Alessandro Corsini

Modeling of rain drop erosion in a multi-MW wind turbine

The actual strategy in offshore wind energy development is oriented to the progressive increase of the turbine diameter as well as the per unit power. Among many pioneering technological and aerodynamic issues linked to this design trend, the wind velocity at the blade tip region reaches very high values in normal operating conditions (typically between 90 to 110 m/s). In this range of velocity, the rain erosion phenomenon can have a relevant effect on the overall turbine performance in terms of power and energy production (up to 20% loss in case of deeply eroded leading edge). Therefore, as a customary approach erosion related issues are accounted for in the scheduling of the wind turbine maintenance. When offshore, on the other hand, the criticalities inherent to the cost of maintenance and operation monitoring suggest the rain erosion concerns to be tackled at the turbine design stage. In so doing, the use of computational tools to study the erosion phenomenon of wind turbines under severe meteorological conditions could define the base-line approach in the wind turbine blades design and verification.In this work, the authors present a report on numerical prediction of erosion on a 6 MW HAWT (horizontal axis wind turbine). Two different blade geometries of different aerodynamic loading, have been studied in a view to explore their sensitivity to rain erosion.The fully 3D simulations are carried out using an Euler-Lagrangian approach. Flow field simulations are carried out with the open-source code OpenFOAM, based on a finite volume approach, using Multiple Reference Frame methodology. Reynolds Averaged Navier-Stokes equations for incompressible steady flow were solved with a k-e turbulence.An in-house code (P-Track) is used to compute the rain drops transport and dispersion, adopting the Particle Cloud Tracking approach (PCT), already validated on large industrial turbomachinery.At the impact on blade, erosion is modelled accounting for the main quantities affecting the phenomenon, which are impact velocity and material properties of the target surface.Results provide the regions of the two blades more sensitive to erosion, and the effect of the blade geometry on erosion attitude.Copyright

Numerical Assessment of Fan-Ducting Coupling for Gas Turbine Ventilation Systems

Gas turbines enclosures entail a high number of auxiliary systems which must be preserved from heat, ensuring therefore the long term operation of the internal instrumentation and of the data acquisition system. A dedicated ventilation system is designed to keep the enclosure environment sufficiently cool and dilute any gas coming from potential internal leakage to limiting explosion risks. These systems are equipped with axial fans, usually fed with air coming from the filter house which provides air to the gas turbine combustion system, through dedicated filters. The axial fans are embedded in a ducting system which discharges fresh air inside the enclosure where the gas turbine is housed. As the operations of the gas turbine need to be guaranteed in the event of fan failure, a backup redundant system is located in a duct parallel to the main one. One of the main requirements of a ventilation fan is the reliability over the years as the gas turbine can be installed in remote areas or unmanned offshore platforms with limited accessibility for unplanned maintenance.For such reasons, the robustness of the ventilation system and a proper understanding of coupling phenomena with the axial fan is a key aspect to be addressed when designing a gas-turbine system. Here a numerical study of a ventilation system carried out with RANS and LES based methodologies will be presented where the presence of the fan is synthetized by means of static pressure discontinuity. Different operations of the fans are investigated by means of RANS in order to compare the different operating points, corresponding to 1) clean and 2) dirty filters operations, 3) minimum and 4) maximum pressure at the discharge section. Large Eddy Simulations of the same duct were carried out in the maximum loading condition for the fan to investigate the unsteady response of the system and validate its correct arrangement.All the simulations were carried out using OpenFOAM, a finite volume open source code for CFD analysis, treating the filters as a porous medium and the fan as a static pressure discontinuity according to the manufacturer’s characteristic curve. RANS modelling was based on the cubic k-e model of Lien et al. while sub-grid scale modelling in LES was based on the 1 equation model of Davidson.Computations highlighted that the ventilation system was able to work in similarity for flow rates between 15 m3/s and 23.2 m3/s and that the flow conditions onto the fan suggest that the aerodynamic stress on the device could be reduced introducing in the duct flow straighteners or inlet guided vanes.Copyright

Large-Eddy Simulation of the Aerodynamic and Aero-Acoustic Performance of an Industrial Fan Designed for Tunnel Ventilation

The development of industrial fans traditionally relies upon the use of empirical correlations and experimental analyses to validate both aerodynamic and acoustic aspects of fan performance. This paper presents the development of a computational based method focused on the prediction of unsteady aerodynamics and modeling of aero-acoustic sources. The authors applied the study to a single fan from a new range of large tunnel ventilation axial flow fans. The fan specification required mechanical and aerodynamic properties that would enable it to operate in the forward direction under ambient conditions to provide cooling air to the tunnel under routine operation, and in the reverse direction at 400°C under emergency conditions in the event of a tunnel fire. The final aerodynamic and mechanical design was additionally required to generate no more than 80 db during reverse operation, to ensure members of the emergency service could still communicate in the event of a fire. The simulations were carried out using the open source code Open-Foam, within which the authors implemented a (Very) Large Eddy Simulation (V)LES based on an one-equation sub-grid scale SGS model to solve a transport equation for the modeled (sub-grid) turbulent kinetic energy. This improvement of the sub-grid turbulence model is here considered as a remedial strategy in VLES of high-Reynolds industrial flows able to tackle the otherwise insufficient resolution of turbulent spectrum. The VLES of the industrial fan permits to detect the flow features such as three-dimensional separation and secondary flows. Predicted noise emissions, in terms of sound pressure level spectra, are compared with experimental results, and found to agree within the uncertainty of the measurements.Copyright

Unsteady Pressure Interaction of an Axial Flow Fan With a Stabilization Ring in Tunnel and Metro Applications

Ventilation fans operating in underground metropolitan tunnels are subjected to abrupt changes in operations due to the pressure wavefronts generated by the passage of the trains, and the magnitude of these pressure waves is increasing due to increasing speed of passing trains in modern mass transport systems. To avoid fans being driven into stall designers can fit fans with a stabilisation ring, i.e. a casing treatment that was found to mitigate the mechanical consequences of being inadvertedly driven into stall due to pressure pulses.A stabilisation ring is a circumferential cavity in the casing of the fan, placed upstream of the rotor in order to allow the fluid to recirculate in stalled operations. A series of fins inside this cavity is used in order to drive the recirculating fluid back into the blade vane with a proper alignment with the leading edge of the rotor.Following a previous RANS investigation that lead to the conclusion that the drive mechanism of the stabilisation ring onto the fan is based on azimuthal pressure unbalance we present here a U-RANS investigation aiming at understanding the dynamics of the interaction of the anti-stall ring with the fan and to provide insight on possible development of the geometry of the casing treatment.The fan selected for this study is a real fan for tunnel and metro applications (9 rotor blades, 1490 rpm) with a real-geometry stabilisation ring (27 fins). Computations account for different operating points (peak efficiency, design point, peak pressure and stalled operations) and rely on the low-Reynolds cubic k-e model of Lien et al. All the simulations were carried out with the open-source OpenFOAM code. Results were validated against available experimental data and then analysed to understand the unsteady interaction between the rotor of the fan and the cavity of the stabilisation ring.Copyright

Aerodynamic simulation of a high-pressure centrifugal fan for process industries

Large centrifugal fans in cement factories operate in an aggressive environment, as the cement particles dispersed in the flow are responsible for strong blade surface erosion that leads to performance degradation.This paper reports on the simulation of the flow field in a large centrifugal fan designed for process industry applications. The aerodynamic investigation, at a preliminary level, highlights the critical regions inside the device and suggests possible modification to increase its duty life.This paper reports on the simulation of the flow field in a large centrifugal fan designed for process industry applications. The aerodynamic investigation, at a preliminary level, serves the aim of highlighting the critical regions inside the device and suggest possible modification to increase its duty life.In the paper we show the results of numerical computations carried out with the finite volume open-source code OpenFOAM using Multiple Reference Frame methodology. Reynolds Averaged Navier-Stokes equations for incompressible flow were solved with standard eddy-viscosity k-e model in order to explore the aerodynamic behaviour of the fan in near-design operations. The incompressible flow hypothesis was adopted even if locally Mach number can exceed 0.5. In fact in this case the pressure-rise does not lead to a variation of the density able to affect the velocity field divergence.Given the high performance of the investigated impeller, the present work has a twofold objective. First, we seek to define an accurate numerical methodology to investigate high-pressure radial fans. Second, we provide detailed analysis of the inlet ring-impeller-volute assembly inner workings under realistic distorted inflow conditions.The results provide the evolution of the pressure field in order to validate the accuracy of the simulation in reproducing the motion inside the fan that was fundamental for credible particle dispersion reproduction. We then investigate the three-dimensional flow field through the impeller in order to provide details about the secondary flow structures that develop within the blade vanes.© 2013 ASME

Synergistic Noise-By-Flow Control Design of Blade-Tip in Axial Fans: Experimental Investigation

The increasing concern on noise emission had recently inspired the definition of a regulatory framework providing standards on eco-design requirements for energy-using products and noise levels, i.e. the European Directive 2005/32/EC (European Parliament, 2005). The compliance to standards within the fan industry appears more stringent than for other sectors because of their use in ventilation systems entailing the direct human exposure to noise emission. This driver demands the elaboration of fan design solutions and technologies in which the exploitation of noise control strategies must not affect aerodynamic performances. To summarise, the main generation mechanisms of aerodynamic noise in low-speed axial fans are: turbulent inflow, self noise (turbulent or laminar boundary layers, boundary layer separation), trailing edge noise, secondary flows (Fukano et al., 1986), and tip leakage related noise. Among these aerodynamic phenomena, Inoue and Kuroumaru (1989), Storer and Cumpsty (1991) and Lakshminarayana et al. (1995) investigated the role of tip leakage flows in compressor rotors and demonstrated the three-dimensional and unsteady nature, and the influence on aerodynamic losses and noise generation. In the context of low-speed turbomachines, Akaike et al. (1991) pointed out that the vortical structure near the rotor tip in industrial fans is one of the major noise generating mechanisms. The tip leakage noise can be one of the most significant sources correlated to the broadband spectral signature (Longhouse, 1978; Fukano et al., 1986; Kameier & Neise, 1997). Kameier and Neise (1997) highlighted that, in addition to the broadband influence, tip leakage flows could be responsible for narrowband tones at frequencies below the blade passing frequency in coincidence with a tip vortex separation. During the last decades, noise control has emerged as a new field of research as the literature demonstrates (Gad-el-Hak, 2000; Joslin et al., 2005), and scholars have proposed noise reduction strategy classifications, which distinguish between passive, active and reactive devices. A number of research programmes envisioned designs tailored to a synergistic noise and flow control by incorporating structural (passive) or flow (active)