The activities encompassed by this proposal consist in the development and exploitation of a methodological approach, suitable for the optimisation of the fan and duct geometries in an electrical machine, with the aim to improve both the aerodynamic and acoustical performance of the system.
The global activity consisted in a first baseline CFD calculation, a screening of different fan designs and finally an optimised fan design in terms of fluid dynamic performance and noise reduction.
The advanced character of the forecast approach is linked to the following aspects:
a) the implementation of the most advanced algorithms and procedures for CFD simulation;
b) the exploitation of Design Of Experiment algorithms during the initial screening phase and for the preliminary selection of the most relevant parameters;
c) the development of a simulation process flow controlled by Multi-Objective Optimisation algorithms, which will allow a complete exploration of the influence of the parameters affecting the system performances, and Response Surfaces algorithms, which will allow the interpolation of the database of virtual (and experimental) data and the definition of an expert system;
d) the ranking of the analysed models, with respect to their key performances, and the selection of the optimal candidate(s), considering the multiple selected objectives, by means of the mostly advanced Multi Criteria Decision methods.
Thus the present proposal is meant to develop a fully integrated software platform based on the most advanced functionalities of the following tools:
- ANSYS-CFX, whose functionalities and capabilities are worldwide recognized for their completeness and robustness;
- modeFRONTIER, one of the most advanced software tool for the optimization and the process integration available on the market.
The overall object of VOCAL-FAN is to develop a sub-assembly dedicated to a new generation of starter/generator for regional aircraft and bizjet. The activities consisted in the development and exploitation of a methodological approach, suitable for the optimisation of the fan and duct geometries in an electrical machine, with the aim to improve both the aerodynamic and acoustical performance of the system.
The device investigated is a machine composed by a 3 stages alternator where the windings of the rotors and stators are cooled by air; the airflow is produced by a fan integrated in the system. After the numerical characterization of the baseline geometry, different options were identified in order to aim for improved performance of the original alternator. The baseline configuration has been firstly modified by means of a Bezier curve for the approximation of the external carcass. Secondly, the baseline configuration has been modified changing the blade angle at the fan outlet. For all of new configurations designed and analyzed the main goals were the increase of fan efficiency and the decrement of the noise level within the machine.
The most advanced algorithms and procedures for CFD simulation has been implemented, in particular for noise evaluation. Design of Experiment algorithms and Optimisation algorithms has been used to identify the influence of the most relevant parameter over the device's performance and to select the optimal shape for the fan and the carcass.
Comparing the results obtained and the costs involved in the manufacturing process, an optimal geometry has been identified with significant improvement in aerodynamic and acoustic performance.
The full numerical characterisation has been developed using Ansys software tools: Ansys Icem-CFD, Ansys-Meshing and Ansys CFX.
Project Context and Objectives:
The overall objective of VOCAL-FAN was to develop a sub-assembly dedicated to a new generation of starter/generator for regional aircraft and bizjet.
The device investigated is a machine composed by a 3 stages alternator where the windings of the rotors and the stators are cooled by air; the airflow is produced by a fan integrated in the system. Different operating conditions characterize the machine (different rotational speeds and air temperature) and the analyses will investigate the fluid dynamic performances and the noise prediction of the system.
Starting from an initial CAD geometry, the computational domain has been developed and appropriate boundary conditions have been applied in order to characterise the fluid dynamic performances of the system.
An ad-hoc numerical procedure has been employed in order to obtain a converged solution such that, with second order highly accurate numerical scheme, asymptotic values of the performances target (such as pressure and torque) are reached with an adequate number of iterations. Different turbulence models for the prediction of the flow, the wakes and the noise generation have been investigated: steady simulations with SST turbulence model and transient simulations with SAS turbulence model.
Post-processing procedures have been established to extract the system performances from the results and to compute the noise level. The noise level has been computed in different locations inside the system (internal noise) using the spectral analysis. Sound pressure level has been investigated for different operating points to correlate the noise to the different operating condition of the system.
After the computational analysis of the original device, different options were identified, in order to aim for improved performance of the baseline alternator. They are:
▪ The modification of the external carcass downstream of the fan.
▪ The modification of the fan blades.
▪ The insertion of stator vanes in the domain downstream of the fan.
As a consequence, the baseline configuration has been firstly modified by means of a Bézier curve, which approximated the external carcass downstream of the fan, and part of the fan shroud.
Secondly, the baseline configuration has been modified changing the blade angle at the fan outlet.
Finally, the insertion of the stator vanes in the external carcass downstream of the fan has not been considered, because of the inherent difficulty of designing such stationary vanes for different operating conditions. As a consequence, this option has not been investigated.
For all of the new configurations designed and analysed in this work, the main goals were the increase of the fan efficiency, and the decrement of the noise level within the machine.
The noise generated by the machine has been studied by comparing the Sound Pressure Level (SPL) for the different configurations. The SPL has been calculated from the pressure fluctuations by means of the Fast Fourier Transform (FFT) analysis.
For both kinds of geometry modification (i.e., Bézier curve and blade angle), firstly steady-state simulations have been performed. Such computations have been carried out in order to allow for the Design of Experiments (DOE) screening of the various modified configurations.
Subsequently, for each kind of modification, transient computations of the best configuration have been performed in order to obtain its performance maps (i.e., efficiency, head, torque and power as a function of the mass flow rate), and its acoustic performance.
An optimal geometry has been identified.
Firstly, a baseline configuration of a 3-stage alternator has been studied by means CFD analyses. Starting from the CAD model supplied by Thales Avionics Electrical System, a fluid dynamic model has been developed: the fluid domain has been extract from the solid geometry and a meshing procedure has been applied including wall refinement. An ad-hoc computational setup and numerical procedure allow us to characterize the fluid dynamic performances of the system (pressure raise, torque, efficiency) and then to predict the noise generated for the different operating conditions. A CAA (Computational AeroAcoustics) method has been used to solve sound sources and propagation in a single comprehensive model using transient Navier-Stokes equations. An advance turbulence model (SAS) has been employed.
Secondly, the baseline configuration has been modified by approximating the external carcass with a Bézier curve, and by changing the blade angle at the fan outlet. Different configurations have been investigated, keeping as target parameters the increase of the fan efficiency and the decrement of the noise level.
The acoustic performance analysis of the best modified geometry has been carried out by means of the unsteady computations. In order to carry out the acoustic investigation, the static pressure has been monitored at internal points within the machine. The recorded static pressure signals at each monitor point have then been utilized to calculate the SPL through the FFT analysis. Only the operating condition corresponding to the maximum mass flow rate has been considered for the acoustic performance analysis.
The main conclusions of the work presented here can be summarized as follows:
• The original device is characterized by high level of internal noise, mainly focalised around the fan. The overall fluid dynamic performance are satisfactory.
• The modification of the external carcass, approximated by a Bézier curve, does not show a significant improvement in the performance of the device. The best design individuated is characterized by an efficiency slightly higher than the original device (less than 2%) and comparable acoustic performance.
• The modification of the blade angle at the fan outlet shows larger improvement in term of efficiency and sound pressure level using backward curved blades and the original carcass. Backward blades are better than the original radial ones with maximum increment of efficiency around 12% and maximum reduction of 8 dBA in sound pressure level.
• A combined modification of external carcass and blade angle at the fan outlet shows an improvement not comparable to the best values founded using only backward blades and the original carcass.
Comparing the performances obtained and the costs involved in the manufacturing process, the best design is a machine where the fan is characterized by backward blades, keeping the original carcass. More interesting, we have noticed that, in one particular operating condition, using this new geometry, the carcass behaves like a reactive silencer with beneficial effect over the noise and the SPL spectrum.
The new design of the fan implemented in the machine is characterised by lower values of noise and better efficiency.
The decrement of noise produce an important impact in the environment and for the psychoacoustics effects.
A better efficiency of the machine means better performance and lower power needed with obvious effects on energy and fuel consumption, with all the benefit related (i.e. CO2 emissions) .