BACKGROUND Micromachines such as micro-gas turbines, combustion power generators, pumps and compressors are described in U. S. Patent No. 5,932,940 (the M. I. T. patent), the disclosure of which is incorporated herein by reference. All of these machines contain a rotor comprising a disc or discs defining either a centrifugal compressor/pump or a radial inflow turbine. The material of construction is characterised by a strength to density ratio enabling a rotor speed of at least 500,000 rotations per minute. The machines are constructed using microfabrication techniques including vapour deposition and bulk wafer etching, the material of construction being common to all the structural elements.

The compressor and the turbine rotors of the devices described in the M. I. T. patent utilise a plurality of radial flow vanes. It is considered that this arrangement of blades is not desirable in micromachines for the following reasons: (a) because the nature of construction involves planar fabrication techniques, fillets on corners are difficult to achieve and, in the absence of adequate fillets, high stress concentration at the blade root attachment decreases the fracture strength of these microelements ; (b) the placement of blades around the periphery

of the discs increases the mass of the structure at the place where centrifugal stresses have the greatest effect ; (c) the plurality of blades tends to set up undesirable turbulence and pulsations in the working fluids, and the cyclic nature of the reaction between fluids & blades results in cyclic stress fluctuations (fatigue stresses) that limit the durability (fatigue life) of the rotor assembly ; (d) the maximum rotor speed is limited in part by the allowable mechanical and thermal stresses that may be imposed on the rotor structure by the plurality of radial flow vanes ; (e) the degree of rotor balance obtainable is affected by the requirement for a plurality of radial flow vanes ; and (f) the rotor disc employs blades only on one side and is subject to a bending moment, caused by centrifugal blade loading.

It is these problems that have brought about the present invention to use a bladeless or vaneless rotor in micromachines.

The use of bladeless rotors has been suggested in the context of"large scale"turbines. Thus, a method for driving turbines by means of viscous drag was taught by Tesla in U. S. Patent 1,061,206 and for fluid propulsion in U. S. Patent 1,061,142. In both disclosures the rotor comprises a stack of flat circular discs with openings in the central portions, these discs being set slightly apart.

In the turbine embodiment the rotor is set in motion by the adhesive and viscous action of the working fluid, which enters the system tangentially at the periphery and leaves it at the center. In the fluid propulsion embodiment, fluid enters the system at the center of the rotating discs and is transferred by means of viscous drag to the periphery where it is discharged tangentially.

For fluid propulsion applications such as pumps and compressors, the fluid is forced into vortex

circulation around a central point where a pressure gradient is created. This pressure gradient is such that an increasing radial distance from the center of rotation leads to an increase in pressure, the density of the fluid and the speed of rotation determining the rate of pressure rise. If an outwardly radial flow is superimposed on the vortex circulation an increasing pressure is imposed on the fluid as it flows outward.

To preserve the vortex circulation an external force must act upon the fluid, and this force must accelerate the fluid in the tangential direction as the fluid moves outward in order to maintain its angular velocity. This function is simply a transfer of momentum from the impeller to the fluid, and with a centrifugal compressor impeller it may be achieved in one of two ways.

A first method is to confine the fluid within a fixed boundary channel and then accelerate the channel. In an impeller of the type utilized in prior art microturbomachinery, the vanes and rotor walls form such a channel, and acceleration occurs as the fluid moves outward towards regions of higher impeller velocity. A second method of momentum transfer is by viscous drag and this is the principe underlying the Telsa arrangement described in the two US patents referred to above. Viscous drag always acts to reduce the velocity difference, so that in the case of a compressor where the channel walls are moving relative and parallel to the fluid, the fluid will accelerate in the direction of the channel motion. Conversely, where the fluid is moving relative and parallel to the channel walls, the channel walls will accelerate in the direction of the fluid motion.

Known bladeless or vaneless rotors have had limited success in large scale turbines. The relatively large number of parts required for their construction and the distortion of the discs that occur under high thermal and mechanical stress conditions have restricted their adoption.

It is these issues that have brought about the present invention.

SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a micromachine including at least one bladeless rotor, said rotor being adapted to impart energy to or derive energy from a fluid.

According to a further aspect of the present invention there is provided a rotor for a micromachine comprising at least a pair of closely spaced co-axially aligned discs defining opposed planar surfaces, at least one disc having at least one aperture whereby a fluid passageway is defined between the aperture, the planar surfaces and the periphery of the rotor, the rotor being formed of a single crystal material.

In accordance with a still further aspect of the present invention there is provided a rotor wherein the rotor is manufactured of a material having a tensile strength to allow the rotor to run at speeds greater than 500,000rpm.

In accordance with a still further aspect of the present invention there is provided a rotor comprising a backing disc and at least one coaxially spaced annular disc supported on the backing disc by a central hub defining at least one aperture, wherein the annular disc defines an unimpeded fluid passage between the aperture and the periphery of the disc.

DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a front elevational view of a first embodiment of a bladeless rotor for use in a micromachine,

Figure 2 is a side elevational view of the bladeless rotor of Figure 1, Figure 3 is a cross sectional view taken along the lines AA of Figure 1, Figure 4 is a front elevational view of a second embodiment of a bladeless rotor, Figure 5 is a sectional view of the rotor, taken through the lines A-A of Figure 4, Figure 6 is a sectional view of the rotor, taken through the lines B-B of Figure 5, Figure 7 is a three dimensional view illustrating two bladeless rotors mounted coaxially on a common shaft, Figure 8 is a front elevational view of a test rig illustrating operation of a radial flow turbine utilising a bladeless rotor, Figure 9 is a cross sectional view taken along the lines CC of Figure 8, Figure 10 is a front elevational view of a test rig illustrating operation of a radial flow turbine utilising a rotor with blades, Figure 11 is a cross sectional view taken along the lines DD of Figure 10, Figure 12 is a graph of rotor speed against plenum chamber pressure utilising the test rigs of Figures 8 and 10, Figure 13 is a graph of rotor speed against mass flow in grams per second, Figure 14 is a front elevational view of a bladeless rotor in accordance with a third embodiment, Figure 15 is a side elevational view of the rotor of Figure 14, and Figure 16 is a sectional view of the rotor taken through the lines BB of Figure 15.

DESCRIPTION OF THE PREFERRED EMBODIMENT In US Patent 5932940 (the M. I. T. patent) there is

disclosure of micromachinery in the form of micro-gas turbines and associated microcomponentry. The components such as the compressor, diffusers, combustion chambers, turbine rotors and stators are all disclosed as being manufactured using microfabrication techniques in a material that is common to all the elements. Suitable materials include a range of ceramics used in the semi- conductor art or in the microelectronic fields, such materials include silicon, silicon carbide silicon nitride.

Other suitable materials include refractory metals and alloys based on nickel, tantalum, iridium and rhenium.

Composite materials such as molybdenum silicide are also envisaged. The materials can also vary depending on whether they are used in the hot and cold regions of the micromachines.

Regardless of whether the engine is a turbine or compressor it includes at least one rotor usually mounted on a shaft. In one embodiment the engine could include a common shaft driving a compressor disc at one end, defining a centrifugal compressor and a turbine disc at the opposite end defining a radially inflow turbine. The componentry is very small with the whole assembly being less that 20mm in diameter. The micromachines are designed to run at very high speeds with a rotational speed of at least 500,000 rotations per minute being typical. In a preferred embodiment the dimensions of the machine embraces compressor and turbine discs of diameters between 1 and 20 mm with a combustion chamber having a height of between 2 to 10 mm and the axial length of the combustion chamber being between 0.5mm and 12mm. The materials that are used to produce the componentry should preferably be able to withstand temperature of at least 1,000°C in the case of turbines.

The micromachine disclosed in the M. I. T. patent utilises bladed or vaned rotors. As discussed in the introduction of the specification it is considered that the use of a bladed or vaned rotor in micromachinery causes a

series of problems, many of which can be solved by the use of bladeless or vaneless rotors.

In the embodiment shown in Figures 1 to 3, a suggested construction of bladeless rotor is illustrated.

The bladeless or vaneless rotor 10 shown in Figures 1 to 3 includes two substantially smooth and planar annular discs or rings 16 and 17 co-axially supported in a close parallel array by a star shaped hub 18 which is attached to a backing disc 19. The hub 18 is provided with openings/apertures 20 that are in connection with the spaces 21 and 22 between backing disc 19, ring 17 and ring 16. In the example shown the rotor has a diameter of about 4mm and a width of about 0.6mm. The rotor is constructed from material such as silicon, silicon carbide or other suitable material and is manufactured preferably as a sub assembly of prior art microturbomachinery and by means compatible with the manufacture of associated microturbomachine components.

The spaces 21 and 22 form fluid passageways from opening 20 to the periphery of the rings 16 and 17. The fluid passageways define four surfaces 5,6,7 and 8 over which the fluid flows namely opposite sides of the ring 17 and the inner surfaces 7 and 8 of the ring 16 and backing disc 19.

In Figures 4,5 and 6, a second embodiment of a micromachine rotor 41 is illustrated in which a backing disc 42, supports a cross shaped hub 43 upon which are supported in close parallel array two smooth and substantially planar annular discs or rings 44 and 45. The hub 43 is provided with openings 46 that are in fluid connection with the spaces 47 and 48 between backing disc 42, ring 44 and ring 45. The spaces 47 and 48 form fluid passageways from openings 46, to the periphery of the rings 44 and 45. The fluid passageways define four surfaces 35, 36,37 and 38, namely opposite sides 35 and 36 of the ring 44, and the inner surfaces 37 and 38 of the ring 45 and the backing disc 42 respectively. The inner diameter 49 of the

spaces 47 and 48, is smaller than the outer diameter 39 of the openings 46. This arrangement allows an unimpeded flow of the vortex circulation of the fluid within the fluid passageways and the openings 46. In this embodiment the rotor has a diameter of about 4mm and a width of about 0.6mm.

Construction of the rotor may be accomplished by means of microfabrication techniques in common usage such as photolithography and masking layers. In the case where silicon is the material of construction deep trench etch processes employing anistropic plasma etching steps alternating with polymerizing steps may also be employed.

Such a process is described in U. S. Pat. No. 5501893 and is available from Surface Technology Systems Ltd. of Imperial Park, Newport U. K. However, other etching techniques can be employed, preferably the etchant and chemistry employed are capable of producing deep trench geometries having high aspect ratios. Other manufacturing techniques may also be employed, particularly when the material of construction is silicon carbide in which case components may be molded by vapor deposition of the selected material into a pre etched mold formed in for instance a silicon wafer. The resulting molded components are then removed from their molds and may be bonded together with other components to produce the finished rotor.

The rotor 10 shown in Figures 1 to 3 may operate either as a compressor/pump or a turbine. In the case where the rotor is defined as the compressor/pump in a microturbomachine, the rotor is driven up to speed within a suitable housing by either electrical or mechanical means (not shown). It should be noted that the rotor 10 will operate with equal efficiency when driven in either a clockwise or counter clockwise direction. Fluid upon entering inlet openings 20 and coming into contact with disc 16 and 17 is subjected to two forces, one acting tangentially in the direction of rotation, and the other radially outward. The combined effect of these tangential

and radial forces is to propel the fluid with increasing velocity in a spiral path until it reaches the perimeter of the rotor where it is ejected. In the case where the rotor is operating as a turbine in a microturbomachine the operation described above is reverse, in other words, if fluid under pressure is admitted tangentially to the perimeter of the rotor disc, the rotor 10 will be set in motion by the viscous drag properties of the fluid which, travelling in a spiral path and with continuously diminishing velocity reaches the openings 20 from where it escapes.

Although a rotor having two discs 16,17 is depicted in Figures 1 to 3, it is understood that a plurality of discs suitably serving particular operating requirements may be utilized. As may be seen from these drawings, stresses set up by centrifugal forces are supported radially by the star shaped hub 18 thus preventing a bending moment on the backing disc 19. Also illustrated in Figures 1 to 3 are the ends 59 of the star shaped hub 18 extending into the space between the backing plate 19 and discs 16 and 17 in order to provide lateral support to the discs.

Compare this with the second embodiment illustrated in Figures 4-6 where the ends of the cross shaped hub 43 terminate below the outer diameter 39 of openings 46 thereby forming inner diameter 49 of spaces 47 and 48. The benefits with this embodiment is that disturbed fluid flow caused by the ends 59 of the first rotor embodiment, is eliminated and the viscous drag flow permitted to continue unimpeded to the openings 46.

A preferred material of construction is silicon carbide, this material possessing the properties of high strength and dimensional stability (creep resistance) at elevated temperatures and a high strength to density ratio.

In the particular case of prior art bladeless turbine rotors where the major problems have always related to internal vibration, high temperatures, high speeds and high

pressures it has been impractical to construct the rotor from silicon carbide thus limiting the high performance potential of turbine rotors operating on the principles of fluid viscous drag. The use of this material in a micro- gas turbine rotor of the present invention minimizes disc distortion and allows higher speeds and therefore improved performance. In addition, because the rotor is made by microfabrication techniques, an advantage is gained from the particular batch production methods available. In the case where microturbomachine rotors may operate at lower temperatures than micro-gas turbines the preferred material of construction may be silicon. This material is already in wide usage in microelectronic componentry and the fabrication techniques are well understood. Ceramics are excellent materials for microfabrication of highly stressed components because they demonstrate high tensile strength at very high temperatures.

Figure 7 is a perspective view of a micro-gas turbine rotor of the present invention constructed according to conventional Brayton cycle gas turbine practice where a centrifugal compressor unit 24 and a radial inflow turbine unit 25 are mounted by their respective support discs 26,27 to each end of a connecting shaft 23.

In some applications of micromachinery, a relatively low level of thermal or mechanical stress may apply in which case the means of supporting the rings 16 and 17 as shown in Figures 1 to 3 may be modified.

In Figures 14 to 16 there is shown an embodiment in which a micromachine rotor 40 comprises a support disc 30 upon which is mounted an array of spacers 31. Each of the spacers 31 is attached by a first face to support disc 30 and by the opposite face to ring 28. On the opposite face of ring 28 is mounted a further array of spacers 32 and these spacers attach to the inner face of ring 29.

Although six spacers 31,32 of a particular size and shape are shown in the drawings it is to be understood that other

numbers, sizes and shapes may be effective. In this particular embodiment of the invention the advantage of the radial support given to the rings by the star shaped or cross shaped hub as shown in Figures 1 to 3 and Figures 1 to 6 respectively, is exchanged for the advantage of an unrestricted opening 33. This embodiment like the first and second embodiments of Figures 1 to 6 defines fluid passageways between the opening 30 and periphery of the rings 28 and 29.

The dimensions of the device as a whole, and the spacings of the discs in any given machine will be determined by the conditions and requirements of the particular application of the micromachine. In general, greater disc spacing is required for larger disc diameters, longer fluid spiral path and greater fluid viscosity. For instance, when the machine is configured as a turbine the torque is directly proportional to the square of the velocity of the fluid relative to the rotor and to the effective area of the discs, and inversely, to the distance separating them. The size and shape of the disc openings will also be determined dependent on application and rotor construction. In a multiple disc rotor, the disc furthest from the backing disc may have larger openings to not only accommodate the fluid out flow through the passage adjacent that disc, but also the fluid outflow from all other discs between the backing disc and furthest disc. Further, the surface finish of the discs is sufficiently smooth to adhere at least one layer of fluid particles to the disc thereby creating a boundary layer in the fluid vortex.

In its preferred forms, the present invention may provide the following advantages over the prior art use of radial flow vanes in microturbomachines: (a) reduced corner stress concentration ; (b) reduced turbulence and pulsation in the working fluids ; (c) higher rotational speeds within the limits of the tensile strength and elastic modulus of the material

due to plain radial loading and absence of sharp section changes; (d) an improved rotor balance; (e) a reduction of the bending moment caused by centrifugal blade loading; and in the case of prior art use of large scale bladeless rotors: (f) there is no requirement for a multiplicity of parts; and (g) minimized disc distortion due to a preferred material construction giving high strength at high temperatures e. g. silicon carbide or silicon.

The reduction or elimination of cyclic stresses that arise from reaction between blades and working fluids in prior art microturbine rotors, has the effect of achieving the advantages outlined in paragraph (b) above and, effectively, extending the fatigue life, or durability of the rotor in the present bladeless configuration.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all aspects as illustrative and not restrictive.

To demonstrate the efficiency of using a bladeless or vaneless rotor in a micromachine a test rig 50 as shown in Figures 8 to 11 was constructed to demonstrate the comparative performance of such machinery when used with a conventional bladed rotor and a bladeless rotor of the kind described above. For practical reasons a decision was made to construct a turbine with 18mm diameter rotors to be driven by compressed air. The use of compressed air meant that the turbine did not require the capacity to embrace high combustion temperatures and thus did not have to be made in high temperature resistant ceramic materials.

Thus, the componentry was constructed of a readily available metal that has excellent qualities of

machineability. An aluminium alloy 2011 was selected due to its characteristics of machineability and its high tensile strength. The choice of an 18mm diameter rotor was selected also for ease of manufacture and to ensure that the rig can still be classed as a micromachine.

The rotor design follows the embodiment as illustrated in Figures 4,5 and 6 but with all dimensions scaled in the ratio of 1: 4.5. The spacing between the backing disc 42, and the discs 44 and 45, was 0.375mm, whilst the thickness of the disc 44, was 0.375mm. The distance between the working surfaces 37 and 38 was 1.125mm.

As shown in Figures 8 to 11 the test rig 50 comprises a housing block 51 having a front face 52 with an annular recess 53, a cylindrical throughway 54 extends through the center of the block from the center of the annular recess to the rear face 55 of the block 51. The throughway 54 supports spaced bearings 56. The rotor 10 which is bladeless in Figures 8 and 9 and bladed (100) in Figures 10 and 11 is mounted at one end of a shaft 60 that is supported within the throughway 54 by the bearings 56 for axial rotation. The rear end 55 of the block 51 is closed off by an end plate 61 which is secured to the block by cap head screws 62. The annular recess 53 at the front of the block 51 supports an annular backing plate 65 that is positioned in close proximity to the rear of the bladeless rotor 10 in Figures 8 and 9 and the bladed rotor 100 in Figures 10 and 11. The bearing plate 65 supports an annular stator 68 having profiled blades 69. The stator is positioned outside but close to the periphery of the rotor 10 or 100 to direct incoming air to the rotor periphery.

The rotor 100 of bladed construction is modelled as closely as possible to the turbine rotors disclosed in the M. I. T. patent. A front cover 70 is secured over the front of the housing by six cap head screws 71. Compresse air is used to drive the turbine and the air inlet 75 is positioned at the lower right hand side of the block as shown in Figure

8. The air initially fills the annular cavity around the periphery of the rotor 10,100 and then in the case of the bladeless rotor 10 flows through the fluid passageway defined by the rotor discs to impart viscous drag to rotate the rotor and then to escape via the apertures at the center of the rotor. The annular space exterior of the rotor is also coupled via a plenum chamber to a pressure sensor (not shown) via a bleed passageway 76 shown in Figure 8 in the top right hand corner of the block 57.

The radial inflow rotor 10,100 that it mounted on the shaft 60 is supported by two high speed (140,000rpm) ball bearing races precisely located with identical preloads in both test rigs. The air is fed tangentially to the rotor by the air inlet. It is also fed to the plenum chamber that includes the pressure sensor. The multi-vaned stator 68 directs the air onto the rotor 10,100 and the stator 68 is again modelled on the stator disclosed in the M. I. T. patent. The rigs 50 have identical exhaust apertures and the shaft 68 includes a bicoloured disc that allows the rotational speed of the shaft 68 to be read using an optical tachometer. The compressed air was regulated with coarse and fine needle valves to ensure final flow control.

Every care was taken to ensure that the two test rigs operated on identical parameters. In one test, the revolutions per minute were measured against the plenum chamber pressure at precise change points to retrieve repeatable data. The pressure was increased slowly to ensure measurements represented stable conditions of air flow and rotor speed. Pressure was progressively increased until a ball bearing rpm limit specification was exceeded.

The results of these test, namely rotor speed against supply pressure were plotted on the graph shown in Figure 12.

The test rigs 50 were then used to conduct mass flow tests where rpm were measured against exhaust air speed at precise change points to retrieve repeatable data.

The pressure was increased slowly to ensure measurements represented stable conditions of air flow and rotor speed.

Pressure was progressively increased until the ball bearing rpm limit specification was exceeded. The mass flow in grams per second was then derived from volume per second of exhaust air and a graph was plotted as shown in Figure 13.

It can be seen from these graphs there is a clear performance advantage in using the bladeless rotor 10. The mass flow graphs diverged from approximately 40,000rpm showing a strong trend to proportionately lower values at increasing rpm. The bladed rotor 100 registered a Mass Flow figure of 30% higher than the bladeless rotor 10 at 100, 000rpm. At maximum test Mass Flow, the bladeless rotor achieved approximately 35% higher rpm than the bladed rotor. The plenum pressure against rpm graph showed a similar strong trend favouring the bladeless rotor 10.

From approximately 50,000rpm the bladeless rotor achieved higher speeds and this divergence increased until 140,400rpm which was just over the specification limit of the bearings. This speed was reached at only 2.75 pounds force per square inch (psi) an improvement of 18.5% over the bladed rotor. Additionally, a 27% higher pressure was required in order for the bladed rotor to reach 100, 000rpm.

The divergent trends of both the graphs are indicative of major performance benefits that would be expected to increase proportionally at higher rpm's.

A further advantage that was noted in using the two test rigs was that the bladeless rotor 10 was considerably quieter than the bladed construction.