Stirring propeller with blades made of sheet bent along two longitudinal bends

Abstract

Stirrer (M) that includes at least two blades and is able to be fixed to a rotary shaft, wherein each blade has a leading edge facing the fluid to be stirred and a trailing edge facing away from the leading edge, and wherein each blade is obtained by bending a flat metal sheet, each blade having two longitudinal bends, the length of each bend being greater than 60% of the maximum radius of the blade.

Claims

1. A stirring member (M), comprising: a hub fastenable to a rotational shaft; and at least two blades attached to the hub, wherein each blade of the at least two blades has a leading edge facing fluid to be stirred and a trailing edge facing away from the leading edge, wherein each blade has an outer edge, with a distal end of the outer edge being remote from the hub and a proximal end that opposite to the distal end and nearest to the hub, wherein each blade is comprised of a bended flat sheet, the bended flat sheet of each blade having two longitudinal bends, a length of each of the longitudinal bends being greater than 60% of a maximum radius of the blade, wherein a radial axis of each blade is defined as through a center of rotation of the stirring member and perpendicular to the outer edge, an angle of incidence is defined as being an angle between the leading edge and the radial axis of the blade passing through the center of rotation and perpendicular to the outer edge, wherein the angle of incidence is positive, so that the distal end of the outer edge meets the fluid before the proximal end when the stirring member is in rotation, and wherein the angle of incidence is between 6 and 15.

2. The stirring member (M) as claimed in claim 1, wherein the length of each bend is greater than 75% of the maximum radius of the blade.

3. The stirring member (M) as claimed in claim 2 wherein the two bends are parallel.

4. The stirring member (M) as claimed in claim 2, wherein at least one of the bends is perpendicular to the outer edge of the propeller.

5. The stirring member (M) as claimed in claim 1 wherein the two bends are parallel.

6. The stirring member (M) as claimed in claim 5, wherein at least one of the bends is perpendicular to the outer edge of the propeller.

7. The stirring member (M) as claimed in claim 1, wherein at least one of the bends is perpendicular to the outer edge of the propeller.

8. The stirring member (M) as claimed in claim 1, wherein the stirring member comprises only two blades.

9. The stirring member (M) as claimed in claim 1, wherein each blade has, on account of the presence of the two bends, a substantially U-shaped cross section in a plane parallel to an axis of rotation of the stirring member and parallel to the outer edge of the blade.

10. The stirring member (M) as claimed in claim 1, wherein each blade has, on account of the presence of the two bends, a substantially Z-shaped cross section in a plane parallel to an axis of rotation of the stirring member and parallel to the outer edge of the blade.

11. The stirring member (M) as claimed in claim 1, wherein, a section plane is defined as a plane transverse to a longitudinal direction of the blade, an intersection line is defined as an intersection between the section plane and a plane orthogonal to an axis of rotation of the stirring member, a departure angle is defined as an angle between the intersection line and a trailing edge face of the blade, and the departure angle is between 30 and 70.

12. The stirring member (M) as claimed in claim 1, wherein, with a width of a distal end of the blade is denoted as l and a width at a base of the blade near an axis of rotation of the stirring member is denoted L, l >0.5 L.

13. The stirring member (M) as claimed in claim 1, wherein, for each blade, an angle of attack (a) between a face of the blade containing the leading edge and a central face of the blade is between 13 and 25.

14. The stirring member (M) as claimed in claim 1, wherein, with a width of a distal end of the blade is denoted as l and a width at a base of the blade near an axis of rotation of the stirring member is denoted L, l >0.75x L.

15. The stirring member (M) of claim 1, wherein a first bend (A) of the two longitudinal bends, being defined as a one of the two longitudinal bends which in rotation of the stirring member will meet the fluid first, is located along an axis passing through an axis of rotation of the stirring member.

16. The stirring member (M) of claim 1, wherein the distal end of the trailing edge is situated forward of the proximal end of said trailing edge and with respect to a direction of rotation of the stirring member, so that said distal end of the trailing edge meets the fluid before said proximal end of the trailing edge.

17. The stirring member (M) as claimed in claim 1, wherein, an angle of departure is defined between i) a face of the blade having the trailing edge, and ii) a plane orthogonal to the rotation axis, said departure angle being between 30 an 70.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention will become apparent from the following description of a preferred embodiment with reference to the appended drawings but without any limiting nature. In these drawings:

(2) FIG. 1 is a side elevation view of a stirrer according to the invention,

(3) FIG. 2 is a schematic perspective view, on a larger scale, of a first embodiment of a blade of a stirring member according to the invention,

(4) FIG. 3 is a top view of the blade in FIG. 2,

(5) FIG. 4 is an end-on view of the blade in FIG. 2,

(6) FIG. 5 and FIG. 6 are perspective views illustrating the introduction of stirring propellers having three and two blades into a container,

(7) FIG. 7 is a schematic perspective view of a second embodiment of a stirring propeller according to the invention,

(8) FIG. 8 is a top view of the propeller in FIG. 7,

(9) FIG. 9 is a view similar to FIG. 7 of a third embodiment of a propeller according to the invention, having three blades,

(10) FIG. 10 is a top view of the propeller in FIG. 9,

(11) FIG. 11 is a view similar to FIG. 7 of a fourth embodiment of a propeller according to the invention, having three blades,

(12) FIG. 12 is a top view of the propeller in another embodiment of a propeller according to the invention, and

(13) FIG. 13 is a graph illustrating the linear speeds at different points on the propellers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(14) Throughout the following description of different embodiments of propellers according to the invention, relative terms such as upper, lower, front, rear, horizontal and vertical should be interpreted as when the propeller according to the invention is installed in an operating situation.

(15) FIGS. 2 to 4 show a first embodiment of a propeller according to the invention, which is produced with two bends, this solution being inexpensive and able to be produced with the aid of tools which are available in most mechanical metal workshops.

(16) Inasmuch as each blade of the propeller has two bends, each blade thus has three faces and, in a cross-sectional view, it is necessary to define three angles in order to define the profile of the blade. These angles are more particularly visible in FIG. 4.

(17) The angle of attack is the angle a between the face having the leading edge and the central face. The angle d is the positioning angle between the central face of the blade and the horizontal when the axis of rotation is vertical. The departure angle f is the angle between the face having the trailing edge and the central face.

(18) This propeller has an angle of attack a and a departure angle f of 21. The first bend A, that is to say the one which will meet the fluid first, is made along an axis passing through the axis of rotation of the propeller. The second bend is denoted B. It may be noted that the distal end of the trailing edge is situated forward of the proximal end of this same trailing edge and with respect to the direction of rotation of the propeller. The distal end will thus meet the fluid first.

(19) The blades are bent so as to obtain a camber coefficient of less than 12%, so as to improve the energy efficiency. The angle of attack is between 13 and 22 so as to have a suitable Cx. Specifically, beyond 30, the radial forces generated will be very high. This then approaches the situation of turbines.

(20) The area of the blade is generous and virtually in the form of a quadrilateral, so as to obtain a high pumping flow since the volume displaced depends on the surface area of the blade.

(21) If the width of the blade at its end is denoted l and the width of the blade at its base at the level of the axis is denoted L, the values of l and L are very close and l>0.5 L and preferably l>0.75 L.

(22) This element has been preferred even if it runs counter to common practice. This is because the majority of propellers have a narrow end, in the form of a trapezoid, so as to limit the torque by narrowing the blade at its end.

(23) Studies have shown that, given the combination of angles chosen, the bends in the blade and the shape of the latter, the performance compared with known propellers is quite acceptable.

(24) If: P=hydraulic pressure P=pressure difference between the inlet and the outlet of the member Q=flow D=diameter of the member N=speed of rotation of the member =density v=speed of the fluid S=area of the member k=constant
the flow of a propeller is given by the following simple equation:
Qp=Nq ND.sup.3
where Nq is the dimensionless number that characterizes the propeller (its shape, the number of blades, etc.).

(25) The power consumed is calculated as follows:
P=Np N.sup.3 D.sup.5
where Np is the dimensionless number that characterizes the propeller (its shape, the number of blades, etc.).

(26) The efficiency is the ratio of the energy that produces the pumping flow and the energy necessary for turning the member.

(27) The efficiency can be expressed simply by the general equation of fluid mechanics and the simplified Bernoulli equation:

(28) General equation of fluid mechanics:
P1=PQ(1)

(29) Pumping flow:
Qp=Nq ND.sup.3(2)

(30) Power needed to rotate the member:
P2=Np N.sup.3 D.sup.5(3)

(31) Simplified Bernoulli equation:

(32) $P=1/2v^2$$v=Q/S=v=\frac{{\mathrm{NqND}}^3}{\frac{D^2}{4}}=\mathrm{kND}$$\frac{P2}{P3}=k\frac{{\mathrm{Nq}}^3}{\mathrm{Np}}$

(33) Note that the calculation is identical when the power consumed in order to generate 1 m.sup.3/h is sought, for example.

(34) The following is noted for example:

(35) TABLE-US-00001 Member type Nq Np Efficiency Novel 3-blade propeller 0.68 0.58 0.54 Novel 2-blade propeller 0.59 0.40 0.50 Prior art 1 0.60 0.41 0.53 Prior art 2 0.61 0.49 0.46 Turbine having blades 0.75 1.20 0.37 inclined at 45 Turbine having 6 0.85 5.5 0.12 straight blades

(36) It can be seen that the efficiencies of the proposed propellers are particularly good compared with the prior art and conventional propellers and turbines such as the marine propeller or the turbine having blades inclined at 45.

(37) The number of blades on the propellers increases the amount of liquid displaced but also the power consumed.

(38) Without being quite proportional, it is often noted that the power consumed increases proportionally with the number of blades by a factor of 0.8.

(39) However, in the present case, given the surface area and the angles, the average speeds of fluid show that with two blades the power decreases by 31% compared with a propeller having three blades, while the flow decreases only by 13%.

(40) There are thus multiple advantages in using a propeller with two blades.

(41) From an economical point of view, manufacturing two blades instead of three allows a 33% saving of material, of labor for forming the blade and welding it on a hub.

(42) It is easier to install the propeller. This is because, depending on the diameter of the shaft, it is sometimes not possible to install three blades around the latter.

(43) In addition, some products are partially destroyed by the shear brought about by the blades. This is because, on each rotation, the blade cuts the product in order to break it (flocs, emulsion, polymers, etc.) and a member equipped with two blades will only shear twice per rotation and not three times.

(44) Finally, the propeller can be made in one piece for different reasons, for example welded to the driveshaft to allow its possible coating for use in a corrosive or abrasive medium or when it is not possible to subsequently fasten it. The three-blade propeller is particularly difficult to introduce into a tube when the member exceeds 500 mm, but a two-blade propeller having the same diameter is easy to introduce, as illustrated in FIGS. 5 and 6.

(45) FIG. 13 shows notably a profile of the range of speeds leaving the blade that is virtually identical for the three propellers proposed, by virtue of the surface area of the blade, of the bends and of the angles combined; a clearly identical axial profile is preserved.

(46) The propeller is desired for its rather axial flow leaving the blade in order to be blown down to the bottom at the axis and to rise again at the wall so as to sweep any deposited particles from the bottom.

(47) Simple propellers made up of blades that are inclined or formed with one bend do not make it possible to bring about a mostly axial flow on account of radial and tangential leaks; for the invention, a mostly axial flow is noted.

(48) The manufacturing of the prior art propellers is complex.

(49) In some cases, it requires a complex machine that can twist the blades for propellers having a diameter of 10 m, this being a single machine that is constantly in production.

(50) Given their curvature, propellers of the saber type require a template for each diameter and shape, and hence a combination of more than one hundred templates.

(51) It is relatively easy to manufacture the proposed propellers with the aid of a bending press; greater competitiveness of subcontractors is thus conceivable, i.e. a greater choice thereof.

(52) The mechanical determination of a stirrer is dictated by its diameter and its speed of rotation for a given operation and consequently the power generated for the rotation of the member.

(53) The saving in power for one and the same pumping flow, this being an essential calculation element for stirring to effect mixing, allows a saving in terms of the motor, the speed reducer transmitting the torque, in terms of the guiding system and in terms of the leaktightness, the shaft supporting the member and the thickness of the member. A saving of 20% in power between the proposed propeller and a marine propeller is noted, for example.

(54) The economic saving that is brought about for the user from the point of view of investment is easily conceivable as the competitive advantage for the constructor.