Franco Rispoli

Using sweep to extend the stall-free operational range in axial fan rotors

Abstract The paper discusses the use of sweep as a remedial strategy to control the aerodynamic limits in low-speed axial fan rotors. In this respect, the present work contributes to the understanding of the potential effect of blade lean on the shifting of the rotor stall margin. Numerical investigations have been undertaken on highly loaded fans of non-free vortex design, with the ideal total head rise coefficient typical of the industrial application range. Two rotors with identical nominal design parameters and, respectively, with 35° forward swept blades and unswept blades have been studied. The investigation has been carried out using an accurate in-house developed multilevel parallel finite element RANS solver, with the adoption of a non-isotropic two-equation turbulence closure. The pay-off derived from the sweep technology has been assessed with respect to the operating range improvement. To this end, the flow structure developing through the blade passages and downstream of the rotors, as well as loss distributions, have been analysed at design and near-peak pressure operating conditions. The analyses of three-dimensional flow structures showed that, sweeping forward the blade, the non-free vortex spanwise secondary flows are attenuated, and a control on the onset of stall is recovered. Moreover, the swept rotor features a reduced sensitivity to leakage flow effects. Consequently, it operates more efficiently approaching the throttling limit.

An les insight into convective mechanism of heat transfer in a wall-bounded pin matrix

We report on an LES (large-eddy-simulations) study of flow and heat transfer in a longitudinal periodic segment of a matrix of cylindrical rods in a staggered arrangement bounded by two parallel heated walls. The configuration replicates the set-up investigated experimentally by Ames et al. (ASME Turbo Expo, GT2007-27432) and mimics the situation encountered in internal cooling of gas-turbine blades. LES have been performed using the in-house finite-volume computational code T-FlowS. Considered are two Reynolds numbers, 10000 and 30000, based on the rod diameter and maximum velocity in the matrix. The unstructured grid contained around 5 and 15 million cells for the two Re numbers respectively. After validating the simulations with respect to the available experimental data, the paper discusses the characteristic vortex and plume structures, streamline and heatline patterns and their evolution along the pin matrix, around individual pins and at the pin-endwall junctions. It is concluded that the convection by organized vertical structures originated from vortex shedding govern the thermal field and play the key role in endwall heat transfer, exceeding by far the stochastic turbulent transport.Copyright

MODELLING OF DEPOSIT MECHANISMS AROUND THE STATOR OF A GAS TURBINE

The analysis of particle laden flow in turbines stages is a very actual topic as deposit can alter the blade cooling due to a partial or total blockage of film cooling holes and the modification of heat transfer coefficient between the internal cooling fluid and the blade surface. A computational tool for predicting particle deposition on a solid surface, developed by the authors, is here applied and validated against literature data. The computational model is based on an Euler-Lagrangian approach with a one-way coupling for the description of the fluid-particles interaction. The deposit model used is based on the paper of Walsh et al., 1990. The prediction of the fluid phase is carried out by using a URANS (Unsteady Reynolds Averaged Navier Stokes) approach on the well-validate open-source code OpenFOAM widely tested and validated by the authors and many other researchers worldwide in a number of turbomachinery relevant cases. The numerical campaign was firstly focused on the analysis of the details of the flow field in order to identify the eventual presence and position of shocks as well as to put in evidence the shock/boundary layer interaction. Then, the trajectories of two class of particles are analyzed in order to determine the influence of drag, pressure and velocity gradient on the particle pattern. Finally, the adhesion on the blade surface and the influence of flow temperature is discussed.

Experimental and Numerical Analysis of Steam-Oxygen Fluidized Gasifier Feeding a Combined SOFC/ORC Power Plant

The aim of this work is to experimentally and numerically analyze the performance of a combined power plant, composed by a steam oxygen fluidized bed biomass gasifier fed by woods, a Solid Oxide Fuel Cell (SOFC) and an Organic Rankine Cycle (ORC). The gasifier model is developed and validated by means of experimental activities carried out with a bench scale gasifier in our lab. Different compounds (Benzene, Toluene, Naphthalene, Phenols) were chosen to analyze the tar evolution in the gaseous stream during the gasification process. Hot gas cleaning (based on catalytic ceramic filter candles inserted in the freeboard of the gasifier - UNIQUE concept) is adopted to remove tar and particulate at high temperature from the fuel gas stream. The numerical analysis is carried out by means of the software Aspen Plus®. In particular, the SOFC and the gasifier are modelled using proper developed Fortran subroutines interfaced to the basic software. The adopted SOFC model was already validated by the authors in previous works. Finally, a realistic ORC is modelled. Different moisture contents in the range of 10–30 % (i.e. in a deviation of 10% around the usual wood moisture content of 20%) are investigated in the simulations, as also the degree of purity of the oxygen used in the power plant (between 25 and 95%, rest N2). The power requirement for the production of pure oxygen for this kind of gasification technology leads to a reduction of the electrical efficiency of the whole power plant. For this reason a sensitivity analysis was conducted to find the optimal operation conditions in order to maximise the syngas content in the produced gas (H2, CO) maintaining a high overall electrical efficiency.Copyright

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

Hybrid Les/Rans Study of Turbulent Flow in a Linear Compressor Cascade With Moving Casing

In this work a robust hybrid LES/RANS model has been applied to the prediction of secondary flows in a linear compressor cascade with moving casing simulating the relative motion between blade and the casing. The hybrid LES/RANS model uses the well established Ζ-f URANS model of Hanjalic et al. (2004) in the near wall region coupled with dynamic Smagorinsky LES. The switch between the two zones is based on a couple of parameters defining the boundary of interface region: the first one is a grid-detection parameter expressed as a function of the ratio between the turbulent and LES characteristic length scales while the switching to pure LES is obtained when the subgrid scale viscosity is greater than eddy viscosity (Delibra et al., 2010). We present hybrid LES/RANS results of a 3D linear compressor cascade with a tip leakage equal to 1.65% of chord. We compare two cascade configurations: with stationary casing and with moving casing. The second simulation allows to scrutinize the exclusive influence of the relative motion between casing and blades on the tip leakage vortex and the turbulent structures developing in the wake. The quality of the results and their agreement with experiments are encouraging in terms of prediction of the main flow characteristics and identification of turbulence structures.Copyright

Unsteady CFD Analysis of Erosion Mechanism in the Coolant Channels of a Rotating Gas Turbine Blade

The two-phase flow in a rotating wedge mimicking the final portion of a blade turbine internal cooling channel is here presented and discussed focusing on unsteady motion and erosion mechanisms. The rotation axis is placed to properly reproduce a configuration with a very strong deviation (90°).The flow field was modelled by using the well known k-e-ζ-f unsteady-RANS model based on the elliptic-relaxation concept. The model was modified by some of the authors to take into account the influence of turbulence anisotropy as well as rotation. The model was applied to the well-established and fully validated T-FlowS code.A systematic comparison of rotating and non-rotating case was carried out to show the influence of Coriolis force on flow and erosion mechanisms.The rotational effects strongly changed the flow behaviour within the channel, affecting both the unsteady flow and the particles trajectories. In the rotating case, there is no recirculation on the tip region; besides, position of the small recirculation regions above each pedestals change. These, and other minor effects, affect the particle motion thus resulting in a different erosion pattern.Copyright

Study of Particles Deposition in Gas Turbine Blades in Presence of Film Cooling

Exhaust entering the gas turbine is usually fed with solid particles produced in the combustion of hydrocarbons (ashes, unburned char, etc.). Then, the interaction between the particles motion and the film cooling jets must be properly addressed.Here an integrated approach based on an Eulerian-Lagrangian scheme for particle-laden flow was applied to a real turbomachinery case. The code was preliminary assessed by simulating two simplified test cases: a) 3-D cooling jet in a channel; b) 2-D turbine cascade with film cooling. These cases were selected to separately validate the main effects here considered: a) interaction of particles trajectories and 3D cooling jets; b) effect of the cooling jets on surface temperature and particles trajectory and possibly on particle deposition, in comparison with the non-cooled case.Finally, 3D simulation of the particle-laden flow around a real E3 gas turbine vane with and without film cooling was performed. Flow features, particles trajectories and deposit on the blade are presented.The compressible flow field was simulated using the OpenFOAM code obtaining credible predictions of the velocity and temperature field.Then the P-Track code developed by the authors was applied for tracking the particles trajectories and determining the deposit on the solid surface. As the temperature are relatively high, the sticking probability method, that is strongly dependent on the temperature itself, was used here.The results showed that the presence of the cooling jets affect deeply the deposit following two main causes: the influence of the jets in removing the fluid from the close-to-the-wall region and the reduction of temperature along the blade.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

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