Numerical Investigation on Aerodynamic Similitude for Axial Microfan

Axial microfan is widely used as cooling equipment for electronic devices such as PCs. Currently, the PC size becomes smaller and smaller while their operating speed becomes faster and faster, which calls for better cooling performance of axial microfan. As an important and effective design approach, the similitude design has been widely used in the design of large and medium fans, but not yet in the design of axial microfan. The traditional similitude design approach may work well in self-similitude area for Reynolds number, such as the flow inside the large and medium fans, where simple geometric similitude can promise the aerodynamic similitude. But the flow inside the microfan locates in non self-similitude area for Reynolds number, where the traditional similitude design for large and medium fans may be out of work on microfans, which results in poor performance of the microfan designed by the traditional similitude approach. In this paper, the aerodynamic similitude conditions for axial microfan are presented according to the similitude principle for aerodynamics. Compared to large and medium fans in self-similitude area, a new criterion called ‘Chord Reynolds Number Criterion’ suitable to the flow in non self-similitude area is proposed and is numerically validated with discussions on its applicability and limitation. Results indicate that ‘Chord Reynolds Number Criterion’ can well realize the aerodynamic similitude for the axial microfan but it requires that the revolution speed of the impeller should be in direct proportion to the squared minification of the model impeller, so the reduced scale of model impeller shall not be too large, otherwise new non similitude factors may occur again due to too high revolution speed.