The Applicant expects this invention to be particularly advantageously applicable to certain aircraft applications.

In accordance with a first aspect of this invention, there is provided a method of mixing gases, the method including introducing a drive gas in a high speed drive gas flow stream annularly, generally longitudinally into a mixing chamber at an inlet end of the mixing chamber; causing a passive gas to be entrained or induced generally longitudinally within the annular drive gas flow stream at said inlet end of the mixing chamber; and causing the drive gas and the passive gas to mix within the mixing chamber.

Causing said passive gas to be entrained or induced into said drive gas flow stream may be from centrally within the annular drive gas flow stream.

Introducing said drive gas flow stream into the mixing chamber may be at an annular position inwardly spaced from a periphery of the mixing chamber. The method may then include causing a passive gas to be entrained or induced also from outwardly of said annular drive gas

METHOD AND DEVICE FOR MIXING GASES THIS INVENTION relates to mixing gases. It relates more specifically to a method of mixing gases and to a gas mixing device.

The Applicant expects this invention to be particularly advantageously applicable to certain aircraft applications.

In accordance with a first aspect of this invention, there is provided a method of mixing gases, the method including introducing a drive gas in a high speed drive gas flow stream annularly, generally longitudinally into a mixing chamber at an inlet end of the mixing chamber; causing a passive gas to be entrained or induced generally longitudinally within the annular drive gas flow stream at said inlet end of the mixing chamber; and causing the drive gas and the passive gas to mix within the mixing chamber.

Causing said passive gas to be entrained or induced into said drive gas flow stream may be from centrally within the annular drive gas flow stream.

Introducing said drive gas flow stream into the mixing chamber may be at an annular position inwardly spaced from a periphery of the mixing chamber. The method may then include causing a passive gas to be entrained or induced also from outwardly of said annular drive gas

flow stream at said inlet end of the mixing chamber. Then said passive gas caused to be entrained or introduced from within the annular drive gas flow stream and said passive gas caused to be entrained or introduced from outwardly of said annular drive gas flow stream, may be the same passive gas from a common source. Said passive gas may be ambient air, the method including placing said inlet end of the mixing chamber in communication with the atmosphere.

Advantageously, the method may include directing said drive gas flow stream into the mixing chamber via an annular, longitudinally directed nozzle at or toward the periphery of the mixing chamber at said inlet end. Said nozzle may be a converging nozzle, the method including accelerating said drive gas flow stream when introducing it into the mixing chamber.

By way of development, the method may include rotating said drive gas annularly to cause said drive gas flow stream introduced generally longitudinally into the mixing chamber to have also a rotational component of flow.

In one method, said drive gas may be hot exhaust gas from a combustion motor, said passive gas may be cold ambient air, and mixing of the gases in the mixing chamber may form a cool mixture having a reduced infra-red signature compared to the hot exhaust gas.

In another method, said drive gas may be high pressure, high temperature air bled off downstream of a compressor of a gas turbine motor of an aircraft, said passive gas may be cold, ambient air, and mixing of the gases in the mixing chamber may produce a mixture

flow stream at said inlet end of the mixing chamber. Then said passive gas caused to be entrained or introduced from within the annular drive gas flow stream and said passive gas caused to be entrained or introduced from outwardly of said annular drive gas flow stream, may be the same passive gas from a common source. Said passive gas may be ambient air, the method including placing said inlet end of the mixing chamber in communication with the atmosphere.

Advantageously, the method may include directing said drive gas flow stream into the mixing chamber via an annular, longitudinally directed nozzle at or toward the periphery of the mixing chamber at said inlet end. Said nozzle may be a converging nozzle, the method including accelerating said drive gas flow stream when introducing it into the mixing chamber.

By way of development, the method may include rotating said drive gas annularly to cause said drive gas flow stream introduced generally longitudinally into the mixing chamber to have also a rotational component of flow.

In one method, said drive gas may be hot exhaust gas from a combustion motor, said passive gas may be cold ambient air, and mixing of the gases in the mixing chamber may form a cool mixture having a reduced infra-red signature compared to the hot exhaust gas.

In another method, said drive gas may be high pressure, high temperature air bled off downstream of a compressor of a gas turbine motor of an aircraft, said passive gas may be cold, ambient air, and mixing of the gases in the mixing chamber may produce a mixture

appropriate for inhalation by humans, the method including introducing the mixture into a cabin of said aircraft.

In accordance with another aspect of this invention, there is provided a mixing device including a mixing chamber having an inlet end and an opposed exhaust end; an annular nozzle toward said inlet end directed longitudinally along the mixing chamber, the inlet nozzle having connection means for connection to a source of drive gas under pressure; an inlet opening annularly adjacent the annular nozzle, said inlet opening being in communication with a source of a passive gas to cause, in use, said passive gas to be entrained or induced into the mixing chamber.

Said inlet opening may be inwardly adjacent the annular nozzle.

Instead, said inlet opening may be outwardly adjacent the annular nozzle.

Instead, said inlet opening may be both inwardly adjacent and outwardly adjacent the annular nozzle.

The inlet opening may be arranged to be open to the atmosphere, said source of passive gas then being the atmosphere and said passive gas then being ambient air.

Advantageously, said nozzle may be a converging nozzle to cause the drive gas to accelerate to be introduced into the mixing chamber at high speed in use.

appropriate for inhalation by humans, the method including introducing the mixture into a cabin of said aircraft.

In accordance with another aspect of this invention, there is provided a mixing device including a mixing chamber having an inlet end and an opposed exhaust end; an annular nozzle toward said inlet end directed longitudinally along the mixing chamber, the inlet nozzle having connection means for connection to a source of drive gas under pressure; an inlet opening annularly adjacent the annular nozzle, said inlet opening being in communication with a source of a passive gas to cause, in use, said passive gas to be entrained or induced into the mixing chamber.

Said inlet opening may be inwardly adjacent the annular nozzle.

Instead, said inlet opening may be outwardly adjacent the annular nozzle.

Instead, said inlet opening may be both inwardly adjacent and outwardly adjacent the annular nozzle.

The inlet opening may be arranged to be open to the atmosphere, said source of passive gas then being the atmosphere and said passive gas then being ambient air.

Advantageously, said nozzle may be a converging nozzle to cause the drive gas to accelerate to be introduced into the mixing chamber at high speed in use.

The mixing device may include an annular plenum upstream of said nozzle, the connection means for the nozzle being on the plenum for connection to the source of drive gas under pressure. Said connection means may include a nipple or conduit extending transversely of an axis of the plenum. By way of development, said nipple or conduit may extend generally tangentially to said plenum, in use to guide said drive gas under pressure correspondinglygenerallytangentially into the plenum to cause rotation of said drive gas in the plenum and to cause said drive gas to be introduced into the mixing chamber in a generally longitudinal drive gas flow stream having a rotational component of flow.

In accordance with a further aspect of this invention, there is provided a combination of a mixing device as herein described, and an aircraft having a combustion motor including an exhaust conduit for exhausting products of combustion, in which combination said combustion motor via said exhaust conduit is said source of drive gas under pressure, said connection means of said inlet nozzle being connected to an outlet of said exhaust conduit, and in which combination said inlet opening is communicated with the atmosphere.

Said mixing chamber may extend longitudinally in the direction of flying of the aircraft, and said exhaust conduit may extend transversely.

IN accordance with yet a further aspect of this invention, there is provided a combination of a mixing device as herein described, and an aircraft having a gas turbine motor including a compressor for compressing ambient air prior to combustion, in which combination said compressor is said source of drive gas under pressure, said connection means of said inlet nozzle being connected to said gas turbine motor to

The mixing device may include an annular plenum upstream of said nozzle, the connection means for the nozzle being on the plenum for connection to the source of drive gas under pressure. Said connection means may include a nipple or conduit extending transversely of an axis of the plenum. By way of development, said nipple or conduit may extend generally tangentially to said plenum, in use to guide said drive gas under pressure correspondinglygenerallytangentially into the plenum to cause rotation of said drive gas in the plenum and to cause said drive gas to be introduced into the mixing chamber in a generally longitudinal drive gas flow stream having a rotational component of flow.

In accordance with a further aspect of this invention, there is provided a combination of a mixing device as herein described, and an aircraft having a combustion motor including an exhaust conduit for exhausting products of combustion, in which combination said combustion motor via said exhaust conduit is said source of drive gas under pressure, said connection means of said inlet nozzle being connected to an outlet of said exhaust conduit, and in which combination said inlet opening is communicated with the atmosphere.

Said mixing chamber may extend longitudinally in the direction of flying of the aircraft, and said exhaust conduit may extend transversely.

IN accordance with yet a further aspect of this invention, there is provided a combination of a mixing device as herein described, and an aircraft having a gas turbine motor including a compressor for compressing ambient air prior to combustion, in which combination said compressor is said source of drive gas under pressure, said connection means of said inlet nozzle being connected to said gas turbine motor to

bleed off a small portion of compressed, high temperature air downstream of the compressor and upstream of a combustion chamber of the gas turbine motor, and in which combination said inlet opening is communicated with the atmosphere, said exhaust end of the mixing chamber being communicated with a cabin of the aircraft to supply the cabin with air appropriate for inhalation by humans.

In accordance with yet another aspect of this invention, there is provided the combination of a mixing device as herein described, and an aircraft having a gas turbine motor including a compressor for compressing ambient air prior to combustion, the aircraft including a filter arrangement in which, in use, a scavenge air flow stream laden with dust or other particles and a filtered air flow stream purged of dust and other particles to provide filtered inlet air for the gas turbine engine are generated, in which combination said compressor is said source of drive gas under pressure, said connection means of said inlet nozzle being connected to said gas turbine motor to bleed off a small portion of compressed, high temperature air downstream of the compressor and upstream of a combustion chamber of the gas turbine motor, and in which combination said inlet opening is communicated with said filter arrangement to collect said scavenge air flow to be entrained or induced into said drive gas flow stream.

The invention is now described by way of example with reference to the accompanying diagrammatic drawings. In the drawings Figure 1 shows, in axial section, a first embodiment of a mixing device in accordance with the invention; Figure 2 shows an end view in a downstream direction of the mixing device of Figure 1;

bleed off a small portion of compressed, high temperature air downstream of the compressor and upstream of a combustion chamber of the gas turbine motor, and in which combination said inlet opening is communicated with the atmosphere, said exhaust end of the mixing chamber being communicated with a cabin of the aircraft to supply the cabin with air appropriate for inhalation by humans.

In accordance with yet another aspect of this invention, there is provided the combination of a mixing device as herein described, and an aircraft having a gas turbine motor including a compressor for compressing ambient air prior to combustion, the aircraft including a filter arrangement in which, in use, a scavenge air flow stream laden with dust or other particles and a filtered air flow stream purged of dust and other particles to provide filtered inlet air for the gas turbine engine are generated, in which combination said compressor is said source of drive gas under pressure, said connection means of said inlet nozzle being connected to said gas turbine motor to bleed off a small portion of compressed, high temperature air downstream of the compressor and upstream of a combustion chamber of the gas turbine motor, and in which combination said inlet opening is communicated with said filter arrangement to collect said scavenge air flow to be entrained or induced into said drive gas flow stream.

The invention is now described by way of example with reference to the accompanying diagrammatic drawings. In the drawings Figure 1 shows, in axial section, a first embodiment of a mixing device in accordance with the invention; Figure 2 shows an end view in a downstream direction of the mixing device of Figure 1;

Figure 3 shows, in side view, another embodiment of a mixing device in combination with a gas turbine motor; Figure 4 shows, in side view, a further embodiment of a mixing device; and Figure 5 shows, in side view, a combination of a gas turbine motor having a filtered inlet, and a mixing device.

It is emphasized that the drawings are schematic to illustrate the principles of the invention. For example, hatching is generally, not shown.

With reference to Figures 1 and 2 of the drawings, reference numeral 10 indicates schematically an extremity of a fuselage of an aircraft which has an exhaust conduit 12 extending transversely from a motor of the aircraft.

Reference numeral 13 generally indicates a mixing device in accordance with the invention. The mixing device 13 includes an elongate mixing chamber 16 arranged longitudinally adjacent the fuselage 10. The mixing chamber 16 may be cylindrical, conveniently round cylindrical. In other embodiments, an exhaust end thereof may be flattened, as is known in the art of infra red signature reduction to present a shallower, albeit wider, target for infra red sensing means.

At an upstream end of the mixing chamber 16, there is provided a plenum 14 which can best be seen in Figure 2 and which is operatively communicated with the exhaust conduit 12. Thus, hot exhaust gas flows via the exhaust conduit 12 into the plenum 14.

Figure 3 shows, in side view, another embodiment of a mixing device in combination with a gas turbine motor; Figure 4 shows, in side view, a further embodiment of a mixing device; and Figure 5 shows, in side view, a combination of a gas turbine motor having a filtered inlet, and a mixing device.

It is emphasized that the drawings are schematic to illustrate the principles of the invention. For example, hatching is generally, not shown.

With reference to Figures 1 and 2 of the drawings, reference numeral 10 indicates schematically an extremity of a fuselage of an aircraft which has an exhaust conduit 12 extending transversely from a motor of the aircraft.

Reference numeral 13 generally indicates a mixing device in accordance with the invention. The mixing device 13 includes an elongate mixing chamber 16 arranged longitudinally adjacent the fuselage 10. The mixing chamber 16 may be cylindrical, conveniently round cylindrical. In other embodiments, an exhaust end thereof may be flattened, as is known in the art of infra red signature reduction to present a shallower, albeit wider, target for infra red sensing means.

At an upstream end of the mixing chamber 16, there is provided a plenum 14 which can best be seen in Figure 2 and which is operatively communicated with the exhaust conduit 12. Thus, hot exhaust gas flows via the exhaust conduit 12 into the plenum 14.

A nozzle 15 forms an integral part of the plenum 14. The nozzle 15 is an annular nozzle which is directed longitudinally into the mixing chamber 16. In this embodiment it is located immediately inward of a periphery of the mixing chamber 16. In a preferred embodiment, the plenum 14, the nozzle 15 and the mixing chamber 16 are integrated.

Thus, in use, the exhaust gas is ejected annularly, longitudinally into the mixing chamber 16 via an annular opening 18 of the nozzle 15.

Centrally within the nozzle 15, there is provided a central opening generally indicated by reference numeral 20 and which is in communication with ambient air. The shape of the nozzle 15 forms an aerodynamic mouth for the central opening 20.

In use, the high speed exhaust gas generally indicated by reference numeral 22 which flows into the mixing chamber 16 via the annular opening 18 creates an annular area of low pressure, in accordance with the principle of Bernoulli, which entrains or induces ambient air to flow in an airflow stream as indicated by reference numeral 24 via the central opening 20 into the mixing chamber 16.

The hot exhaust gas which enters the mixing chamber annularly peripherally is mixed with the cold ambient air which enters the mixing chamber longitudinally centrally to form a cool mixture which is exhausted from the mixing chamber 16 as indicated at 26. The cool mixture reduces the infra-red signature of the aircraft as mentioned above.

It is important to appreciate that the exhaust conduit 12, in the aircraft of Figures 1 and 2, extends transversely. It is thus highly

A nozzle 15 forms an integral part of the plenum 14. The nozzle 15 is an annular nozzle which is directed longitudinally into the mixing chamber 16. In this embodiment it is located immediately inward of a periphery of the mixing chamber 16. In a preferred embodiment, the plenum 14, the nozzle 15 and the mixing chamber 16 are integrated.

Thus, in use, the exhaust gas is ejected annularly, longitudinally into the mixing chamber 16 via an annular opening 18 of the nozzle 15.

Centrally within the nozzle 15, there is provided a central opening generally indicated by reference numeral 20 and which is in communication with ambient air. The shape of the nozzle 15 forms an aerodynamic mouth for the central opening 20.

In use, the high speed exhaust gas generally indicated by reference numeral 22 which flows into the mixing chamber 16 via the annular opening 18 creates an annular area of low pressure, in accordance with the principle of Bernoulli, which entrains or induces ambient air to flow in an airflow stream as indicated by reference numeral 24 via the central opening 20 into the mixing chamber 16.

The hot exhaust gas which enters the mixing chamber annularly peripherally is mixed with the cold ambient air which enters the mixing chamber longitudinally centrally to form a cool mixture which is exhausted from the mixing chamber 16 as indicated at 26. The cool mixture reduces the infra-red signature of the aircraft as mentioned above.

It is important to appreciate that the exhaust conduit 12, in the aircraft of Figures 1 and 2, extends transversely. It is thus highly

advantageous to terminate the exhaust conduit in the annular plenum from which the exhaust gas can be ejected annularly longitudinally. It is an advantage that a change in direction of flow of the exhaust gas takes place via the plenum into the nozzle. The arrangement has the added advantages that the exhaust gas conduit need not include a corner or bend, that the exhaust gas conduit does not obstruct flow of ambient air into the central opening, that the hot exhaust gas is not present in the form of a concentrated hot core but rather in the form of a thin annular layer of large diameter, that the arrangement is structural simple, and that the nozzle forms an integral part of the mixing device thus obviating problems associated with interfacing between the nozzle and mixing chamber as experienced in conventional arrangements.

With reference to Figure 3, another embodiment of a mixing device is generally indicated by reference numeral 113. It is similar, even identical, in many respects to the mixing device 13 of Figures 1 and 2.

Like reference numerals are used to denote like features or components, and the mixing device 113 is not again described in detail. It is emphasized that, like the other Figures, Figure 3 is schematic. For example, the same scale is not used for the gas turbine motor and the mixing device.

The mixing device 113 is particularly suitable to provide warm ventilation air to a cabin of an aircraft such as a helicopter. The mixing device 113 is used in combination with a gas turbine motor 100 having a compressor 102.

Hot air at high pressure is bled off immediately downstream of the compressor 102 of the gas turbine motor 100 of the aircraft. Such bleed

advantageous to terminate the exhaust conduit in the annular plenum from which the exhaust gas can be ejected annularly longitudinally. It is an advantage that a change in direction of flow of the exhaust gas takes place via the plenum into the nozzle. The arrangement has the added advantages that the exhaust gas conduit need not include a corner or bend, that the exhaust gas conduit does not obstruct flow of ambient air into the central opening, that the hot exhaust gas is not present in the form of a concentrated hot core but rather in the form of a thin annular layer of large diameter, that the arrangement is structural simple, and that the nozzle forms an integral part of the mixing device thus obviating problems associated with interfacing between the nozzle and mixing chamber as experienced in conventional arrangements.

With reference to Figure 3, another embodiment of a mixing device is generally indicated by reference numeral 113. It is similar, even identical, in many respects to the mixing device 13 of Figures 1 and 2.

Like reference numerals are used to denote like features or components, and the mixing device 113 is not again described in detail. It is emphasized that, like the other Figures, Figure 3 is schematic. For example, the same scale is not used for the gas turbine motor and the mixing device.

The mixing device 113 is particularly suitable to provide warm ventilation air to a cabin of an aircraft such as a helicopter. The mixing device 113 is used in combination with a gas turbine motor 100 having a compressor 102.

Hot air at high pressure is bled off immediately downstream of the compressor 102 of the gas turbine motor 100 of the aircraft. Such bleed

air is conducted in a bleed air conduit 112 into a plenum 114 forming part of the mixing device 113. The plenum 114 incorporates an annular nozzle 115 which is directed longitudinally via an annular nozzle opening 118 into a mixing chamber 116.

Ambient air, which is cold, is entrained or induced as indicated by reference numeral 124 via a central opening 120 centrally within the annular nozzle 115, into the mixing chamber 116. The hot bleed air (acting as the drive gas 122) and the cold ambient air 124 are mixed in the mixing chamber 116 to leave the mixing chamber 116 as a warm ventilation air mixture 126 suitable for inhalation by occupants of the cabin of the aircraft.

One particularly important advantage is that the nozzle opening 118 is provided at a pitch circle having a diameter, thus ensuring that a mixing interface between the annular flow stream 122 of the hot air and the periphery of the opening 120 is of a large area or along a long linear dimension i. e. corresponding to the circumference of the diameter. Thus, it is believed that the arrangement of Figure 3 is much less noisy than conventional arrangements.

It is an advantage that the arrangement provides a large (i. e. long, equal to the circumference of the annulus) interface with the ambient air which the Applicant expects to reduce noise, thus reducing an acoustic signature of the aircraft and enhancing the comfort of the occupant (s). This is particularly important bearing in mind the high pitched or high frequency, piercing or penetrating nature of the air when it leaves a small opening at high speed. Other advantages of this arrangement are

structural simplicity, easy locating of the nozzle, and no obstruction in front of the central opening inhibiting flow of the ambient air.

With reference to Figure 4, a developed embodiment of a mixing device in accordance with the invention is generally indicated by reference numeral 213. The mixing device 213 is similar in many respects to the mixing devices of Figures 1 and 3 described above, like reference numerals refer to like components or features and emphasis will merely be placed on three important developments embodied in the device of Figure 4, without describing it in detail.

First, the conduit 212 by means of which drive gas under pressure is conducted to the device 213, enters the annular plenum 214 tangentially as opposed to radially as shown in Figure 2 for the device 13. This has the effect that the drive gas is rotated annularly, ie it is swirled, within the plenum 214 which rotational component of flow is retained while flowing through the nozzle 215 and into the mixing chamber 216.

The Applicant does not wish to be bound by theory, but a possible explanation of the advantage of the rotational flow component in the drive gas is that the drive gas has a longer residence time in the mixing chamber 216 while still maintaining high kinetic energy which enhances mixing.

A second advantage is that the annular nozzle 215 is a converging nozzle thus accelerating the drive gas in the drive gas flow stream as it is directed into the mixing chamber. This enhances the Bernoulli effect

structural simplicity, easy locating of the nozzle, and no obstruction in front of the central opening inhibiting flow of the ambient air.

With reference to Figure 4, a developed embodiment of a mixing device in accordance with the invention is generally indicated by reference numeral 213. The mixing device 213 is similar in many respects to the mixing devices of Figures 1 and 3 described above, like reference numerals refer to like components or features and emphasis will merely be placed on three important developments embodied in the device of Figure 4, without describing it in detail.

First, the conduit 212 by means of which drive gas under pressure is conducted to the device 213, enters the annular plenum 214 tangentially as opposed to radially as shown in Figure 2 for the device 13. This has the effect that the drive gas is rotated annularly, ie it is swirled, within the plenum 214 which rotational component of flow is retained while flowing through the nozzle 215 and into the mixing chamber 216.

The Applicant does not wish to be bound by theory, but a possible explanation of the advantage of the rotational flow component in the drive gas is that the drive gas has a longer residence time in the mixing chamber 216 while still maintaining high kinetic energy which enhances mixing.

A second advantage is that the annular nozzle 215 is a converging nozzle thus accelerating the drive gas in the drive gas flow stream as it is directed into the mixing chamber. This enhances the Bernoulli effect

and enhances entrainment or inducement of the passive gas into the mixing chamber.

It is a further advantage that the nozzle 215 is annularly spaced inwardly of an outer periphery of the mixing chamber 216. Thus, passive gas is induced both radially outwardly of the nozzle 215 and inwardly of the nozzle 21 5, thus nearly doubling the interface between the drive gas and the passive gas (it is borne in mind that the diameter and thus also the circumference of the nozzle outlet 218 is smaller than those of the periphery of the mixing chamber 216 and thus of those which a nozzle at the periphery of the mixing chamber would have).

With reference to Figure 5, a mixing device 313 is combined with a gas turbine motor 300 including a compressor 302 and having a filter arrangement 304 to filter inlet air.

Axially downstream of the filter arrangement 304, an inlet stream of filtered air, generally indicated by reference numeral 306, is conducted into the compressor 302.

The filter arrangement 304 filters dust grit and other undesirable particulate matter which are carried by means of a scavenge stream 308 away from the filter arrangement 304 and away from the gas turbine motor 300 via a conduit 312.1.

In accordance with the invention, the mixing device 313 having a mixing chamber 316 is arranged such that high pressure air bled off from downstream of the compressor 302 via a conduit 312 is entered as a

and enhances entrainment or inducement of the passive gas into the mixing chamber.

It is a further advantage that the nozzle 215 is annularly spaced inwardly of an outer periphery of the mixing chamber 216. Thus, passive gas is induced both radially outwardly of the nozzle 215 and inwardly of the nozzle 21 5, thus nearly doubling the interface between the drive gas and the passive gas (it is borne in mind that the diameter and thus also the circumference of the nozzle outlet 218 is smaller than those of the periphery of the mixing chamber 216 and thus of those which a nozzle at the periphery of the mixing chamber would have).

With reference to Figure 5, a mixing device 313 is combined with a gas turbine motor 300 including a compressor 302 and having a filter arrangement 304 to filter inlet air.

Axially downstream of the filter arrangement 304, an inlet stream of filtered air, generally indicated by reference numeral 306, is conducted into the compressor 302.

The filter arrangement 304 filters dust grit and other undesirable particulate matter which are carried by means of a scavenge stream 308 away from the filter arrangement 304 and away from the gas turbine motor 300 via a conduit 312.1.

In accordance with the invention, the mixing device 313 having a mixing chamber 316 is arranged such that high pressure air bled off from downstream of the compressor 302 via a conduit 312 is entered as a

drive gas via an annular inlet nozzle 315 into the mixing chamber 316.

The scavenge stream 308 is communicated with a central nozzle having a nozzle opening 320 inwardly of the annular drive gas nozzle 31 5. The drive gas induces the scavenge flow stream 308 to flow into the mixing chamber 316, thus to be extracted from the filter arrangement 304.