Design of a Low-Noise Axial Fan for Heat Pump Applications

In this paper the design of an axial fan for a heat pump is presented. This technology is experiencing a big growth in the number of applications in the residential heating sector and investments are expected to keep increasing in the near future. Heat pumps are characterised by the exploitation of thermal energy from the external environment and represent a green alternative to traditional fossil fuel systems capable of reducing CO2 emissions. In recent years research has mainly focused on maximising the efficiency of these products. However, nowadays the effort of manufacturers must shift to improving acoustic performance, a feature that is increasingly demanded by the market. Therefore, the designed axial fan must not only have a high efficiency in the desired working points but also low noise levels. These are the targets of the project, while the constraints are imposed by the costumers in terms of case diameter and rotation speed. An approach based on vibro-acoustic simulations has been adopted to obtain an already efficient and functional prototype. Firstly, a 2D design of the airfoil and blade has been performed in order to get to an initial fan geometry. This has then been optimized with subsequent and more accurate CFD simulations. Structural resistance has been ensured through FEA simulations and successive reinforcements. Great attention has been paid to the study of noise which, given the particular configuration of axial heat pumps, represents a critical issue for this kind of products. Specific regulations that set limits in emissions, market analyses and an experimental benchmark on competitors' products have been considered to define the design requirements. Starting from these data and the obtained geometries a study of structure and air borne noise has been carried out. Acoustic analyses have been performed in order to investigate both tonal and broad-band noise due to vortex shedding which required transient CFD and hybrid aero-acoustic simulations to identify the main noise sources. Suitable turbulence models and solver parameters have been chosen in order to ensure high quality results and acceptable computation times. Fan resonance issues have been avoided through vibration and modal simulations for the structure of the impeller. The goodness of the results has been verified through literature study on aero-acoustic simulations, still few in number, and an experimental benchmark.