Bullnose bellmouth for jet fan

Abstract

A bellmouth that enhances the effective longitudinal thrust of a jet fan, by deflecting the flow on the discharge side, minimising flow separation on the inlet side, and reducing or eliminating the underhang of the bellmouth beneath the thickness of the 5 silencer The bellmouth comprises a curve with a variable radius at the distal end of the fan; a curve that is flat or nearly flat; a curve with a variable radius that blends smoothly with the rest of the bellmouth; a straight curve, which is used to deflect the flow on the discharge side in order to reduce the Coanda effect; and a radiused curve which smoothly turns the inlet flow towards the lower part of the silencer.

Description

BULLNOSE BELLMOUTH FOR JET FANFIELD OF THE INVENTION

[0001] The preset invention relates to a bullnose bellmouth for a jetfan. More especially, the invention relates to a bellmouth formed with a contiguous series of curves.BACKGROUND OF THE INVENTION

[0002] A previous patent application number GB2562263 filed by the present Applicant described a bellmouth for a jetfan having the purpose of enhancing the thrust of jet fans while promoting the deflection of the discharged jet, by specifying the design of the bellmouths attached to one or both sides of the fans. Typically, silencers would be bolted on either side of a fan casing, and a bellmouth would be attached to each of the silencers at the distal ends of the fan assembly. The internal surfaces of the silencers effectively form nozzles for the flow of air.

[0003] The application discloses a bellmouth wherein the cross-sectional area of the bellmouth throughbore decreases from the location of its attachment to the nozzle in the direction away from the fan, to a minimum cross-sectional area. The cross-sectional area of the bellmouth throughbore decreases from the distal end from the fan in the direction towards the fan, to the minimum cross-sectional area.

[0004] Although the invention disclosed in the application has been proven via Computational Fluid Dynamics and experiments to deliver significant benefits in terms of aerodynamic thrust and reduction in power consumption, it fails to deliver a compact design. In particular, the disclosed design typically exhibits an underhang of the bellmouth below the underside of the silencers. Such an underhang is undesirable (and may be unacceptable) in tunnels with limited clearances and in car parks.

[0005] Based upon the above-mentioned considerations, the Applicant believes that there is significant scope for improvements in the design of bellmouths for jet fans, based upon automatic optimisation using Computational Fluid Dynamics.SUMMARY OF THE INVENTION

[0006] According to one aspect of the invention, there is provided a fan assembly for installation in a tunnel or car park to provide ventilation in the tunnel or car park, the fan assembly comprising: a fan rotor for generating a ventilating flow, the inflow into the fan rotor being substantially parallel to the outflow from the fan rotor; the nozzle has a trailing edge at the distal end from the fan; the fan assembly is arranged or arrangeable such that a ventilating flow generated by the fan will pass through the nozzle throughbore before exiting the assembly to enter a space to be ventilated; the angle made between the nozzle trailing edge and a centreline of the fan is not perpendicular; a bellmouth is attached to the nozzle trailing edge; the cross-sectional area of the bellmouth throughbore decreases from the location of its attachment to the nozzle in the direction away from the fan, to a minimum cross-sectional area, wherein: the bellmouth includes: a first curved section at the distal end of the fan that has a variable radius; a second curved section that is flatter than the first curved section; and a third curved section that has a variable radius and which is generally parallel to the profile of the rest of the bellmouth, at its point of attachment to the rest of the bellmouth.

[0007] The present invention provides a solution to the technical issue of how to avoid flow separation at the lower part of an inclined inlet to a jet fan silencer, while reducing or eliminating any underhang of the bellmouth beneath the level of the silencer. The lower part of the inclined inlet is connected to the shortest edge of the nozzle throughbore.

[0008] The Applicant’s automatic optimisation using Computational Fluid Dynamics has indicated that the optimum shape of the bellmouth at the distal end from the fan comprises the following connected parts: (1) a curved part with avariable radius at the distal end of the fan; (2) a part that is flatter than the first part; (3) a curved part with variable radius that blends smoothly with the rest of the bellmouth; (4) an optional part exhibiting a constant-area cross-section, which is used to deflect the flow on the discharge side in order to reduce the Coanda effect (please see the paragraph below); and (5) an optional radiused part which smoothly turns the inlet flow towards the lower part of the silencer (which comprises a shaped nozzle). All transitions between the different portions of the bellmouth must be smooth, with no discontinuities that may disturb the flow.

[0009] Although the turning of the discharge flow, and the subsequent overcoming of the Coanda effect, would occur even without the optional part (4) mentioned in the paragraph above, our Computational Fluid Dynamics calculations suggest that it is preferable to include part (4) in the design of the bellmouth, to reduce the pressure drop through the bellmouth.

[0010] The relatively flat part of the bellmouth (part (2) described above) is useful from a production perspective, because it can be used to hold onto the bellmouth while the rolling or spinning of the component is undertaken. Due to the presence of this part, our invention may be colloquially termed a “bullnose bellmouth”.

[0011] The optimum dimensions of the five parts of the bellmouth described above are associated with aerodynamic and fan parameters, namely the boundary layer thickness (which affects the propensity for the flow to separate from the bellmouth) and the fan diameter, which affects the required length of bellmouth to deliver a certain jet deflection on the discharge side, and is controlled by part (4) of the bellmouth as described above.

[0012] The Applicant observed from their Computational Fluid Dynamics simulations that the flow in the vicinity of the lower edge of the intake silencer, and its propensity or otherwise to separate, is essentially two-dimensional, and is not significantly influenced by the flow-field elsewhere. The minimum required vertical height of a bellmouth generally scales up and down in proportion to the jet fan diameter.

[0013] The bellmouth is preferably arranged to be rotationally symmetrical about its own central axis. Such a geometry is readily manufactured using standard spinning and rolling production techniques.

[0014] The above discussion is predicated upon the assumption that bellmouths are installed on both sides of a fan. However, benefits can also be obtained by installing the bellmouth only on one side of the fan, since the bellmouth is effective at reducing the inlet pressure drop on the inlet side, and in reducing the Coanda effect on the discharge side.

[0015] While the description refers to both “nozzle” and “silencer”, it will be appreciated that the terms are essentially interchangeable in this context, as discussed previously.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] A number of preferred embodiments of the present invention will now be described by way of example only, and with reference to the accompanying drawings, in which like reference numerals are used for like components throughout the figures, in which:

[0017] Figure 1 shows a vertical section through an embodiment of this invention with a bellmouth as described in our description connected to a silencer (comprising a shaped nozzle), where the silencer is designed to be attached to a fan casing;

[0018] Figure 2 shows an embodiment of this invention with a bellmouth as described in our description, with labels on each distinct part of the bellmouth;

[0019] Figure 3 shows an embodiment of this invention with a bellmouth as described in our description, with radii indicated along the curves;

[0020] Figure 4 shows further geometrical details of the bellmouth described in this invention; and

[0021] Figure 5 shows the angles subtended by different parts of the bellmouth curves.DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0022] Referring to Figure 1, this shows a sectional side view of an embodiment of the present invention, with a bellmouth (12) attached to a silencer (7). The silencer’s centreline (9) is designed to coincide with the centreline of the fan housing, to which it is intended to be attached. The angle (11) between the centreline and the normal to the trailing edge of the silencer (13) is arranged such that the angle between the centreline and the trailing edge of the silencer is non-perpendicular.

[0023] In the embodiment shown in Figure 1, the bellmouth is arranged to be rotationally symmetrical about its own axis (14).

[0024] Due to the tilted trailing edge (13) of the silencer, the internal surface of the silencer (6) forms a non-cylindrical nozzle. The air is designed to flow through the nozzle throughbore (8).

[0025] The bellmouth geometry (12) is designed such that the lower parts of the bellmouth exhibit either no flow separation, or minimal flow separation, when the silencer is attached to the fan intake.

[0026] The bellmouth (12) is arranged such that there is no vertical underhang, or only minimal vertical underhang, of the bellmouth beneath the silencer thickness (10).

[0027] In the depicted cross-section, the invented bellmouth (12) can be described as a contiguous series of curves (1, 2, 3, 4 and 5), the last of which (5) smoothly guides the flow to the lower part of the intake silencer (6).

[0028] Referring to Figure 2, this figure provides more detail on the curves comprising the invented bellmouth, in the context of the cross-sectional view presented.

[0029] The bellmouth comprises a first curved section (1) with a variable radius at the distal end of the fan. The first curved section leads to a second curved section (2) that is flatter than the first section (1). The second curved section leads to a third curved section (3) with a variable radius that blends smoothly with the rest of the bellmouth. The third curved section (3) leads to a straight or flat section (4). The straight section is dimensioned to deflect the flow on the discharge side in order to reduce the Coanda effect. Finally, the straight section (4) leads to a fourth radiused curve section (5) which smoothly turns the inlet flow towards the lower part of the intake silencer (6). Curved section (5) is parallel to the lower part of the silencer (6) at the location where they connect.

[0030] Each of the above-mentioned curved sections (1 to 5 inclusive) is rotated about the bellmouth centreline (14) to form a corresponding surface, and the combination of these surfaces forms the inner surface of the bellmouth.

[0031] Figure 3 is a detailed view of one example embodiment of a variable radius bullnose bellmouth. In this embodiment, the fan has a diameter of 720 mm and the silencer has a thickness of 100 mm. The calculated radii values along the bellmouth curves are shown in the figure. The trailing edge angle is 20 degrees.

[0032] In Figure 3, it may be noted that the second curved section of the bellmouth has a peak radius of 275 mm, compared to a peak radius of 53 mm for the first curved section. Our analysis has indicated that in general, the peak radius of the second curved section should be at least twice the peak radius of the first curvedsection. The larger radii in the second curved section imply that it is flatter than the first curved section.

[0033] In Figure 3, the peak radius of the third curved section (42 mm) is somewhat less than the peak radius of the first curved section (53 mm). Our analysis has shown that in general, the peak radius of the third curved section should be equal to, or up to 50% below, the peak radius of the first curved section.

[0034] Figures 4 and 5 are detailed views of another example of a variable radius bullnose bellmouth, In this embodiment, the fan has a diameter of 400 mm and the silencer has a thickness of 50 mm. The calculated radii and minimum radii values along the bellmouth curves are shown in the figure. The approximate angle of curvature of the bellmouth B is 8 degrees. The trailing edge angle in this case is 17 degrees. The approximate angles of the respective relevant curved sections are shown in figure 5.

[0035] It would be possible to modify an existing fan assembly in order to fit silencers and bellmouths as described in this invention to one or more sides of a fan, and hence reap the benefits of improved performance.

[0036] It will be appreciated that the foregoing are merely an examples of embodiments and just some examples of their use. The skilled reader will readily understand that modifications can be made thereto without departing from the true scope of the inventions.

Claims

CLAIMS:

1. A fan assembly for installation in a tunnel or car park to provide ventilation in the tunnel or car park, the fan assembly comprising: a fan rotor for generating a ventilating flow, the inflow into the fan rotor being substantially parallel to the outflow from the fan rotor; the nozzle has a trailing edge at the distal end from the fan; the fan assembly is arranged or arrangeable such that a ventilating flow generated by the fan will pass through the nozzle throughbore before exiting the assembly to enter a space to be ventilated; the angle made between the nozzle trailing edge and a centreline of the fan is not perpendicular; a bellmouth is attached to the nozzle trailing edge; the cross-sectional area of the bellmouth throughbore decreases from the location of its attachment to the nozzle in the direction away from the fan, to a minimum cross-sectional area; wherein: the bellmouth includes: a first curved section at the distal end of the fan that has a variable radius; a second curved section that is flatter than the first curved section; and a third curved section that has a variable radius and which is generally parallel to the profile of the rest of the bellmouth, at its point of attachment to rest of the bellmouth.

2. A fan assembly according to claim 1, wherein the bellmouth includes a further section that has a constant throughbore cross-sectional area, with a value equal to the minimum throughbore cross-sectional area.

3. A fan assembly according to any of claims 1 and 2, wherein the bellmouth is rotationally symmetrical about its own central axis.

4. A fan assembly according to any one of claims 1 to 3, wherein the bellmouth throughbore is arranged to be parallel to the shortest edge of the nozzle throughbore, at its intended point of attachment to the nozzle.

5. A fan assembly according to any preceding claim, wherein the bellmouth throughbore is arranged as a radiused curve from the point of its intended point of attachment to the nozzle, to the rest of the bellmouth profile.

6. A fan assembly according to any preceding claim wherein two bellmouths are provided, one installed on each side of the fan assembly.

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