1,721,031 research outputs found
The α-prototype of an ultra-micro-gas turbine at the University of Roma 1. Final assembly and tests
No full textThe paper describes the realization of the alpha-prototype of a portable power device consisting of an electrical generator with a power output of about 300 W driven by a small gas turbine set. The device is so small that it can be properly defined an ultra micro device, capable of supplying electric power in stand alone conditions and for prolonged periods of time (up to 24 hours continuously). In practice the device can be used as a convenient substitute (or replacement) for all current battery storage systems and is significantly smaller, lighter and most likely more reliable than the few existing internal combustion engines of comparable power output. The particular nomenclature is UMGTG-UDR1 (Ultra-Micro Gas Turbine Generator). The final configuration of the prototype (for which a patent is pending) is described in the paper as well, together with some of the results of the final operational tests
A filter-less LES model based on the local minimization of the entropy generation rate: model description
No full textThe paper discusses a novel LES turbulent model, first presented in a linearized version in 2019. The model, named “ESS-LES” (for Entropy Smoothing at Sub-grid-scales LES), is based on the local minimization of the entropy generation rate in each single cell. We present here the theoretical background in some detail. First, the exact entropy generation equation is written in each cell in terms of the instantaneous velocities U,V,W. Then, a set of explicit differential equation for the subgrid scale velocities u,v,w is obtained from the Lagrangian minimization of the entropy generation rate, and a closed form solution is derived in the form of an infinite sinh*sin series. By solving them and truncating the series at the relevant Kolmogorov scales, a formal integration of the uu, vv, ww within each cell provides the subgrid stress tSGS terms that can be added into the resolved Stokes-Navier equations that are in turn solved iteratively. The physical correctness of the model is first analyzed by studying the effect of the model on a number of externally imposed wavelike velocity solutions, to verify the congruency of the resulting shapes of the subgrid stresses. Some preliminary results (not described here in detail) indicate that the model is in a good agreement with the DNS solution and appears to be competitive with other “classical” LES models. A fully-3D simulation is being developed to validate the model on realistic geometries against available LES and DNS results, to verify that it properly reproduces the energy dynamics of turbulence
Design and Performance Prediction of a Ultra-Micro Gas Turbine for Portable Power Generation
No full textIn the last decades, the enormous growth in portable electronics applications has stimulated research in lightweight and reliable power sources. In this technological niche, the idea of developing miniaturized turbomachinery to serve as portable power sources is attractive, especially in view of the much higher power density of a small turbogas generator in comparison with conventional chemical batteries. The potential social impact of an eventual commercialization of such "nano-devices" is enormous, but the technological and phenomenological complexity of the design and manufacturing of ultra-small machines (with a rotor diameter of the order of 1 cm) poses difficult problems, because some of the well-established design procedures for large and medium scale machines do not seem to be applicable as such at these extremely reduced scales. In this work the design and performance of a miniaturized gas turbine with a tip diameter of about 10 mm is examined from a thermo-fluiddynamic point of view. An extensive series of CFD simulations allows for the quantification of the losses at the resulting low Reynolds numbers (∼4600 in the stator and ∼1500 in the rotor) on the single-curvature geometry imposed by manufacturing constraints. The geometry of the blades is "optimized" on the basis of an entropy generation rate analysis, and the capabilities and limitations of conventional design strategies are assessed. Copyright © 2007 by ASME
A general model for the evolution of non-equilibrium systems
No full textThe paper addresses the problem of the evolution of systems that are initially in a state of non-equilibrium. The model we propose leads to an equation of motion that starts from a rephrasing of the classical non-equilibrium Ginzburg-Landau equation by reinterpreting in the sense of an exergy evolution paradigm. This paper may be considered as a logical corollary to -and at the same time as an e conceptual extension of-the solution to the problem of the existence and quantification of a non-equilibrium exergy presented in previous articles by the present Authors. In previous papers it was shown that, if both energy and exergy are considered a priori concepts, the evolution of the exergy of a solid body subject to a sufficiently smooth relaxation process can be calculated for arbitrary initial temperature or concentration distributions with an accuracy that depends only on the information about the initial distribution of the system properties at the initial time and on the availability of proper material relations. It was shown that the non-equilibrium exergy, i.e., the extra ideal work that can be extracted from the body, relaxes to zero as the system tends to its equilibrium state, so that the total exergy content (given by the sum of non-equilibrium and equilibrium exergy) attains the value given by its classical definition. The evolution history depends of course on the imposed b.c. and on the “gradient” that drives the relaxation. In this paper, we formalize the dependence of the non-equilibrium exergy on its possible drivers (pressure, temperature or concentration gradients) and derive a general “equation of motion” that links the former to the latter. The solution is analytical, and therefore there is no need to postulate local equilibrium, as long as we are dealing with a continuum (scales sufficiently removed from the atomic ones). A few applications to ideal and real processes are presented and discussed, while the application of the method to more complex and industrially relevant cases is left for later studies. The paradigm is theoretically simple and the resulting model of relatively easy implementation: we therefore hope that applications of the proposed framework may be systematically developed in the fields of engineering and natural science, to gain a better insight into real non-equilibrium processe
Progress in the development of a prototype “Ultra-Micro” Gas Turbine Set for Portable Power Generation
No full textThe advancement in the design and prototype testing of an ultra-micro gas turbine set for portable electrical generation is reported in this paper. The extremely small power requirements (300 W) pose severe restrictions both to the Designer's and to the Technologist's choices: the present specifications lead to rotor diameters of about 10 (compressor) and 13 mm (turbine) and as a consequence to extremely high rotational speeds (4-500000 rpm). Previous studies have shown that the attainable cycle efficiency is rather low (below 10%): therefore, a regenerative cycle has been adopted here. The turbine entry temperature is much lower than current industrial standards for large-scale machines, because the selected material (SiC) cannot exceed 1000°C and more advanced ceramic materials have raised concerns about their fragile behaviour especially during on-off transient operation. A back-to-back radial/radial configuration is presented here: the blade geometry is specifically studied to attain the maximum possible fluid dynamic efficiency and to be easily manufactured (no blade twisting: the profiles are simply extruded from the hub to the tip, and their shape has been "optimized" in a previously published series of CFD simulations). A thermo-mechanical stress analysis of the rotating assembly has been performed, and some critical issues resolved, so that neither rotor is near failure under realistically "hot" operating conditions. A mockup has been built to undergo cold flow tests (not reported here)
Study, development and prototyping of a novel mild hybrid power train for a city car: Design of the turbocharger
Full text linkWithin a large, state-funded, Italian National Project aimed to test the feasibility of an on-the-road prototype of a mild hybrid city vehicle, one of the tasks was to conceive, design and implement an innovative turbocharger that would allow for some energy recovery. The selected vehicle is propelled by a 3-cylinder, 998 cc turbocharged engine (the 66 kW Mitsubishi-Smart W451). The idea is to implement two types of energy recovery: one via the new turbocharger and one through a standard braking energy recovery (also known as KERS). The study of the former is the object of this paper. The proposed turbocharger configuration consists of mechanically separated, electrically coupled compressor and turbine, possibly mounting only slightly modified commercial equipment to reduce construction costs. This paper reports the results of the calculation of the behavior of the new turbocharging group across the entire engine operating range and describes the preliminary design of the unit. An accurate simulation of a mixed (urban and extra-urban) driving mission demonstrates that a net saving of about 5.6% can be attained by the installation of the novel turbocharger unit
- …
