Showing papers by "Vincenzo Dossena published in 2017"

Preliminary characterization of an expanding flow of siloxane vapor MDM

Abstract: The early experimental results on the characterization of expanding flows of siloxane vapor MDM (C8H24O2Si3, octamethyltrisiloxane) are presented. The measurements were performed on the Test Rig for Organic VApors (TROVA) at the CREA Laboratory of Politecnico di Milano. The TROVA test-rig was built in order to investigate the non-ideal compressible-fluid behavior of typical expanding flows occurring within organic Rankine cycles (ORC) turbine passages. The test rig implements a batch Rankine cycle where a planar converging-diverging nozzle replaces the turbine and represents a test section. Investigations related to both fields of non-ideal compressible-fluid dynamics fundamentals and turbomachinery are allowed. The nozzle can be operated with different working fluids and operating conditions aiming at measuring independently the pressure, the temperature and the velocity field and thus providing data to verify the thermo-fluid dynamic models adopted to predict the behavior of these flows. The limiting values of pressure and temperature are 50 bar and 400 °C respectively. The early measurements are performed along the nozzle axis, where an isentropic process is expected to occur. In particular, the results reported here refer to the nozzle operated in adapted conditions using the siloxane vapor MDM as working fluid in thermodynamic regions where mild to medium non-ideal compressible-fluid effects are present. Both total temperature and total pressure of the nozzle are measured upstream of the test section, while static pressure are measured along the nozzle axis. Schlieren visualizations are also carried out in order to complement the pressure measurement with information about the 2D density gradient field. The Laser Doppler Velocimetry technique is planned to be used in the future for velocity measurements. The measured flow field has also been interpreted by resorting to the quasi-one-dimensional theory and two dimensional CFD viscous calculation. In both cases state-of-the-art thermodynamic models were applied.

Development of a background-oriented schlieren technique with telecentric lenses for supersonic flow

Abstract: Background oriented schlieren (BOS) is a quantitative optical technique which exploits light deflection occurring in non-homogeneous transparent media. It allows to indirectly measure the density gradients by analysing the apparent displacement of features of a background pattern when imaged through the investigated flow. Thanks to its simple set-up and to the consolidated data reduction technique based on cross-correlation algorithms the BOS technique has progressively attracted the interest of the researchers. In this work a BOS system using a telecentric lens system has been set up in order to improve measurement accuracy and to avoid 3D effects arising from using conventional entocentric lenses. The design of the telecentric lens system is reported along with an analysis of its performance in term of spatial resolution. Some preliminary tests on a supersonic flows are also reported.

Comparison of experimental and numerically predicted three-dimensional wake behaviour of a Vertical Axis Wind Turbine

Abstract: The evolution of the wake of a wind turbine contributes significantly to its operation and performance, as well as to those of machines installed in the vicinity. The inherent unsteady and three-dimensional (3D) aerodynamics of vertical axis wind turbines (VAWT) have hitherto limited the research on wake evolution. In this paper, the wakes of both a troposkien and a H-type VAWT rotor are investigated by comparing experiments and calculations. Experiments were carried out in the large-scale wind tunnel of the Politecnico di Milano, where unsteady velocity measurements in the wake were performed by means of hot wire anemometry. The geometry of the rotors was reconstructed in the open-source wind-turbine software QBlade, developed at the TU Berlin. The aerodynamic model makes use of a lifting line free-vortex wake (LLFVW) formulation, including an adapted Beddoes-Leishman unsteady aerodynamic model; airfoil polars are introduced to assign sectional lift and drag coefficients. A wake sensitivity analysis was carried out to maximize the reliability of wake predictions. The calculations are shown to reproduce several wake features observed in the experiments, including blade-tip vortex, dominant and minor vortical structures, and periodic unsteadiness caused by sectional dynamic stall. The experimental assessment of the simulations illustrates that the LLFVW model is capable of predicting the unsteady wake development with very limited computational cost, thus making the model ideal for the design and optimization of VAWTs.

Numerical Wind Tunnel Tests of an Open Data IPC-VAWT

Abstract: The evaluation of the performance of a Darrieus Vertical Axis Wind Turbine is performed using an actuator line code. This allows to investigate the aerodynamics of the turbine and the interaction with the boundary conditions. Blade Pitch control, tip-effects modelling and blockage effects in an open-jet wind tunnel test section are studied and investigated. Pitch control allows to enhance the power extraction, tip effects allow to generate a more realistic model, while the modelling of the open-jet test section allows to validate the numerical approach on experimental data intrinsically characterized by not negligible blockage effects.

Reynolds number effects on the performance of safety valves operating with incompressible flows

Abstract: The goal of the sizing procedure of safety valve operating in liquids is the selection of the valve flow area, taking into account the effect of the viscosity variation by means of a correction factor ( K v ) computed on the basis of the flow Reynolds number In the most commonly applied reference standards (ISO 4126-7, and similarly API 520, part 1), questionable assumptions are often used and this may lead to unpractical solutions or, in a wide range of applications, to untenable paradoxes The paper describes a numerical and experimental investigation of the behavior of a 15″ G 3″ safety valve operating in water and oil at different temperatures Computational results have been obtained using an high-order accurate CFD discontinuous finite element research code Experimental tests have been carried out on two test rigs: the first one operating with water and a second one with ISOVG46 oil The results obtained have shown that whilst the discharge coefficient decreases for increasing values of the fluid viscosity, the force acting on the valve disc may exhibit a non monotonic behavior Variations in the force exerted by the flow affect the operating performance and lead in some cases to an additional reduction of the valve flowing capacity The critical evaluation of the results allows to suggest a modification of the sizing procedure proposed by the reference standards

Experimental and numerical analysis of supersonic blade profiles developed for highly loaded impulse type steam turbine stages

Abstract: The paper describes the results of a numerical and experimental research program addressing the aerodynamic investigation on the performance of blade profiles specifically developed for application in highly loaded impulse type turbine stages. The industrial requirements driving toward the adoption of highly loaded stage solutions are presented, along with an estimation of the profiles operating parameters. Two stator vanes and one rotor blade profile have been developed and extensively tested by means of flow field measurements and schlieren visualization in a transonic blow-down wind tunnel for linear cascades. Experimental results for the relevant operating conditions are presented, providing validation data for the CFD model used for blade design and evidencing that the main goals of the design optimization procedure have been achieved.

A review based on evaluation experiences with ten-years activity in VAWT experimental wind tunnel testing

Abstract: The purpose of this paper is to provide guidance on the practice gained on vertical-axis wind turbine (VAWT) testing by a combined group of researchers from the Universita di Trento (IT) and the Politecnico di Milano (IT), from early experiences dating back to 2007 up to the present day. The adopted operating procedures are discussed with particular care set to the achievement of high precision measurements to be used both as a benchmark for the validation of numerical codes and as a contribution to a deeper understanding of the flow field around VAWTs, to be converted into novel measurement procedures. As a matter of fact, by providing historical and technical information from the ongoing research activity, the lessons learned about the main obtained results could serve as a valuable tool for use by other research groups who are facing similar activities, providing information on which to help base their project plans.

Wake Measurements of Small-Scale Vertical Axis Wind Turbines at Politecnico Di Milano: A Critical Review

Abstract: In the last ten years, four measurement campaigns were performed at Politecnico di Milano on two Darrieus Vertical Axis Wind Turbines (VAWT) for micro-generation of different architecture (H-shaped vs troposkien), but sharing the blade number (3), the blade profile (NACA 0021), and the swept area (1.5 m2). The experiments, carried out in the large-scale wind tunnel of Politecnico di Milano, included detailed wake measurements. This paper presents a review of the research activities related to velocity and turbulence measurements in the wake, proposing an analysis of both the technical aspects and the scientific outcomes of the investigation. In particular, the wakes of these turbines were measured on several surfaces downstream of the rotors for different tip speed ratios and different Reynolds numbers, searching for corresponding conditions between the two rotors. The paper first presents the technical issues involved in measuring the flow velocity in the wake of VAWT rotors with intrusive techniques such as hot wire anemometers and pressure probes. The second part of the paper proposes a comprehensive analysis of the wakes shed by the tested models. The wakes appear asymmetric and roughly follow the shape of the rotor, their width and velocity deficit being strongly dependent on the tip speed ratio. Flow angle measurements show the onset of large-scale tip vortices, for both the H-shape and the troposkien rotors, even though resulting from different aerodynamic mechanisms in the two architectures. A discussion on the impact of the wake features on the implementation of VAWTs in the urban environment concludes the paper.