Elizabeth S. Gusyentsova, Inna K. Nasonkina, Yana A. Gusyentsova, Konstantin N. Andriychuk

East Ukrainian National University named after Vladimir Dahl

ANTISTALL DEVICES FOR AXIAL FANS

The questions of stability of fan’s operations modes belong to the number of most difficult and insufficiently known. It is caused by complication of solution hydrodynamics equations and determination of one or another turbulence hypothesis. In addition, work on unstable modes causes growth of dynamic tensions in the elements of construction and often results the fan’s fail [1-4].

It is possible to consider grid and rotating stall the most studied forms of unstable fan’s operation. However there is no admitted theory yet, which would most fully describe these phenomena. There is little quantity of devices, which can fail-safely remove harmful influence of these phenomena on axial turbo-machine work.

The surge origin depends not only on the fan’s operations mode but also on network features. Therefore it is impossible to talk about a point on pressure characteristics of fan, in which the surge appears. This point can shift depending on parameters of joint to fan network. Transition of fan to the mode of rotating stall takes place at certain mode on pressure characteristics and does not depend on the network capacity. It is determined by structural and geometrical sizes of flowing part and machine blade row. At transition on the rotating stall mode (on the left-hand from pressure maximum) the pressure developed by fan decreases abrupt, the characteristics discontinuity appears and typical noise change occurs, created at machine work.

It is determined that surge occurs at machine work in area of rotating stall, at the discontinuity of pressure characteristics and rather big reverse inclination of pressure characteristic on the left-hand from pressure maximum.

Obviously, for solution the question about extension of steady fan work area, above all things, it is necessary to prevent possibility of appearance of rotating stall mode or separate the stall area from basic stream part.

It is possible to consider decreasing the aerodynamic loadings of workings grid of fan the simplest solution of the question of unsteady modes of work elimination. Thus pressure characteristics have monotonic falling view without maximums and minimums of pressure. Usually it is achieved at the small blade angles setting of fan impeller (less than 10 - 15°). In this case neither rotating stall nor surge do not occur in whole productivity diapason from maximum to zero. However decrease of aerodynamic loadings of workings grid conduces increase of sizes and rise in prices of machine that in majority of cases does not prove its value. For such grids appearance of the modes with rotating stall and surge is inevitable, if special measures are not foreseen. It is shown [4], that vane separators with the number of vanes of z ~ 200 effectively work at the comparatively low aerodynamic loadings on one stage, with angle of rotor vane setting no more then 35°, at the developing coefficient of pressure 0,18—0,2. Efficiency factor decreases on 1—2%. Sharply expressed frequency constituents appear, conditioning high audio-frequency fan noise, because of big number of separator vanes in noise spectrum. For fans with meridian acceleration at developed pressure to 0,35-0,4 more expedient is antistall device of type “air separator”.

Air separator, as well as vane separator, works on principle of reverse flow localization, accompanying the rotating stall at the small productivity in peripheral vane part of rotor wheel. Here reverse flow is run off through circular channel, located above inlet edges of rotor wheel vanes, in space before the vanes of inlet guide-vane.

Under centrifugal forces and pressure difference the antistall part of boundary layer on periphery and disturbed part of flow, that forms stall areas, sucks off through this channel (but not thrown out before rotor wheel, as in vane separator), not overload the inlet section on periphery of rotor wheel. Due to such diversion of disturbed part of flow normal work of whole wheel on the modes is provided, according pressure maximum. Hereupon growth and development of areas of derangement as far as diminishing of the productivity ceases and all harmful phenomena related to him, including formation of cavity on characteristic pressures, eliminates.

Because taking off the disturbed part of flow is realized within the limits of inlet part of body and flow is not thrown out outside the fan, such device works efficient on sucking and on forcing at condition of similar flow to the fan. The air separator (fig. 1) consists of shell 1, set before rotor wheel on its external diameter; guide-vane body 2, connected with fan body 3 through conical transition 4; vanes 5 of guide-vane, set on shell, and inlet collector 6, having internal diameter, equal to the shell diameter. Inlet edges of vanes of rotor wheel on periphery come forward in transition 4 so that there is circular channel 8 above vanes of guide-vane, and between shell and collector circular passage-way 9appears [3,4].

Fig.1. Air separator scheme

The basic stream moves through the guide-vane grate and partly sucks through channel 8 towards rotor wheel at fan work of on antistall modes. Flow rate in channel 8 becomes close to zero at the optimum mode. At the further fan productivity decreasing swirling flow begins moving in direction from rotor wheel through channel 8. The disturbed flow goes out through passage-way 9 and unites with basic stream.

Fig. 2. Characteristics of fan К-9Г-12 at work with separator and without it

On a fig. 2 the characteristics of fan К-9Г-12 are shown at work with inlet collector and without separator (scheme 1), with inlet collector and air separator (scheme 2), with circular ledge and air separator (scheme 3). Characteristic comparison shows that in the range of the modes = 0, 21- 0, 36 application of separator with inlet collector results the decline of pressure and coefficient of efficiency approximately on 1% (compared with scheme 1). At < 0, 2 separator provides removal of trough and considerable efficiency factor increase. Pressure characteristics at fan work on schemes 3 and 2 coincide practically [4].

The antistall device of axial fan has the defect, that shells are located on the greater diameter of flowing part, which substantially decreases the plane for air flow, increases hydraulic resistance and decreases the axial fan productivity and piles the construction of axial fan in whole.

It is possible to remove this defect if make the antistall device in a form vanes, set between the vanes of guide-vane, which will decrease hydraulic resistance of flowing part, will increase plane for air flow and will increase the axial fan productivity [5]. On fig. 3 it is represented antistall device of axial fan, in form of vanes 1, set between vanes of guide-vanes 3.

There is vortex, which hinders the motion of air and decreases the fan productivity at rotor wheel rotation, especially at large charges in gap between guide-vane and rotor wheel.

The axial fan device works like this. Vanes 1 are executed in thin plates form, the chord of which is equal to 0, 4 from the chord of guide-vane, and height – 0.4 from its height. Such vanes localize the areas of reverse currents and simultaneously untwist them in axial direction. It results in the substantial weakening of co-operation of basic and reverse streams, removes the stall areas along guide-vane.

It prevents the stall of flow that conduces diminishing of hydraulic resistance of fan flowing part, increases the plane of flowing part and promotes the axial fan productivity.

Fig. 3. Antistall device of guide-vane of axial fan

Analogical method can be applied to the rotor wheel (fig. 4). Here the antistall device is done also in form of thin bent plates [6], angles of inlet and outlet of which coincide with streamline, the chord of plates is equal 0,4 from the chord of vane of rotor wheel, the height of plates is equal 0,4 from the height of vanes of rotor wheel. Such construction of antistall device, as well as in previous case, decreases hydraulic resistance flowing part of axial fan, increases the area for basic stream and increases the fan productivity, prevents stream stall on vanes 4 of rotor wheel 2, which decreases hydraulic resistance of fan flowing part, increases the plane of flowing part and raises the axial fan productivity.

Fig. 4. Antistall device of driving wheel of axial fan.

We will underline once again, that shown structural solutions increase the plane of flowing part, decrease the hydraulic resistance and increase the axial fan productivity comparing with air separator.

References

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