CFD Analysis of the Non-Uniform Velocity Distribution Generated by a Large Axial Flow Fan

The air flow propelled by an axial flow fan is often assumed to be uniform across the tube bundle of an industrial air-cooled heat exchanger, but the fact is that the air flow is non-uniform in practice to the extent that as much as 35% performance of air-cooled heat exchangers is cut off. Besides the structure of tube bundle, the high flux and low pressure head fan with 1.5 m to 4.5 m in diameter is the primary reason of the air flow maldistribution. CFD was employed to numerically simulate the non-uniform air flow distribution produced by the fan in the radial and flow direction. One axial flow type fan with 3 blades about 2 m in diameter was investigated under the angular speed of 80, 350 revolutions per minute. Given that the uniform velocity is the default for the velocity or mass flow boundary, the boundary of the total pressure inlet and the static outlet was adopted in the present work for axial flow fan simulation. Between the rotating zone and the other zones is the fluid to fluid type interface with frozen rotor. The result showed that the air velocity rises when it comes nearer to the fan either in the upstream or in the downstream direction. The upstream flow decreases more rapidly than the downstream flow, the upstream velocity air at Z=−1.5 m is almost the same with the ambient while the downstream velocity at Z=5.8 m is still 2, 6 times larger than the ambient for 80, 350 RPM respectively. In each plane, the air flow distribution from the rotating center of the fan to the circumference is different from each other plane by the boundary conditions. And the maximum velocity locates at about the radial middle part of the blades. The hub at the central part near the fan causes the concave velocity distribution of the downstream flow. It is the driver mechanism geometry that causes the radial velocity change of the upstream flow. Finally, 3 methods involving the upstream or downstream type and the blade angle of inclination are provided to reduce the air flow non-uniformity.