Faber Spa, which has designed and manufactured ventilation hoods for the past 50 years, holds a key global position both in terms of production and technology. It has achieved this position through strategic investments in research and development, which have just recently been further strengthened through the addition of Finite Element Method (FEM) tools, particularly the ANSYS software platform. In this technical article, the engineers describe how they applied FEM methods – including fluid dynamics, structural analyses and thermal analyses – to refine a hood’s ventilation systems, predict the appliance’s performance in a drop test, and predict the behaviour of the plastic molded parts under normal operating conditions.
In addition to these performance improvements, further objectives for the FEM approach were to reduce the time to market and the product costs while increasing the quality and the structural reliability of the hoods. Faber explains that this integrated development approach to product planning, using simulation, enabled them to produce more solutions in less time and at less cost compared to an experimental approach.
Faber Spa has designed and manufactured hoods for the past fifty years and holds a key global position both in terms of production and technology. Research and development has played a central role in achieving this objective and has recently been strengthened through the implementation of Finite Element Method (FEM) tools; in particular the ANSYS software platform.
The desire for ever more complex and attractive designs plus low energy consumption has led to the development of high-efficiency fan units with a reduced size compared to other hoods currently on the market. Therefore, the objective for this activity was to develop a platform of fan units with a suitable size to meet the new requirements for aesthetics, better energy efficiency and volumetric flow rate, and reduced noise to out-perform current state-of-the-art hoods.
To achieve this, the product development plan included new activities using fluid dynamics, structural and thermal analysis. The fluid dynamic analysis was aimed at developing or updating the ventilation systems; the structural analysis was used to predict the appliance’s behaviour during a drop test; and thermal analysis was used to predict the behaviour of the plastic molded parts under normal operating conditions.
In addition to increased performance, reductions in the time to market and the product cost and an increase in quality and structural reliability were the other project objectives.
The integrated development approach, using simulation, produced more solutions in less time and at less cost compared to an experimental approach. Through a Design of Experiment (DOE) with a high number of input parameters, it was possible to investigate the parameter interactions and evaluate several technical solutions. This activity therefore optimized the outputs with a high degree of confidence. This enabled a mathematical Response Surface Methodology (RSM) model of the different analyses to be trained, validated and inspected which allowed output variation demand to be analysed as a function of the input parameters thus showing how they interact. The RSM was generated using results obtained from hydrodynamic and structural simulations.
The simulations of the diffuser outside the hood were carried out using the ANSYS CFX tool to determine the torque curve under exit pressure conditions of 0 and 100 Pa. Moreover, further simulations were carried out to replicate the real working conditions of the fan and all the fluid dynamic quantities to obtain the efficiency curves using Finite Difference Equations (FDE). The results of the simulations were used both in qualitative and numerical terms: the qualitative analyses verified the flows trend inside the fan unit and consequently allowed the shapes and geometries to be modified to minimize turbulence; the numerical results determined the efficiency values and helped to detect an error of 3% when analyzing the simulation results and the experimental data.
A series of static analyses under normal operating conditions (temperature reached on the cooktop) were performed to determine the stress state of the plastic product. The mechanical characteristics of the material were defined using experimental tests, normalized at increasing temperatures with respect to the environment, in order to verify the maximum stress state tolerable under different conditions of use. In this case, the material behavior was assumed to be linear elastic and isotropic. By analyzing the results, it was possible to verify the presence of a specific safety margin in relation to the tolerable stress values and deformations that could compromise the product’s integrity.
In addition to the normal operating conditions, a packaging drop test proved to be a very critical condition from a structural point of view for considering the component’s resistance. An optimal design of the component, therefore, would determine a limited number of modifications on the mold (or none, in the best case). To define the stress state generated by the drop test, a static analysis of the product was carried out using the same deceleration applied during the drop test. A static analysis doesn’t consider the product’s kinetic reaction, although it’s advisable to proceed like this to check the stress state reached. If this value is lower than the maximum tolerable stress (considering all safety coefficients), then it’s likely that the product will not incur any breakage.
A preliminary drop test was performed on the product to evaluate its real deceleration. The hood used for the test weighed 10 kg (including the packaging) and was dropped from a height of 40 cm. A uniaxial accelerometer was positioned on the engine to measure the crash force perpendicularly to the fall plane. The deceleration measured was 40 g and this value was then used for the structural analysis where it was set in three falling directions.
The simulation enabled Faber to reduce the maximum stress values by introducing the following modifications:
Because of the structural analyses, product quality was improved by reducing the time and cost usually caused by mold modifications (to be applied in case no simulations were run).
The challenge was to find the optimal aerodynamic design while substantially reducing the costs associated with traditional CFD modeling.
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