WO2013021366A2

WO2013021366A2 - Centrifugal fan with an anti-vortex fin for reducing vibrations and noise levels

Info

Publication number: WO2013021366A2
Application number: PCT/IB2012/054087
Authority: WO
Inventors: Stefano Tiribuzi
Original assignee: Enel Ingegneria E Ricerca S.P.A.
Priority date: 2011-08-11
Filing date: 2012-08-10
Publication date: 2013-02-14
Also published as: WO2013021366A3; EP2742244A2; ITFI20110177A1

Abstract

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).

Description

CENTRIFUGAL FAN WITH AN ANTI-VORTEX FIN FOR REDUCING VIBRATIONS AND NOISE LEVELS

DESCRIPTION

Field of the invention

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.

Subject of the invention

The general subject of the present invention is to provide a double inlet centrifugal fan having reduced fluid dynamic pulses downstream of its outlet section, so as to reduce the low frequency mechanical vibrations which occur in centrifugal fans according to the known art.

A further particular purpose of the present invention is to reduce pulses of any frequency that excite the noise emissions generated inside the fan and the ducts connected to it.

This is achieved with the double inlet centrifugal fan according to the present invention whose essential characteristics are defined in claim 1. Further important characteristics are defined in the dependent claims.

Brief description of the drawings

The characteristics and advantages of the centrifugal fan according to the invention will be apparent from the following description of an embodiment thereof, given as a non-limiting example with reference to the attached drawings, in which: figure 1 shows a schematic section view of a double inlet centrifugal fan according to the present invention;

figures 2A and 2B schematically show the alternation of wake vortices downstream of a double inlet centrifugal fan according to the prior art;

figures 3A and 3B respectively show the curve over time of the transverse thrust on a dumper's louver arranged downstream of a double inlet centrifugal fan according to the prior art and the spectrum of the thrust on the louver itself;

figures 4A and 4B schematically show the alternation of wake vortices downstream of the centrifugal fan according to the present invention;

figures 5A and 5B respectively show the curve over time of the transverse thrust on a dumper's louver arranged downstream of a centrifugal fan according to the present invention, and on the anti-vortex fin of this fan.

Detailed description of the invention

With reference to figure 1 , a centrifugal fan having a double inlet (shown in a side section view) intended, in the specific example, to be installed in a smoke evacuation system of a thermal power plant, is generically indicated with reference numeral 1 . Reference numeral 2 indicates a dumper equipped with louvers 3 connected to the outlet opening 4 (indicated with a broken line) of the fan. Downstream of the dumper 2 a smoke exhaust manifold extends communicating with a chimney (both not shown). The fan 1 is formed by a housing 5 made up of two flat side walls 6 (only one of which is visible in the section view) united with a back 7, in its turn made up of a curved plate with a spiral-shaped profile. Between the two side walls 6 an impeller 8 is positioned, which defines a central chamber 8a thereof wherein the sucked fluid is conveyed. The impeller comprises two side plates 1 1 , to which a double ring of suitably shaped blades 9 is fixed, whose rotation gives a radial thrust to the fluid coming out from the outer cylindrical surface of the impeller 8 and is collected in an outer volume, called volute, indicated with reference numeral 10, enclosed in the housing 5 of the fan.

As generally provided in double inlet fans, the inner volume of the impeller 8 is divided in half by a central wall 13 arranged on the median plane of the impeller. Two semi-impellers are thus created, each equipped with an array of blades. This makes it possible to angularly offset the arrays of the two semi-impellers so as to obtain a more uniform flow at the outlet.

In order to provide a sufficiently wide outlet section 4, the side walls 6 of the fan housing 5 are spaced apart from the impeller 8, so that part of the fluid re-circulates inside the housing 5 around the impeller itself. The housing 5 has an opening that is typically rectangular, and coincides with the outlet opening 4, from which the fluid comes out with a dynamic pressure that is increased with respect to the inlet, thus overcoming the hydrostatic losses of the remaining part of the circuit.

The back 7 radially delimiting the volute 10 is shaped so as to increase the distance between the periphery of the impeller and the volute itself starting from a minimum value at the beginning of the volute, to a maximum value at the end of the volute. The beginning and the end of the volute are joined through the opening 4. The beginning of the volute is joined through a profile, called tongue, indicated with reference numeral 12, which separates the flow directed towards the opening 4 from the small part which re-circulates going back in the volute 10.

According to the invention, at the outlet section 4 there is a two-dimensional element 14, called anti-vortex fin, which is substantially plate-shaped, with median lying plane that is parallel to the average direction of the fluid flow, indicated with the arrow F in figure 1. Since the outlet section and the downstream ducts generally have a quadrilateral section, and in particular square or rectangular section, the anti-vortex fin 14 extends between opposite sides of said section and its median lying plane is substantially orthogonal to the axis of the impeller 8. Preferably its median lying plane is coplanar to the med ian plane of the i m pel ler 8 and , i n the example under consideration, it is coplanar to the central wall 13 of the impeller.

From the verification carried out it has been found that the anti-vortex fin 14 arranged in the aforementioned position has the effect of hindering the transverse component of the speed of the wake vortices formed downstream of the outlet section 4, reducing in a substantial manner the fluctuation intensity. Moreover, it should be noted that its orientation parallel to the outlet flow direction of the smoke does not produce a significant load loss.

The efficiency of the anti-vortex fin 14 for the reduction of fluctuations was numerically verified through CFD (computational fluid dynamics). I n particular this method was applied first to a centrifugal fan operating according to the prior art and then to a centrifugal fan operating according to the present invention.

In the first case the CFD simulation method was applied to a calculation domain which reproduces the real configuration present in a thermal power plant. The calculation domain also comprises the two louvers of an insulation dumper arranged at a short distance from the outlet of the fan. This allows to evaluate the impact of the vortices generated by the fan on said louvers in terms of total thrust acting on the surfaces of the same louvers. The simulated operative conditions are those relative to a low load level of the plant which, in the case under examination, were found to be the most serious in terms of development of oscillation phenomena.

The simulation in the starting conditions of a plant (figures 2A, 2B) highlighted the formation of alternating wake vortices which separate from the impeller projecting towards the outlet duct. The intensity of the vortices is such that in the central region of the duct there are areas in which the flow flows in a direction that is opposite to the average one. The periodical vortex structures can take up a symmetrical configuration, like that illustrated in the figures, or they can overlap to a flow field which is highly asymmetric and unbalanced towards one side of the duct.

Figures 3A, 3B show graphs that refer to the aforementioned simulated condition.

Figure 3A shows the curve over time of the transverse component, with respect to the axis of the exhaust duct, of the overall thrust acting on one of the two louvers of the dumper. The thrust is always positive, i.e. directed towards the middle section of the duct, but has a strong fluctuation component (F1 ), which adds on to the oscillation, of greater frequency but with lower amplitude (F2), due to the BPF.

Fig. 3B shows the frequency spectrum of the previous graph. The two peaks relating to the BPF (indicated with F2) and the much more intense peak relating to the alternated vortices (indicated with F1 ) are highlighted. The latter have a frequency of a few Hz, i.e. about two orders of magnitude lower than the BPF.

The simulation was then repeated by introducing the anti-vortex fin 14 in the calculation domain previously used, and observing the time evolution of the fluid dynamic field starting from the conditions reached in the calculation without said element.

As shown in figures 4A and 4B, there is still the generation of alternated wake vortices, but their extension towards downstream is now reduced and hits the louvers of the dumper with a much lower intensity.

The curve of the transverse thrust on the louver is shown in figure 5A. It is worth noting the rapid dampening of the low frequency fluctuation component (median curve), overlapped to an almost stationary component acting again towards the centre of the duct. This behaviour of the thrust stresses the mechanical resistance of the materials much less.

Of course, the same anti-vortex fin 14 is immersed in the fluctuating fluid dynamic field and is therefore subjected to the thrusts deriving from it. Figure 5B shows the thrust that would act upon the anti-vortex fin 14. This thrust has a wide low frequency fluctuating component (median curve); however, the magnitude of this thrust is lower than the magnitude of the thrust that acted on the dumper louver when the anti-vortex fin was not present. Moreover, since the latter is a structure without moving parts, it can be manufactured so as to better resist these mechanical stresses.

In addition to the aforementioned advantage concerning mechanical stress on the structures downstream of the fan outlet, it has also been noted for there to be a considerable lowering, of around one order of magnitude, of the fluctuations of pressure in the whole calculation domain, both inside the volute of the fan, and in the ducts downstream of the outlet.

The anti-vortex fin 14 is, in its most simple form, made up of a rectangular plate extending between two opposite walls of the fan outlet. It may have a profile similar to that of a wing and have rounded-off edges in the flow direction. The width of the plate along the height may be uniform, easier to make, or vary so as to be wider at the opposite end of the tongue, thus having a greater surface for anchoring to the housing of the fan.

The mounting position of the anti-vortex fin 14 according to the present invention is at the outlet section of the fan. Practically this can be obtained by arranging it directly on the outlet of the volute housing of the fan or by pre-mounting it on a frame having the same dimensions as the exhaust duct to be placed against and fixed to the outlet of the fan.

The present invention refers to a double inlet centrifugal fan, that is a fan wherein the sucked fluid enters the fan coming from the two opposite walls thereof; a double inlet fan has therefore a geometrical form with a symmetry plane, contrary to a single inlet centrifugal fan wherein the sucked fluid may enter the fan from one of the side on ly. The tech nical problem that the present invention solves is linked to th is symmetrical structure of the double inlet centrifugal fan: as a matter of fact, at the outlet of this kind of centrifugal fan, particular vortices are produced, which alternatively have configu rations that are opposite and anti-symmetrical between each other, as described above. These alternate vortices cause vibrations producing high noise levels and potentially having serious consequences on the fan's components, and are now substantially reduced by the present fan equipped with the above described anti-vortex fin.

Variants and/or modifications can be made to the centrifugal fan according to the present invention without for this reason departing from the scope of protection of the invention itself as defined in the following claims.

Claims

1 . A centrifugal double inlet fan (1 ) comprising a housing (5) delimiting a volute (10) and housing an impeller (8) to convey a fluid in said volute and through an outlet section (4) thereof having a quadrilateral shape, characterized in that an anti-vortex fin (14) is provided at said outlet section (4), said anti-vortex fin (14) being coplanar to the median plane of said impeller (8) and extending on a lying plane parallel to the fluid flow direction (F) between two opposite sides of said outlet section (4).

2. The centrifugal fan (1 ) according to claim 1 , wherein said anti-vortex fin (14) is a plate-shaped element having quadrilateral section.

3. The centrifugal fan (1 ) according to claim 1 , wherein the width of said anti- vortex fin (14) is variable with the distance from one of said opposite sides.

4. The centrifugal fan (1 ) according to any one of the previous claims, wherein said anti-vortex fin (14) has edges rounded-off in the fluid flow direction (F).

5. The centrifugal fan (1 ) according to any one of the previous claims, wherein said anti-vortex fin (14) is mounted in said outlet section (4).

6. The centrifugal fan (1 ) according to any one of claims 1 -4, wherein said anti- vortex fin (14) is pre-mounted on a frame having the same dimension of said outlet section (4) and applicable to the outlet of said fan.