Blade design offers superior efficiency

An Australian company has developed an innovative fan impeller that has facilitated the creation of quieter and more efficient fan and pump systems. Initial applications for the Jetfan are as electric motor cooling fans, automotive and building ventilation systems, as well as industrial water pumps, low noise signature water pumps for shipping and automotive engine cooling pumps. The Jetfan concept has been developed by Queensland based technology firm New Fluid Technology which conducts research on fluid dynamics that can improved, or be applied to entirely new, devices and systems. As the name suggests, the Jetfan has much in common with the compressor blades of a jet engine. For a start, it has a lot more blades than a conventional fan. But the really innovative aspect of the design is the helical blades. This makes the trailing edges much closer together than those of an axial, mixed flow or centrifugal fan, which results in a reduction in the cross-sectional area of the passageways between the blade trailing edges compared to the leading edges. This greatly reduces recirculation and, in combination with the high solidity - meaning you cannot see between the fan blades facing directly on - results in a stall free high efficiency impeller. The layout also gives rise to another desirable aerodynamic effect. Since a fluid is compressed as it passes through the blades, there is no need for stator blades or a diffuser downstream, which is normally essential for efficiency improvements. "The Jetfan has already proven to be the highest efficiency fan possible for heating, ventilating, and air conditioning (HVAC) systems with the lowest noise," says technical director of New Fluid Technology, Terry Day. "And the same principle is being applied to water pumps. Conventional water pumps must employ noisy diffusers to get their pressure gain and efficiency," says Day. "But our impeller enables a gentle turning path for the water which minimises the effects of direction and momentum change. As a result our pumps are much quieter." Many centrifugal fans have a relatively large distance between the trailing edge blades which creates a pressure difference. This forces air to migrate across the blade to the lower pressure areas found along the adjacent blades. This creates turbulence. In some cases air drawn into the fan reverses direction caused by downstream resistance and blade divergence. Conventional fans are prone to turbulence and recirculation which is not only inefficient but is also a primary cause of noise. Despite the performance improvements, the manufacture of the innovative impeller created somewhat of a headache for the designers. "After six months of investigation in Asia no one could work out a way to manufacture The Jetfan, short of welding individual metal blades onto the hub due to the blade overlap which would preclude its viability," says Day. "We thickened the blade material at certain areas to get the required fluid cross sectional area reduction needed for performance and to keep within the spirit of the patent. "The injected liquid plastic, upon hardening, results in each blade becoming a cam when the fan is pulled linearly thus causing the fan to autorotate as it disengages from the mould in a total time of 16 seconds. It took mostly mental gymnastics to solve the problem and when the tooling was built to enable manufacture it worked perfectly first time." Pointers * Helical bladed fans with trailing edges closer together than leading edges run much quieter and more efficiently than conventional designs with flat blades * A patented moulding process has been developed that allows the fans and impellers to be manufactured at low cost