GB2084719A - Measuring fluid flow - Google Patents

Abstract

A vortex-shedding flowmeter for measuring the velocity of gas flow in a pipe (2) includes a bluff body (1) from the edges of which vortices are generated at a rate dependent on the gas flow velocity. Also in the pipe (2) there is an optical sensor (3) whose output to a measuring device is a light beam modulated with a frequency dependent on the gas velocity. Preferably the sensor is an optical microphone whose diaphragm is caused to vibrate at the vortex rate, but a pivoted flap which is oscillated by the gas stream could also be used. Vibrations of the diaphragm or flap are sensed by a light source and detector coupled by optical fibre means to a reflective surface of the diaphragm or flap. <IMAGE>

Description

SPECIFICATION Measuring fluid flow This invention refers to vortex-shedding flowmeters, such as used for measuring gas flow in a pipe or the like.

Such flowmeters depend on the fact that if a bluff body is located in a pipe through which gas flows, vortices are generated and are shed from the edges of the bluff body. The rate at which such vortices are generated is dependent on the velocity of flow of the gas in the pipe. This invention has as its object the production of a simple way of responding to such vortices and of giving an output representative of the gas flow velocity.

According to the invention, there is provided a vortex-shedding flowmeter, for measuring the velocity of flow of a gas in a pipe, which includes a bluff body located in the pipe so that gas flow therein produces vortices which are shed from the bluff body, a diaphragm or flap located in the pipe so as to be caused to move in response to vortices due to gas flow in the pipe, said diaphragm or flap having a reflective surface on one of its faces, a light source coupled by an optical fibre to said reflective surface so as to apply a beam of light to that surface, a light detector coupled via an optical fibre to said reflective surface so that the light reaching said detector is modulated in accordance with the rate at which vortices influence said diaphragm or flap, and measuring means associated with said light detector and response to the modulations on the light reaching the detector.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figure 1 is a simple diagram explanatory of the invention, Figures 2 and 3 are two forms of sensor for use in the arrangement of Figure land Figure 4 shows explanatory graphs.

In Figure 1, there is a bluff body 1 in a gas flow pipe, so that vortices are generated due to the presence of the bluff body in the gas flow stream.

These vortices are shed alternately from the top and bottom edges of the bluff body, and the frequency of occurence of these vortices is proportional to, or dependent on, the gas flow velocity.

To measure the vortex rate an optical sensor 3 is installed in the pipe downstream of the bluff body.

This sensor is fitted into the pipe at a specific position relative to the size and shape of the bluff body 1 and the pipe diameter. The optimum position for the sensor can be determined by experiment.

However, although shown as separate from the bluff body 1, it could be situated within or on the body 1, or it could be an integral part of that body.

In one embodiment, see also Figure 2, the sensor is an optical microphone to which light is applied from a light source such as a light-emitting diode (not shown) via one branch 5 of a Y coupler of optical fibres. The microphone has a diaphragm 6, Figure 2, whose inner surface is silvered. The light which reaches the diaphragm is reflected back via the other branch 7 to a light detector (not shown). The sensor is installed in the pipe so that its diaphragm is influenced by the gas flow, and vibrates at a a rate defined by the vortex flow rate. Thus the modulations of the light is dependent on the vortex flow rate, and thus on the velocity of the gas in the pipe.

In an alternative, Figure 3, instead of a diaphragm as used in an optical microphone, we have a flap 8, which is so influenced by the gas stream as to oscillate according to the vortex frequency. This flap is silvered on its inner surface, so its frequency is similar in principle to that of Figure 2.

Experimental results detailed for a 3" air duct at various flow rates, and using an optical microphone as a sensor are shown in Figure 4, from which it will be seen that linearity is maintained at very low flow rates. Note that in Figure 4 the calibrations curver do not pass through gas. This is because the Strouhal number, which in effect determines flow rate, does not remain constant at low Reynolds number. If this is inconvenient it could be sorted out electronically using a microprocessor, in which case very low flow rate measurement is possible.

The above system has the following advantages: (a) Vortex detection is possible at very low flow rates.

(b) Remote sensing using fibre optics is useful when flow rates have to be measured in hazardous conditions.

(c) The sensors used have very low inertia, which improves the detection of very low flow rates.

1. A vortex-shedding flowmeter, for measuring the velocity of flow of a gas in a pipe, which includes a bluff body located in the pipe so that gas flow therein produces vortices which are shed from the bluff body, a diaphragm or flap located in the pipe so as to be caused to move in response to vortices due to gas flow in the pipe, said diaphragm orflap having a reflective surface on one of its faces, a light source coupled by an optical fibre to said reflective surface so as to apply a beam of light to that surface, a light detector coupled via an optical fibre to said reflective surface so that the light reaching said detector is modulated in accordance with the rate at which vortices influence said diaphragm on flap, and measuring means associated with said light detector and responses to the modulations on the light reaching the detector.

2. Aflowmeter as claimed in claim 1, and in which the sensor is an optical microphone whose diaphragm is subjected to the gas flow.

3. A vortex-shedding flowmeter substantially as described with reference to Figures 1, 2 and 4 or Figures 1 and 3 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Measuring fluid flow This invention refers to vortex-shedding flowmeters, such as used for measuring gas flow in a pipe or the like. Such flowmeters depend on the fact that if a bluff body is located in a pipe through which gas flows, vortices are generated and are shed from the edges of the bluff body. The rate at which such vortices are generated is dependent on the velocity of flow of the gas in the pipe. This invention has as its object the production of a simple way of responding to such vortices and of giving an output representative of the gas flow velocity. According to the invention, there is provided a vortex-shedding flowmeter, for measuring the velocity of flow of a gas in a pipe, which includes a bluff body located in the pipe so that gas flow therein produces vortices which are shed from the bluff body, a diaphragm or flap located in the pipe so as to be caused to move in response to vortices due to gas flow in the pipe, said diaphragm or flap having a reflective surface on one of its faces, a light source coupled by an optical fibre to said reflective surface so as to apply a beam of light to that surface, a light detector coupled via an optical fibre to said reflective surface so that the light reaching said detector is modulated in accordance with the rate at which vortices influence said diaphragm or flap, and measuring means associated with said light detector and response to the modulations on the light reaching the detector. Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figure 1 is a simple diagram explanatory of the invention, Figures 2 and 3 are two forms of sensor for use in the arrangement of Figure land Figure 4 shows explanatory graphs. In Figure 1, there is a bluff body 1 in a gas flow pipe, so that vortices are generated due to the presence of the bluff body in the gas flow stream. These vortices are shed alternately from the top and bottom edges of the bluff body, and the frequency of occurence of these vortices is proportional to, or dependent on, the gas flow velocity. To measure the vortex rate an optical sensor 3 is installed in the pipe downstream of the bluff body. This sensor is fitted into the pipe at a specific position relative to the size and shape of the bluff body 1 and the pipe diameter. The optimum position for the sensor can be determined by experiment. However, although shown as separate from the bluff body 1, it could be situated within or on the body 1, or it could be an integral part of that body. In one embodiment, see also Figure 2, the sensor is an optical microphone to which light is applied from a light source such as a light-emitting diode (not shown) via one branch 5 of a Y coupler of optical fibres. The microphone has a diaphragm 6, Figure 2, whose inner surface is silvered. The light which reaches the diaphragm is reflected back via the other branch 7 to a light detector (not shown). The sensor is installed in the pipe so that its diaphragm is influenced by the gas flow, and vibrates at a a rate defined by the vortex flow rate. Thus the modulations of the light is dependent on the vortex flow rate, and thus on the velocity of the gas in the pipe. In an alternative, Figure 3, instead of a diaphragm as used in an optical microphone, we have a flap 8, which is so influenced by the gas stream as to oscillate according to the vortex frequency. This flap is silvered on its inner surface, so its frequency is similar in principle to that of Figure 2. Experimental results detailed for a 3" air duct at various flow rates, and using an optical microphone as a sensor are shown in Figure 4, from which it will be seen that linearity is maintained at very low flow rates. Note that in Figure 4 the calibrations curver do not pass through gas. This is because the Strouhal number, which in effect determines flow rate, does not remain constant at low Reynolds number. If this is inconvenient it could be sorted out electronically using a microprocessor, in which case very low flow rate measurement is possible. The above system has the following advantages: (a) Vortex detection is possible at very low flow rates. (b) Remote sensing using fibre optics is useful when flow rates have to be measured in hazardous conditions. (c) The sensors used have very low inertia, which improves the detection of very low flow rates. CLAIMS

1. A vortex-shedding flowmeter, for measuring the velocity of flow of a gas in a pipe, which includes a bluff body located in the pipe so that gas flow therein produces vortices which are shed from the bluff body, a diaphragm or flap located in the pipe so as to be caused to move in response to vortices due to gas flow in the pipe, said diaphragm orflap having a reflective surface on one of its faces, a light source coupled by an optical fibre to said reflective surface so as to apply a beam of light to that surface, a light detector coupled via an optical fibre to said reflective surface so that the light reaching said detector is modulated in accordance with the rate at which vortices influence said diaphragm on flap, and measuring means associated with said light detector and responses to the modulations on the light reaching the detector.

2. Aflowmeter as claimed in claim 1, and in which the sensor is an optical microphone whose diaphragm is subjected to the gas flow.

3. A vortex-shedding flowmeter substantially as described with reference to Figures 1, 2 and 4 or Figures 1 and 3 of the accompanying drawings.