Reduced Tip Clearance Losses in Axial Flow Fans

Background of the Invention

[0001] This invention relates generally to axial flow fans and, more particularly, to a method and apparatus for reducing their clearance flow losses.

[0002] Axial flow fans are used in a wide variety of applications, including

HVAC, refrigeration, automotive, power systems and aerospace. In each of these applications, efficiency and space limitations are especially important considerations.

[0003] Significant efficiency loss occurs in axial flow fans due to backflow in the clearance region between the fan rotor and the casing. The rotor may utilize conventional blades that extend outward with blade tips approaching the casing, or it may utilize blades that include a rotating shroud attached to the blade tips. In either case backflow is driven from the high pressure side of the rotor to the suction side across the clearance gap, leading to reduced performance, increased noise level and reduced stability and stall-margin.

[0004] Various designs have been proposed for increasing fan efficiency by reducing or controlling clearance flows. The designs generally involve an interruption or decrease in the size of the gap. One approach is the use of a tip seal structure wherein a circumferentially extending groove in the casing circumscribes the tips of the blades as shown and described in U.S. Patent 4,238,170. In another approach, an axial fan is provided with a casing having a bellmouth, and the shroud is so formed as to create a separation bubble between the bellmouth and the shroud in order to limit the circulation flow as shown in U.S. Patent 7,086,825 assigned to the assignee of the present invention.

[0005] Fan stability is affected by rotating flows within the clearance gap.

These flows tend to develop into organized rotating cells which can lead to strong through-flow oscillations and excessive noise.

[0006] Various designs have been proposed to improve fan stability by controlling these rotating flows. These designs are generally classified as casing treatment.

Summary of the Invention

[0007] Briefly, in accordance with one aspect of the invention, a sharp, forward facing step is provided in the fan casing which, when combined with an overlapping rearward facing step in the fan blade tips, tends to disrupt the backflow so as to thereby restrict clearance flow loss.

[0008J In accordance with another aspect of the invention, each of the blades has an attached vane on its suction side, with the vanes having a rearward facing step that overlaps the casing forward facing step.

[0009] In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention.

Brief Description of the Drawings

[0010] FIG. 1 is a perspective view of an axial fan assembly in accordance with the present invention.

[0011] FIG. 2 is an enlarged view of a portion thereof.

[0012] . FIGS. 3A and 3B are respective front and end views of a normal blade tip.

[0013] FIG. 3C is an axial cross sectional view thereof in relationship to the casing.

[0014] FIGS. 4A and 4B are respective front and end views of a blade tip with a step in accordance with the present invention.

[0015] FIGS. 5 A and 5B are respective front and end views of a blade tip with a vane in accordance with the present invention.

[0016] FIG. 6 is a suction side view of a blade tip and vane in accordance with the present invention.

[0017] FIG. 7 is a pressure side view of a blade tip and vane in accordance with the present invention.

[0018] FIG. 8 is a radially inward view of a blade tip and vane in accordance with the present invention.

[0019] FIG. 9 is an axial cross sectional view of the Figs. 4A and 4B embodiment of the blade tip in relationship to the casing.

[0020] FIG. 10 is an axial cross sectional view as seen along lines 10-10 of

Fig. 2.

[0021] FIG- 11 is an axial cross sectional view as seen along lines 11-11 of

Fig. 2.

[0022] FIG. 12 is a partial view thereof showing the flow of air therein.

[0023] FIG. 13 is an axial cross sectional view of the apparatus as shown in

Fig. 11 but with an added inlet bellmouth insert.

Detailed Description of the Invention

[0024] Referring to Figs. 1 and 2, the invention is shown generally at 10 as applied to an axial fan assembly 11 that includes in serial airflow relationship an axial fan 12 and a stator 13. The axial fan 12 includes a rotatable hub 14 and a plurality of fan blades 16. The stator 13 includes a stationary hub and a plurality of radially extending stationary vanes 17 having their radially outer ends integrally connected to a cylindrical outer housing 18. In operation, the fan 12 is rotated at relatively high speeds to induce the flow of air through the casing 18, and in the process it creates a swirl in the direction of the fan rotation. The stator vanes 17 are so disposed and shaped as to substantially remove the swirl from the main airflow stream such that the flow at the downstream end is substantially axial in direction. [0025] As is well known in the art, the dimensions of the axial fan 12 are such that the radial clearance between the ends of the fan blades 16 and the inner diameter of the casing 18 are as small as possible but without engagement between the two elements. Because of this necessary radial clearance, there is a tendency for the air within the casing 18 to flow back through the radial gap to the forward side of the fan 12. This results directly in reduced pressure rise and efficiency. The present invention is intended to significantly reduce the backfiow.

[0026] Referring now to Figs. 3 A and 3B, a normal blade is shown at 16A, with a generally planar tip being shown in Fig. 3B. That is, the blade tip is slightly curved to accommodate the curved inner diameter of the casing ISA, but is of a substantially constant radius throughout the length of the blade tip. The blade tip of blade 16A in combination with a standard casing 18A is shown in Fig. 3C.

[0027] In Figs. 4 A and 4B, the blade 16B is shown to have a blade tip with a rearwardly facing (i.e. toward the downstream or pressure side of the blade 16B) step as shown at 19. That is, that portion 21 of the blade tip nearest the leading edge is of one fixed radius and that portion 22 thereof nearest the trailing edge is of a constant reduced radius. The face of the step 19 is generally planar in form and is aligned tangentially (i.e. normal to the fan axis).

[0028] Referring now to Fig. 9, where the blade 16B is shown with its blade tip profile that includes the rearwardly extending step 19 and the leading edge portion 21 and trailing edge portion 22. Here it will be seen that the casing 18B includes a matching forward facing step 23 which interconnects a larger radius portion 24 and a smaller radius portion 26 of the casing 18. The forward facing step 23 is a generally planar surface and is aligned tangentially such that the rearwardly facing step 19 is generally parallel with and in close proximity to the fowardwardly facing step 23. Similarly, the blade tip leading edge portion 21 is closely radially spaced from the larger radius portion 24, and the trailing edge portion 22 of the blade tip is closely radially spaced from the smaller radius portion 26 of the casing 18. This combination is provided for the purpose of reducing the backflow and its associated swirl that would otherwise result in a normal blade tip and casing relationship as shown in Figs. 3 A and 3B.

[0029] Referring now to Figs. 5A and 5B, a blade 16C is shown with a rearwardly facing step 19, leading edge portion 21 and trailing edge portion 22 as shown in Figs. 4 A and 4B. However, the blade 16C is further modified to include a vane 27 which is attached to the suction side of the blade as shown in Fig. 2 and which forms part of the blade tip as shown in Figs. 5A and 5B. [0030] The vane 27 can best be seen in Figs. 6, 7 and 8 where it is shown as being attached to the blade 16C. Fig. 6 shows the blade 16 from the suction side, Fig. 7 shows it from the pressure side and Fig. 8 shows it from the radially inward direction as shown in Fig. 8. As will be seen, the vane 27 forms a part of the blade tip and is placed approximately in the middle of the suction side of the blade 16C and extends approximately one-third of the way across. The size and shape of the vane 27 can be selectively varied to meet the particular axial fan assembly and operating requirements.

[0031] An important feature of the vane 27 is that it too includes a rearwardly extending step 28 as will be seen in Fig. 7. This step 28 also interfaces with the forward facing step 23 of the casing 18B in a manner similar to the rearwardly facing step 19 of the blade tip as discussed hereinabove to provide a further reduction of backflow that would otherwise occur around the blade tips. This can be seen in Fig. 11 wherein the rearwardly facing step 28 of the vane 27 is closely aligned with the forward facing step 23 of the casing 18B. In order to understand the structure of the blade tip of blade 16C, Figs. 10 and 11 should be referred to in combination. Fig. 10 is a sectional view through the stepped tip at a point forward of the vane 27, whereas Fig. 11 is a sectional view thereof at a point that includes both the stepped tip and the vane 27.

[0032] The design of both the casing and the fan rotor are such that they can be produced using straight-pull tooling (e.g. injection molding or die casting). [0033] In operation, as will be seen in Fig. 12, the relationship of the stepped blade tip and casing produces a convoluted path for the tip clearance leakage flow, which is highly restrictive. The effect is essentially similar to a labyrinth seal where the backflow and recirculation is forced to turn abruptly multiple times. Each flow turning produces a pressure drop which then enables the flow system to withstand a higher differential pressure and a lower leakage loss.

[0034] The embodiment of Fig. 11 can be used as shown without the use of inlet bellmouth insert. It will operate similarly but will benefit from the further use of an inlet bellmouth insert 29 as shown in Fig. 13.

[0035] It should be understood that the present invention can be used by itself for the reduction of backflow, or it may be used in combination with the wedges that are shown and described in the patent application being filed concurrently herewith and assigned to the assignee of the present invention.