Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Patent
1989-08-07
1991-01-15
Goldstein, Herbert
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
G01F 132
Patent
active
049844709
DESCRIPTION:
BRIEF SUMMARY
This invention relates to vortex-shedding flowmeters.
A vortex-shedding flowmeter consists essentially of two components: a "bluff body", the function of which is to generate a double stream of Karman vortices in the flowing fluid and an electrical device for detecting those vortices and transmitting an electric pulse to an electronic counter every time it senses the passage of a vortex. Ideally, the rate of emission of vortices and hence of electric pulses is directly proportional to the fluid flowrate past the bluff body. The extent to which this ideal is attained is known as the "linearity" of the flowmeter.
In conventional commercially-available vortex-shedding flowmeters, the bluff body takes the form of a metal bar extending diametrally across the pipe. These are generally made with a "blockage ratio"--i.e. the fraction of the cross-sectional area of the pipe that is obstructed by the bluff body--of around 30 to 40 percent. This results in a rather high pressure loss, which is disadvantageous, but attempts to construct such meters with substantially lower blockage ratios have not been successful, because their performance deteriorates when the blockage ratio is reduced.
Another disadvantage of such meters is that the flow past the bluff body is not axi-symmetric. This causes flow instabilities which result in irregular shedding of vortices. Measurements with several conventional meters have shown that the intervals between successive pulses vary so widely that their standard deviation is generally at least 10 percent of their mean value. This results in the rather poor repeatability which is characteristic of such meters.
Attempts have been made to overcome these disadvantages of the bar-shaped bluff body by using a ring-shaped bluff body, which it was thought would preserve the axi-symmetric nature of the flow past it, when installed co-axially in a pipe, and several patent applications for such meters were filed. But the ring-shaped bluff body was found not to give the performance that had been hoped for, and it is therefore still not utilised in practical meters.
With a ring-shaped bluff body, vortices are shed alternately from the inside and outside surfaces of the ring, thereby forming two streams. The essence of the invention is the discovery that the performance of a bluff body in the form of a vortex-shedding ring can be greatly improved if steps are taken to keep these two streams of vortices separate during the early stages of their formation, thus preventing them from mutual interference. This separation is accomplished according to the invention by providing a physical barrier between the two vortex streams. This barrier may be solid or it may be a fluid barrier, as will be explained hereinafter.
According to the invention we therefore provide a vortex-shedding flowmeter including an annular or ring-shaped vortex-generating bluff body, characterised by means to provide a barrier which at least delays mutual interference between the two streams of vortices generated respectively at the inside and outside surfaces of the bluff body.
The said means may provide either a solid or a fluid barrier.
In the case of a solid barrier the means preferably comprises a tubular or ring-shaped vortex separator so arranged that it keeps the two vortex streams physically separate. A fluid barrier, on the other hand, is preferably provided by a second ring downstream of the bluff body and coaxial with it. A stagnation zone in the flowing fluid forms between the two rings and if the relative inside and outside diameters of the two rings and the spacing between them are correctly chosen, this stagnation zone will lie between the two streams of vortices and act as a fluid barrier to keep them apart.
The ring-shaped bluff body is preferably circular when viewed in the direction of flow, so that its inside and outside circumferential surfaces are cylindrical, but it may be of other shapes, e.g. square, octagonal, hexagonal, or elliptical. Where a solid vortex separator is provided it is also preferably circular w
REFERENCES:
patent: Re31217 (1983-04-01), Rodely
patent: 3927564 (1975-12-01), Ketelsen
patent: 3972232 (1976-08-01), Miller et al.
patent: 4455877 (1984-06-01), Blechinger et al.
patent: 4584883 (1986-04-01), Miyoshi et al.
Cousins Terence
Hayward Alan T. J.
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