AXIAL FAN

Abstract

The invention relates to an axial fan for delivering cooling air, in particular for an internal combustion engine of a motor vehicle, comprising fan blades which are fastened to a hub and which have a pressure side and a suction side and a trailing edge and a blade depth, on the pressure side of which fan blades is arranged a hub ramp which rises counter to the direction of rotation of the axial fan, wherein the trailing edge has an outer region situated radially outside the hub ramp and has an inner region situated radially within the hub ramp.

Description

The invention relates to an axial fan for delivering cooling air, in particular for an internal combustion engine of a motor vehicle, as per the preamble of patent claim 1.

A generic axial fan is disclosed in the prior patent application of the applicant with the official file reference 2010 042 325.4. The axial fan has fan blades which are fastened to a hub ring and which, on their pressure side, have a hub ramp, and on their suction side, have air-guiding elements, also referred to as stabilizers, which serve for influencing the fan flow. The fan blades have in each case a leading edge, also referred to as flow impingement edge, and a trailing edge, also referred to as flow-off edge. The trailing edge of the fan blade has substantially two radially extending portions, specifically an outer portion arranged outside the hub ramp and an inner portion arranged within the hub ramp. The inner portion of the trailing edge is, for weight-saving reasons, angled inward, that is to say in the direction of the hub ring, so as to form a recess for the trailing edge and thus a reduction of the width of the fan blade. It has been found that, as a result of said recess of the trailing edge, a transverse flow and/or backflow of the fan blade flow occurs, which adversely affects the flow on the pressure side of the adjacent fan blade. Said backflow and/or transverse flow gives rise, in the region of the hub ramp, to a vortex structure which results in a decrease in efficiency.

EP 0 515 839 A1 discloses an axial fan having fan blades, on the pressure side of which is arranged a hub ramp which rises counter to the flow direction. The hub ramp in effect fills out the stagnant area in the region of the vane root and thus prevents a loss-afflicted vortex flow.

DE 199 29 978 B4 discloses an axial fan having fan blades, on the suction side of which are arranged air-guiding elements, and on the pressure side of which are arranged hub ramps. In this way, a flow duct is formed which effects stable guidance of the flow in the region of the vane root.

It is an object of the present invention, in the case of an axial fan of the type specified in the introduction, to improve the flow conditions and in particular prevent a loss-afflicted vortex formation.

The object of the invention is achieved by means of independent patent claim 1. Advantageous refinements emerge from the subclaims. According to the claim, the fan blade has a trailing edge with two portions, wherein a first, outer portion is situated radially outside the hub ramp and a second, inner portion is situated radially within the hub ramp. According to the invention, it is advantageous for the trailing edge to have, in the outer region radially outside the hub ramp, a profile which continues in substantially unchanging fashion radially inward across the radial position of the hub ramp into the inner region, and which, in the radially innermost region, runs toward the hub. This yields the inventive result that a stabilization of the fan blade flow is attained in the hub ramp region, that is to say a transverse flow and/or backflow around the trailing edge of the fan blade is at least reduced or is prevented. This leads to a significant increase in the efficiency of the fan and to a considerable increase in the volume flow delivered by the fan at the operating point of the fan. Furthermore, the specific sound pressure level is reduced.

Furthermore, it is advantageous for the radially innermost region to be the radially inner component of the radius of the radially inner region.

It is also advantageous for said component to be approximately one third, approximately one quarter or preferably less than approximately one fifth of the radius of the inner region of the trailing edge.

It can thus also be understood that three regions are present, the outer region and the inner region, wherein the inner region is itself in turn divided into a so-called intermediate region and the innermost region. According to the invention, therefore, it is advantageous for the trailing edge to have, in the outer region radially outside the hub ramp, a profile which continues in substantially unchanging fashion radially inward across the radial position of the hub ramp into the so-called intermediate region of the inner region, and which, in the radially innermost region, runs toward the hub. Here, said “running toward the hub” may mean curved or angled etc.

The fan blade advantageously also has substantially the same blade depth in the inner region as in the outer region, that is to say in particular the blade trailing edge merges substantially rectilinearly from the outer portion into the inner portion, such that overall, a straight trailing edge is formed as far as into the vane root region. Here, a certain curvature of the leading edge is not detrimental; for simplicity, however, said leading edge has been assumed in this examination as being straight, wherein a non-straight, curved leading edge would likewise be admissible.

In relation to the fan of the prior patent application, therefore, the blade depth and also the blade width in the region within the hub ramp have advantageously been increased.

The expressions “fan blade” and “fan vane” are used synonymously within the context of the present application. The expression “blade depth” is to be understood to mean the axial extent of the fan blade. The blade depth is the projection of the blade width in the circumferential direction, wherein the blade width is the spacing between the blade leading edge and blade trailing edge, measured in the direction of the chord.

In one preferred embodiment, the trailing edge, in the innermost region, is rounded. A reduced-stress transition of the blade trailing edge into the hub region is made possible in this way.

In a further preferred embodiment, the blade trailing edge, in its innermost region, merges—via a rounding—into the free edge of the hub ramp. An increase of the strength in the vane root region for the connection of the fan blade to the hub is attained in this way. Furthermore, a streamlined duct is formed between the suction side and the pressure side in the vane root and hub ramp region. The free edge of the hub ramp is to be understood to mean that edge of the hub ramp which faces away from the fan blade and which projects from the fan blade.

In a further preferred embodiment, stabilizers are arranged on the suction side of the fan blades, which stabilizers are preferably situated radially within the hub ramp. The downstream region of the stabilizer thus issues into the inner portion of the blade trailing edge. The stabilizers in conjunction with the hub ramps between the two vanes yield a further stabilization of the flow in the vane root region.

In a further preferred embodiment, the hub is in the form of a hub ring which has a significantly smaller axial extent than the fan blades. There is thus no longer a cylindrical hub in the classic sense. As mentioned above, the axial extent of the fan blades as represented by a projection of the blade width in the circumferential direction is referred to as the blade depth. The fan blades project both with their leading edges and also with their trailing edges beyond the end faces of the hub ring. In this respect, the trailing edge which runs rectilinearly as far as into the innermost region forms an axial projecting length of the fan blade with respect to the hub ring.

In a further preferred embodiment, the axial fan has a hub ratio D_i/D_a of greater than 42%, wherein the hub ratio is the quotient of hub diameter and outer diameter of the fan blades. The axial projecting length of the blade trailing edge in the inner region has a particularly advantageous effect in the case of fans with a relatively large hub ratio, because this has an adverse effect on the efficiency and on the volume flow rate delivered by the fan—in this respect, compensation is generated here. The relatively large hub ratio may arise here owing to a relatively small outer diameter if the fan blades are shortened owing to performance gradation.

In a further preferred embodiment, the axial fan is fixedly connected via its hub ring to a liquid friction clutch, which in turn is driven by the internal combustion engine and drives the fan with a regulated drive output rotational speed. In the case of relatively high power ratings, the diameter of the liquid friction clutch and thus the hub diameter increases, which may lead to a larger hub ratio.

The projecting blade length according to the invention, which leads to an increase in efficiency and in volume flow rate, has a particularly positive effect here.

In a further preferred embodiment, the fan blades are angled in the region of the blade root, whereby the form of a vane for the fan blade is defined. Advantages here are a low material accumulation in the region of the connection of fan blade and hub ring, and an increased strength.

Exemplary embodiments of the invention are illustrated in the drawing and will be described in more detail below, wherein further features and/or advantages may emerge from the description and/or from the drawing, in which:

FIG. 1 shows a fan blade design according to the prior art,

FIG. 2 shows a fan blade design according to the invention with stabilized flow,

FIG. 3 shows a detail of a fan according to the invention in a three-dimensional illustration,

FIG. 4 shows a radial section through the fan hub,

FIG. 5 shows a partial view of the axial fan in the section plane VI-VI, and

FIG. 6 shows a sectional illustration as per section plane VI-VI in FIG. 5.

FIG. 1 shows an arrangement of fan blades 1, 2 of an axial fan according to the prior art. The direction of rotation of the fan is indicated by an arrow D. The fan blades 1, 2 have hub ramps 3, 4 in each case on their pressure side, and have blade trailing edges 1a, 2a. The trailing edges or flow-off edges 1a, 2a each have cutouts or recesses 5, 6 in their radially inner region, that is to say within the hub ramp 3, 4. Such recesses and shortenings of the fan blade width were implemented in the prior art because they firstly yield a weight saving, and secondly, it was believed that no further increase in performance of the fan blade is attained in the vane root region. It was however found to be disadvantageous that there was a transverse flow and/or backflow indicated by the arrows W. The flow around the blade trailing edge in the region of the recess 5 yields a wake indicated by the arrows W, which leads to a reduction in fan efficiency, a reduction in volume flow rate and to increased noise generation.

FIG. 2 schematically shows a design according to the invention of fan blades 7, 8 and the blade trailing edges 7a, 8a thereof. The direction of rotation of the axial fan is again denoted by an arrow D. On the pressure side of the fan blades 7, 8 there are arranged hub ramps 9, 10 which divide the blade trailing edges 7a, 8a into a radially outer region and a radially inner region. According to the invention, the radially inner region of the blade trailing edge 7a runs substantially straight, that is to say there is a substantially straight profile from the transition from the outer region to the inner region.

In other words, the vane width of the fan blade 7 is increased in the radially inner region in relation to the vane width of the fan blade 1 according to the prior art, such that no recess is provided.

Said enlarged region is highlighted by a contour 7b illustrated in bold. The effect of the increased vane width in the region 7b is a prevention of the loss-afflicted transverse flow and/or backflow illustrated in FIG. 1. In particular, the flow radially outside the hub ramp 10 at the fan blade 8, as illustrated by arrows S, is substantially not disrupted. Secondly, it is also the case that a relatively stable and turbulence-free flow is generated below the hub ramp 10, as indicated by arrows P. The elongation of the blade trailing edge in the radially inner region 7b, that is to say the enlargement of the vane width, yields a significant increase in volume flow rate and in efficiency, and a noise reduction.

The expressions “vane width” or “blade width” are to be understood to mean the distance between the leading edge and trailing edge or the length of the chord of the vane or of the blade. The depth of the vane (blade depth) is to be understood to mean the projection of the vane width in the circumferential direction.

FIG. 3 shows a detail of an axial fan 11 according to the invention in a three-dimensional illustration. The illustration shows a hub ring 12 on which fan blades 13, 14, are fastened, that is to say integrally formed by injection molding. The fan blades 13, 14, 15 have, on their pressure side in each case, hub ramps 13a, 14a, 15a which rise counter to the direction of rotation indicated by an arrow D. For reasons of strength, the hub ramps 13a, 14a, 15a are connected on their underside to the hub ring by ribs. The fan blades 13, 14, 15 each have trailing edges 16, 17, 18, also referred to as flow-off edges 16, 17, 18, which run substantially rectilinearly from radially outside to radially inside. The downstream end of the hub ramps 13a, 14a, 15a divides the trailing edges 16, 17, 18 into two portions, specifically radially outer portions or regions 16a, 17a, 18a and radially inner portions or regions 16b, 17b, 18b. The inner portions 16b, 17b, 18b of the blade trailing edges 16, 17, 18 merge via a radius or a rounding R into the free edges of the hub ramps 13a, 14a, 15a, wherein the reference lines of the reference numerals 13a, 14a, 15a extend from the free edges. A fan structure is thus created which is optimized with regard to strength and which is capable of absorbing the forces, in particular centrifugal forces, that arise during the operation of the fan. The suction sides of the fan blades have fin-like stabilizers 19.

FIG. 4 shows a radial section (section in a radial plane) through the hub ring 12 of the axial fan 11, wherein the same reference signs as those in FIG. 3 are used for identical parts. The air flow direction is indicated by an arrow L. The hub ring 12 has an axial extent a and the fan blade 14 has a depth t which—as mentioned above—is defined as a projection of the vane width in the circumferential direction. From the diagrammatic illustration, it is clear that the blade depth t is considerably greater than the axial extent a of the hub ring 12. In a preferred exemplary embodiment, the blade depth t is approximately twice as large as the axial extent a of the hub ring 12. The trailing edge 17 of the fan blade 14 runs substantially rectilinearly in the radial direction, wherein the innermost portion of the trailing edge 17 is rounded. The hub ramp appears as a sectioned surface denoted by 14a.

FIG. 5 shows a view of an incompletely illustrated axial fan 20, viewed in the direction of its front side or the suction sides of the fan blades 21, on which air-guiding elements 22 are arranged. The axial fan 20 comprises a metallic carrier ring 23 which is connected at one side to the plastic hub of the axial fan 20 and which can be fastened at the other side to a clutch (not illustrated), preferably a liquid friction clutch. The fan blades denoted by the reference signs 21a, 21b, 21c have been cut away in the section plane VI-VI.

FIG. 6 is a sectional illustration of the axial fan as per section plane VI-VI. The illustration shows different sectioned surfaces of the fan blades 21a, 21b, 21c. As shown by the section plane VI-VI in FIG. 5, the fan blades 21a, 21b, 21c are sectioned in different planes in relation to their radial central line, wherein the section plane for the central blade 21b can be regarded as a tangential section and is situated radially within the hub ramp. The sectioned surface of the blade 21c in FIG. 6 is situated above the hub ramp, which in this case is denoted by the reference sign 24 and is not visible in FIG. 5 because it is arranged on the rear side of the fan blades 21. The sectioned surface of the middle fan blade 21b shows the vane width b, that is to say the distance between the leading and trailing edges. The projection of the vane width b in the circumferential direction yields the vane depth t (not indicated) which is approximately constant over the entire radial range, specifically in the case of an approximately rectilinearly running blade trailing edge.

Further features and preferred embodiments will emerge from the applicant's prior patent application, mentioned in the introduction, with the official file reference 10 2010 042 325.4—said prior patent application is incorporated in its entirety into the content of disclosure of the present application. According to said document, it may be advantageous for the fan blades to be kinked in the direction of their blade roots and for the angled, inner region to be drawn downward onto the hub ring. The angled formation yields a vane-like form of the fan blade and a transition, which is optimized in terms of stresses, between the fan blade and hub ring.

Claims

1. An axial fan for delivering cooling air, in particular for an internal combustion engine of a motor vehicle, comprising fan blades which are fastened to a hub and which have a pressure side and a suction side and a trailing edge and a blade depth, on the respective pressure side of which fan blades is arranged a hub ramp which rises counter to the direction of rotation of the axial fan, wherein the trailing edge has an outer region situated radially outside the hub ramp and has an inner region situated radially within the hub ramp, wherein the trailing edge has, in the outer region radially outside the hub ramp, a profile which continues in substantially unchanging fashion radially inward across the radial position of the hub ramp into the inner region, and which, in the radially innermost region, runs toward the hub.

2. The axial fan as claimed in claim 1, wherein the radially innermost region is the radially inner component of the radius of the radially inner region.

3. The axial fan as claimed in claim 2, wherein the component is approximately one third, approximately one quarter or preferably less than approximately one fifth of the radius of the inner region of the trailing edge.

4. The axial fan as claimed in claim 1, wherein the blade depth in the inner region corresponds to the blade depth in the outer region.

5. The axial fan as claimed in claim 1, wherein the trailing edge is formed as a substantially straight edge in the outer and in the inner region but not in the innermost region.

6. The axial fan as claimed in claim 1, wherein the trailing edge, in the innermost region, is rounded and merges into the hub.

7. The axial fan as claimed in claim 1, wherein the trailing edge, in its radially innermost region, merges via a rounding into a free edge of the hub ramp.

8. The axial fan as claimed in claim 1, wherein air-guiding elements in the form of stabilizers are arranged on the suction side of the fan blades.

9. The axial fan as claimed in claim 8, wherein the stabilizers extend as far as into the inner region of the trailing edges.

10. The axial fan as claimed in claim 1, wherein the hub is in the form of a hub ring with an axial extent which is significantly smaller than the fan blade depth.

11. The axial fan as claimed in claim 1, wherein the axial fan has a hub ratio Di/Da of greater than 42%, wherein Di is the outer diameter of the hub and Da is the outer diameter of the fan blades.

12. The axial fan as claimed in claim 1, wherein a liquid friction clutch is arranged within the hub ring and is fixedly connected to the hub ring.

13. The axial fan as claimed in claim 1, wherein the fan blades are angled in the direction of their blade root, wherein the angled region of the fan blade is at least partially drawn downward onto the hub or the hub ring.