GB2055726A - Fluid flow device - Google Patents

Abstract

A fluid flow device suitable for producing a jet of fluid for use, for example, in the jet propulsion of a ship, comprises a primary fixed Coanda outlet with a pair of flaps 8 directing the primary flow either side of the central profile 9. The flaps 8 extending downstream of the outlet to form secondary outlets and are movable in a direction transverse to the fluid flow. During passage of fluid through the primary outlet at an increasing rate the flaps a tend to reduce the effective cross sectional area of the secondary outlets. The fluid flow around the profile 9 draws a further fluid flow in through a throat 10. Other applications include a Coanda eductor (Figure 2, not shown), a heat exchanger (Figure 4, not shown) and fluid mixing in bulk storage tanks. <IMAGE>

Description

SPECIFICATION Fluid flow device The present invention relates to fluid flow devices.

It is known to use narrow slots to produce jets of fluid which may be used for propulsion purposes. It is usually found that the exit slot of the nozzle is very sensitive to surface irregularities and in general the quality of manufacture in respect of condition, shape and geometry has a marked effect on the performance of the nozzle apart from adding to manufacturing costs. Furthermore in the case of narrow slots producing high velocity fluid jets, total or partial blockage by foreign matter in the fluid due to the small cross section can cause a considerable disturbance to fluid flow.

The present invention provides a flow device in which a slot or orifice is modified by means of an additional component to give improved fluid flow.

Thus, according to the present invention there is provided a fluid flow device suitable for producing a jet of fluid comprising a primary fixed outlet capable of passing a fluid and having one or more flaps attached at or near the outlet, the flaps being capable of extending downstream of the outlet to form a secondary outlet or outlets, at least one flap being movable in a direction transverse to the fluid flow, whereby during passage of fluid through the primary outlet at an increasing rate the flap or flaps tend to reduce the effective cross sectional area of the secondary outlet or outlets.

The device may be used for a variety of purposes including its use as a nozzle producing a jet of fluid, as a means for propulsion of a vessel such as a boat or on a hydrofoil and as means for mixing the contents of a vessel, in the latter case the absence of moving parts being advantageous.

The flaps are fixed at one end and the other end may move relatively, for example, by use of a flexible flaps or by use of a rigid flap which is hinged or otherwise movably attached to a fixed surface, for example, the fluid supply line. At higher flow rates it is preferred to use a rigid flap which is more robust and less susceptible to sympathetic vibrations with the flow. The materials of construction of the flaps may comprise, for example, plastics materials and metals such as stainless steel which are corrosion resistant. Preferably the hinge or attachment portion of the flap is positioned at the primary outlet although in certain applications it may be positioned at a distance away from the primary outlet.

In a homogeneous system for example where a liquid passes out from the primary outlet into a surrounding liquid phase, then it appears that the high speed outlet jet has sufficient kinetic energy to decrease the pressure such that the flap tends to narrow the secondary outlet gap and give a smoother flow. In a non homogeneous system, if the primary outlet flow is of greater density than the surrounding phase e.g. with liquid primary outlet flow into surrounding air, then the flap or flaps are biassed, for example, by springs in orderto modify the closure characteristics of the flaps.

It is known that when the extension of one lip of the mouth of a slot through which a fluid emerges under pressure, prog ressively.diverges from the axis of the slot, the stream of fluid emerging through the slot tends to stick to the extended lip thus creating a pressure drop in the surrounding fluid thus causing fluid flow towards the low pressure region. The physical phenomenon is known as the Coanda effect and a body exhibiting this effect is known as a Coanda body. A Coanda nozzle may be defined as a nozzle capable of discharging a fluid at high pressure into another fluid of low pressure through a narrow slot of chosen dimensions having a surface of a Coanda body substantially contiguous with one wall of said slot. A suitable form of Coanda nozzle is described in our British Patent No. 1278577.The Coanda body usually is of (a) the internal Venturishaped type in which the pressurised fluid emerges from an orifice near the throat of the venturi - and passes towards the mouth of (b) the external type in which the pressurised fluid emerges from an orifice and passes outwards over an external director surface of a Coanda body. When using a Coanda nozzle, a single flap may be used, the fluid emerging from the primary outlet between the flap and the Coanda body.

The primary outlet gap width is dependent upon the application. It has been noted that as the flap size decreases there is less frictional resistance and a suitable ratio of primary outlet gap width to length of the flap is 1:5.

Apart from their use in propulsion applications, it is also envisaged that the flow devices may be used where a linear fluid jet is required, for example, in heat exchange applications where liquid flow is necessary at the liquid/solid interface, fluid mixing in bulk storage tanks, aero foil or hydrofoil sections where controlled flows of different velocities are required.

The invention will now be described by way of example only.

Figure 1 shows a nozzle arrangement having a pair of surrounding flaps according to the invention.

Figure 2 shows an internal Coanda ejector layout having flaps according to the invention.

Figure3shows an external Coanda nozzle using twin flaps and slots in a propulsion unit.

Figure 4 shows a use of the flap for providing a linear jet over a surface.

Figure 5 shows a form of water/air nozzle with spring compensation.

Figure 1 shows a simple linear nozzle having primary outlet of a row of five circular orifices 2 one above the other. A pair of flaps 3 are mounted parallel and spaced apart from the row of orifices 2.

The flaps 3 are rigid and hinged to the surface of the nozzle 1 so that they can move freely toward and away from the nozzle orifices. During use of the nozzle when immersed in a liquid, liquid emerging from the orifices tend to draw the flaps 3 towards each other due to the relative balance between the dynamic and static pressures acting on the flaps, thereby producing a linearjet of liquid at the secondary outlet.

In Figure 2 a Coanda eductor is shown with hinged flaps 4, one on each side of primary outlet in the form of a Coanda slots 5, as the velocity of the primary fluid increases from Coanda slots 5, the flaps 4 are drawn in and this results in a high speed linear jet that clings to the walls of the eductor and passes through the throat 6 of the eductor. The resultant high speed primary linear jets cause secondary fluid to be drawn through the eductor. As the primary flow increases, the flaps 4 are drawn in further, thus maintaining efficient and frictional control of the jet.

In Figure 3 the slot 7 and flaps 8 show another arrangement using twin flaps within a single device which acts as a propulsion unit. The primary outlet in the form of slot 7 and flap nozzles 8 direct the primary flows either side of the central profile 9 of the device. In the example shown, the flow passes through a 'throat' 10 which simulates the conditions in the throat of an eductor as in Figure 1 and protects the resulting low pressure in the throat which is important to the operation of drawing through the secondary flow.

Figure 4 shows how the flaps 11 can be applied to provide a linear jet close to a solid surface 12 for purposes such as heat exchange requirements.

Figure 5 shows how a linear nozzle can be used in a water in air' situation with balance between fluids being achieved by an added spring force 13 applied to the outside of the flaps 14. In the example shown, water is the fluid emerging from the intake 15 nozzle into air.

Claims (11)

1. Afluid flow device comprising a primary fixed outlet capable of passing a fluid and having one or more flaps attached at or near the outlet, the flaps being capable of extending downstream of the outlet, to form a secondary outlet or outlets, at least one flap being movable in a direction transverse to the fluid flow, whereby during passage of fluid through the primary outlet at an increasing rate the flap or flaps tend to reduce the effective crosssectional area of the secondary outlet or outlets.

2. A fluid flow device according to claim 1 comprising a primary outlet in the form of orifices in a fluid supply line, there being a pair of flaps mounted on the fluid supply line and on opposite sides of the orifices

3. Afluid flow device according to claim 2 in which one or both flaps are hinged to the supply line.

4. Afluid flow device according to claim 2 in which one or both flaps are fabricated from a flexible material.

5. Afluid flow device according to claim 1 in which the primary outlet is in the form of an elongate outlet in a fluid supply line, there being a flap or flaps mounted along an edge of the outlet.

6. A fluid flow device according to any of the preceding claims in which the primary outlet comprises an outlet of a Coanda nozzle.

7. A fluid flow device according to any of the preceding claims in which the flap or flaps are biassed so as to modify the flap closure characteristics.

8. Afluid flow device according to claim 7 in which the flap or flaps are biased by a spring or springs.

9. Afluid flow device as hereinbefore discussed with reference to Figures 1 to 5 of the accompanying drawings.

10. Vehicle or boat propulsion means comprising fluid flow devices according to any of claims 1 to 9.

11. Fluid mixers comprising fluid flow devices according to any one of claims 1 to 9.