Cavitation and noise reduction in axial flow rotors

Abstract

A propeller, impeller or mixer comprising at least one blade, the blade having a suction surface and a pressure surface which extend from a leading edge to a trailing edge of the blade and a radially-outer tip region, wherein five to one hundred duct openings are provided extending through the at least one blade from the pressure surface to the suction surface, the duct openings being grouped in the tip region of the blade.

Claims

1. An axial flow rotor configured to accelerate a flow of a liquid while reducing cavitation thereof, the axial flow rotor characterized as a selected one of a propeller, an impeller for a pump, or an impeller for a mixer, the axial flow rotor comprising: a blade having a suction surface and a pressure surface each of which extend from a leading edge to a trailing edge of the blade and a radially-outer tip region, the radially-outer tip region being a radially-outer third of the blade adjacent to a blade tip of the blade, and the suction surface generating a lower pressure in the flow of the liquid adjacent the suction surface and the pressure surface generating a higher pressure in the flow of the liquid adjacent the pressure surface, wherein: the blade comprises a plurality of duct openings each extending through the blade from the pressure surface to the suction surface, the duct openings being limited to and disposed in the radially-outer tip region of the blade, the plurality of duct openings being from five (5) to fifty (50) arranged into two (2) to five (5) radially-spaced, chordally-extending rows, and the duct opening of the plurality of duct openings closest to the leading edge of the blade in the row of the duct openings closest to the blade tip is further from the leading edge than the duct opening of the plurality of duct openings closest to the leading edge of the blade in the row of duct openings furthest from the blade tip.

2. An axial flow rotor according to claim 1, wherein the blade is a first blade affixed to and radially extending from a central hub, and the axial flow rotor further comprises a second blade adjacent the first blade affixed to and radially extending from the central hub, the second blade having no duct openings extending through the second blade from opposing pressure and suction surfaces thereof.

3. An axial flow rotor according to claim 1, wherein the duct openings are each aligned along a duct axis opening that is skewed with respect to a rotor axis about which the blade rotates.

4. An axial flow rotor according to claim 1, wherein the duct openings are limited to within a radially-outer tenth of the blade.

5. An axial flow rotor according to claim 1, wherein the duct openings are limited to within a radially-outer twentieth of the blade.

6. An axial flow rotor according to claim 1, wherein the number of duct openings is from ten (10) to fifty (50).

7. An axial flow rotor according to claim 1, wherein the blade is a first blade, the axial flow rotor further comprises a second blade with the first and second blades rotatable about a central axis and defining an outermost diameter of the rotor, each of the plurality of duct openings having an outermost duct opening diameter, and a ratio of the outermost diameter to the outermost duct opening diameter being from between 100:1 to about 1000:1.

8. An axial flow rotor according to claim 1, wherein the duct openings each have a diameter of from 0.5 mm to 50 mm.

9. An axial flow rotor according to claim 1, wherein the duct openings are arranged into at least one of: even spacing of the duct openings; a combination of even spacing and uneven spacing of the duct openings; or spaced pairs of the duct openings, the spacing between each of the duct openings of a pair of the spaced pairs being less than the spacing between neighbouring pairs of the spaced pairs.

10. An axial flow rotor according to claim 1, wherein the duct openings each have the same internal diameter.

11. An axial flow rotor according to claim 1, wherein the axial flow rotor is a propeller comprising five (5) blades including the blade, each of the five (5) blades comprising thirty-three (33) of the duct openings, wherein the duct openings are grouped in the radially-outer tenth of each of the five blades, and wherein the duct openings are grouped into three radially-spaced, chordally-extending rows.

12. An axial flow rotor according to claim 1, wherein the axial flow rotor is a propeller comprising four (4) blades including the blade, each of the four (4) blades comprising seventeen (17) to fifty (50) of the duct openings, wherein the duct openings are grouped in the radially-outer tenth of each of the four (4) blades, and wherein the duct openings are grouped into three (3) radially-spaced, chordally-extending rows.

13. An axial flow rotor, comprising: a central hub rotatable about a central axis; and a plurality of blades affixed to and extending radially from the central hub, wherein each of the plurality of blades comprises: a leading edge; a trailing edge; a blade tip opposite the central hub; a suction surface that extends between the leading edge and the trailing edge to the blade tip; a pressure surface opposite the suction surface that extends between the leading edge and the trailing edge to the blade tip; and a plurality of duct openings fluidically interconnecting the pressure surface and the suction surface through a thickness of the blade to reduce cavitation effects in a flow of a liquid established during rotation of the axial flow rotor, the plurality of duct openings being a number from five (5) to fifty (50) duct openings, the plurality of duct openings arranged in two (2) to five (5) radially-spaced, chordally-extending rows adjacent the blade tip, wherein the duct openings are retained within a radially-outer tip region of the blade and the duct opening of duct openings closest to the leading edge of the blade in the row of the duct openings closest to the blade tip is further from the leading edge than the duct opening of duct openings closest to the leading edge of the blade in the row of duct openings furthest from the blade tip, the radially-outer tip region being a radially-outer third of the blade adjacent to the blade tip.

14. An axial flow rotor of claim 13, characterized as a selected one of a marine propeller, an impeller for a pump, or an impeller for a mixer.

15. An axial flow rotor of claim 13, wherein the plurality of blades define a first diameter of the rotor extending through the central hub, a largest duct opening of the plurality of duct openings has a second diameter, and a ratio of the first diameter to the second diameter is from 100:1 to 1000:1.

16. An axial flow rotor of claim 13, wherein the plurality of duct openings each have the same diameter.

17. An axial flow rotor according to claim 13, wherein the blade comprises ten (10) to fifty (50) duct openings.

18. An axial flow rotor configured to accelerate a flow of a liquid while reducing cavitation thereof, the axial flow rotor characterized as a selected one of a propeller, an impeller for a pump, or an impeller for a mixer, the axial flow rotor comprising: a blade having a suction surface and a pressure surface each of which extend from a leading edge to a trailing edge of the blade and a radially-outer tip region, the radially-outer tip region being a radially-outer third of the blade, and the suction surface generating a lower pressure in the flow of the liquid adjacent the suction surface and the pressure surface generating a higher pressure in the flow of the liquid adjacent the pressure surface, wherein: the blade comprises a plurality of duct openings each extending through the blade from the pressure surface to the suction surface, the duct openings being limited to and disposed in the radially-outer tip region of the blade, the plurality of duct openings being from five (5) to fifty (50) arranged into two (2) to five (5) radially-spaced, chordally-extending rows, and the duct opening of the plurality of duct openings furthest from the leading edge of the blade in the row of the duct openings closest to the blade tip is further from the leading edge than the duct opening of the plurality of duct openings furthest from the leading edge of the blade in the row of the duct openings furthest from the blade tip.

19. An axial flow rotor according to claim 18, wherein the blade comprises ten (10) to fifty (50) duct openings.

20. An axial flow rotor, comprising: a central hub rotatable about a central axis; and a plurality of blades affixed to and extending radially from the central hub, wherein each of the plurality of blades comprises: a leading edge; a trailing edge; a blade tip opposite the central hub; a suction surface that extends between the leading edge and the trailing edge to the blade tip; a pressure surface opposite the suction surface that extends between the leading edge and the trailing edge to the blade tip; and five (5) to fifty (50) duct openings fluidically interconnecting the pressure surface and the suction surface through a thickness of the blade to reduce cavitation effects in a flow of a liquid established during rotation of the axial flow rotor, the duct openings arranged into two (2) to five (5) radially-spaced, chordally-extending rows adjacent the blade tip, wherein the duct openings are retained within a radially-outer tip region of the blade and the duct opening of the duct openings furthest from the leading edge of the blade in the row of the duct openings closest to the blade tip is further from the leading edge than the duct opening of the duct openings furthest from the leading edge of the blade in the row of the duct openings furthest from the blade tip, and the radially-outer tip region being a radially-outer third of the blade adjacent to the blade tip.

21. An axial flow rotor according to claim 20, wherein the blade comprises ten (10) to fifty (50) duct openings.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a propeller according to a first embodiment of the invention;

(2) FIG. 2(A) shows a known propeller blade with no duct openings;

(3) FIGS. 2(B) to (E) show propeller designs according to further embodiments of the invention with a prior art propeller for comparison shown in FIG. 2(A);

(4) FIG. 3 shows a perspective view of the Guardian propeller;

(5) FIGS. 4(A) to (D) are schematic diagrams of four further embodiments of the invention;

(6) FIG. 5(A) shows a propeller with five blades, each with four rows of radially-spaced, chordally-extending rows;

(7) FIG. 5(B) shows a close up of the tip region of the propeller of FIG. 6 (A);

(8) FIG. 6 shows an experimentally obtained graph of noise against frequency produced by the propeller of FIGS. 5A and 5B, compared to a propeller with no ducts; and

(9) FIGS. 7(A) to 7(F) show six further embodiments of the invention.

DETAILED DESCRIPTION

(10) FIG. 1 shows a single blade 2 attached to the hub 1 of a propeller. Duct openings 3 are shown on the face of the blade 2, generally located in the tip region 4 of the blade. In this embodiment seven relatively large duct openings 3 are shown.

(11) FIG. 2 illustrates in FIGS. 2(B) to (E) further embodiments of the invention, compared with a normal, prior art propeller blade illustrated in FIG. 2(A).

(12) In FIG. 2(B) each blade of the propeller has seventeen duct openings in the form of holes 3, each hole being of 1 mm diameter drilled from the pressure side to the suction side of the propeller blade, all positioned in the tip region of the blade 2. As illustrated, the duct openings 3 are arranged in three chordally-extending, radially-spaced rows and are confined to the radially-outer third of the blade, preferably the radially-outer quarter of the blade adjacent to the blade tip 4. In the embodiment illustrated in FIG. 2(B) the outer row consists of three duct openings, with the other two rows each having seven duct openings.

(13) FIG. 2(C) illustrates another embodiment of the invention with twenty-five duct openings, again arranged in three chordally-extending, radially-spaced rows in the tip region of the propeller blade. Again in this embodiment the duct openings are 1 mm in diameter. The radially-outer row has five duct openings, with the other two rows each containing ten duct openings. As illustrated, the rows may comprise a set of equally-spaced duct openings, with one or more unequally-spaced openings, in FIG. 2(C) with the duct opening nearest the leading edge of the first and third rows being spaced an increased distance from the remaining duct openings in the row. As with the FIG. 2(B) embodiment, the duct openings are all confined in the tip region of the blade, that is to say in the radially outer third, or more preferably radially-outer quarter, of the blade.

(14) FIG. 2(D) illustrates another embodiment of the invention in which thirty-four duct openings are provided in the tip region of the blade 2. Again, the duct openings 3 are grouped into three chordally-extending, radially-spaced rows and in the radially outer and innermost rows, the duct openings are set in pairs so that the distance between duct openings in a row alternates between a greater and lesser spacing. FIG. 2(E) illustrates another embodiment of the invention in which fifty duct openings are provided in the tip region of a blade 2, again in three radially-spaced, chordally-extending rows with ten duct openings in the radially-outermost row, seventeen in the middle row and twenty-three in the radially innermost row. In this embodiment the duct openings are evenly spaced in the three rows.

(15) In the above embodiments the duct openings are circular of 1 mm diameter, though these embodiments may be modified to use duct openings of 0.5 mm diameter.

(16) Table 1(a) below illustrates the results of CFD simulations of the propeller blades of FIGS. 2(A) to (E). The propeller was a four-bladed propeller, of the Guardian type, the details of which are shown below in Table 1(a). FIG. 3 provides an overall perspective view of this type of propeller, as used in the CFD simulation.

(17) TABLE-US-00001 TABLE 1(a) Diameter, D 350 mm Pitch ratio, P/D 0.699 Expanded Blade Area Ratio, A.sub.E/A.sub.O 0.524 Number of blades, Z 4 Direction of rotation Right-handed Scale ratio, ? 19.57 Hub diameter, D.sub.hub 56 mm

(18) TABLE-US-00002 TABLE 1(b) FIG. 2(A) 2(B) 2(C) 2(D) 2(E) BASE 17 Holes 25 Holes 34 Holes 50 Holes Thrust (N) 288.49 287.02 286.51 286.11 287.35 Torque (Nm) 14.78 14.83 14.72 14.73 15.10 Cavitation 2.15E?06 2.05E?06 1.98E?06 1.93E?06 1.84E?06 Volume (m.sup.3) Efficiency 58.70% 58.21% 58.56% 58.44% 57.24% ? % Thrust ?0.51% ?0.69% ?0.82% ?0.39% ? % Torque 0.32% ?0.45% ?0.39% 2.14% Efficiency Loss 0.48% 0.14% 0.25% 1.46% (%) ? % Cavitation ?4.44% ?8.03% ?10.42% ?14.58% Volume

(19) It can be seen from Table 1(b) that the presence of the duct openings in the tip region does not detract substantially from the thrust and torque performance of the propeller, nor from its efficiency. However, significant reductions in cavitation volume are achieved of between 4% and 15%.

(20) FIGS. 4(A) to (D) illustrate further alternative embodiments of the invention. In FIG. 4(A) seventeen circular duct openings of 1 mm diameter are positioned in the tip region of the blade, and in a group in the leading half of the blade. Thus the duct openings are grouped in the radially outer quarter, or more preferably radially outer fifth of the blade, and in the leading two-thirds of the region of the blade tip. In FIG. 4(B) the duct openings 3 are again in the leading part of the tip region, but the group extends further around the leading edge. The embodiment of FIG. 4(C) has a group of twenty-five duct openings in the leading half of the tip region and the embodiment of FIG. 4(D) has a group of twenty-five duct openings spread across tip region of the blade.

(21) The following results were obtained by CFD simulations of the four arrangements shown in FIG. 4(A) to 4(D), and are compared to a CFD simulation of the same propeller with no duct openings (referred to as BASE in Table 1). The CFD simulations used a STAR-CCM+ finite volume stress solver, Detached Eddy Simulation (DES) and a Schnerr-Sauer cavitation model. The propeller was the Guardian type propeller, the details of which are set out in Table 1(a) above.

(22) TABLE-US-00003 TABLE 2 FIG. BASE 4(A) 4(B) 4(C) 4(D) Thrust (N) 290.57 290.76 288.93 288.05 288.58 Torque (Nm) 14.78 14.86 14.79 14.78 14.72 Cavitation 2.44E?06 2.10 E?06 2.32 E?06 2.27 E?06 2.26 E?06 Volume (m.sup.3) Efficiency 51.93% 58.84% 58.76% 58.63% 58.96% ? % Thrust 0.07% ?0.57% ?0.87% ?0.68% ? % Torque 0.57% 0.07% ?0.01% ?0.40% Efficiency Loss 0.50% 0.64% 0.86% 0.29% (%) ? % Cavitation ?13.83% ?4.71% ?6.92% ?7.09% Volume

(23) It can be seen from Table 2 that, in all of the embodiments illustrated in FIG. 4, a significant decrease in cavitation is observed, with only a small decrease in thrust, torque and efficiency.

(24) FIGS. 5A and 5B illustrate a further embodiment of the invention, in which a propeller comprising five blades is provided with thirty-three duct openings in the tip region of each blade. The duct openings 3 are arranged in four chordally-extending, radially-spaced rows and are confined to the radially-outer third of the blade, preferably the radially-outer quarter of the blade adjacent to the blade tip 4.

(25) The propeller is the Princess Royal propeller, which is a subcavitating propeller (i.e. the majority of the blade area operates under cavitating conditions, and is hence prone to noise). This propeller is the benchmark propeller for carrying out noise trials, and is recognised as such by the Specialist Committee on Hydrodynamic Noise in the 28th ITTC (International Towing Tank Conference).

(26) In this embodiment, the duct closest to the leading edge of the blade in the row of duct openings closest to the tip is further from the leading edge than the duct closest to the leading edge of the blade in the row of duct openings furthest from the tip. Likewise, the duct furthest from the leading edge of the blade in the row of duct openings closest to the tip is further from the leading edge than the duct furthest from the leading edge of the blade in the row of duct openings furthest from the tip. In other words, the row of ducts which is radially innermost is positioned closer to the leading edge of the blade than the row of ducts which is radially outermost. The rows in between the radially innermost and outermost rows are positioned such that the position of the duct closest to the leading edge is between that of the radially innermost and outermost rows.

(27) FIG. 6 shows the noise produced at various frequencies by a propeller as shown in FIGS. 5A and 5B (Modified Propeller), and by a propeller identical to that of FIGS. 5A and 5B but without any ducts (Original Propelleri.e. solid blades). These tests were carried out in a water tank with a vessel speed of 10.5 and 15.1 kn that corresponds to engine rotational speed of 1500 RPM and 2000 RPM respectively with a reduction gear ratio of 1.75, thus mirroring the conditions used in a typical ship propeller.

(28) In FIG. 6, it can be seen that, for frequencies from 10 Hz to just over 10 kHz, the noise produced by the Modified Propeller (i.e. a propeller according to the arrangement of FIGS. 5A and 5B) is lower than that of the Original Propeller. These are the noise frequencies which are typically most harmful to marine life. For some frequencies, the reduction is of the order of 15-20 dB. Although the noise at higher frequencies is higher for the Modified Propeller than for the Original Propeller, these higher frequencies are of less concern so an increase in noise at these frequencies is acceptable, given that the noise at lower, more harmful frequencies is being reduced.

(29) FIGS. 7A to 7F show further variations of the embodiment shown in FIGS. 5A and 5B. Each variation has a different number of holes (i.e. duct openings) and/or size of holes drilled in the tip region, in the radially outermost 10% of the blade. The number of holes and hole diameter for each embodiment is set out in Table 3 below. Table 3 also shows the results for each embodiment when tested in a water tank. The BASE propeller is the same propeller with no ducts (i.e. the same as the Original propeller referred to in relation to FIGS. 5 and 6 above).

(30) TABLE-US-00004 TABLE 3 Figure BASE 7(A) 7(B) 7(C) 7(D) 7(E) 7(F) Number of n/a 41 60 33 92 17 23 holes Hole diameter n/a 1.0 1.0 1.0 0.6 1.0 1.0 (mm) Thrust (N) 586.64 578.71 574.94 578.86 579.59 582.04 580.96 Torque (Nm) 17.11 17.95 18.16 17.79 17.77 17.47 17.60 Cavitation 8.11E?06 6.02E?06 5.14E?06 6.47E?06 6.17E?06 7.18E?06 6.89E?06 Volume (m.sup.3) Efficiency 61.38% 57.73% 56.69% 58.24% 58.41% 59.67% 59.09% ? % Thrust ?1.35% ?1.99% ?1.33% ?1.20% ?0.78% ?0.97% ? % Torque 4.89% 6.11% 3.99% 3.82% 2.06% 2.87% Efficiency 5.95% 7.64% 5.11% 4.84% 2.79% 3.73% Loss (%) ? % Cavitation ?25.77% ?36.67% ?20.19% ?23.97% ?11.5% ?15.04% Volume

(31) It can be seen that each of the above configurations results in a significant decrease in cavitation volume, with a much smaller decrease in thrust and efficiency. Further, the loss in thrust is offset by an increase in torque.

(32) It will be appreciated that the number of ducts in each blade need not be the same, or that one or more of the blades may not include any ducts. For example, the ducts could be provided on only one of the blades of the propeller, or on a subset of the blades.

(33) It will also be appreciated that the axis of the ducts may be in any direction through the blade. For example, it may be normal to the blade mean line, it may be parallel to the axis of the shaft to which the propeller is mounted, or at any other suitable angle.

(34) Although the above embodiments illustrate propeller designs with particular numbers of duct openings in particular rows or arrangements, the precise number is not critical and can be varied. Thus the distribution of the duct openings between the rows, and the number of rows can be varied without substantially affecting the performance.

(35) It will also be noted that different types of axial flow device may be designed to work in different fluids. For example, an impeller which is used in a pump may be used to pump a fluid with a viscosity different to that of water, which may require a different size of duct opening to be used. A higher viscosity of fluid may require a larger size of duct opening (hole).

(36) The above embodiments are described to illustrate the invention, and are not intended to be limiting. The skilled person will be readily able to devise alternative embodiments without departing from the scope of the claims.