GB2418980A - A flow arrangement for a heat exchanger - Google Patents

Abstract

A flow arrangement for a heat exchanger 3 comprises an entrainment path 7 extending to a draught region 9, with the entrainment path 7 arranged to direct a gas flow 6a across the draught region 9 to entrain airflow 5 upstream of the draught region 9. Airflow 5 may be drawn through an inlet duct 4 or apertures (105, fig 7), passes through heat exchangers 3, which may be finned, along a conduit 12 communicating with the draught region 9 and mixes with the gas 6a supplied via a nozzle 8 in a mixing zone 13. After mixing the gases may be further entrained into a high flow rate gas by entering a diffuser 10 and passing out through an exit 11. The heat exchanger 3 may be used to cool electronic components used to control and monitor internal combustion engines or gas turbine engines and the gas 6a may be compressed gas from the gas turbine or exhaust gas from the internal combustion engine or a turbine stage.

Description

24 1 8980 A Flow Arrangement The present invention relates to flow

arrangements and more particularly to flow arrangements utilised with respect to heat exchangers or heat sinks in order to dissipate heat from electronic components.

Gas turbine engines and internal combustion engines are utilised in a wide number of situations with respect to acting as a prime mover. In order to improve efficient operation or performance of such engines, electronic devices are utilised for adjustment and control. The application of electronic devices offers considerable opportunities to enhance the performance of an engine.

Projected benefits of electronically optimised engines include improved functionality, reliability and maintainability; and reduced size, weight and fuel consumption. Due to the rapidly growing technical demand on electronics, it is envisaged that the heat flux generated from the silicon-based devices will increase from the current lOOW/cm2 to lOOOW/cm2 over the next ten years.

Heat dissipation has therefore become a major concern for future development of electronic devices. If heat is inadequately dissipated it will be understood that eventually the electronic component devices will fail or their operation may be compromised.

Clearly, natural heat dissipation will increasingly become inadequate for the increased level of heat flux generated. In such circumstances, more forced cooling regimes must be employed. Heat exchangers are common means for dissipating heat from the electronic components to the heat sink (i.e. atmosphere). While heat exchangers can be compact when liquid cooled, gas-cooled, normally air, heat exchangers are usually bulky and heavy as a much larger heat transfer surface area is needed to recompense the low heat transfer capacity of the fluid gas. This is undesirable in certain cooling applications, such as that in an aero gas turbine engine where weight and space must be controlled tightly.

In such circumstances, heat exchangers can achieve desired cooling, but generally there is a relationship between heat exchanger size and level of cooling achievable without forcing air flow. Thus, achievement of desired cooling rates with the smallest feasible heat exchanger has particular advantages in space/weight driven environments such as those associated with aircraft engines. In such circumstances, means for improving coolant airflow through a heat exchanger will be advantageous provided there is limited or no adverse effects upon weight or excessive requirements for space.

In accordance with the present invention there is provided a flow arrangement for a heat exchanger, the arrangement comprising an entrainment path extending to a draught region, the entrainment path arranged to direct a gas flow angularly across the draught region to entrain airflow upstream of the draught region to stimulate airflow through the arrangement.

Generally, the draught region narrows immediately downstream of the entrainment path to form a contraction area.

Typically, the draught region is associated with a diffuser to further stimulate airflow through the arrangement.

Normally, the entrainment path includes a nozzle to the draught region, the nozzle formed between an overhang surface and a ramp surface to form an annulus between them to provide angular presentation of the gas flow across the draught region. Alternatively, the pressurised gas is fed coaxially into the entrained flow. Further alternatively, the pressurized gas is fed at an angle into the entrained flow through dedicated nozzles.

Generally, the entrainment path is indirect before the nozzle.

Also, in accordance with the present invention there is provided a heat exchanger assembly for cooling electronic components on a chassis, the assembly comprising a flow arrangement as described above and a heat exchanger associated upstream of the arrangement through a conduit path whereby the airflow is drawn from the heat exchanger.

Typically, the conduit path narrows towards the draught region to form a contraction area.

Typically, one side of the conduit path is provided by an entrainment wall with a part of that entrainment wall forming part of the entrainment path.

Further in accordance with the present invention there is provided an engine comprising a source of hot gas flow and a flow arrangement as described above wherein the gas flow is taken from the source of higher pressure gas flow in order to stimulate the relatively cooler ambient air flow through the flow arrangement.

Generally, the engine is a gas turbine engine or internal combustion engine, producing any kind of exhaust gas at temperatures and/or pressures higher than ambient.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which Fig. 1 is a schematic cross-section of a heat exchanger assembly incorporating a flow arrangement in accordance with the present invention; Fig. 2 is a front perspective view of an engine mounting configuration incorporating a flow arrangement in accordance with the present invention; Fig. 3 is a schematic cross-section of one side of a flow arrangement in accordance with the present invention; Fig. 4 is a schematic front view of a first embodiment of a heat exchanger assembly incorporating a flow arrangement in accordance with the present invention; Fig. 5 is a second embodiment of a heat exchanger assembly incorporating a flow arrangement in accordance with the present invention and, Fig. 6 is a schematic front view of a third embodiment of a heat exchange assembly incorporating a flow arrangement in accordance with the present invention, Fig. 7 is a schematic illustration whereby the heat source is situated on an opposite side to a heat exchanger; and Fig. 8 is a schematic illustration of an alternative nozzle feed arrangement in accordance with the present invention.

Heat exchangers, whether they be of a heat sink type or coolant flow type generally incorporate fins through which a coolant air flow passes in order to provide cooling. The heat exchanger is associated normally through a chassis with the source of heat generation or thermally connected via conductive paths or a secondary fluid flow circuit. Thus the airflow can generically be used for other cooling requirements in order to cool a liquid flow, which could be made up of water, refrigerants, glycol, kerosene, oil, or any other suitable liquid in liquid phase for the ambient aircraft temperature range. Of particular concern with respect to the present invention is heat flux generated by electronic components utilised for control and monitoring of an engine such as an aircraft gas turbine engine. Clearly, if coolant airflow can be stimulated then there is a greater heat exchange capacity per unit time and a smaller heat exchanger may be utilised for the same level of heat flux generation. Nevertheless, normal means for generating enhanced coolant airflow such as fans may be difficult to accommodate in situations such as within an aircraft where weight is a significant factor and space is at a premium.

Within engines such as a gas turbine engine there are generally relatively strong airflows generated by normal thrust operation. Unfortunately, these airflows may be gases from compressor (pre- combustion) and gases from turbine hot exhaust combustion gases and so unsuitable for cooling. Similarly, it will be understood with regard to an internal combustion engine that the exhaust flow from that internal combustion engine will generally be relatively forceful. Thus, if this hot but forceful gas flow is utilised as a motive force, it is possible through entrainment to draw cooler ambient airflow through a heat exchanger.

Referring to Fig. 1 illustrating as a schematic cross- section a heat exchanger assembly 1 in accordance with the present invention. Thus, the assembly 1 incorporates heat exchangers 3 with fins extending into ducts 4. These heat exchangers 3 are associated with electronic packages 2 which generate a heat flux as a result of their operation.

In order to stimulate and draw a coolant airflow in the direction of arrowhead 5, a gas flow in the direction of arrowhead 6 is presented through a feed path 7 in order that the gas flow is angularly presented through nozzles 8 into a draught region 9. The gas flow 6a projected from the nozzles 8 creates a low pressure zone in the draught region 9 which draws air through the ducts 4 and so past the heat exchanger 3 fins for greater cooling effect.

In the above circumstances, the gas flow 6, although relatively hot, never impinges upon the heat exchangers 3 and is merely utilised in order to create the necessary draw for the coolant airflow 5 past the heat exchangers 3.

Subsequent to the draught region 9, a diffuser 10 is provided in order to further stimulate combined gas flow 6 and airflow 5 through the assembly 1. The diffuser 10 stimulates such flow through an expanding crosssection downstream towards a duct exit 11. It will also be understood that this exit 11 may also be into a relatively high flow rate gas stream and so in itself can be entrained flow through the duct exit 11.

As can be seen, generally the coolant airflow 5 through the heat exchangers 3 passes along a conduit path 12 which limits the available flow cross-section and therefore the effectiveness of the entrainment drawing effect caused by the gas flow 6a. The length of this conduit path 12 will be chosen such that the drawing effect created by the low pressure in the draught region 9 is still effective along the conduit length. It will be understood that the low pressure effect in terms of strength diminishes with distance and so the conduit path 12 in terms of length and cross-section is chosen to maximise coolant air flow for effective operation.

Fig. 2 provides a further schematic illustration of the heat exchanger assembly depicted in Fig. 1 as a front perspective. Similar reference numerals have been used for comparison. Thus, an inlet duct 4 acts as a scoop for ambient air flow drawn by hot gas flow through the entrainment path 7. In a draught region 9, a low pressure zone is created in order to draw the air through the duct 4 and so past heat exchangers (not shown) for cooling effect.

As can be seen, immediately downstream of the draught region there is a mixing zone 13. This mixing zone 13 narrows to further stimulate low pressure and therefore entrainment to draw airflow (coolant airflow) 5 through the conduit path 12 and therefore over the heat exchanger fins 3. As indicated previously, the objective is to provide through an injection pumping action of the hot gas flow 6a, a coolant airflow drawing effect past the heat exchangers and therefore to create a high heat exchange effect relative to heat exchanger size.

As seen in Fig. 2 the heat exchanger assembly with the flow arrangement in accordance with the present invention are generally mounted upon a mounting wall 20 of an engine as an exterior structure. In such circumstances, and as depicted, the mounting wall 20 will generally be circumferential about a gas turbine engine with the present heat exchanger assembly and its flow arrangement secured externally of the rotating shafts for compressor stages and turbine stages. The hot motive gas flow 6 will be presented through the entrainment path by appropriate bleed from within the engine and passage to the entrainment path 7 as required.

Clearly, care must be taken with respect to the manner and presentation of the motive gas flow in order to stimulate drawing of coolant air flow through the fins of a heat exchanger. The gas flow can be co-axial (in the direction of the entrained flow) or angularly presented to cause entrainment and therefore pressure reduction effect drawing coolant airflow through the conduit path. Gas flow can feed entrained gas parallel to the entrained flow also to ensure it is not possible for the hot motive gas flow to impinge upon the heat exchanger which would clearly have a detrimental effect upon cooling.

Fig. 3 illustrates in more schematic detail a flow arrangement and heat exchanger assembly in accordance with the present invention. Similar reference numerals have been used for consistency and clarity. This side view further illustrates the essential narrowing of the flow arrangement between the relatively wide cross section between the duct 4 side and the diffuser 10 side. As indicated previously, this narrowing is to facilitate entrainment by the projected motive gas flow 6a in order to stimulate coolant air drawn flow through the conduit path 12. This coolant flow 5 as indicated previously enhances the heat exchange capacity of the heat exchanger through fins 3 such that the assembly, on a like for like basis, has a higher heat exchange capacity than without such coolant flow. Injection of the motive gas flow 6a can be angled but must point into flow direction in order to stimulate a localized low pressure in the draught region 9 which creates a bias causing the coolant flow 5 through the conduit path 12 and subsequently out of the diffuser 10 following mixing in the mixing zone 13.

It will be understood that the angled presentation of the motive gas flow 6a is dependent upon the nozzle 8 projection into the draught region 9. This angle will depend upon desired operational performance but clearly the more perpendicular the flow 8 to the path conduit 12 generally the less entrainment will be achieved.

Typically, the motive gas flow 6a will be projected to a point somewhere within the mixing region 13 in order that this flow 6a entrains through Venturi effects cooler air within the conduit path 12 to sustain a notional lower pressure in the draught region 9 and therefore stimulate further replacement coolant flow 5 into that conduit path 12 and into draught region 9 and into mixing zone 13 from about the heat exchanger.

The nozzle 8 can generally be formed by a wall portion 30 of the conduit path 12 and an opposed ramp surface 31 in order that the surfaces overlap in order to create an annulus of sufficient length and cross section to project the motive gas flow 6a as required for entrainment and stimulation of coolant air flow through the conduit 12 as required. Alternatively, injector nozzles of circular, triangular, rectangular, etc. geometry can be used to inject flow 6a into draught region 9.

In order to regulate and control the motive gas flow 6 it will be understood that as illustrated in Fig. 3 generally the entrainment path 7 to the nozzle 8 will be indirect, that is to say, will incorporate baffles or bends in order that the flow 6 can be appropriately orientated for projection through the nozzle 8 at an angle suitable for stimulating coolant air flow 5 through the fins 3 via the conduit path 12 and using nozzles of circular, rectangular, ellipsoidal and similar geometric exits.

The present heat exchanger assembly and its flow arrangement will be particularly used with regard to achieving adequate cooling of electronic components utilised for converting electric power and operating electric engine components engines. Figs. 4 to 7 illustrate as front schematic cross-sections alternative configurations for engines incorporating heat exchanger assemblies with their flow arrangements in accordance with the present invention.

Fig. 4 illustrates a first configuration consistent with the heat exchanger assemblies depicted in Figs. 1 and 2. Thus, a mounting wall 40 supports a cowling within which heat exchangers 43 are presented in order to dissipate heat from electronic components in modules 41 supported upon a respective chassis 42 between the heat exchangers 43 and electronic component modules 41. The heat exchangers 43 present fins in a conduit 44 which as described previously leads into a conduit path 12 and subsequent flow arrangement in accordance with the present invention. In such circumstances, when a motive gas flow is presented through that flow arrangement than coolant air is drawn past these heat exchangers 43 in a direction perpendicular to the plane of Fig. 4 in order to stimulate greater heat exchange effect relative to the natural capability of that heat exchanger 43.

Fig. 5 illustrates a second embodiment of a heat exchanger arrangement incorporated in an engine in accordance with the present invention. Thus, upon a mounting wall 50, heat exchangers 53 are secured with electronic modules 51 secured through a respective chassis to those heat exchangers 53 such that heat flux generated by the electronic modules 51 is dissipated by the heat exchangers 53. Each heat exchanger 53 is essentially compartmentalized such that it has a proportion of a duct 54 which leads to a conduit path and subsequent flow arrangement in accordance with the present invention.

These flow arrangements operate as described previously in order to stimulate coolant air flow through the heat exchanger 53 and therefore create an enhanced heat exchanger operation in comparison with the natural capacity of that heat exchanger 53. By provision of the arrangement illustrated in Fig. 5, it will be appreciated that the requirements of each individual heat exchanger and therefore associated electronic module can be accommodated by varying the motive gas flow 6 to stimulate drawn coolant airflow through the conduit 54 to the requirements of that particular heat exchanger either consistently or transiently dependent upon coolant requirements.

Fig. 6 illustrates a third embodiment of a heat exchanger assembly incorporating a flow arrangement in accordance with the present invention for an engine. Thus, each heat exchanger 63 is associated with a respective electronic component 61 through a chassis such that heat flux generated by those electronic components 61 is dissipated by the heat exchanger 63. These heat exchangers 63 are formed in ducts 64 which lead to conduit paths and subsequent flow arrangements in accordance with the present invention. Again, the individual ducts 64 will generally have their own nozzle for entrainment through the motive gas flow as described previously such that the individual heat exchanger 63 can be subject to differing stimulated coolant air flows drawn through the ducts 64.

Fig. 7 illustrates an alternative arrangement where heat exchangers 103 are located outside of electronic packages 102 such that a hot motive gas flow 106 enters through nozzles 107 in order to draw entrained air flow 108 through the heat exchangers 103 in order to cool those heat exchangers 103 and therefore the electronic packages 102.

In such circumstances it will be appreciated that operation of the arrangement depicted in Fig. 7 is in a similar fashion to that described previously in that by use of the high pressure hot gas flow 106, an entrained cooler flow 104 is drawn through apertures 105 in order to cool the heat exchangers 103.

Fig. 8 illustrates an alternative nozzle arrangement in accordance with the present invention. Thus, a nozzle 206 presents a hot gas flow to a contraction area 200 in a duct 207. Thus, a heat exchanger 203 has an entrained flow 205 drawn through it as a result of the hot gas flow 201 projected from the nozzle 206. The entrained flow 205 as well as the projected hot gas flow 201 then proceeds further through the ducts 207 as a combined flow 210. The heat exchanger 203 is coupled to an electronic package 202 in order to provide cooling for that package 202.

Generally, the duct 207 may be circular, rectangular, conical, triangular or octagonal provided that a constriction in the contraction area 202 created in order to facilitate and drive through a drawing effect upon the entrained flow 205 presented to the heat exchanger 203 for cooling effect.

As indicated above, the present invention utilises the motive airflow from a relatively hot gas flow typically taken anywhere appropriately in a turbine engine from the high pressure gas flow through that turbine engine.

Conventionally, coolant air has been taken through a ram effect via an inlet duct. The present invention may supplement that prior approach.

Unlike conventional ejector pump systems, the present flow arrangement is designed for energy efficient drive of exhaust gas or machines and to minimise the requirement for space in an engine (e.g. aero gas turbine). A duct entry to the flow arrangement has a much larger aspect ratio (width/height), which is desirable for improving the aerodynamic performance inside the arrangement.

Essentially, the flow arrangement can be formed with two sheets of metal which confine the coolant air flow and motive gas flow. The upper wall of the arrangement is mounted directly onto a rigid wall 20 of a main engine.

The wall 20 may be a plane rather than a curved surface as depicted. Compressed motive gas 6 is fed through the ejector nozzle(s) from a lower wall, which has a varying profile characterizing a contraction, into the draught region 6a, a mixing section 13 and a diffuser 10. It should be noted that the arrangement can be modified to fit the shape of the mounting wall 20 in an engine. For instance, an arrangement having a rectangular frontal cross-section but a varying profile in the streamwise direction can be used for mounting onto a flat wall in an engine. Other possible embodiments of flow arrangements are such as that having triangular and polygonal cross-sections (as shown in Figs. 5 and 6).

Single or multiple nozzle ejectors can be used for feeding motive gas 6 into drawn flow conduit path 12, depending on the specifications for the associated pressure losses and noise level. The inlet duct to the arrangement may protrude from the lower wall and interact directly with the cooling air flows, that is to say as a scoop. An aerodynamic fairing can be used in conjunction with the inlet duct and the feed nozzles to minimise pressure losses of the cooling air flows. Alternatively, the inlet duct may be located coaxially with and within the lower wall of the structure.

Heat exchangers 3 are attached to the electronic devices 2 from which heat is generated and conducted to the convective fin surface. The electronic devices 2 and heat exchangers 3 are fixed into position within the heat exchanger assembly. It is possible to utilise an engine housing wall as a chassis or heat spreader plate for the electronic devices 2. The electronic devices 2 are positioned in-line and circumferentially around the lower wall of the assembly and sufficiently away from the compressed hot motive gas flow 6. This is to avoid any possible transfer of heat from the hot motive gas flow 6 to the upstream electronics 2. The in-line arrangement is used to facilitate a uniform cooling air flow 5 temperature across the different electronics devices. This is important for efficient operations and a prolonged life for the electronic devices.

The motive gas flow 6 discharges through the nozzle 8 into the draught region 9 of the assembly. Consequently, a low pressure zone is created at the exit plane nozzle.

This resulting pressure difference helps to entrain cold gas (i.e. ambient air) through the heat exchangers 3 which are located near the inlet for the flow arrangement. It is important to note that the compressed gas temperature increases with pressure. Heated motive gas flow 6 is only used to entrain coolant air flow and draw it through the heat exchangers 3 to improve heat transfer. This invention is thus able to utilise hot gas and/or exhaust gas to promote cooling. The contraction in the draught region 9 and diffuser 10 are usually required to produce a specific exit velocity or pressure or simply act as the connection between the ejector nozzle 8 and the downstream duct of a different radius. The entrained cooling air flow is accelerated through the contraction of the draught region 9 which also promotes the mixing of the motive flow 6 and cooling air flow 5. The use of cold entrained gas instead of the hot motive gas for cooling widens the temperature difference with respect to the heat source (i.e. electronics). The temperature differences lead to enhanced heat transfer on the heat exchange surface. Hence, the heat exchangers 6 can be re-sized accordingly for cooling a particular heat load to satisfy the requirements for minimal system weight and space. The integration of the nozzle 8 into the arrangement structure ensures the reduced size and weight of the heat exchangers 3 are not offset by the weight and space caused by the ejector nozzle. Air can alternatively be injected by dedicated nozzles with a range of outlet nozzle geometries. Air can also be transiently injected from a compressed gas cylinder for operation, in which RAM pressure driven air flows are supplemented with ejector driven airflow. The compressed air cylinder can be recharged at regular intervals from the engine compressor.

The supply of motive gas flow 6 from the main engine eliminates the use of a rotatable compressor fan to drive the cooling gas through the heat exchangers 3. This not only minimises the size and space of the cooling system but also avoids vibrations and noises caused by a compressor.

Sources of motive gas flow include compressed gas from a gas turbine engine, and exhaust gas from a conventional internal combustion engine or turbine stage and a separate compressor, which may be rechargeable by engine powered means (i.e. only a high pressure gas is needed to drive coolant flow - the source can be a gas cylinder with high pressure gas). The use of hot gas from the turbine stage could offer considerable advantages for land-based gas turbines. It is possible to modify the assembly such that a condensable vapour (e.g. steam) or other liquids can be used as the primary gas flow 6 for entraining a secondary liquid coolant stream (like an eductor, injector and siphon).

The assembly can be modified to compensate for changes in the temperature, density or velocity of the secondary cooling stream for the heat exchangers. This is especially important in an aircraft engine where ambient conditions change as the aircraft changes speed. In such applications it is possible to set the penalty of the power consumed to drive the ejected motive gas flow 6 for short periods against the benefit of being able to employ a smaller heat- exchanger. Free and cold ambient air is ducted into the assembly inlet when the velocity of the moving primary body (e.g. aircraft) is high. Once the dynamic head of the ducted ambient flow is sufficient to cool a designed heat load on the electronics, the motive gas supply is switched off. Such modulation of the ejector ensures an economic running of the cooling system.

As an alternative to the motive gas flow 6 generating entrainment and therefore drawing flow 5 as described above, it will be appreciated that in the alternative, motive air flow may be presented through the duct 4, conduit path 12 and out of the diffuser 10 via the draught region 9 such that air or gas flow is then drawn through the nozzle 8 by this high pressure flow through the arrangement. Thus, annular nozzles can provide the high pressure driving air in a larger annular arrangement, in which the high pressure air points into the overall direction, into which a larger low pressure air flow is to be entrained or transported.

A number of small nozzles can entrain air in a larger circular or rectangular duct, by this alternative described above. In this patent application the heat exchanger can be between the entry and the low pressure region (say 4 to 9), or alternatively it could be situated in between position 7 and 9 but this would require careful design, but is possible to attain the same effect of drawing air through a finned or otherwise porous heat exchanger. The heat exchanger could also be an arrangement of conductive pipes containing oil or kerosene fuel for transferring heat from inside an aero- engine to the outside ultimately to the surrounding ambient heat sink.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect ofany patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims (18)

1. A flow arrangement for a heat exchanger, the arrangement comprising an entrainment path extending to a draught region, the entrainment path arranged to direct a gas flow angularly across the draught region to entrain airflow upstream of the draught region to stimulate airflow through the arrangement.

2. An arrangement as claimed in claim 1 wherein the draught region narrows immediately downstream of the entrainment path to form a contraction area.

3. An arrangement as claimed in claim 1 or claim 2 wherein the draught region is associated with a diffuser to further stimulate airflow through the arrangement.

4. An arrangement as claimed in any of claims 1, 2 or 3 wherein the entrainment path includes a nozzle to the draught region, the nozzle formed between an overhang surface and a ramp surface to form an annulus between them to provide angular presentation of the gas flow across the draught region.

5. An arrangement as claimed in any of claims 1, 2 or 3 wherein the gas flow is fed co-axially into the entrained flow.

6. An arrangement as claimed in any of claims 1, 2 or 3 wherein the gas flow is fed at an angle into the entrained flow.

7. An arrangement as claimed in claim 6 wherein the gas flow is directed by a specific nozzle.

8. An arrangement as claimed in any preceding claim wherein the entrainment path is indirect before the nozzle.

9. An arrangement as claimed in any preceding claim wherein the arrangement includes means for ejecting pressurized gas to supplement the gas flow directed angularly across the draught region.

10. A flow arrangement for a heat exchanger substantially as hereinbefore described with reference to the accompanying drawings.

11. A heat exchanger assembly for cooling electronic components on a chassis, the assembly comprising a flow arrangement as claimed in any preceding claim and a heat exchanger associated upstream of the arrangement through a conduit path whereby the airflow is drawn from the heat exchanger.

12. An assembly as claimed in claim 11 wherein the conduit path narrows towards the draught region to form a contraction area.

13. An assembly as claimed in claim 11 or claim 12 wherein one side of the conduit path is provided by an entrainment wall with a part of that entrainment wall forming part of the entrainment path.

14. A heat exchanger assembly for cooling electronic components on a chassis substantially as hereinbefore described with reference to the accompanying drawings.

15. An engine comprising a source of hot gas flow and a flow arrangement as claimed in any of claims 1 to 9 wherein the gas flow is taken from the source of hot gas flow in order to stimulate the relatively cooler ambient air flow through the flow arrangement.

16. An engine comprising a source of compressed hot gas flow or exhaust flow and a heat exchanger assembly as claimed in any of claims 11 to 14.

17. An engine as claimed in claim 15 or claim 16 wherein the engine is a gas turbine engine or internal combustion engine.

18. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.