GB2183046A - A flowmeter - Google Patents

Abstract

A fluid flowmeter comprises a sensor in the form of a plain disc (2) and a jet (1) which directs fluid adjacent the disc, this acts on the disc via the fluid boundary layer. Drag on the rotor is very small enabling the flowmeter to measure low flow rates. A plurality of discs and jets may be used. Either the disc(s) rotates and its speed is sensed or the disc(s) is stationary and the force exerted on it is measured. <IMAGE>

Description

SPECIFICATION A flowmeter FIELD OF THE INVENTION The present invention relates to fluid flowmeters.

BACKGROUND OF THE INVENTION In general, known paddle or axial flowmeters comprise a metering chamber in which is housed a turbine in the form of a rotor having radially directed, axially extending blades or vanes.

Fluid to be metered is fed into the chamber through a nozzle or jet to impinge upon the blades or vanes to drive the turbine, the consequent rotation thereof being detected to provide the metering measurement required.

Many flowmeters of the above type have been produced but none to-date that successfully operate at extremely low fluid flow rates.

At low fluid flow rates the amount of energy available from the liquid being metered to rotate the turbine is very small leading inter alia to fluid "slip" around the blades of the turbine with consequent lack of rotation thereof. In this regard the viscosity of the fluid, the quality of the bearings and the fluidic efficiency of the turbine itself all have a part to play.

To counteract this problem flowmeters are available which employ multi-inlet jets or nozzles directed at the turbine blades but unfortunately, at low flow rates, this becomes undesirable as the volume of the liquid entering the metering chamber thereby becomes many time the volume of the chamber itself which is counter-productive in its desired result.

Thus the art has had to be content with flowmeters with single inlet jets the amount of available energy from which, at low flows, is expected to "paddle" the turbine in a large metering chamber containing a relatively viscous fluid and so move quite large volumes of the liquid. There is also the inertia of the turbine which has to be overcome, and the static friction of the bearings of the turbine which is a product of the mass of the turbine.

Flowmeters have been produced which reduce the bearing friction and the volume of the chamber. However it has not been possible to produce a turbine that does not have a relatively high drag at low velocities of fluid flow.

SUMMARY OF THE INVENTION It is an object of the invention to obviate the difficulties of the prior art as outlined above.

To this end it is to be noted that if a plain disc (or other object) is in movable contact with a fluid the fluid at the disc surface travels with the disc so creating a boundary layer around the disc. This boundary layer can be quite thick at low velocities, and it has been discovered that this can act as a very useful "key" to rotate a flowmeter turbine if a jet of fluid is directed at it thus reducing drag particularly at low flow velocities. The fast stream of fluid leaving the jet, which is placed very close to the disc, hits the fluid which is in contact with the surface and so rotates the disc.

According to the present invention there is provided a flowmeter comprising a metering chamber, metering-detector means in the chamber, and means for directing fluid to be metered in the form of a jet at an area adjacent a surface of said metering-detector means in which a fluid boundary layer is formed during operation of the flowmeter.

The metering-detector means is preferably in the form of a rotatable plain disc and to increase the efficiency of the device multiple discs may be used, these being spaced to suit the viscosity of the fluid.

The jet or jets in this case consist of a fine tube (similar to a hypodermic needle) located between the discs. This ensures that all of the fluid leaving the jet(s) hits the turbines and therefore none is "split".

Additionally as the discs are plain and have a good surface finish, they offer very little viscous drag as they do not displace any fluid rather only "shear" two layers. Sufficient clearance however is to be allowed around the discs to ensure that the stationary boundary layer in the housing is not directly disturbed by the "moving" boundary layer on the disc.

The discs themselves are preferably of the lowest practical mass and should be mounted on very low friction bearings.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1 shows on a microscope level for purposes of illustration the velocity gradient of a fluid adjacent a disc spinning in the fluid; Figure 2 shows in side and end view a single turbine disc being rotated by a single jet of fluid as an embodiment of the invention; Figure 3 shows in side and end view a jet of fluid directed between two turbine discs as a further embodiment of the invention.

Figure 4 shows in side and end view a multiple disc turbine with fluid delivering jets interposed between the discs according to a still further embodiment of the invention; and Figure 5 shows in detail a preferred form of turbine blade for use with the invention.

BEST MODES OF CARRYING OUT THE INVEN TION In Figure 1 the velocity gradient adjacent to a rotating disc D within a fluid, is depicted.

This shows that the fluid at a distance x from the disc D is stationary while the fluid in contact with the disc D is travelling at the same velocity a) as the disc D.

The thickness of R and the gradient of the remaining slope are dependent upon the Reynolds number of the fluid in which the disc D is rotating. This includes such variables as the velocity of the disc D and the viscosity and density of the fluid.

The invention makes use of this phenomenon as illustrated in Figure 2. Here the fluid whose flow rate is to be metered is introduced into the chamber of the flowmeter (not shown) by means of a single jet 1.

The turbine of the flowmeter in this embodiment comprises a disc 2 rotatable about an axis 3 within the flowmeter chamber.

The direction of the jet 1 is such that it directs the fluid into those areas around the disc where forms the fluid boundary layer as the disc rotates, and not directly at and normally to, its rotating surfaces in contrast to the paddle wheel turbine discussed herein with reference to the prior art.

Two positions 4 and 5 are shown for the jet in Figure 2, position 4 directing the jet into the boundary layer at the sides of the disc 2 while position 5, shown in dotted outline, directs the jet against its peripheral edge of the disc 2.

The possibility of having a pair of jets 4 and 5 acting together in the disc 2 to provide rotation with reduced drag at low fluid velocities, must not of course be precluded as an operating possibility.

It has been shown however that the arrangement of Figure 2, while being operationally suitable, lacks efficiency since some of the jet stream is diffused into the stationary fluid in the chamber.

This disadvantage is removed by the arrangement shown in Figure 3, wherein two coaxially mounted discs 2 are used with the jet 1 being interposed between the discs as shown. The jet 1 is positioned such that it lies between the common rotating axis and respective peripheries in a plane parallel to both discs 2 and with its stream directed into the fluid boundary layers at adjacent surfaces of the discs.

This avoids the diffusion effects referred to with respect to the Figure 2 embodiment.

To improve the drive effect still further multiple discs 2 may be employed as shown in Figure 4, with jets in between each disc pair in the manner described with reference to Fig ure 3.

To construct the flowmeter in an ideal world, the discs 2 would have zero mass and thickness and be supported on bearings with zero friction.

The above is obiousiy impossible to achieve. However the turbine construction shown in Figure 5, is an attempt to come as nearly as practically possible to these ideal conditions.

This comprises a pair of discs 6 shown in Figure 5 rotating in a fluid 7. The material of the discs 6 is as close as practical to the density of the fluid 8. They are mounted on saphire bearings 9 which provides the lowest possible contact area with the hardest possible contact surface.

The discs 6 are connected by a central core 10 having a concave peripheral surface 11, the core 10 being of lesser diameter than the discs 6.

The discs 6 taper to a peripheral edge 12, and a jet 13 for delivering the driving liquid is positioned between the disc 6 adjacent the surface of the core 10.

As with the other preferred embodiments herein described, the position of the outlets of the jets 1, 13 is chosen such that a maximum rotational force or torque is imparted to the discs 2, 6 by interaction of the jet stream with the liquid boundary layer around the discs 2, 6.

This position will correspond to that farthest distance from the axis of the discs 2, 6 but within their peripheral boundary, whereat all the fluid issuing from the jets 1, 13 is used to interact with the boundary layer.

Although the flowmeter employs a rotatable turbine in the examples given here nevertheless it is to be understood that this component of the meter could be a stationary element. In this case fluid flow would be measured by detecting the force of reaction set up in the disc caused by the interaction between the fluid boundary layer and the jet stream. This is in contrast to a rotating turbine where rotation would be detected by any convenient zero drag detector leading to a determination of the velocity of fluid flow.

Variations to the preferred embodiments of the invention as herein disclosed will be evident to those skilled in the art without the need to exercise further inventive ingenuity.

Moreover the invention may also be used in situations where it is necessary to measure high fluid flows and is not limited to low flow applications.

Claims (8)

1. A flowmeter comprising a metering chamber, metering-detector means in the chamber, and means for directing fluid to be metered in the form of a jet at an area adja cent a surface of said metering-detector means in which a fluid boundary layer is formed during operation of the flowmeter.

2. A flowmeter as claimed in claim 1 wherein the said metering-detecting element is in the form of at least one rotatably mounted planar disc.

3. A flowmeter as claimed in claim 2 wherein there are a pair or more pairs of said planar discs, a said jet being interposed between a said pair of discs.

4. A flowmeter as claimed in claim 3 wherein the discs have peripheries which taper to an edge, each pair of discs being joined by a central core of lesser diameter than the discs.

5. A flowmeter as claimed in claim 4 where a said jet is interposed between the discs adjacent the surface of the core, the outlet of the jet being in a plane containing a diameter of each of the discs.

6. A flowmeter as claimed in claim 5 wherein the radial position of said outlet with respect to the common axis of a disc pair is such as to impart maximum rotational torque to said pair by interaction as between the stream of fluid from the jet and said boundary fluid layer around each disc of the pair.

7. A flowmeter as claimed in any one of claims 2 to 6 wherein the disc or discs are mounted on sapphire bearings.

8. A flowmeter substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.