GB1587713A - Fluidic oscillators - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO FLUIDIC OSCILLATORS (71) We, NORMALAIR-GARRETT (HOLD INGS) LIMITED, of Westland Works, Yeovil, in the County of Somerset, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to acoustic type fluidic oscillators.

An acoustic type fluidic oscillator of suitable design will produce an acoustic output, having a frequency that varies as a function of the speed of sound in the gas or gas mixture employed to drive it. By means of a suitable pressure transducer, this acoustic output can be converted to a signal capable of being counted against a time base and processed electronically as required for control or display purposes. For good accuracy of measurement to be obtained it is desirable that the output signals generated by the oscillator should be at the highest frequencies practicable.

An acceptably functional high frequency acoustic oscillator is one in which the output frequency for a given driving gas or a gas mixture/temperature combination remains substantially constant within a usefully large range of pressure differences across the oscillator, above some minimum pressure differences required to operate it. It has therefore been proposed to provide such an oscillator with a driving gas outlet in the form of an orifice of suitable size and configuration to produce sonic or choked flow conditions therein during operation of the oscillator at a pressure differential above a required minimum, whereby variations in upstream pressure over a suitably extended range will not alter the Mach number of the driving gas or gas mixture in the oscillator inlet to affect the output frequency of the oscillator.

However, oscillators as hitherto proposed, embodying this principle, require an undesirably high minimum pressure differential for many purposes, the reason for this being, of course, that choked flow in a simple orifice requires a pressure ratio of at least 1-89: 1 when the driving gas or gas mixture is mainly diatomic. If the pressure downy stream of the orifice is normal atmospheric pressure, such a device therefore requires a minimum upstream pressure of about 2 bar.

It is an object of the present invention to reduce the overall minimum pressure differential required to operate an acoustic type fluidic oscillator whilst maintaining constant the Mach number of the gas flowing through an inlet jet to the oscillator, within a wide range of absolute upstream pressures of a driving gas, or gas mixture and of changes in the differential pressure across the oscillator.

It is a further object of the invention to provide an acoustic type fluidic oscillator that is suited for use in the discernment of a constituent of a binary gas mixture.

For convenience hereinafter we use the term 'gas' to include both a single gas and a mixture of gases in appropriate contexts.

One aspect of the invention provides an acoustic type fluidic oscillator including a jet entry for driving gas and an outlet for such gas, characterised in that said outlet comprises a pressure recovery venturi adapted to operate with choked sonic flow and having a substantial length of parallel section downstream of a divergent section.

The invention is generally applicable to gas-driven acoustic type fluidic oscillators.

However it is preferably embodied in oscillators of the type constructed to operate by the generation of edge tones, that is, on the vortex-shedding principle. Devices of this type can be of simple and compact construction and can also be made especially insensitive to changes in driving gas pressure by utilising resonance effects to maintain the output frequency under given conditions of driving gas composition and temperature.

Thus, in preferred embodiments, the invention consists in an acoustic type fluidic oscillator comprising a pair of resonance chambers symmetrically disposed on opposite sides of a flow-splitter edge facing a driving gas entry jet, for edge tone generation, at least one of said chambers connecting with an gas outlet including a pressure recovery venturi adapted to operate with choked sonic flow and having a substantial length of parallel section downstream of a divergent section.

Preferably, the resonance chambers are of elongated form and disposed with their major axes parallel with one another and with the extended axis of the entry jet, being conveniently separated from one another by a dividing wall an end of which is wedgeshaped to constitue the said flow-splitter edge.

It has been found to be advantageous to interconnect the resonance chambers at a location remote from the said flow-splitter edge. In the preferred configuration this interconnection is conveniently constitued by a hole or passageway traversing said dividing wall.

Whilst each resonance chamber may be connected to an individual driving gas outlet of the said venturi form, it is of advantage to use only one such outlet because this facilitates the construction of an oscillator with very small dimensions to achieve very high frequency output signals. A single gas outlet may connect with only one resonace chamber; however, we have found that a symmetrical flow path to the outlet from both resonance chambers is to be preferred.

Accordingly, a preferred form of acoustic oscillator has its resonance chamber interconnected at the end of the dividing wall remote from the flow-splitter by a passageway that connects with a driving gas outlet arranged on the axis of the dividing wall so as to be connected with the two resonance chambers by substantially symmetrical flow paths.

The oscillator may be of unitary construction, consisting of a body with cavities and passages formed therein, or it may be formed from a plurality of parts located together and fastened by suitable means. It may be cast in ceramic materials, preferably by a method according to U.K. Patent Speci location No. 1,414,485 issued in the names of the United Kingdom Atomic Energy Authority and the present applicants.

The acoustic output signal may be sensed and converted to an electrical signal by a suitable pressure transducer appropriately arranged to respond to the acoustic signal.

In the preferred embodiments such a transducer conveniently communicates with one of the resonance chambers to respond to changing pressures therein resulting from the acoustic signal. However, because in certain applications of the oscillator pressures may occur in the resonance chambers that would be damaging to a convenient form of transducer arranged to sense the chamber pressure relative to the ambient pressure, the transducer may be protected from damaging differential pressures by being connected to both chambers so as to sense the pressure difference between the chambers, while being isolated from ambient pressure.By this means, the transducer is exposed only to differential pressures of the magnitude of the acoustic signal to be sensed, so that a transducer sensitive to pressure changes of a few inches of water may be used in conjunction with an oscillator in the resonance chambers of which absolute pressures may attain values of two or more orders of magnitude greater than the differential pressure to be sensed.

The pressure transducer may be positioned remote from the oscillator and be connected thereto by suitable conduits. However, it is preferably positioned at a pressure path distance, from the or each resonance chamber, of not more than one wavelength of the highest frequency acoustic signal to be produced by the oscillator. This distance is predetermined by the physical design of the oscillator and the intended operating con -ditions, including the nature of the driving gas or gas mixture to be used.

An oscillator in accordance with the invention may be utilised to monitor or display any characteristic of a driving gas the variation of which characteristic produces a related change in the speed of sound in the gas and, hence, a change in the output signal frequency of the oscillator.

However, an important application of the oscillator of the invention is to the monitoring of changes in the composition of a gas used for driving the oscillator.

Thus, for instance, the oscillator may be driven by a sample of a stream of gas in, say, some production process, the pressure of the stream being utilised to cause the sample gas to flow through the oscillator or, if there is insufficient pressure to force the gas through the oscillator (as may also be the case when the oscillator is applied in monitoring the ambient atmosphere of a laboratory) the sample gas may be drawn through the oscillator by means of reduced pressure effective at the driving gas outlet, as created in one preferred arrangement by jet pump means.

As will be understood, the invention utilises the phenomenon that where a gas flow passes through two orifices placed in series and the flow through the downstream orifice is sonic then a choked condition exists which causes a constant velocity flow condition to exist through the upstream orifice over a wide range of pressure variance of the supply gas, above some minimum pressure sufficient to maintain the sonic flow at the downstream orifice.However, by emp.oying a downstream orifice in the form of a pressure recovery venturi, the pressure ratio required to produce choking of the downstream orifice is reduced from a figure as high as 1-89 to one approaching 1-20, thereby greatly reducing the total required pressure ratio across the oscillator as compared with the total pressure ratio required for operation of previously proposed acoustic type fluidic oscillators. This is of great practical value in applications where toxic or corrosive gases need to be discerned, identified, monitored or analysed, because it reduces to a minimum the mass of gas required to pass through the oscillator in unit time.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic longitudinal sectional view of an oscillator, with diagrammatic representation of associated frequency indicating apparatus and means for drawing gas through the oscillator, and Figure 2 is a diagrammatic representation of an oscillator and associated frequency indicating apparatus in communication with a pressurised gas supply.

An acoustic type fluidic oscillator in accordance with one embodiment of the invention, as shown in Figure 1, comprises a body portion 11 providing a filter (not shown) opening to a gas entry chamber 12 communicating by way of a jet stream nozzle 13 into two elongated resonance chambers 14, 15, having their major axes parallel to the axis of the nozzle 13 and separated by a wall 16 having one end arranged as a splitter 17 facing the nozzle 13. The resonance chambers 14, 15, are interconnected by a passageway 18 formed between that end of the wall 16 remote from the splitter 17 and the internal wall of the body portion 11.The passageway 18 is connected to an outlet chamber 19 by way of a sonic flow outlet 20 of high efficiency venturi form that is aligned with the longitudinal axis of the separating wall 16 and includes a substantial length of parallel section downstream of a divergent section.

Connections are provided in a body enclosure wall or cover plate 21 for a gas entry 22, a gas outlet 23 and a pressure transducer connection 24. The transducer connection 24 provides accommodation for a transducer 32 and positions it effectively within one wavelength distance of its associated resonance chamber 15.

The configuration and physical relationship of the jet stream nozzle 13 and the splitter 17 are to known considerations.

In operation, for example when monitoring for a toxic gas in an environment 30, a sampling gas flow is induced to enter the oscillator by means of operation of a jet pump 31, being drawn in through entry connection 22 and discharged through the outlet connection 23 in the known manner of jet pump arrangements. The sampling gas flow, after entering the oscillator, issues as a jet from the nozzle 13 towards the splitter 17 (in the region of which an inherently unstable condition results) and deflects substantially towards one resonance chamber more than the other until pressure builds sufficiently to switch the jet substantially towards the other chamber and thus a repetitive switching action develops at a frequency dependent upon the gas composition and its temperature.The gas passes from the resonance chambers 14, 15, to the outlet chamber 19 by way of the space 18 and the sonic flow outlet 20 and thence to the throat of the jet pump 31 and associated toxic dispersion or removal means (not shown). The transducer connection 24 accommodates the pressure transducer 32 which is connected by way of an electronic circuit 33 of suitable known form, to some appropriate apparatus 34, such as indicator, warning or control apparatus according to a required end function.

If one assumes that the acoustic output signal frequency of an exemplary device for dry air at 0 C is 10 KHz, then the corresponding frequency for Chlorine is 6215 Hz, (the velocity of sound in the Chlorine being 62-15% of that in the air), whilst the frequency for Hydrogen is 38798 Hz. If the sampled gas is contaminated by Chlorine, or Hydrogen contaiminated by air, the device driven by the sampled gas will produce oscillations at a frequency between, respectively, 10000 and 6215 Hz, or between 38798 and 10000 Hz, appropriate to the concentration of the contaminant.It has been found that the presence of a contaminant gas in which the velocity of sound is considerably lower than in a gas it might contaminate is readily discernable acoustically when only a small contamination occurs, but that where sonic velocity in each of the two gases is nearly the same a larger degree of contamination is required to give a similar degree of response. Thus, the higher the working frequency of the oscillator when driven by a given gas, the finer the discernment obtainable.

Fine discrimination in a system for identifying an unknown gas can be obtained and the effect of temperature substantially eliminated if the beat frequency between two matched sensing oscillators is measured when one of them is supplied with a subject gas and the other with a reference gas of known quality at the same, temperature as the subject gas.

Referring now to Figure 2, an oscillator 40 of similar construction to that hereinbefore described with reference to and shown in Fig ure 1, is arranged, to receive a pressurised gas such as, say, a mixture of superheated steam and oxygen from a high pressure line 41 passing the gas at, for example, a pressure of 500 p.s.i.g. and a temperature of 260"C. An insulated sampling line 42 connects the high pressure line 41 to the gas entry of the oscillator 40 by way of a shut-off valve 43 positioned upstream of a pressure reducing valve 44, whilst the outlet from the oscillator 40 may be arranged to vent by way of an optional adjustable throttle valve 45. A thermo-couple 46 is provided to sense the temperature in the osicllator 40 and is connected to suitable gauge means 47.A pressure transducer 48 is arranged to sense pressure oscillations within the oscillator 30 and is connected to a digital type frequency indicator 49 by way of a suitable electronic circuit 50. Two pressure gauges 51, 51, are tapped into the sampling line 42 upstream and downstream, respectively, of the oscillator 40. An optional muff arrangement (shown in broken line) may be fitted which comprises a muff 53 that encloses the oscillator 40 and provides a chamber thereabout which is conduitly connected to the sampling line 42 upstream of tie oscillator 40 by way of an on/off valve 54. A conduit 55 is provided to drain and vent the muff.

These means of heating the oscillaotr are to be preferred to other means of heating, in view of the explosive nature of the exemplified sample gas.

In operation, a sampling flow of pressur ised gas comprising the steam and oxygen is passed to the oscillator 40 through the sampling line 42 by way of the shut-off valve 43 being reduced in pressure in passing through the pressure reducer 44 to provide a pressure ratio across the oscillator slightly above, say, 1 5, at which the oscillator outlet is designed to choke sonically. The oscillator functions in similar manner to that previously described. When using dry steam sufficient heat must be held in the structure of the oscillator to ensure that steam super heat is maintained and is not lost in transfer ring heat to the oscillator as the gas passes through it; and to this end optional adjustable throttle valve 45 may be employed as follows.

The pressure reducer 44 is set at a pressure which is capable of producing a pressure ratio across the oscillator of say, 2 5 (above which in this example clarity of the output signal deteriorates) which then allows the throttle valve 45 to be adjusted so that it creates a back-pressure which controls the pressure ratio at a value intermediate 2-5 and 1-5 where there is sufficient mass flow and consequently heat to maintain the steam superheat. However, where required, supple mentary heat can be given to the oscillator by means of the optional muff arrangement, taking sampling gas by way of the on/off valve 54 to the muff 53 and thence returning it to the sampling line downstream towards venting.When the muff arrangement is incorporated it obviates need for the optional adjustable throttle valve 45. The two pressure and temperature gauges 51, 52 and 47, respectively, enable an operator to observe the relevant conditions that exist in the system and to make appropriate valve adjustments to maintain them as desired.

The invention may be combined with various apparatus for many applications such as, for example, in chemical control processes where the quality of gaseous products needs monitoring for control and safety purposes; in the medical field for breathable gas regulation; for the warning of toxic gases appearing in confined spaces; for gas leak testing; in the area of underwater activity for monitoring and controlling the helium/oxygen mixture supplied to divers; whilst in the field of aviation it may be used in monitoring the quality of the gas supporting a dirigible. It is to be understood that the invention is suitable for combination with apparatus which discerns any characteristics of a gas by determination of sonic frequency and is not solely for gas identification.

The embodiments have been described by way of example only, and it will be understood that various modifications and alternatives may be employed without departing from the scope of invention. One modification may concern the necessity to install the pressure sensing element remotely of the oscillator in which case tuning means may be incorporated to compensate for the volume of the sensing line connecting the element to one of the resonance chambers. The tuning means can conveniently comprise a tubular element projecting from the chamber to which the sensing line is connected and be provided with a threaded adjustable plug.

Another modification concerns the economical use of pressure transducers where the pressure, ambient to the external side thereof, may peak to excessively high pressures relative to the capabilities of the transducer to withstand pressure. A pressure transducer may be arranged to receive pressure across it from both resonance chambers and so isoate its sensing member from the ambient pressure, thereby permitting use of lower quality and less expensive transducers than would otherwise be required to withstand an occasional high pressure excursion.

WHAT WE CLAIM IS:- 1. An acoustic type fluidic oscillator including a jet entry for driving gas and an outlet for such gas, characterised in that said outlet comprises a pressure recovery venturi adapted to operate with choked sonic flow and having a substantial length

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

Claims (15)

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

   ure 1, is arranged, to receive a pressurised gas such as, say, a mixture of superheated steam and oxygen from a high pressure line 41 passing the gas at, for example, a pressure of 500 p.s.i.g. and a temperature of 260"C. An insulated sampling line 42 connects the high pressure line 41 to the gas entry of the oscillator 40 by way of a shut-off valve 43 positioned upstream of a pressure reducing valve 44, whilst the outlet from the oscillator 40 may be arranged to vent by way of an optional adjustable throttle valve 45. A thermo-couple 46 is provided to sense the temperature in the osicllator 40 and is connected to suitable gauge means 47.A pressure transducer 48 is arranged to sense pressure oscillations within the oscillator 30 and is connected to a digital type frequency indicator 49 by way of a suitable electronic circuit 50. Two pressure gauges 51, 51, are tapped into the sampling line 42 upstream and downstream, respectively, of the oscillator 40. An optional muff arrangement (shown in broken line) may be fitted which comprises a muff 53 that encloses the oscillator 40 and provides a chamber thereabout which is conduitly connected to the sampling line 42 upstream of tie oscillator 40 by way of an on/off valve 54. A conduit

   55 is provided to drain and vent the muff.

   These means of heating the oscillaotr are to be preferred to other means of heating, in view of the explosive nature of the exemplified sample gas.

   In operation, a sampling flow of pressur ised gas comprising the steam and oxygen is passed to the oscillator 40 through the sampling line 42 by way of the shut-off valve 43 being reduced in pressure in passing through the pressure reducer 44 to provide a pressure ratio across the oscillator slightly above, say, 1 5, at which the oscillator outlet is designed to choke sonically. The oscillator functions in similar manner to that previously described. When using dry steam sufficient heat must be held in the structure of the oscillator to ensure that steam super heat is maintained and is not lost in transfer ring heat to the oscillator as the gas passes through it; and to this end optional adjustable throttle valve 45 may be employed as follows.

   The pressure reducer 44 is set at a pressure which is capable of producing a pressure ratio across the oscillator of say, 2 5 (above which in this example clarity of the output signal deteriorates) which then allows the throttle valve 45 to be adjusted so that it creates a back-pressure which controls the pressure ratio at a value intermediate 2-5 and 1-5 where there is sufficient mass flow and consequently heat to maintain the steam superheat. However, where required, supple mentary heat can be given to the oscillator by means of the optional muff arrangement, taking sampling gas by way of the on/off valve 54 to the muff 53 and thence returning it to the sampling line downstream towards venting.When the muff arrangement is incorporated it obviates need for the optional adjustable throttle valve 45. The two pressure and temperature gauges 51, 52 and 47, respectively, enable an operator to observe the relevant conditions that exist in the system and to make appropriate valve adjustments to maintain them as desired.

   The invention may be combined with various apparatus for many applications such as, for example, in chemical control processes where the quality of gaseous products needs monitoring for control and safety purposes; in the medical field for breathable gas regulation; for the warning of toxic gases appearing in confined spaces; for gas leak testing; in the area of underwater activity for monitoring and controlling the helium/oxygen mixture supplied to divers; whilst in the field of aviation it may be used in monitoring the quality of the gas supporting a dirigible. It is to be understood that the invention is suitable for combination with apparatus which discerns any characteristics of a gas by determination of sonic frequency and is not solely for gas identification.

   The embodiments have been described by way of example only, and it will be understood that various modifications and alternatives may be employed without departing from the scope of invention. One modification may concern the necessity to install the pressure sensing element remotely of the oscillator in which case tuning means may be incorporated to compensate for the volume of the sensing line connecting the element to one of the resonance chambers. The tuning means can conveniently comprise a tubular element projecting from the chamber to which the sensing line is connected and be provided with a threaded adjustable plug.

   Another modification concerns the economical use of pressure transducers where the pressure, ambient to the external side thereof, may peak to excessively high pressures relative to the capabilities of the transducer to withstand pressure. A pressure transducer may be arranged to receive pressure across it from both resonance chambers and so isoate its sensing member from the ambient pressure, thereby permitting use of lower quality and less expensive transducers than would otherwise be required to withstand an occasional high pressure excursion.

   WHAT WE CLAIM IS:- 1. An acoustic type fluidic oscillator including a jet entry for driving gas and an outlet for such gas, characterised in that said outlet comprises a pressure recovery venturi adapted to operate with choked sonic flow and having a substantial length

   of parallel section downstream of a divergent section.
2. 2. An acoustic type fluidic oscillator comprising a pair of resonance chambers symmetrically disposed on opposite sides of a flow-splitter edge facing a driving gas entry jet for edge tone generation, at least one of said chambers connecting with a gas outlet including a pressure recovery venturi adapted to operate with choked sonic flow and having a substantial length of parallel section downstream of a divergent section.
3. 3. An oscillator according to claim 2, wherein said resonance chambers are of elongated form and disposed with their major axes parallel with one another and with the extended axis of the entry jet.
4. 4. An oscillator according to claim 3, wherein said resonance chambers are separated from one another by a dividing wall an end of which is wedge-shaped to constitute the said flow-splitter edge.
5. 5. An oscillator according to claim 3 or 4, wherein said resonance chambers are interconnected at a location remote from said flow-splitter edge.
6. 6. An oscillator according to any one of claims 2 to 5, having a single said gas outlet connected to both resonance chambers by symmetrical flow paths.
7. 7. An oscillator according to claims 5 and 6, wherein said resonance chambers are interconnected by a passageway that connects with the said gas outlet, the latter being arranged on the axis of the dividing wall.
8. 8. An oscillator according to any preceding claim, of unitary construction and consisting of a body with cavities and passages formed therein.
9. 9. An oscillator according to claim 2 or any claim dependent thereon, having a pressure transducer communicating with at least one of said resonance chambers and responsive to the acoustic output of the oscillator to generate an electrical signal of corresponding frequency.
10. 10. An oscillator according to claim 9, wherein said pressure transducer communicates with both resonance chambers and is responsive to the pressure difference therebetween.
11. 11. An oscillator according to claim 9 or 10, wherein the said transducer is positioned at a pressure path distance, from the or each resonance chamber, of not more than one wave-length of the highest frequency acoustic signal to be produced by the oscillator.
12. 12. A gas monitoring system comprising an oscillator in accordance with claim 9, 10 or 11, in combination with means for continually passing a sample of gas through the oscillator and means connected to the pressure transducer for indicating the output signal frequency thereof.
13. 13. A system according to claim 12, wherein said means for passing a gas sample through the oscillator comprise a jet pump arranged to reduce the pressure at said gas outlet.
14. 14. An acoustic type fluidic oscillator substantially as described with reference to and as shown in Figure 1 of the accompanying drawings.
15. 15. A gas monitoring system substantially as described with reference to and as shown in Figure 2 of the accompanying drawings.