GB2549948A

GB2549948A - Vortex finder for a cyclonic separator

Info

Publication number: GB2549948A
Application number: GB1607670.5A
Authority: GB
Inventors: Dodgson Oliver; Streeter Robert
Original assignee: Dyson Technology Ltd
Current assignee: Dyson Technology Ltd
Priority date: 2016-05-03
Filing date: 2016-05-03
Publication date: 2017-11-08
Also published as: GB201607670D0

Abstract

A vortex finder for a cyclonic separator that comprises a tube 20 and a plurality of vanes 30 spaced around the inner surface of the tube 20. Each vane 30is curved and comprises a leading edge 31 and a trailing edge 32. The leading edge 31 of each vane 30 is spaced circumferentially from the trailing edge 32 of an adjacent vane 30 such that a circumferential gap (33, fig 4) exists between adjacent vanes 30. Preferably the circumferential gap (33, fig 4) subtends a central angle (θG, fig 4) of between 10 and 40 degrees and each vane 30 extends radially from the inner surface of the tube 20 by at least 10% and by no more than 30% of the inner diameter of the tube 20. The angle of incidence (αL, αT, fig 5) of each vane 20 relative to the longitudinal axis 22 of the tube 20 is preferably greater than 45 degrees at the leading edge 31 and less than 15 degrees at the trailing edge 32. The height of each vane 30 is preferably less than half the height of the tube 20. The leading edge 31 of each vane 30 is preferably adjacent the inlet 21 of the tube 20 and the trailing edge 32 is spaced from the outlet 24 of the tube 20. The vortex finder can be used in a cyclonic separator of a vacuum cleaner.

Description

Vortex Finder for a Cyclonic Separator

The present invention relates to a vortex finder for a cyclonic separator.

The performance of a cyclonic separator is generally characterised by its separation efficiency and its pressure drop. A significant proportion of the pressure drop arises from dissipation of the rotational energy of the swirling airflow within the vortex finder and the downstream ducting. The cyclonic separator may therefore include means for converting part of the rotational energy back into static pressure. For example, the cyclonic separator may comprise a diffuser located on top of the vortex finder, or the vortex finder may include vanes for straightening the airflow. Unfortunately, the means for recovering pressure are often difficult to manufacture (this is particularly true where the cyclonic separator is relatively small) or provide poor pressure recovery.

The present invention provides a vortex finder for a cyclonic separator, the vortex finder comprising a tube and a plurality of vanes spaced around the inner surface of the tube, wherein each vane is curved and comprises a leading edge and a trailing edge, and the leading edge of each vane is spaced circumferentially from the trailing edge of an adjacent vane such that a circumferential gap exists between adjacent vanes.

The vanes act to recover static pressure from a swirling airflow by straightening the airflow as it passes through the tube. By employing vanes that are curved, the swirling airflow is straightened gradually. This then helps reduce flow separation and thus increase pressure recovery.

The leading edge of each vane is spaced circumferentially from the trailing edge of an adjacent vane. As a result, there is a circumferential gap between each pair of adjacent vanes. This then has the advantage that the vortex finder may be manufactured in a relatively simple manner. In particular, in spite of the provision of curved vanes, the vortex finder may be manufactured by a moulding process using a sliding core, thus avoiding the need for a rotating or collapsible core.

As the size of the circumferential gap decreases, it becomes increasingly difficult to manufacture the vortex finder using a sliding core or the like, this is particularly true if the vortex finder is relatively small. Conversely, as the size of the gap increases, the curvature of each vane must increase in order to achieve the same turning angle. As a result, flow separation is likely to increase. Accordingly, the circumferential gap may subtend a central angle of between 10 and 40 degrees. This then provides a relatively good balance between manufacturing ease and pressure recovery.

Each vane may extend radially from the inner surface of the tube by no more than 30% of the inner diameter of the tube. As a result, a vaneless core is established through the centre of the tube. This vaneless core is relatively large and provides a path for residual dirt to pass through the tube. As a result, the vortex finder is less susceptible to blockages. In spite of the size of the vaneless core, relatively good pressure recovery is observed in comparison to a vortex finder having no vanes.

As the radial extent or width of each vane decreases, so too does the likelihood of blockages. However, the pressure recovered by the vanes also decreases. Accordingly, each vane may extend radially from the inner surface of the tube by at least 10% of the inner diameter of the tube. The width of each vane may vary along its length. Nevertheless, the width of the vane at any point along its length is at least 10% of the inner diameter of the tube. Consequently, the leading edge and the trailing edge of each vane extend radially by at least 10% of the inner diameter of the tube. This then ensures relatively good pressure recovery.

The angle of incidence of each vane relative to the longitudinal axis of the tube may be greater than 45 degrees at the leading edge and less than 15 degrees at the trailing edge. The vanes therefore act to turn the swirling airflow through an angle of at least 30 degrees. As a result, the vanes may be used to straighten and thus recover pressure from airflows having a relatively high swirl angle.

The angle of incidence of each vane may be between 5 and 15 degrees at the trailing edge. Employing an angle of incidence of at least 5 degrees has the advantage of simplifying the manufacture of the vortex finder by acting as a draft angle. For example, when manufactured by a moulding process, the vortex finder may be ejected from the mould without the trailing edge scraping against the mould, which might otherwise damage the trailing edge and adversely affect the performance of the vortex finder. The angle of incidence at the trailing edge may be larger than that required for draft alone. This then has the advantage that the vanes are required to turn the airflow through a smaller angle and thus a shallower curvature may be employed. Whilst an angle of incidence of 0 degrees is perhaps ideal from a de-swirling viewpoint, the effect of terminating the vanes prematurely was found to give only a negligible drop in pressure recovery when the angle of incidence was no greater than 15 degrees.

When the vortex finder is manufactured using a moulding process, the height of the vanes may influence the lifespan of the mould. In particular, where the vortex finder is relatively small, excessively tall vanes may require delicate features on the mould that are more susceptible to wear. Accordingly, in order to prolong the lifespan of the mould, the height of each vane may be less than half the height of the tube.

The leading edge of each vane may be adjacent the inlet of the tube and the trailing edge may be spaced from the outlet of the tube. By positioning the vanes at the inlet of the tube, the vanes act immediately upon the airflow entering the tube. As a result, pressure recovery is likely to be greater. By contrast, if the vanes were positioned further along the tube, the spiralling airflow entering the tube would first move through a vaneless section. Losses will then arise from dissipation of the rotational energy of the swirling airflow within the vaneless section. By positioning the vanes at the inlet of the tube, the vaneless section is moved downstream of the vanes. As a result, the airflow moving through the vaneless section has less swirl and thus losses are likely to be smaller.

In order that the present invention may be more readily understood, an embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a side view of a vacuum cleaner;

Figure 2 is a perspective view of a vertical section through part of the vacuum cleaner, the view illustrating two cyclonic separators;

Figure 3 is a perspective view of a vertical section through a vortex finder that forms part of the cyclonic separators of Figure 2;

Figure 4 is a plan view of the vortex finder;

Figure 5 is a cylindrical projection of the vortex finder; and

Figure 6 is a perspective view of part of a mould for manufacturing the vortex finder.

The vacuum cleaner 1 of Figure 1 comprises a main body 2 to which a dirt separator 3 is removably attached. The dirt separator 3 comprises a plurality of cyclonic separators 10 that separate dirt from an airflow drawn through the dirt separator 3.

Referring now to Figure 2, each of the cyclonic separators 10 comprises a cyclone body 11, an inlet 12, and a vortex finder 13. With the exception of the vortex finder 13, the cyclonic separator 10 is conventional in design. In particular, the interior of the cyclone body 11 defines a cyclone chamber 14 into which an airflow is admitted tangentially via the inlet 12. An opening located at the bottom of the cyclone body 11 serves as a dirt outlet 15, and the vortex finder 13 extends centrally through the top of the cyclone body 11 and into the cyclone chamber 14. During use, a dirt-laden airflow enters the cyclone chamber 14 via the inlet 12. The airflow swirls within the cyclone chamber 14 and moves downwards around the outer region of the cyclone chamber 14.

As the airflow moves downwards, the airflow is forced into the inner region of the cyclone chamber 14 and moves upwards. The dirt, on the other hand, continues to move downwards around the outer region of the cyclone chamber 14. The airflow then exits the cyclone chamber 14 via the vortex finder 13, whilst the dirt exits the cyclone chamber 14 via the dirt outlet 15.

Figures 3 to 5 illustrate the vortex finder 13 in isolation. The vortex finder 13 comprises a tube 20 and a plurality of vanes 30 spaced around the inner surface of the tube 20. In the embodiment illustrated in the figures, the vortex finder 13 comprises four vanes 30, but could equally comprise a greater or smaller number of vanes.

The tube 20 is cylindrical and has a constant diameter. Alternative geometries for the tube 20 are, however, possible. The leading edge 21 of the tube 20 is rounded such that less turbulence is generated by the swirling airflow on entering the tube 20.

Each vane 30 is curved and comprises a leading edge 31 and a trailing edge 32. The leading edge 31 has of an angle of incidence oil of 53 degrees and the trailing edge 32 has an angle of incidence αχ of 10 degrees relative to the longitudinal axis 22 of the tube 20. As explained below, the angle of incidence at the leading edge 31 is chosen such that a small positive angle of attack is established between the swirling airflow and the leading edge 31. The angle of incidence at the trailing edge 32, on the other hand, is chosen primarily to aid manufacture.

The leading edge 31 of each vane 30 is spaced circumferentially from the trailing edge 32 of an adjacent vane. As a result, a circumferential gap 33 exists between each pair of adjacent vanes 30. The vanes 30 are spaced evenly around the inner surface of the tube 20, with each vane 30 subtending a central angle ()v of 62 degrees, and each gap 33 subtending a central angle Og of 28 degrees.

The vanes 30 are positioned within the tube 20 such that the leading edge 31 is adjacent the inlet 23 of the tube 20. The height or axial extent (i.e. in a direction parallel to the longitudinal axis 22) of each vane 30 is approximately one third of the height of the tube 20. Consequently, the trailing edge 32 is spaced considerably from the outlet 24 of the tube 20.

The width or radial extent (i.e. in a direction normal to the longitudinal axis 22) of each vane 30 is approximately 20% of the inner diameter of the tube 20. As a result, a vaneless core 40 is established through the centre of the tube 20. As explained below, this vaneless core 40 provides a path for residual dirt to pass through the vortex finder 13.

Figure 5 illustrates a cylindrical projection of the vortex finder 13 in which the tube 20 is unrolled and appears flat. This figure has been provided in order to better illustrate the geometry and spacing of the vanes 30.

During use, the vanes 30 act to recover static pressure from the airflow by de-swirling or straightening the airflow as it passes through the tube 20. As a result, the pressure drop associated with the dirt separator 3 is reduced. The vanes 30 also provide acoustic benefits by reducing the noise generated by the airflow moving through the dirt separator 3.

The vanes 30 are curved and thus the swirling airflow is straightened gradually as it moves through the tube 20. This then helps reduce flow separation and increase pressure recovery. The design of the cyclonic separators 10 is such that the airflow entering the tube 20 has a swirl angle of 58 degrees. For the avoidance of doubt, the swirl angle corresponds to the angle between the velocity of the airflow and the longitudinal axis 22 of the tube 20. Consequently, an airflow having a swirl angle of 0 degrees moves linearly through the tube 20 without any swirl. The angle of incidence aL of the leading edge 31 of each vane is 53 degrees. Consequently, a positive angle of attack of 5 degrees is established between the airflow and the leading edge 31 of each vane 30. Whilst an angle of attack of 0 degrees would be preferable, a small positive angle of attack ensures that a negative angle of attack is avoided at the worst case tolerance condition.

The height of each vane 30 is defined by the central angle θν subtended by the vane 30, the angles of incidence ol and αχ at the leading and trailing edges 31,32, and the curvature of the vane 30. With the geometry described above, the height of each vane 30 is approximately one third of the height of the tube 20. Vanes of greater height may be employed. However, as will now be explained, when the vortex finder 13 is manufactured using a moulding process, increasing the height of the vanes 30 may reduce the lifespan of the mould. Figure 6 illustrates two parts of a mould 40 suitable for manufacturing the vortex finder 13. The lower part 41 comprises four prongs 42 that extend vertically upwards. The very top of each prong 42 is relatively thin and is defined by the size of the circumferential gap 33. As the height of the vanes 30 increases, so too does the height of the prongs 42. As a result, the top, relatively thin section of each prong 42 increases in length. The prongs 42 are therefore more delicate and thus more susceptible to wear with repeated use; this is particularly true when the mould 40 is used to manufacture a vortex finder 13 that is relatively small.

The vanes 30 are positioned within the tube 20 such that the leading edge 31 is adjacent the inlet 23 of the tube 20. As a consequence, the trailing edge 32 is spaced from the outlet 24 of the tube 20 by a distance of about two thirds of the height of the tube 20. By positioning the vanes 30 at the inlet 23 of the tube 20, the vanes 13 act immediately upon the swirling airflow as it enters the tube 20. As a result, pressure recovery is likely to be improved. By contrast, if the vanes 30 were positioned further along the tube 20, the spiralling airflow would first move through a vaneless section of the tube 20. Losses will then arise from dissipation of the rotational energy of the swirling airflow within the vaneless section. By positioning the vanes 30 at the inlet 23 of the tube 30, the vaneless section is moved downstream of the vanes 30. Consequently, the airflow moving through the vaneless section has less swirl and thus losses are likely to be smaller.

The width of each vane 30 is approximately 20% of the inner diameter of the tube 20. A vaneless core 40 is therefore established through the centre of the tube 20. This vaneless core 40 is relatively large (i.e. approximately 80% of the inner diameter of the tube 20) and provides a path for residual dirt to pass through the tube 20. As a result, the vortex finder 13 is less susceptible to blockages. In spite of the size of the vaneless core 40, relatively good pressure recovery is observed in comparison to a vortex finder having no vanes. As the width of each vane 30 increases, the pressure recovered by the vanes 30 increases, but so too does the likelihood of a blockage. Additionally, the size of the gap 33 at the inner edges of the vanes 30 decreases. In order to maintain the same minimum gap size (e.g. for the purposes of manufacturing), each gap 33 must span a larger central angle 9g- Consequently, each vane 30 must span a smaller central angle θν and thus the curvature of the vanes 30 must increase in order to achieve the same turning angle. Vanes 30 having a width of between 10% and 30% provide a good balance of these factors, particularly in view of the size of vortex finder that is likely to be used with the dirt separator 3, as well as the type of dirt that is likely to be carried through the dirt separator 3 of the vacuum cleaner 1.

The circumferential gap 33 between each pair of vanes 30 simplifies the manufacture of the vortex finder 13. In particular, in spite of the fact that the vanes 30 are curved, the vortex finder 13 may be manufactured by injection moulding (or another moulding process) using a sliding core, such as that illustrated in Figure 6. By contrast, if the vanes 30 were to overlap, a rotating or collapsible core would be required in order to manufacture the vortex finder 13, which would naturally increase the cost. As the size (i.e. the central angle 9g) of the gap 33 decreases, it becomes increasingly difficult to manufacture the vortex finder 13 using a sliding core, this is particularly true if the vortex finder 13 is relatively small, e.g. if the diameter of the tube 20 is less than 10 mm. Conversely, as the size of the gap 33 increases, each vane 30 will subtend a smaller central angle 9y and thus the curvature of each vane 30 must increase in order to achieve the same turning angle. A good balance may therefore be struck with a circumferential gap 33 that subtends a central angle of between 10 and 40 degrees.

The trailing edge 32 of each vane 30 has an angle of incidence ατ of 10 degrees relative to the longitudinal axis 22 of the tube 20. This then has the advantage of further simplifying the manufacture of the vortex finder 13 by acting as a draft angle. In particular, when manufactured by a moulding process, the vortex finder 13 may be ejected from the mould without the trailing edge 32 scraping against the mould, which might otherwise damage the trailing edge 32 and adversely affect the performance of the vortex finder 13. The angle of incidence at the trailing edge 32 may be larger than that required for draft alone. This then has the advantage that the vanes 30 are required to turn the airflow through a smaller angle and thus a shallower curvature may be employed. Whilst an angle of incidence of 0 degrees is perhaps ideal from a de-swirling viewpoint, the effect of terminating the trailing edge 32 prematurely was found to give only a negligible drop in pressure recovery when the angle of incidence was no greater than 15 degrees. An angle of incidence of between 5 and 15 degrees therefore provides a good balance between manufacturing ease and pressure recovery.

Claims

1. A vortex finder for a cyclonic separator, the vortex finder comprising a tube and a plurality of vanes spaced around the inner surface of the tube, wherein each vane is curved and comprises a leading edge and a trailing edge, and the leading edge of each vane is spaced circumferentially from the trailing edge of an adjacent vane such that a circumferential gap exists between adjacent vanes.

2. A vortex finder as claimed in claim 1, wherein the circumferential gap subtends a central angle of between 10 and 40 degrees.

3. A vortex finder as claimed in claim 1 or 2, wherein each vane extends radially from the inner surface of the tube by no more than 30% of the inner diameter of the tube.

4. A vortex finder as claimed in any one of the preceding claims, wherein each vane extends radially from the inner surface of the tube by at least 10% of the inner diameter of the tube.

5. A vortex finder as claimed in any one of the preceding claims, wherein the angle of incidence of each vane relative to the longitudinal axis of the tube is greater than 45 degrees at the leading edge and less than 15 degrees at the trailing edge.

6. A vortex finder as claimed in any one of the preceding claims, wherein the angle of incidence of each vane is between 5 and 15 degrees at the trailing edge.

7. A vortex finder as claimed in any one of the preceding claims, wherein the height of each vane is less than half the height of the tube.

8. A vortex finder as claimed in any one of the preceding claims, wherein the leading edge of each vane is adjacent the inlet of the tube and the trailing edge is spaced from the outlet of the tube.

9. A vacuum cleaner comprising a dirt separator, wherein the dirt separator comprises a plurality of cyclonic separators, and each cyclonic separator comprises a vortex finder as claimed in any one of the preceding claims.