Impact of Anti-vortex Ring on Centrifugal Fan Performance
As the constraints of the shape and size of volute,there is a large number of vortex and secondary flow in the flow field of a centrifugal fan,which is the main reason for low efficiency and high noise.We present three-dimensional numerical simulation of a whole centrifugal fan of a certain type in different conditions,using the CFD commercial software Numeca.We analysed the impact of anti-vortex ring on centrifugal fans′ internal flow before and after the installation of anti-vortex ring.We explored how the anti-vortex ring improved the centrifugal fan performance.The results show that the anti-vortex ring has a good effect for breaking large scale vortices,reducing the crushing strength and improving the rotation within the flow field.Get more news about Vortex Centrifugal Fan
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A double inlet centrifugal fan (1), in particular for the evacuation of combustion fumes in thermal power plants, producing less mechanical vibrations and lower noise levels, thanks to the presence of an anti-vortex fin (14) at the fan outlet section (4) extending on a lying plane parallel to the fluid flow direction (F).
The invention refers in general to the field of double inlet centrifugal fans, in particular to those used in smoke evacuation systems in thermal power plants. More precisely the invention relates to a double inlet centrifugal fan that is equipped with a device that is suitable for reducing vibrations, for limiting the noise emitted and for reducing mechanical stress on the components hit by the flow of outlet fumes.
State of the art
Centrifugal fans for aeriform substances (especially air and smoke) are plant components that are very common and applied in numerous fields. Their purpose is to provide a thrust to the fluid so as to overcome the hydrostatic losses that the flow generates in the circuits upstream and downstream of the fan.
There are centrifugal fans of all sizes. The greater sizes, used for instance in the smoke evacuation circuit of thermal power plants, are usually of the type having a double inlet. In this case the inlet of the fluid inside the impeller occurs symmetrically from both sides of the volute housing and the impeller is usually divided into two adjacent semi-impellers.
It is known that these centrifugal fans produce high noise and vibration levels even under normal operation conditions. The noise is annoying and, beyond certain levels, it is dangerous for people and may also jeopardise the integrity and the normal operation of the fans themselves and of the surrounding components. Periodical movements and pressure fluctuations can lead to fluid dynamic oscillations or pulses having a great magnitude when their frequency coincides with one of the resonance frequencies of the structural parts of the fan or of the surrounding components, for example a duct or a dumper.
According to the prior art, it is commonly thought that the main source of the pulses, for the centrifugal fans, is the interaction between the rotating and the fixed parts, i.e. the interaction between the blades of the impeller on one hand and the fixed parts of the housing on the other, in particular the interaction with the tongue, i.e. with the connecting element between the beginning of the volute and the outlet duct of the fan. These pulses have a well-defined characteristic frequency value: the Blade Pass Frequency (BPF). The BPF value depends, through a simple relationship, on the rotation speed of the impeller and on the number of blades of the impeller (or of a semi- impeller) and it is normally in the order of hundreds of Hz. These pulses at the BPF can be reinforced by structural resonance that is generated in the ducts downstream of the fan or they can excite other frequencies corresponding to the acoustic modes of these ducts.
As mentioned later on, fluid dynamic studies through CFD showed that in addition to pulses at BPF frequency, large low frequency pulses (few Hz) can be generated in the area of the fan outlet, ascribable to secondary fluid dynamic instability overlapping to the average flow.
The large variation of the thrust produced by these pulses leads to fatigue stress of the structures hit by these non stationary phenomena, which is particularly serious also due to the low frequency of the phenomenon and therefore determines the possible breaking of these structures. Moreover, such pulses can excite or supply acoustic energy of the typical harmonics of the duct system downstream of the fan, amplifying the acoustic emissions.
The problem of the noise and pressure pulses downstream of the centrifugal fans, limited just to those mainly due to the interaction between rotating and fixed parts of the fan, has already been tackled. For instance, EP 1378668 describes a double inlet centrifugal fan comprising a stabilizing element arranged between the volute and the outlet and essentially arranged on a median plane that is perpendicular to the flow direction so as to allow the outlet flow to become homogeneous. EP 2182220 describes a centrifugal fan with a perforated plate which radially divides the space of the volute so as to form two separate channels which extend for a portion inside the exhaust duct.