This invention relates generally to flowmeters of the vortex-shedding type, and more particularly to an obstacle assembly for such flowmeters which is adapted to enlarge the effective operating range of the meter and to improve its accuracy and repeatability.
It is well known that under certain circumstances the presence of an obstacle in a flow conduit will give rise to periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed. These patterns are known as Karman vortex streets. The frequency at which vortices are shed in a Karman vortex street is a function of flow rate. It is this phenomenon which is exploited to create a flowmeter. Flowmeters of this type are disclosed in the Bird patent, U.S. Pat. No. 3,116,639, and in the White patent, U.S. Pat. No. 3,650,152.
In a vortex-shedding flowmeter, the frequency of shedding is precisely related to the velocity of fluid passing through the flow tube containing the shedding body, but only as long as the separation point from which shedding takes place remains fixed and the feedback mechanism causing shedding to transfer from one side of the body to the other remains constant.
In its most elementary form, the shedding body is a simple cylinder mounted across the flow tube. The difficulty experienced with this type of shedding body is that the separation point (i.e., the location at which vortices leave the body) shifts with Reynolds numbers. As a consequence, the vortex trail tends to meander down the flow tube behind the shedding body. If the angle of this vortex trail changes, the feedback mechanism causing shedding to take place from alternate sides of the shedding body also undergoes change, thereby giving rise to deviations from the predicted frequency of the shedder. As a result, meter accuracy and meter repeatability are poor.
Vortex meters are commercially available having shedding bodies which are designed to overcome these drawbacks by optimizing the shedding body width and geometry in relation to the flow tube size. The U.S. Pat. No. 3,572,117 to Rodely discloses a bluff body flowmeter having a prescribed geometric configuration designed to minimize irregularities in the oscillating wake. These meters constitute an improvement over meters having cylindrical shedding bodies. However, under less-than-ideal operating conditions, the vortex wake or trail created by these non-cylindrical shedding bodies will still, on occasion, become intermittent or meander, to produce the same disadvantages encountered with cylindrical bodies.
The Burgess patent, U.S. Pat. No. 3,589,185 discloses an improved form of vortex-type flowmeter wherein the signal derived from the fluid oscillation is relatively strong and stable to afford a favorable signal-to-noise ratio insuring accurate flow-rate information over a fairly broad range. In this meter, an obstacle assembly is mounted in the flow conduit, the assembly being constituted by a block positioned across the conduit with its longitudinal axis at right angles to the direction of fluid flow, a strip being mounted across the conduit behind the block and being spaced therefrom to define a gap which serves to trap Karman vortices and to strengthen and stabilize the vortex street. This street is sensed to produce a signal whose frequency is proportional to flow rate.
In another Burgess patent, U.S. Pat. No. 3,888,120 dealing with a vortex-type flowmeter, there is disclosed an obstacle assembly constituted by a fixed front section contoured to cause flow separation of the incoming fluid stream whose flow rate is to be measured, and a rear non-streamlined section which is shaped to interfere with the vortex street in the wake of the front section and is cantilevered from the front section to define a gap. The rear section is slightly deflectable relative to the front section whereby it is excited into minute vibrations by the vortex street. These vibrations are sensed by a strain gauge to produce a signal proportional to flow rate.
The liquid vortex flowmeter Model 10 LV 1000, manufactured by the Fischer & Porter Company of Warminster, Pennsylvania, the assignee herein, operates in accordance with the principles set forth in Burgess, U.S. Pat. No. 3,888,120. This liquid vortex flowmeter constitutes a commercially successful version of a vortex meter utilizing a two section shedder to create a vortex street. It is an excellent flowmeter whose rate accuracy on low viscosity fluids, such as water, within a broad operating range is about 2%.
However, some flow rate measuring applications require a higher order of accuracy and a still broader operating range. Also, in some applications the fluid being measured is subject to viscosity changes, turbulence and other disturbances which adversely affect the accuracy of the readings obtained with meters of the 10 LV 1000 type.