WO1986005558A1

WO1986005558A1 - Improvements in pressure-exchangers

Info

Publication number: WO1986005558A1
Application number: PCT/GB1986/000169
Authority: WO
Inventors: Ronald Denzil Pearson
Original assignee: University Of Bath
Priority date: 1985-03-22
Filing date: 1986-03-24
Publication date: 1986-09-25
Also published as: GB8507540D0; EP0215091A1

Abstract

A rotary pressure-exchanger acting as a prime mover, or ''gas-wave turbine'', has a high-pressure scavenging stage (28 and 47) that supplies compressed air to a combustion chamber (25) and receives compressed hot gas from it. Part of the expanding hot gas is supplied to an intermediate-pressure scavenging stage that supplies compressed air to a high-pressure pre-scavenge inlet (24) and optionally also to low-pressure pre-scavenge inlets (19 and 32). Low pressure scavenging steps (11 and 34, 21 and 45), alternate with scavenging steps at higher pressures.

Description

Improvements in pressure-exchangers

The invention relates to rotary pressure- exchangers, and especially to rotary pressure- exchangers capable^" of acting as prime movers.

A rotary pressure-exchanger typically comprises a rotor that defines a ring of cells open at both ends, and is located between a pair of stators defining ports that are opened and closed to each cell in turn by virtue of the motion of the rotor. The changes in pressure within the cells are largely determined by waves of compression and rarefaction propagating along the cells from the edges of the ports. A rotary ■pressure exchanger may be caused to act as a prime mover by arranging that gases predominantly flow into the cells at one end and out of the cells at the other end, and seρarating_> adjacent cells by blades that are oblique to the axis of the rotor so that the net current of gases through the rotor can cause the rotor to turn like a turbine. For those reasons, rotary pressure-exchangers acting as prime movers are sometimes known as gas-wave turbines, and it is with such gas-wave turbines that the present invention is especially concerned.

The invention provides a pressure-exchanger comprising: a rotor defining a ring of cells open at both axial ends thereof; inlet and outlet stators covering respective ends of the cells and so defining permanently-open ports that as the rotor rotates each cell is in communication at one end with each port in the inlet stator in turn in a predetermined sequence and is in communication at the other end with each port in the outlet stator in a predetermined sequence, the said ports comprising first and second low-pressure scavenge inlet ports arranged to be in communication with a source of a first gas at a low pressure^', first and second low-pressure scavenge outlet ports associated with the first and second low-pressure scavenge inlet ports, respectively, and arranged to be in communication with a sink for a second gas at a low pressure, an intermediate-pressure scavenge inlet port after the first and before the second low-pressure scavenge inlet port with respect to one sense of rotation of the rotor, an intermediate-pressure scavenge outlet port associated with it, a high-pressure scavenge inlet port after the second and before the first low-pressure scavenge inlet port arranged to be in communication with a source of the second gas at high pressure, a high-pressure scavenge outlet port associated with the high-pressure scavenge inlet port arranged to be in communication with a ^*sink for the • first gas at high pressure, each said scavenge inlet port and its associated scavenge outlet port being in opposite ones of the two stators and being so disposed that in normal operation of the pressure exchanger a gas can enter cells through the scavenge inlet port and displace the cell contents through the associated scavenge outlet port, and a second high-pressure outlet port between the high-pressure scavenge outlet port and the first low-pressure scavenge outlet port, the second high-pressure Outlet port being in communication with the intermediate-pressure scavenge inlet port.

The invention also provides a method of operating a pressure-exchanger that comprises a rotor defining a ring of cells, which method comprises performing cyclically for each cell the steps of: introducing a first gas to the cell and scavenging a second gas from the cell with it at a low pressure; scavenging the first gas from the cell with the second gas at an intermediate pressure higher than the said low pressure; scavenging the second gas from the cell with the first gas at a low pressure lower than the said intermediate pressure; scavenging the first gas from the cell with the second gas at a high pressure higher than the said intermediate pressure; and permitting the second gas to expand and conveying a portion of it to the intermediate-pressure scavenging step.

Throughout this Specification, the portion of the pressure-exchanger carrying the cells is referred to as the "rotor" and the portions carrying the ports as the "stators". Although it is in general more convenient to have the ports stationary, there are circumstances in which it is preferable for them to move and the present invention includes arrangements in which the "stator" rotates about the axis of the rotor ^'relative to an external .frame of reference. The "rotor" may then be stationary relative to the external frame of reference.

Although it is preferred to have a single complete operating cycle around the entire circumference of the rotor, there may- instead be two or more substantially identical cycles each occupying a fraction of the circumference, and the invention includes such configurations.

Although it is preferred to have all parts of the cycle on a single rotor, the high-pressure and intermediate-pressure steps may be performed on separate rotors, which are preferably mounted on a common shaft.

In order to achieve turbine action it is preferred to cause gases to flow through the cells predominantly from an "inlet stator" to an "outlet stator", but the use of those terms and the corresponding terms "cell inlet" and "cell outlet" does not exlude the presence of a minority of inlet ports on the outlet stator and/or outlet ports on the inlet stator.

At least part of the first gas from the intermediate-pressure scavenging step may be delivered through a pre-scavenge inlet port immediately before the high-pressure scavenge inlet port. The power that is extracted from the expansion step after the high- energy scavenging is thus returned, at least in part, to the high-energy cycle.

Pre-scavenge inlet ports are preferably so arranged at a high angle to the axis of the rotor, and a low angle to the local direction of motion of the rotor past it, as to start the gas in the cell moving towards the scavenge outlet before the scavenging begins. As is explained in more detial by way of example below, it is preferred for a compression wave from the opening edge of the pre-scavenge inlet (that is to say, the edge at which each cell first opens to the port) to propagate along the cell followed by another compression wave of similar height from the opening edge of the scavenge inlet, and for the two waves to reach the cell outlet on either side of the opening edge of the scavenge oulet. The two compres¬ sion waves and their reflections can then give condi¬ tions at the opening edge of the cell outlet that lead to little back-flow of gas from the outlet into the cell and little generation of new waves from the edge.

Any high-pressure or intermediate-pressure scavenge outlet may be provided in the land defining its opening edge with a reverse-flow duct in the form of an arcuate duct that connects points in the outlet port adjacent to and at a short distance from the rotor, so that if there is a transient reverse flow of gases from the outlet port into the cell outlets as they open that reverse flow can be gas from the reverse-flow duct with a significant velocity component parallel to the direction of motion of the rotor.

The second gas may be allowed to expand out of the cells as a last stage after each of the high-pressure and intermediate-pressure scavenging steps and immediately re-introduced at pre-scavenge inlet ports for the subsequent low-pressure scavenging steps. Advantageously, the second gas is allowed to expand into an expansion port in the outlet stator that generates an expansion wave-pair and is re-introduced through a pre-scavenge inlet port in the inlet stator. Instead, the second gas may be allowed, to expand into and return frmo a pocket formed in the face of the inlet stator; that is a simpler arrangement to construct bu may give less efficient conversion of the energy in the gas to thrust on .the rotor .

Instead, some of the first gas from the intermediate-pressure scavenge outlet port may be supplied to low-pressure pre-scavenge inlet ports. In that case, the pre-scavenge gas wil actually initiate the scavenging operation and the length of the scavenge inlet port in the direction of motion of the rotor can be correspondingly reduced. Each low- pressure scavenge inlet port is then preferably as long as the progression of a wave-pair, that is to say, as long circumferentially as the distance travelled by any point on the rotor in the time taken for a wave from the opening edge of the inlet to propagate to the cell outlet and the reflected wave to propagate back to the cell inlet, which is about half as long as the inlet would otherwise be.

There may be additional intermediate-pressure scavenging steps, supplied with gas from the expansion stages of other cycles, and there are then advantage¬ ously three intermediate-pressure scavenging steps, the first and second supplied with the second gas from successive expansion stages after the high-pressure scavenging step and the third from an expansion stage after the first intermediate-pressure scavenging step. The first gas from the first intermediate-pressure scavenging step is then preferably supplied to a high- pressure pre-scavenge inlet, and that from each of the other two intermediate-pressure scavenge outlets is then preferably supplied to two low-pressure pre- scavenge inlets, one for a low-pressure scavenging step immediately before the intermedate-pressure scavenging step in question and the other for a low-pressure scavenging step before a respective one of the high- pressure and first intermediate-pressure scavenging steps. Such an arrangement with multiple pressure cycles enables the available energy from expansion of the second gas to extracted efficiently, while the large number of exchanges of gas leads to very efficient cooling of the rotor when one of the gases starts hot and the other cool.

Further intermediate-pressure cycles may be included, for example, there may be four further cycles each driven from a further expansion stage of a respective one of the cycles mentioned in the last . paragraph.

The cells of the rotor are advantageously separated by blades that are helically shaped and aligned with their edges at the cell inlets leading their edges at the cell outlets in such a manner that the gases flowing through the cells tend to impel the rotor to rotate. The blades may be curved more nearly parallel to their direction of motion at the cell outlets, restricting the outlets somewhat so that when the cell outlets are open to outlet ports waves propagating along the cells are not strongly reflected at the outlets. The cell outlets may be divided by radially-extending dividers which both increase the degree of constriction of the cell outlets and decrease the tendency for a jet of gas to leave the cell in a direction relative, to the cell having a component in the direction of movement of the cell when the cell first begins to open to an outlet port at a lower pressure than itself.

The first gas may be a combustion-supporting or combustible gas, and is advantageously air, and the second gas may be a gas produced by combustion involving the first gas. The first gas from the high- pressure scavenge outlet is then advantageously supplied to a combustion chamber from which gaseous combustion products are supplied as the second gas to the high-pressure scavenge inlet. The pressure exchanger may then be included as the highest-pressure stage of a gas-turbine or the like, replacing the usual combustion chamber. Even the "low-pressure" stages will then be at quite a high pressure, but it will be appreciated that their pressure will still be low compared to that of the "high-pressure" stages.

Various forms of rotary pressure-exchanger constructed in accordance with the invention will now be described by way of example only with reference to the accompanying drawings, in which:

Fig. 1 is an axial elevation view of a rotor;

Fig. 2 is a fragmentary developed sectional view of a first form of pressure-exchanger including a rotor as shown in Fig. 1;

Fig. 3 is a cross-section through part of a rotor;

Fig. 4 is a view similar to Fig. 2 with the rotor omitted;

Fig. 5 is a view similar to Fig. 4 of a second form of pressure-exchanger;

Fig. 6 is a view similar to Fig. 4 of a third form of pressure-exchanger; and

Fig. 7 is a detail view corresponding to part of Fig. 4 and showing a modification of the form of pressure-exchanger shown in Fig. 4, to a larger scale than Fig. 4.

Referring to the accompanying drawings, and initially to Figs. 1 to 3, one form of rotary pressure-exchanger comprises a rotor indicated generally by the reference numeral 1 disposed between an inlet stator indicated generally by the reference numeral 2 and an outlet stator indicated generally by the reference numeral 3.

The rotor 1 comprises a hub 4, radially and axially extending blades 5., and a shroud 6, between which are defined cells 7 that are open at both axially-facing ends. The .radially extending edges of the blades 5 define radially-extending notional surfaces facing the stators 2 and 3, and the cells 7 open out through those surfaces at inlets B towards the inlet stator 2 and outlets 9 towards the outlet stator 3. Each cell may be provided in its outlet end portion with a divider 10. The number of cells 7 is preferably a prime number to reduce the possible resonant vibrations of the rotor 1.

Although the cells 7 and blades 5 are shown in Fig. 1 as being so angled that their radially outer extremities are offset relative to their radially inner extremities by about half the width of a cell, it will be appreciated that they may be at a different angle, for example, they could extend radially with no offset. In Figs. 2 to 7, any appearances of the blades 5 outside the plane of section that might arise from any such offset have been omitted in the interests of clarity.

As may be seen from Fig. 2, the stators 2 and 3 define a number of ports that open through the surfaces of the stators facing the rotor 1. Any appearances of the ports outside the plane of section have been omitted. The surfaces of the stators through which the ports open and the (largely notional) surfaces defined by the leading and trailing edges of the blades 5 are separated by a gap, the width of which is determined by the need to provide a running clearance between the rotor and stators and the need to minimise the amount of gas that flows along the gap from cell to cell or form port to port. Contact seals are not provided.

Referring to Fig. 2, the inlet stator 2 defines a first low-pressure scavenge inlet 11 which is here divided by a first nozzle blade 12, although such division of the inlet is not essential. The first low- pressure scavenge inlet 11 is followed (going in the direction of motion of the rotor 1) by a land 13 that is large enough to cover the inlet port 8 of a cell 7 completely. Next is an intermediate-pressure pre- scavenge inlet 14, which is separated by a thin wall 15 from an intermediate-pressure scavenge inlet 16. There is then a large land 17 provided internally with cooling pipes 18, followed by a first low-pressure pre- scavenge inlet 19, which is separated from a second low- pressure scavenge inlet 21 by a wall 20 that forms a land narrower than the width of a cell. The first low- pressure pre-scavenge inlet 19 is highly angled, forming an angle of only about 20° with the local direction of motion of the rotor 1. The second low- pressure scavenge inlet 21 may be divided internally by a second nozzle blade 22.

There is then a land 23 about one and a half times as long as the cell pitch, followed by a high-pressure pre-scavenge inlet 24 at a low angle to the local direction of motion of the rotor 1. A combustion chamber 25 has a fuel inlet 26, and a jacket 27 that in operation contains air to which no fuel has been added. The combustion chamber exhausts through a high- pressure scavenge inlet 28 in the inlet stator 2 and the jacket 27 through jacket scavenge inlets 29 and 30 before and after, respectively, the high-pressure scavenge inlet 28. After the inlets 28 to 30 is a long land 31 provided internally with cooling pipes 18. After the land 31 is a second low-pressure pre-scavenge inlet 32 which is separated from the first low-pressure scavenge inlet 11 by a wall 33 which forms a land that is narrower than the inlet of a cell. The second low- pressure pre-scavenge inlet 32 is at a low angle to the local direction of motion of the rotor 1. The two low- pressure scavenge inlets 11 and 21 are in communication with a source of fresh^* air.

The outlet stator 3 defines a first low-pressure scavenge outlet 34 which is in communication with a sink for gas, for example, the external atmosphere. The sink may be a turbine, in which case the low- pressure scavenge inlets and outlets may be at a pressure higher than atmospheric pressure. After the first low-pressure scavenge outlet 34 is a large land 35, then an intermediate-pressure scavenge outlet 36 a minor portion 36a of which adjacent to the closing edge (that is to say, the edge that a cell reaches after traversing the outlet) is separated off from the main portion of the outlet by a partition 37. The main portion of the intermediate-pressure scavenge outlet 36 is connected by a duct 38 to the high-pressure pre- scavenge inlet 24; the minor portion 36a is connected by a duct 40 to the intermediate-pressure pre-scavenge inlet 14. The ducts are- omitted from Fig. 2 in the interests of clarity but are shown by double-chain- dotted lines in Fig. 4. A reverse-flow duct 39 is formed in the land 35, and comprises arcuate duct means which opens through the face of the land 35 defining the opening edge of the intermediate-pressure scavenge outlet 36 at one end immediately adjacent to the rotor 1 and at the other end of the duct means at a short distance from the rotor. A minor portion of the outlet 36 adjacent to the land 35 may be divided off by a partition 39a.

After the intermediate-pressure scavenge outlet 36 is a land 41 large enough to cover a cell 7, then a boost extraction port 42, which is connected by a duct

43 to the first low-pressure pre-scavenge inlet 19, and to a sink (not shown) , or to means (not shown) for extracting work from it as it expands, for example, a turbine. After the boost extraction port 42 is a land

44 similar to the land 41. After the land 44 is a second low-pressure scavenge outlet 45 which is connected to the sink. After the second low-pressure scavenge outlet 45 is a large land 46 followed by a high-pressure scavenge outlet 47, which is connected by a duct 48 to a combustion air inlet of the combustion chamber 25 and to the jacket 27. A reverse-flow duct 49 is formed in the land 46 and opens into the outlet 47, which may have a partition 49a. A land 50 that is large enough to cover a cell 7 is followed by an intermediate-pressure extraction port 51 that is connected by a duct 52 to the intermediate-pressure scavenge inlet 16, and then by a land 53 that is somewhat larger than the land 50. The land 53 is followed by a low pressure extraction port 54 that is separated from the first low-pressure scavenge outlet 34 by a land 55 that is somewhat larger than the land 53, and may be up to twice the width of a cell 7. The low-pressure extracton port 54 is connected by a duct 56 to the second low-pressure pre-scavenge inlet 32 and to the sink or external expansion device.

As may be seen from Fig. 2, the ports and lands described form a complete cycle between the lines A in Fig. 2, which are at the same point on the circum¬ ference of the pressure-exchanger, so that Fig. 2 shows the entire circumference of the stators 2 and 3 with some overlap.

The sizes of the inlet and outlet^* ports and their interrelations will be described with reference to this operation.

Referring to Fig. 4, the operation of the first form of pressure-exchanger is as follows. In opera¬ tion, fresh air is supplied to the low-pressure scavenge inlets 11 and 21, and exhaust gas is removed from the low-pressure scavenge outlets 34 and 45. The pressure in the inlets 11 and 21 is preferably main¬ tained slightly higher than that in the outlets 34 and 45 by the use of a fan, compressor, or the like (not shown) , which may be driven from the rotor 1 or from a separate turbine (not shown) . Exhaust gas is also extracted from the ducts 43 and 56 and, since that gas is substantially above the pressure of the sink to which the low-pressure outlets 34 and 45 are connected, it is preferably exhausted through an external turbine (not shown) to extract as much useful work from it as possible. The rotor 1 is caused to rotate, with the cells 7 and blades 5 moving upwards as seen in Fig. 1. Although the blades 5 are shown in Fig. 1 as extending parallel to the axis of the rotor, they are in fact preferably staggered, with the inlet end leading the outlet end, as is shown in Fig. 3, so that the flows of gas from inlet to outlet will tend to propel the rotor. If the cells 4 are staggered then the inlet stator 2 and the outlet stator 3 must be offset relative to the positions shown in Figs. 2 and 4 in such a way that points on the two stators that are shown level in Figs. 2 and 4 are passed by the ends of any cell 7 simultaneously. The pressure exchanger shown in Fig. 4 is especially suited to use as a prime mover, that is to say, as an apparatus in which power generated by combustion in the combustion chamber 25 is extracted primarily as mechanical power from the rotor 1.

In Fig. 4, waves of rarefaction have been shown by dashed lines, and waves of compression by solid lines, and contact surfaces between different gases by chain- dotted lines. Except where the contrary is stated, the lines show the points at which the mid-height of the wave meets the leading wall of the cell as the cell moves up the drawing. The lines may thus be regarded either as instantaneous positions with the stepping caused by the fact that the wicith of the cells is no.t small enough to be ignored smoothed out, or as tracks of waves in a given cell over time.

A cell 7 enters Fig. 4 at the bottom, past the land 55, containing exhaust gas at low pressure, but above the pressure of the first low-pressure scavenge outlet 34. As the cell 7 clears the land 55 and opens to the outlet 34, gas starts to flow out of the outlet and a wave of rarefaction 57 propagates along the cell towards the inlet. The wave 57 is somewhat spread out, because it builds up gradually as the outlet of the cell opens, and spreads out more as it propagates because it is a rarefaction wave and thus its beginning is in denser and warmer air, and thus is faster-moving, than its end. The wave 57 is shown in Fig. 1 by two diverging dashed lines, and in fact occupies the region between those lines. The rarefaction wave reaches the inlet of the cell shortly before that passes the wall 33 between the second low-pressure pre-scavenge inlet 32 and the .first low-pressure scavenge inlet 11, at least when the rotor is moving at a design speed, and the wall- is so shaped that the effective boundary condition at the cell inlet is midway between those of a completely closed cell and a completely open cell, and there is no significant reflected wave. The land at the end of the wall 3 is between one third and one fifth of the.width of a cell 7; if it were larger it would tend to cause a reflected wave pulse, while if it were narrower it would not prevent a back-flow into the low-pressure scavenge inlet 11 of gas at a higher pressure from the pre-scavenge inlet 32 or from cells 7 that have not yet had their pressure reduced by the wave 57.

After passing the wall 33, the cell is open at its inlet end to the first low-pressure scavenge inlet 11 and at its outlet to the first low-pressure scavenge outlet 34, and fresh air from the inlet flows into ^*the cell and exhaust gas flows out of the cell to the outlet. The boundary between fresh air and exhaust gas is shown approximately by the chain-dotted line 58. It will be seen that the cell is preferably slightly over-scavenged so that a little fresh air reaches the outlet 34, in order to ensure that no exhaust gas remains in the cell. The length of the inlet 11 between the wall 33 and the first nozzle blade 12 is approximately equal to the progression of a wave-pair, that is to say, to the distance that the rotor moves at the said design speed in the time that a wave takes to travel the length of the cell and back. The first nozzle blade 12 has an aerofoil shape, and causes a region of reduced pressure_. between it and the rotor so that a rarefaction wave 59 propagates along the cell from the upstream end of the nozzle blade, followed by a compression wave 60 from the downstream end of the blade. The rarefaction wave 59 meets the outlet of the cell while the cell is still open to the first low- pressure scavenge outlet 34,^' and is reflected as a compression wave 61. The compression wave from the nozzle blade 12 meets the outlet of the cell when the cell is closed by the land 35 and is reflected as a compression wave 62. Between the compression wave 60, the reflected wave 61, and the cell outlets is a region in which the cell contents are substantially at rest and at the same pressure as the outlet 34. The closing edge of the outlet 34 may therefore be anywhere between the points of reflection of the two waves without serious flow-reversal through the cell outlet and undesired waves resulting. There is thus significant tolerance in the rotor speed, which determines where the points of reflection fall. The two reflected compression waves 61 and 62, propagating to meet the inlet of the cell as it passes the land 13, cause fresh air from the second portion of the inlet 11, between the nozzle blade 12 and the land 13, to enter the cell, raising the pressure in the cell while leaving the cell contents substantially at rest. The length of the second portion of the inlet 11 is approximately equal to the progression of a wave-pair. The compression wave 61 is reflected from the inlet 11 as a rarefaction wave 63 and the compression wave 61 is reflected from the land 13 as a compression wave. Between the rarefaction wave 63, the compression wave 62, and the inlet end of the rotor is a region of substantially stationary air (which appears triangular in Fig. 4) and, provided that the edge where the land 13 closes the inlet 11 falls alongside that stationary region, no significant new pulse is generated from the closing edge. The length of the stationary region therefore gives a tolerance of variations in the speed of the rotor .

While the outlet of the cell is still closed by the land 35, the inlet passes the intermediate-pressure pre-scavenge inlet 14. The^' pre-scavenge inlet 14 is steeply, angled relative to the axis of the rotor and injects a mixture of gases under pressure into the cell with a fairly high tangential velocity, causing a compression wave 64 to propagate along. the cell towards the outlet. The cell then passes the wall 15 and opens to the intermediate-pressure scavenge inlet 16, which is supplied with exhaust gas at fairly high temperature and at substantially the same pressure as the gas in the pre-scavenge inlet 14. A second compression wave 65 therefore propagates along the cell. The pre- scavenge inlet 14 and the scavenge inlet 16 together span the progression of a wave pair.

Although the. inlets 14 and 16 are- supplied with gas at equal pressures, the high angle of the pre-scavenge inlet 14, a throttling action in the inlet, and the fact that it delivers cooler gases than the scavenge inlet 16, make it possible to achieve the double compression wave 64 and 65, with some of the energy of the pre-scavenge air being converted into thrust on the rotor 1.

The exhaust gas entering from the scavenge inlet 16 expels the fresh air in the cell to the intermediate- pressure scavenge outlet 36 which is connected by the duct 39 to the high-pressure pre-scavenge inlet 24, the boundary surface between the fresh air and exhaust gas being shown by a chain-dotted line 66. The inter¬ mediate-pressure scavenging thus serves to convert compressed exhaust gas expanding from the high-pressure stage to compressed fresh air which is to be supplied -to the high-pressure stage. The outlet 36 is about as long as the progression of a wave pair. The cell is slightly over-scavenged, and near the closing end of the outlet 36 some exhaust gas is mixed with the fresh air. The mixed gas is received in the partitioned-off minor portion 36a of the scavenge outlet 36 and is returned through the duct 40 to the pre-scavenge inlet 14. Because of the way in which the inlet 14 and the outlet 36a_ are connected, the inflow from the pre-scavenge inlet is maintained at a pressure such that the two compression waves 64 and 65 are produced, and the pressure increase between the cell pressure after the low-pressure inlet 11 and the pressure during the intermediate-pressure scavenging is divided between those two compression waves, with a plateau at a half-way pressure between them. As may be seen from Fig^'. 4, the mixed gas from the pre-scavenge inlet 14 passes along the boundary surface and is collected again gby the duct 40. As a result, very little exhaust gas contaminates the compressed air in the duct 38.

The first compression wave 64 reaches the outlet end of the cell before the cell opens to the intermediate-pressure scavenge outlet 36, and is reflected as a compression wave 67 from the land 35. Between the reflected compression wave 67, the second compression wave 65, and the outlet end of the rotor is a region of stationary gas at the outlet pressure (which appears triangular in Fig. 4) and, provided that the opening edge of the outlet 36 falls alongside that stationary region, no significant new wave will be caused by the edge. Because the outlet 9 of the cell 7 is restricted both by its being curved to form a higher angle with the rotor axis, as is shown in Fig. 3, and by the divider 10, the second compression wave 65 is reflected only weakly from the outlet 36.

The cells 7 are of non-zero circumferential width. and the waves are spread out over the width of each cell. As a cell reaches the outlet 36, the outlet may begin to open before the pressure in the cell reaches the pressure in the outlet 36. There is then a transient back-flow of gas from the outlet 36 into each cell 7 followed by a transient surge of gas from the cell into the outlet as the cell pressure rises. The presence of the reverse-flow duct 39 enables the back- flow to be drawn from the reverse-flow duct with significant tangential speed, reducing the retarding impulse on the rotor that would result from a back flow of gas from the outlet at low tangential speed, and the subsequent transient surge into the outlet flows into the passage between the land 35 and the partition 39a, reducing the disturbance to the flow elsewhere in the outlet 36.- The dividers 1.0 in the cell- outlets also serve to reduce these transient phenomena by hindering the transient flows, which tend to enter and leave the cell obliquely to the plane of the divider 10.

The reflected compression wave 67 will lead to a pressure at the scavenge outlet 36 higher than that at the inlet 16, and reflects again from the closing end of the scavenge inlet as a rarefaction wave that brings the contents of the cell to rest.

The land 17 after the intermediate-pressure scavenge inlet 16 is exposed continuously to the heat of the combustion gas from the inlet 16, and is provided with the cooling pipes 18 to prevent its becoming too hot. While the inlet of the cell is closed by the land 17, the outlet opens to the boost extraction port 42. The gas in the cell expands into the boost extraction port 42, from which some of it is returned to the cell through the first low pressure pre- scavenge inlet 19, almost opposite. The boost extraction port 42 is about as long as the progression of a wave pair, so that the expansion wave 6.9 that propagates from its opening edge reflects from the land 17 and returns to its closing edge, leaving the gas in the cell more rarefied but at rest. The pre-scavenge inlet 19, can also be as long as the progression of a wave-pair, but is preferably substantially shorter. The outlet end of the cell then opens to the second low- pressure scavenge outlet 45, through which the gas from the cell further expands and escapes to the sink.

The part of the cycle just described, above the line A and below the line B in Fig. 4, forms a boost cycle. The part of the cycle above the line B forms a main cycle which is in many ways simliar to the boost cycle and will therefore be described more briefly.

The inlet end of the cell passes the narrow land of the wall 20 and opens to the second low-pressure scavenge inlet 21, with the second nozzle blade 22, and passes the inlet 21, the second low-pressure scavenge outlet 45 and the land 46. The structure, function, and inter-relation of these are the same as for the corres¬ ponding features 11, 12, 34 and 35 described above.

The inlet of the cell then passes the high- pressure pre-scavenge inlet 24 and the high-pressure scavenge inlet 28, which is the outlet from the combustion chamber. The pre-scavenge inlet 24 is supplied with compressed fresh air from the inter¬ mediate-pressure pre-scavenge outlet 36. Between the • pre-scavenge inlet 24 and the scavenge inlet 28 is one inlet 29 from the combustion chamber jacket 27, which acts to some extent as a further pre-scavenge inlet, while the other inlet 30 from the jacket 27 provides a stream of less hot air between the very hot gas emerging from the main combustion chamber and the land 31. The inlets 24 to 30 span approximately the progression of a wave-pair. The exhaust gas from- the combustion chamber 25, entering the cell through the high-pressure scavenge inlet 28, expels the compressed fresh air from the cell to the high-pressure scavenge outlet 47, from which it is supplied by the duct 48 to the combustion chamber. As in the intermediate-pressure scavenging described above, the high-pressure pre-scavenge inlet 24 introduces air at a high angle to the axis of the rotor. The air from the pre-scavenge inlet 24 is at a pressure higher than the pressure previously present in the cell and lower than the pressure at the scavenge inlet 29 so that a double compression wave is produced in the cell, with a plateau between the two waves at a. pressure roughly half way between the pressure before the first wave and the pressure after the second. The first wave^'„of the double compression wave is reflected off the land 46 as a compression wave propagating towards the inlet^* against the high-pressure scavenging flow. Because of that extra compression, the air at the high-pressure scavenge outlet is at the highest pressure in the system unless a form of combustor that gives a pressure gain is used.

After the high-pressure scavenge outlet 47, the cell passes the intermediate-pressure extraction port 51, into which the exhaust gas in the cell expands to provide the gas supply for the intermediate pressure scavenging stage. The gas then expands further into the low-pressure extraction port 54, from-which part is removed to the external turbine and part is immediately re-introduced into the cell through the second low- pressure pre-scavenge inlet 32. Each of the extraction ports 51 and 54 spans the progression of one wave pair, for the reasons explained above with reference to the boost extraction port 42.

While the cell is open at its inlet to the second low-pressure pre-scavenge inlet 32, its outlet opens to the first low-pressure scavenge outlet 34 and the boost cycle starts again with a compression wave from the opening edge of the pre-scavenge inlet 32 reflecting at the scavenge outlet 34 near its opening edge as an expansion wave and combining with an expansion wave starting from that edge to form the expansion wave 57.

Although the pressure-exchanger shown in Fig. 4 ahs a limited tolerance for variations in the rotor speed giving efficient operation, a considerably wider speed range can be achieved if less efficient operation is accepted, and it is believed that the pressure exchanger will start operation at a speed as low as one sixth of its optimum operating speed and accelerate itself to the full operating speed.

Referring now. to Fig. 5, the second form of pressure-exchanger is similar to t e first form shown in Fig. 4. Features that are substantially the same have been given the same reference numerals and the description of them will not be unnecessarily repeated.

Fig. 5, like Fig. 4, shows some overlap top and bottom, with one complete circumference of the pressure exchanger between the lines C. The cycle may be divided into a main cycle, above the line D and below the line C, and an expansion-scavenging cycle below the line D.

On the inlet stator 2, the first low-pressure scavenge inlet 11a is only the progression of a single wave pair in length and corresponds generally to the second portion of the inlet 11 of the first form of pressure-exchanger, and the first low-pressure scavenge outlet 34a facing it is correspondingly shorter than the outlet 34 of the first form of pressure-exchanger. The second low-pressure pre-scavenge inlet, immediately before the first low-pressure scavenge inlet 11a, is divided into a first inlet portion 70 and a second inlet portion 71 by a partition 72. The second low- pressure scavenge inlet 21a and the second low-pressure scavenge outlet 45a are also only a single wave-pair progression long.

The large land 46 on the outlet stator 3 opposite the high-pressure pre-scavenge inlet 24 may be divided into a land 73, wider than a cell 4, adjacent to the second low-pressure scavenge outlet 45a, and a land 74 narrower than a cell, adjacent to the high-pressure scavenge outlet 47. Between the lands 73 and 74 is a compression inlet 75 which is provided with a plurality of nozzle blades 76 arranged to ensure that any gas entering the cells 4 through the compression inlet has a direction of motion at a low angle to the local -direction of motion of the rotor 1.

The high-pressure, scavenge outlet 47 may have a minor portion 77 at its closing end separated off by a partition 78, or the intermediate-pressure extraction port 51 may have a minor portion 79 at its opening end separated off by a partition 80, or both. The second form of pressure-exchanger does not have any port corresponding to the boost extraction port 42 or to the low-pressure extraction port 54, and the lands 17a and 31a are correspondingly shorter than the lands 17 and 31 in the form of pressure-exchanger shown in Fig. 4.

As previously described, the intermediate-pressure extraction port 51 is connected by a duct 52 to the intermediate-pressure scavenge inlet 16. The intermediate-pressure scavenge outlet 36 is either connected by a duct 39 to the high-pressure pre- scavenge inlet 24 or connected by a duct 81 to the compression inlet 75. The intermediate-pressure scavenge outlet 36 is also connected by ducts 82 to 84 to the second portion 71 of the second low-pressure scavenge inlet preceding it, to the intermediate- pressure pre-scavenge inlet 14, and to the first low- pressure pre-scavenge inlet 19, respectively. The minor portion 36a at the closing end of the intermediate- pressure scavenge outlet 36 is preferably connected by a duct 85 to the first portion 70 of the preceding second low-pressure pre-scavenge inlet, although it may instead be connected to the intermediate-pressure pre- scavenge inlet 14, replacing the duct 81. Thus, the second low-pressure pre-scavenge inlet is supplied with compressed air and mixed gas from the expansion- scavenging cycle after it, instead of being supplied with gas from an expansion outlet of the high-pressure stage before it.

The minor portion 77 of the high-pressure scavenge outlet 47 (if present) and/or the minor portion 79 of the intermediate-pressure expansion port 51 (if present) is or are connected by a duct or ducts 86 to the high-pressure pre-scavenge inlet 24 and/or to the compression inlet 62. The connections provided by the duct or ducts 86 may be determined in the light of the exact operating cycle that is proposed to be carried out.

The operation of the second form of pressure- exchanger is simlar to that of the first form except as follows:

As with the first form of pressure exchanger, fresh air entering through the first low-pressure scavenge inlet 11a expels exhaust gas at low pressure from the cells 4 through the first low pressure scavenge outlet 34a, and the fresh air is then expelled at an intermediate pressure through the intermediate- pressure scavenge outlet 36 and 36a. It is thus fresh air and perhaps some mixed gas at intermediate pressure that is supplied through the ducts 82 and 85 to the second low-pressure pre-scavenge inlet 70 and 71, so that the scavenging of the cells 4 is actually started at the pre-scavenge stage. Because the scavenging of the cells starts sooner than in the first form of pressure exchanger, the low-pressure scavenge inlet 11a and outlet 34a do not need to remain open for so long, and the amount of fresh air taken in at the low- pressure scavenging stage is approximately two thirds of the corresponding amount for a scavenging stage as shown in Fig. 4. Similar considerations apply to the second low-pressure scavenging stage, for similar reasons.

Because the cells are slightly over-scavenged at the intermediate-pressure scavenging stage, the gas collected by the partitioned-off minor portion 36a of the intermediate-pressure scavenge outlet 36 is a mixture of exhaust^* gas and fresh air. When that- mixed gas is supplied by the duct 85 to the first portion 70 of the second low-pressure pre-scavenge inlet, where it is introduced into the cells 7 at the interface between scavenged and scavenging gases. Because the cells 7 are slightly over-scavenged at this stage also, the mixed gas is then expelled to the sink through the first low-pressure scavenge outlet 34a. There is thus very little contamination by exhaust gas of the compressed fresh air supplied through the duct 39.

In order to raise the pressure in the cells 7 before high-pressure scavenging, and thus to minimise the compression wave from the opening edge of the high-presure scavenge inlet 28, gas may be introduced into the cells through the compression inlet 75. In order that the gas being introduced shall not itself cause unacceptable waves, the nozzle blades 76 cause it to enter the cells 7 with a very low speed relative to the cells. It is preferred not to use all of the ducts 39, 81, and 86 simultaneously. Thus, compressed air from the intermediate-pressure scavenge outlet 36 may be supplied to the high-pressure pre-scavenge inlet 24 or to the compression inlet 75, but not to both. Air or mixed gas from the high-pressure scavenge outlet 77, if any, would be supplied to the pre-scavenge inlet 24, but not usually to the compression inlet 75. Exhaust gas from the extraction port 79, if any, may be supplied either to the pre-scavenge inlet 24 or to the compression inlet 75, but not usually to both at once and not usually to the pre-scavenge inlet if air from the scavenge outlet is being supplied to the pre- scavenge inlet.

Referring now to Fig. 6, the third form of pressure-exchanger is also shown as a complete circumference, between the li-nes E, together with some overlap at both ends. As shown in Fig. 6, it may be. divided into four cycles, between the lines E, F and G, G and H, and H and E, respectively. The uppermost cycle, between the lines H and E, is a main cycle with a combustion chamber 25, similar to those shown in Figs. 4 and 5. The cycle between the lines F and G is a boost cycle similar to that shown in Fig. 4. It will be seen, however, that the last expansion stage of each of the main and boost cycles does not provide gas for a low-pressure pre-scavenging stage immediately after it, as in Fig. 4, but instead supplies gas to an intermediate-pressure scavenge inlet of one of the other two cycles, which are expansion-scavenging cycles.

Thus, each of the first expansion-scavenging cycle E-F and the second expansion-scavenging cycle G-H has a low-pressure pre-scavenge inlet 32, a low-pressure scavenge inlet 11a, and intermediate-pressure pre- scavenge inlet 14, an intermediate-pressure scavenge inlet 16, and a first low-pressure pre-scavenge inlet 19, on the inlet stator 2, and a low-pressure scavenge outlet 34a and an intermediate-pressure scavenge outlet 36 on the outlet stator 3. The intermediate-pressure scavenge outlet 36 is connected by ducts 82 to 84 to all three of the pre-scavenge inlets. The intermediate- pressure scavenge inlet 16 of the first expansion- scavenging cycle E-F is connected by a duct 87 to a boost extraction port 42 of the boost cycle F-G. The intermediate-pressure scavenge inlet 16 of the second expansion-scavenging cycle G-H is connected by a duct 88 to a low-pressure extraction port 54 of the main cycle H-E.

The boost cycle F-G has a low-pressure scavenge inlet 21a and outlet 45a, with which the first pre- scaverige inlet 19 of the first expansion scavenging stage E-F_is associated, an intermediate-pressure pre- scavenge inlet 14 which is^' connected by a duct 40 to an intermediate-pressure scavenge outlet 36, an intermediate-pressure scavenge inlet 16, which is connected by a duct to an intermediate-pressure extraction port 51 of the main cycle, and the boost extraction port 42. A duct 39 connects the intermediate- pressure scavenge outlet 36 of the boost cycle to the high pressure pre-scavenge inlet 24 of the main cycle.

The main cycle H-E has a low-pressure scavenge inlet 21a and outlet 34a, with which the first low- pressure pre-scavenge inlet 32 of the second expansion scavenging cycle is associated, the high-pressure pre- scavenge inlet 24, a high-pressure scavenge inlet 28 and outlet 47, the outlet supplying air to the combustion chamber 25 and the inlet delivering the exhaust gas from the combustion chamber to the rotar 1, and the extraction ports 51 and 54.

As may be seem from Fig. 6, in which the principal waves of compression and rarefaction and the boundary surfaces between fresh air and exhaust gas are shown as they were in Figs. 4 and 5, the operation of the third form of pressure-exchanger is exactly analogous to that of the first two forms previously described.

Because of the extra expansion-scavenging stages, the third form of pressure exchanger has the advantages, as composed with the first form, that the additional flows of fresh air give more effective cooling of the rotor 1, and that the exhaust gas is more completely expanded, and its energy extracted, without the need for an external turbine as mentioned with reference to Fig. 4. The second form of pressure exchanger, as shown in Fig. 5, has cooling and energy extraction to an extent that falls in between the other two, while having a construction nearly as ε _mρle as that shown in Fig. 4. .

Referring now to Fig. 7, it has been found that the compressed air supplied by the boost cycle to the main cycle through the duct 39 may not have sufficient energy to provide an effective pre-scavenging action. Instead, therefore, in the main cycle of any of the three forms of pressure-exchanger shown in Figs. 1 to 6, the inlet 29 from the combustion chamber jacket may be replaced by a larger inlet 29a that delivers sufficient fresh air at a sufficiently high pressure to generate the desired double compression wave. The inlet 24 is then replaced by a high-angle nozzle 89 with nozzle blades 90 extending over an entire wave-pair progession. As may be seen from Fig. 7, the nozzle 89 generates a compression wave 91 from its opening edge that reflects off the land 46 and returns to the closing edge of the nozzle. The nozzle 89 thus serves to increase the pressure of the air in the cells while leaving the air substantially at rest. The nozzle 89 may extend over only half of the radial extent of the . cells 7, in order to reduce its area and thus increase the speed of the air flowing through it.

The nozzle blades 12 and 22 in the low-pressure scavenge inlets 11 or 11a and 21 or 21a may be omitted. If they are omitted then some other means is then preferably provided to generate the double compression wave 61 and 62 or its equivalent. For example, the lands 35 and 46 or 73a may be formed with portions at their edges with the ports 34 or 34a and 45 or 45a that comprise sections of land and sections of port alternating over the radial extent of the edge.

Claims

Claims :

1. A pressure-exchanger comprising: a rotor defining a ring of cells open at both axial ends thereof; inlet and outlet stators covering respective ends of the cells and so defining permanently-open ports that as the rotor rotates each cell is in communication at one end with each port in the inlet stator in a pre¬ determined sequence and is in communication at the other end with each port in the outlet stator in a predetermined sequence, the said ports comprising first and second low-pressure scavenge inlet ports arranged to be in communication with a source of a first gas at a low pressure, first and second low-pressure scavenge outlet ports associated with the first and second low- pressure scavenge inlet ports, respectively, and arranged to be in communication with a sink for a second gas at a low pressure, an intermediate-pressure scavenge inlet port after the first and before the second low-pressure scavenge inlet port with respect to one sense of rotation of the rotor, an intermediate- pressure scavenge outlet port associated with it, a . high-pressure scavenge inlet port after the second and before the first low-pressure scavenge inlet port, the high-pressure scavenge port being arranged to be in communication with a source of the second gas at high pressure, a high-pressure scavenge outlet port associated with the high-pressure scavenge inlet port and arranged to be in communication with a sink for the first gas at high pressure, each said scavenge inlet port and its associated scavenge outlet port being in opposite ones of the two stators and being so disposed that in normal operation of the pressure-exchanger a gas can enter cells through the scavenge inlet port and displace the cell contents through the associated scavenge outlet port, and a second high-pressure outlet port between the high-pressure scavenge outlet port and the first low-pressure scavenge outlet port, the second high-pressure outlet port being in communication with the intermediate-pressure scavenge inlet port.

2. A pressure-exchanger as claimed in claim 1, which comprises a boost inlet port between the second low- pressure scavenge inlet port and the high-pressure scavenge inlet port, the boost inlet port being in communication with the intermediate-pressure scavenge outlet port.

3. A pressure-exchanger as claimed in claim 1, comprising at least one duct in communication with a respective one .of the high-pressure and intermediate-- pressure scavenge outlet ports and .with a pre-scavenge inlet port that is immediately adjacent to the respective scavenge inlet port and is between the second low-pressure scavenge inlet port and the high pressure scavenge inlet port, or between the first low- pressure scavenge inlet port and the intermediate- pressure scavenge inlet port, respectively.

4. A pressure-exchanger as claimed in claim 1, comprising an expansion port immediately before at least one low-presure scavenge outlet port, with respect to the said sense of rotation of the rotor, which expansion port is in communication with a pre- scavenge inlet port that is immediately before and on the same stator as the scavenge inlet port associated with that scavenge outlet port and is so shaped as to tend to ensure that the velocity of a gas entering the cells of the rotor through it is at a low angle to the local direction of motion of the rotor rotating in the said sense .

5. A pressure-exchanger as. claimed in claim 1 comprising a further low-pressure scavenge inlet port and associated scavenge outlet port, and a further intermediate-presure scavenge inlet port and associated scavenge outlet port after the further low-pressure scavenge inlet and outlet ports and before any subsequent scavenge inlet and outlet ports, the further intermediate-pressure scavenge inlet port being in communication with an outlet port that is after the aforesaid high-pressure outlet port or another intermediate-pressure outlet port and is before any scavenge outlet port subsequent to the last-mentioned outlet port.

6. A pressure-exchanger as claimed in claim 1^' or claim 5, wherein at least one intermediate-pressure scavenge outlet port is in communication with at least one low-pressure pre-scavenge inlet port that is immediately before a respective low-pressure scavenge inlet port adjacent to the said intermediate-pressure scavenge outlet port, and is so angled as to tend to ensure that gas entering the cell through the said pre- scavenge inlet port has a velocity component towards the respective low-pressure scavenge inlet port.

7. A pressure-exchanger as claimed in claim 1, wherein at least one high-pressure or intermediate- pressure scavenge outlet port is provided in the face defining its edge at which rotor cells open to it, in the said sense of rotation of the rotor, with duct means opening at one end through that face adjacent to the said edge and at the other end through the same face at a location spaced from that edge and so arranged that if a cell begins to open to the outlet port with a pressure in the cell below equilibrium with the outlet port then gas can^' be drawn from the duct means into the cell outlet with a low speed relative to the cell.

8. A pressure-exchanger as claimed in claim 6, wherein each said respective low-pressure scavenge inlet port has an extent in the direction of motion of the rotor such that under normal operation a wave emitted from the edge of the inlet port at which cells open to that port and reflected at the outlet of a cell returns to the cell inlet substantially at the other edge of the inlet port.

9. A pressure-exchanger as claimed in claim 1,- wherein the cells are separated by radially-extending blades with their edges near the inlet stator leading in the said sense of rotation and their trailing edge portions curved towards a circumferential direction of the rotor.

10. A pressure-exchanger as claimed in claim 9, wherein the blades are so curved from their edges near the inlet stator to their edges near the outlet stator that flows of gases through the cells in use tend to impart to the rotor a thrust greater than is necessary to maintain the rotation of the rotor.

11. A pressure-exchanger as claimed in claim 1 which comprises a combustor and wherein the said high- pressure scavenge outlet is in communication with an intake of the combustor and the said high-pressure scavenge inlet is in communication with an outlet for combustion gases from the combustor.

12. A pressure-exchanger as claimed in claim 11 in combination with a gas-turbine, the low-pressure scavenge inlets being arranged to received the first gas from the compressor stages of the gas turbine and the low-pressure scavenge outlets being arranged to supply the second gas to the turbine stages of the gas turbine.

13. A method of operating a pressure-exchanger that comprises a rotor defining a ring of cells, which method comprises performing cyclically for each cell the steps of: introducing a first gas to the cell and scavenging a second gas from the cell with it at a low pressure; scavenging the first gas from the cell with the second gas at an intermediate pressure higher than the said low pressure; scavenging the second gas from the cell with the first gas at a low pressure lower than the said intermediate pressure; scavenging the first gas from the cell with the second gas at a high pressure higher than the said intermediate pressure; and permitting the second gas to expand and conveying a portion of it to the intermediate-pressure scavenging step.

14. A method as claimed in claim 13, wherein the gases are conveyed to and from the rotor through permanently open ports in stators adjacent to the rotor and wherein each step is performed for every cell as the cells pass a corresponding portion of the- stators.

15. A method as claimed in claim 13, which comprises introducing the first gas from the intermediate- pressure scavenging step into the cell after the second low-pressure scavenging step and before the high- pressure scavenging step.

16. A method as claimed in claim 15, wherein the said step of introducing the first gas from the intermediate- pressure scavenging step into the cell comprises introducing the gas with a direction of motion at a low angle to the local direction of motion of the rotor.

17. A method as claimed in claim 13, which comprises permitting a portion of the second gas - to expand out of the cell after the intermediate-pressure scavenging step and introducing that gas into the cell again before the second low-pressure scavenging step.

18. A method as claimed in claim 13,. which comprises permitting a further portion of the second gas to expand out of the cell after the said step of permitting the second gas to expanδ and introducing that gas into the cell again before the first low-pressure scavenging step.

19. A method as claimed in claim 13., which comprises introducing a portion of the gas from at least one of the said intermediate-pressure scavenging step and the said high-pressure scavenging step into the cell immediately before the respective scavenging step.

20. A method as claimed in claim 19, wherein the portion of the gas from the scavenging step that is introduced into the cell immediately before the scavenging step is the last portion that is scavenged from the cell and consists of a mixture of the first and second gases.

21. A method as claimed in claim 19, wherein each of the said step of introducing a portion of gas and the subsequent intermediate-pressure or high-pressure scavenging step causes a compression wave to propagate along the cell, the two waves being of similar height and one arriving at the cell, outlet before and one after an outlet port opens.

22. A method as claimed in claim 13, which emprises at least one further step of scavenging the second gas from the cell with the first gas at a low pressure and at least one further step of scavenging the first gas from the cell with the second gas at an intermediate pressure higher than that low pressure after the further low-pressure scavenging step and before the next low-pressure scavenging step, the second gas for the further intermediate-pressure scavenging step being extracted from cells after^' the high-pressure scavenging step or after another intermediate-pressure scavenging step.

23. A method as claimed in claim 13 or claim 22, which comprises introducing at least some of the first gas from at least one intermediate-pressure scavenging step into the cell at the beginning of a respective low- pressure scavenging step next before and/or next after that intermediate-pressure scavenging step.

24. A method as claimed in claim 23, wherein at each said respective low-pressure scavenging step, after the said step of introducing first gas from an intermediate- pressure scavenging step, first gas is introduced from an external source for a period substantially equal to the time taken for a wave to travel from the cell inlet to the cell outlet and back.

25. A method as claimed in claim 13, wherein the first gas is air and the second gas is a gas formed by burning a fuel in air, and which comprises burning a fuel in air from the high-pressure scavenging step and supplying gas from the combustion to the high-pressure scavenging step as the second gas.