GB2102953A

GB2102953A - Flow meters and resistance elements therefor

Info

Publication number: GB2102953A
Application number: GB08218686A
Authority: GB
Inventors: Jurgen Bohm
Original assignee: Bopp and Reuther GmbH
Current assignee: Bopp and Reuther GmbH
Priority date: 1981-06-27
Filing date: 1982-06-28
Publication date: 1983-02-09
Also published as: NL8201316A; FR2508634A1; IT1190771B; GB2102953B; IT8220721A0; DE3125381A1; DE3125381C2; FR2508634B3

Abstract

A residance element 1 for a vortex flowmeter has a cross-sectional contour comprising a rectangular portion 3 followed by a step 6, 7 and then a trapezoidal portion 4 situated beyond the former in the direction of flow and narrowing in the direction of flow, the ratio between the thickness d of the rectangular portion and its height h amounting to 0.3 to 0.6, and the height h of the end of the trapezoidal portion 4 delimiting the rectangular portion 3 being in the ratio of approximately 3:4 to the height H of the rectangular portion 3. <IMAGE>

Description

SPECIFICATION Flow meters and resistance elements therefor The present invention relates to flow meters incorporating a resistance element situated transversely to the direction of flow in the pipe and generating vortexial flow, of the kind which have a section in cross-section made up of different geometrical individual surfaces, the frequency of the detached Kármám vortices being detected by a scanning system. Hereinafter such resistance elements will be referred to as "of the kind described".

In the case of vortex counters, it is known that resistance units may be utilised for separation of the Kármán vortices, which have either a circular, triangular, rectangular, trapezoidal or analogous geometrical cross-section. It was observed that the separation of vortices and thus also the error graph of a vortex counter, may be affected by the geometrical cross-sectional shape of the resistance element. An error graph linearity adequate for precise measurements could not however be obtained up to now with the known resistance element outlines. A comparatively early rise of the error graph may be recorded at low Reynolds numbers in particular.

Vortex counters are also known, comprising resistance elements which have a cross-sectional outline built up from different separate geometrical surfaces. For example the German Patent Specification AS 24 58 901, thus shows a resistance element section having a trapezoidal surface situated at the flow impingement side, and a rectangular surface lying behind the same in the direction of flow. Within the range of moderate Reynolds numbers, the error graph co ordinated with this resistance element still display an excessive curvature, so that no very precise measurements may be performed even with this sectional element. The other known resistance element sections made up of different separate surfaces were also not suitable to provide any satisfactory precision.

It is an object of the invention to obtain an error characteristic for the vortex counter which extends horizontally throughout the mensuration range is possible, and in particular also in the region of the lower mensuration range limit.

Accordingly, the invention consists in a flow meter resistance element of the kind described whose cross-sectional contour comprises a rectangular portion followed by a step, and then a trapezoidal portion situated beyond the former in the direction of flow and narrowing in the direction of flow, the ratio between the thickness of the rectangular portion and its height amounting to 0.3 to 0.6, and the height of the end of the trapezoidal portion delimiting the rectangular portion being in the ratio of approximately 3:4 to the height of the rectangular portion.

The invention also consists in a flow meter incorporating such a resistance element.

The finding taken as a basis in the invention was that a rectangular element acting as a resistance element draws comparatively little energy for the by-pass flow action from the flow as compared to resistance elements having more acute edges inducing the change from laminar to turbulent flow, and nevertheless generates a sufficiently defined vortex separation. It was discovered however that a purely rectangular element of this kind utilised as a resistance element has a Strouhal number depending to a comparatively great extent on the direction of flow impingement.To this end, the variation of the impingement flow direction is caused by travel of the ram point on the impingement flow surface of the resistance element, the oblique flow impingement varying with the Reynolds number because the current lines (or tubes) also have different curvatures at different Reynolds numbers in the approach flow before the resistance element.

To balance this disadvantage of the varying Strouhal number, the rectangular portion of the resistance element is followed by a trapezoidal portion the end of which deiimiting the rectangular portion is narrower than the height of the rectangle, so that a discernible jump occurs at the two points of transition. The dependence of the Strouhal number variation on the impingement flow direction and thus on the Reynolds number, is reduced considerably by this trapezoidal portion.

The "jump" between the rectangular portion and the trapezoidal portion initially allows of an increase of the volume moved by the vortex, and the constriction of the throughflow cross-section is reduced gradually by the sloping surface of the trapezoidal portion, a flow back-up being assured by the departing vortex for the period of passage along the trapezoidal portion. The curvature of the flow lines in the approach flow before the resistance element is kept to a lesser and also approximately constant value by this back-up effect. Furthermore, the trapezoidal portion assures that the vortices detached alternately at the resistance element cannot obstruct each other.

Since the ratio between the thickness of the rectangular portion and its height H amounts to 0.3 to 0.6, it is assured that a minimum of energy is drawn from the vortex during its nascency by the comparatively long rectangular side. An increase of the horizontal extension of the error graph may be obtained for a low flow energy expenditure thanks to the co-operation of the rectangular portion with the trapezoidal portion, i.e. by reduction of the Strouhal number variation as a result of the more protracted stagnation effect caused by the trapezium. It was observed in particular that a remarkable increase of the linear section of the error graph could be obtained in the range of low Reynolds numbers.

Advantageously the two sloping sides of the trapezoidal portion are endowed with the same angle of inclination of 10 to 250 with respect to the axis of the ducting pipe, and the extension of the trapezoidal portion in the direction of flow is dimensioned at 0.5 to 0.7 of the overall longitudinal extension of the resistance element.

An optimum course of the error graph may be set up in this form of trapezoidal portion by virtue of the particularly low-loss vortex guiding action.

In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which show one embodiment thereof by way of example, and in which: Figure 1 shows a vortex counter within the cross-section of the ducting pipe, comprising the resistance element, of the invention, and Figure 2 shows the vortex counter in crosssection along the line lI-Il of Figure 1.

Referring now to the drawings, a resistance element 1 is secured in the centre of a ducting pipe 2. It extends transversely throughout the diameter of the ducting pipe 2 and in its crosssectional outline, shown in Figure 2, comprises a rectangular portion 3 situated in front in the direction of flow, followed by a trapezoidal portion 4. The end 5 of the trapezoidal portion 4 adjacent to the rectangular portion 3 is smaller than the height H of the rectangular portion 3, so that steps 6 and 7 are formed as a result on the resistance element 1. The ratio between the thickness dof the rectangular portion 3 and its height H then amounts to 0.3 to 0.6 and the height h of the end 5 of the trapezoidal portion 4 adjacent to the rectangular portion 3 has a ratio of approximately 3:4 with the height H of the rectangular portion 3.Moreover, the two sloping sides 8 and 9 of the trapeboidal portion 4 have the same angle of inclination cy of 10 to 25 with respect to the axis of the ducting pipe, and the extension 1 of the trapezoidal portion 4 in the direction of flow amounts to 0.5 to 0.7 of the total longitudinal extension L of the resistance element 1.

The Kármán vortices 12 and 13 the frequency of which is a measure for the fluid throughflow in the ducting pipe 2, separate alternately at the edges 10 and 11 of the resistance element 1. In the area of the resistance element 1 are situated two sensor bores 14 and 1 5 in the wall of the ducting pipe 2, which transmit the pressure fluctuations caused by the Kármán vortices 12 and 1 3 into a mensuration passage 1 6 which a sensor 1 7 sensing pressure or speed is located.

This sensor 1 7 transmits the measurement signals, i.e. the frequency of the detached Kármán vortices 12 and 13 to an electronic evaluator system of any desired kind so that it is not shown in the drawings.

Claims

1. A flow meter resistance element of the kind described, whose cross-sectional contour comprises a rectangular portion followed by a step and then a trapezoidal portion situated beyond the former in the direction of flow and narrowing in the direction of flow, the ratio between the thickness of the rectangular portion and its height amounting to 0.3 to 0.6, and the height of the end of the trapezoidal portion deliming the rectangular portion being in the ratio of approximately 3:4 to the height of the rectangular portion.

2. A resistance element as claimed in claim 1, wherein two sides of the trapezoidal section have the same angle of inclination of 10 to 250 with respect to the axis of the ducting pipe, and wherein the extension of the trapezoidal portion in the direction of flow amounts to 0.5 to 0.7 of the total longitudinal extension of the resistance element.

3. A flow meter resistance element substantial as hereinbefore described with reference to the accompanying drawings.

4. A flow meter incorporating a resistance element as claimed in claim 1, 2 or 3.