level of details. The fast quasi-2D approach is used to select channel geometries showing the same pressure loss. Discussion is made on results obtained with the more accurate CHT model. Results identify, for the selected test case, an ideal aspect ratio which optimizes cooling performance at large values of channel aspect ratio © 2012 by M. Pizzarelli, F. Nasuti, M. Onofri
Numerical study of heat transfer in film cooled thrust chambers
No full textFilm cooling as a thermal protection for the walls of liquid rocket engines is studied numerically for hydrogen and methane thrust chamber tests. The aim is to verify the capability of the Reynolds Average Navier-Stokes model to capture the basic characteristics of film-cooled thrust chambers, considering a simplified approach, named pseudo-injector approach, which does not model propellant injection and combustion. This assumption allows a great saving in computational time, in particular when considering 3D simulations. The present study takes its origin from the European Community In-Space Propulsion 1 (ISP-1) program where, among various projects, an experimental campaign has been designed to study the film cooling technique in an oxygen/methane thrust chamber and to provide a database for computational fluid dynamics validation. The results show that the present approach gives good results in terms of heat flux characterization, in particular when dealing with test cases of high chamber pressure. © 2012 by B. Betti, E. Martelli, F. Nasuti, M. Onofri
Coupled numerical simulation of wall heat conduction and coolant flow in liquid rocket engines
No full textCoolant-flow modeling in regeneratively cooled rocket engines fed with turbo-machinery is a challenging task because of the high wall temperature gradient, the high Reynolds number, the high aspect ratio of the channel cross section and the heat transfer coupling with the hot-gas flow and the solid material. In this study the effect of wall heat conduction on the coolant flow is analyzed by means of coupled computations between a validated Reynolds Averaged Navier-Stokes Equations solver for the coolant flow-field and a Fourier's equation solver for the thermal conduction in the solid material. Computations of supercritical-hydrogen flow in a straight channel with and without coupling with the solid material are performed and compared in order to understand the role played by the coupling on the coolant flow evolution. Finally, the whole cooling circuit of the Space Shuttle Main Engine Main Combustion Chamber is analyzed in detail; in this case, the hotgas flow is described by means of experimental data of the heat transfer coefficient while the wall structure and the coolant flow behaviors are described by the coupled procedure presented in this work. © 2011 by M. Pizzarelli, F. Nasuti and M. Onofri
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