Experimental Studies of the Axial Guide Vane Assembly of the VDN-8 Centrifugal Fan

Yu. A. Trifonova

(Presented by Prof. Yu. N. Sokolov)

Regulation of the capacity of centrifugal fans at a constant rotational speed by means of an axial guide vane assembly (AGVA) has become widespread. In this connection, it is considered that, when the AGVA blades are turned, capacity regulation is achieved by imparting swirl to the flow approaching the impeller.

From Euler’s equation

\[ H_t = \rho \left( u_2 \cdot c_{2u} - u_1 \cdot c_{1u} \right) \ [2] \]

it is evident that an increase in the circumferential component of the absolute velocity at the inlet, \(c_{1u}\), reduces the head developed by the machine. The change in \(c_{1u}\) is ensured by decreasing the setting angle of the AGVA blades.

Any swirling flow tends to form according to the law, natural for it, of constancy of circulation, \(c_u \cdot r = \mathrm{const}\). The axial guide vane assemblies currently used do not make it possible to swirl the flow according to this law, since they employ flat blades and blades outlined along radii. The width of these blades, decreasing toward the center, also reduces the swirl of the flow; that is, as the radius decreases, \(c_{1u}\) will not increase, but will decrease.

Consequently, downstream of an AGVA of the usual type and before entry into the fan impeller, a reshaping of the flow must inevitably occur, associated with energy losses.

In connection with this, Yu. N. Sokolov and I. Yu. Salanina put forward a proposal for such changes in the structural forms of the AGVA of a centrifugal fan as could ensure swirl of the gas flow according to the law of a centrifugal vortex, \(\Gamma = c_u \cdot r = \mathrm{const}\).

Since such swirl of the flow is easier to provide in an annular cross section that is relatively small in radial dimensions, it was recommended to increase the hub ratio \(d/D\) of the AGVA, where \(d\) denotes the diameter of the fairing and \(D\) the diameter of the AGVA casing.

At the Department of Hydraulics and Hydraulic Machines of the Tomsk Polytechnic Institute, control tests were carried out on the main model of the VDN-8 fan, made according to the MO TsKTI 0.5—150° scheme. Capacity regulation in this fan was accomplished by an AGVA of the usual type, with a hub ratio of 0.15 and flat, radially outlined blades. The maximum efficiency of this fan is fairly high (79%), but its regulation characteristic decreases sharply as the capacity \(Q\) is reduced. At capacities

below 84%, the normal efficiency of the VDN fan becomes lower than that of fans with forward-curved blades (according to the \(0.7\)—\(37^\circ\) arrangement).

In connection with the foregoing, the task arose of investigating changes in the structural forms of the AGV and of its blades that could provide an improvement in the control characteristics of the centrifugal fan.

As the first variant, the AGV blades were kept flat, but their outlines were varied along the radius with such a calculation that, at a blade setting angle \(\beta_l = 45^\circ\), the flow swirl would be ensured according to the law \(c_u \cdot r = \mathrm{const}\). The blade width required for this at various radii was determined from the basic equations of the theory of flow around a cascade of flat plates [1]. For design reasons, the width of the peripheral element of the blades (at \(D = 560\ \mathrm{mm}\)) was in all cases taken equal to \(70\ \mathrm{mm}\). Hereafter, for brevity, such flat blades, outlined according to the law \(c_u \cdot r = \mathrm{const}\) at \(\beta_l = 45^\circ\), will be called “calculated.”

Fig. 1. Experimental model of the AGV for testing the VDN-8 centrifugal fan.

In the experimental model using the calculated blades, the hub of the guide-vane assembly was replaced by a pear-shaped body 1 (Fig. 1). This body was fastened and centered in the AGV casing by means of tie rods 2. The inner journals of the blades were fastened between the pear-shaped body and the removable fairing 1a. The central part of the pear-shaped body (together with the fairing) had a spherical surface, making it possible to rotate the blades.

For the calculated blades, a series of experiments was carried out with hub ratios of 0.60, 0.55, and 0.50. In addition, experiments were carried out with flat-

blades contoured along the radius at a hub ratio of 0.50. For each hub ratio, the experiments were carried out at various setting angles of the ONA blades, corresponding to fixed positions of the mechanism for rotating them from 0 to 5. The blade setting angles for these fixed positions were as follows:

Table 1

The results of the experiments showed that the use of calculated blades that do not completely cover the flow section of the ONA at $\beta_l = 90^\circ$ is inexpedient, since in this case the limits of regulation of the fan output are sharply reduced. Increasing the hub ratio of the ONA with flat blades contoured along radii, however, slightly reduces the maximum efficiency of the fan, but improves its regulating characteristic.

Fig. 2. Regulating characteristics of an ONA of the usual type and an ONA of the modified type.

Figure 2 presents the regulating characteristics $\eta_{reg} = \varphi(Q_{reg}/Q_{opt})$ for an ONA of the usual type (curve 1) and for a hub ratio of 0.50 with flat blades contoured along the radius (curve 2). Despite a certain decrease in the maximum efficiency (from 79% to 77.7%), curve 2 lies above curve 1 at $Q_{reg}/Q_{opt} < 0.95$.

Based on the results of the tests carried out, an attempt was made to make an approximate distinction between the influence of swirl and throttling in the process of regulating the fan capacity by means of the axial guide-vane assembly.

Fig. 3. Separation of the effects of swirl and throttling when regulating the fan capacity by the axial guide-vane assembly.

Curve 2 in Fig. 3 determines the fan characteristic obtained by theoretical calculation for the swirl of the flow supplied to the impeller, corresponding to the setting of the AGVA blades at an angle \(\beta_l = 45^\circ\) for a hub ratio of 0.50. In this case the circulation of the central vortex \(\Gamma = c_u \cdot r\), produced by the mean annular element of the AGVA, was determined in accordance with the flow conditions around a cascade of flat plates of the corresponding parameters. Assuming that this circulation remains unchanged up to the inlet to the fan impeller, it proved possible, for each calculated operating condition, to determine the product \(c_{1u} \cdot u_1\) entering into Euler’s equation. To estimate the corresponding values \(u_2 \cdot c_{2u}\), the experimental fan characteristic at full opening of the AGVA (curve 1), that is, with radial inlet, was used.

Curves 3 and 4 in the same figure correspond to the experimental characteristics of the fan for the same hub ratio and with the same setting angle of the blades of an axial guide vane assembly (curve 3) and of blades outlined along a radius (curve 4).

The close agreement of characteristics 2 and 3 indicates that the blades of an axial guide vane assembly of the design type do indeed provide regulation of the fan capacity by swirling the flow supplied to the impeller. The considerable reduction in the characteristic with radially outlined blades shows that, in this case, regulation is achieved not only by swirl but also by its substantial throttling in the axial guide vane assembly.

The experimental studies carried out have thus shown that there are real possibilities for improving the control characteristic of a centrifugal fan with an axial guide vane assembly. To this end, one should strive to create such an axial guide vane assembly as would provide intense swirl of the flow entering the impeller with only slight throttling of it. In reconstructing the axial guide vane assembly, it is possible to increase its hub ratio considerably (to 0.50–0.55 as against 0.15–0.20 in existing types), but the use of flat blades that do not completely cover the flow passage does not produce positive results. Attention should therefore be directed toward finding other forms of axial-guide-vane blades or other methods of swirling the flow during the regulation process.

REFERENCES

1. A. N. Sherstyuk. Axial Compressors. GEI, 1955.
2. L. A. Rikhter. Draft and Blast in Thermal Power Stations. GEI, 1962.
3. I. M. Levin, I. A. Botkachik. Smoke Exhausters and Fans of High-Capacity Power Stations. GEI, 1962.