How the air stream flows through the airfoil
Axial, diagonal, radial?
In short, a fan should move air, generate a certain pressure, and do it as efficiently and quietly as possible. In order to cool electronic components, for example, you need cold air to absorb the heat and pressure to dissipate it against the resistance through the component. For many applications, fans are the best option in terms of noise and efficiency. In addition, they have few moving parts and generate a continuous flow of air in the smallest of spaces. In order to understand whether an axial, radial or diagonal fan is optimal for a particular application, the basic functionality should be briefly described.
The pressure decides
The pressure build-up in axial fans is created by the inflowing air being deflected by the blades and leaving the fan on spiral paths. The pressure build-up depends on the angle that the air flow forms relative to the blade profile. If more pressure is to be achieved, this angle must be increased. This principle has its limits: If the angle of attack is too large, the profile flow breaks off and the fan works inefficiently and with more noise.
If more pressure is required, fans are used that use centrifugal forces in addition to the effects described. As in any rotating system, the air in the fan wheel is exposed to centrifugal forces, which force it to the outside. If axial fans are operated with small volume flows, part of the air blocks the blade channel and forces the air flowing through it on a radial path through the fan. The centrifugal forces are then increasingly involved in the build-up of pressure. The axial fan behaves similarly to a radial fan in this operating range.
Correspondingly, diagonal or radial fans are used when more pressure is required relative to the volume flow. In the case of pure radial fans, the centrifugal effect is even the dominant mechanism that needs to be implemented in the best possible way. With the same outer wheel diameter and the same speed, centrifugal fans can achieve significantly higher pressures than axial fans, whose area of application is always where relatively large amounts of air have to be moved with minimal effort.
New measurement technology
With these basic considerations, the fan can then be designed and optimized aerodynamically. For this purpose, experimental methods were developed and refined in the past, which together with mathematical models still form the basis of fan development today.
Today, computer-aided methods are increasingly used that allow so-called numerical experiments to be carried out. Computational Fluid Dynamic (CFD) is used wherever mass and heat transport tasks have to be solved.
In principle, with CFD, a fan can only be designed with the specifications for volume flow and pressure. The optimization of individual fan components as well as the overall system are the design goals for quieter and more efficient products today. Examples of this are the twisted blades of modern fans or specific blade tip shapes. Today it is possible to design fans for specific applications and to implement correspondingly optimized systems. The strengths of CFD are the simple change of variants and the detailed analysis of the flow. This has also changed the requirements for measurement technology. Today, experiments are not only required in the design process, but also serve as evidence and as a selection criterion for previously calculated variants of new fan concepts.
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