Patent Description:
The present disclosure relates, generally to turbomachines and other mechanisms and, more particularly, to blade-like element arrangements for cavities within a turbomachine adapted for reducing rotor disk excitation using intentional mistuning.

Turbomachines, such as centrifugal flow compressors, axial flow compressors, and turbines may be utilized in various industries. Centrifugal flow compressors and turbines, in particular, have a widespread use in power stations, jet engine applications, gas turbines, and automotive applications. Centrifugal flow compressors and turbines are also commonly used in large-scale industrial applications, such as air separation plants and hot gas expanders used in the oil refinery industry. Centrifugal compressors are further used in large-scale industrial applications, such as refineries and chemical plants.

With reference to <FIG>, a multi-stage, centrifugal-flow turbomachine <NUM> is illustrated in accordance with a conventional design. In some applications, a single stage may be utilized. In other applications, multiple stages may be utilized. Such a turbomachine <NUM> generally includes a shaft <NUM> supported within a housing <NUM> by a pair of bearings <NUM>. Turbomachine <NUM> shown in <FIG> includes a plurality of stages to progressively increase the fluid pressure of the working fluid. Each stage is successively arranged along the longitudinal axis of turbomachine <NUM> and all stages may or may not have similar components operating on the same principle.

With continued reference to <FIG>, an impeller <NUM> includes a plurality of rotating blades <NUM> circumferentially arranged and attached to an impeller hub <NUM> which is in turn attached to shaft <NUM>. Blades <NUM> may be optionally attached to a cover disk <NUM>. A plurality of impellers <NUM> may be spaced apart in multiple stages along the axial length of shaft <NUM>. Rotating blades <NUM> are fixedly coupled to impeller hub <NUM> such that rotating blades <NUM> along with impeller hub <NUM> rotate with the rotation of shaft <NUM>. Rotating blades <NUM> rotate downstream of a plurality of stationary vanes or stators <NUM> attached to a stationary tubular casing. The working fluid, such as a gas mixture, enters and exits turbomachine <NUM> in the axial direction of shaft <NUM>. Energy from the working fluid causes a relative motion of rotating blades <NUM> with respect to stators <NUM>. In a centrifugal compressor, the cross-sectional area between rotating blades <NUM> within impeller <NUM> decreases from an inlet end to a discharge end, such that the working fluid is compressed as it passes across impeller <NUM>.

Referring to <FIG>, working fluid, such as a gas mixture, moves from an inlet end <NUM> to an outlet end <NUM> of turbomachine <NUM>. A row of stators <NUM> provided at inlet end <NUM> channels the working fluid into a row of rotating blades <NUM> provided at outlet end <NUM> of turbomachine <NUM>. Stators <NUM> extend within the casing for channeling the working fluid to rotating blades <NUM>. Stators <NUM> are spaced apart circumferentially with equal spacing between individual struts around the perimeter of the casing. A diffuser <NUM> is provided at the outlet of rotating blades <NUM> for homogenizing the fluid flow coming off rotating blades <NUM>. Diffuser <NUM> optionally has a plurality of diffuser vanes <NUM> extending within a casing. Diffuser blades <NUM> are spaced apart circumferentially, typically with equal spacing between individual diffuser blades <NUM> around the perimeter of the diffuser casing. In a multi-stage turbomachine <NUM>, a plurality of return channel vanes <NUM> are provided at outlet end <NUM> of a fluid compression stage for channeling the working fluid to rotating blades <NUM> of the next successive stage. In such an embodiment, the return channel vanes <NUM> provide the function of stators <NUM> from the first stage of turbomachine <NUM>. The last impeller in a multi-stage turbomachine typically only has a diffuser, which may be provided with or without the diffuser vanes. The last diffuser channels the flow of working fluid to a discharge casing (volute) having an exit flange for connecting to the discharge pipe. In a single-stage embodiment, turbomachine <NUM> includes stators <NUM> at inlet end <NUM> and diffuser <NUM> at outlet end <NUM>.

An important concern in designing turbomachines is controlling the vibration of the rotating blades and the hub throughout the operating range of the turbomachine. Rotating blades and disks in turbomachinery are excited into resonant vibrations by a) upstream stator strut and/or vane wakes and potential flow interaction with downstream struts and vanes, b) other inhomogeneities in the flow stream formed by non-uniform circumferential pressure distribution, c) acoustic pulsations either at rotating blade passing frequency and/or d) vortex shedding from stationary vanes, in turn causing coincident acoustic resonance of the gas within the casing. For example, Tyler/Sofrin modes may occur due to sound waves at blade passing frequency reflecting off vanes giving spinning modes. (Ref.<NPL>. ) The acoustic pulsations at the spinning mode in turn can match the mode shape and frequency of the acoustic mode of the cavity at the sides of the impeller, and also match the mode of the impeller structure. This is termed triple coincidence. The acoustic pulsations reflect differently off of the stator struts set back further from the impeller and reduce the effective amplitude of the spinning modes. For example, in an impeller having <NUM> rotating blades and <NUM> stator struts, there is a <NUM>-diameter spinning mode at blade passing frequency <NUM> times the rotor speed. If the <NUM>-diameter structural mode is equal to <NUM> times the rotating speed, the blade excitation can be lowered by setting half of the stator struts downstream about one-half an acoustic wave length, as wave reflections would result in phase cancellation. One example of such an arrangement is disclosed in <CIT>.

These excitations cause cyclic stress, resulting in potentially high cycle fatigue and failure in impellers either at rotating blades, the hub, or the cover. The impeller can be excited to a large amplitude when a modal frequency corresponds to shaft rotational frequency multiplied by the harmonic number of the flow inhomogeneity seen by the blades. Typically, the number of resonances with an amplitude large enough to cause high cycle fatigue is limited. Since the damage rate from fatigue occurs only if infinite endurance strength of the material is breached, a modest reduction in the vibration amplitude often will eliminate high cycle fatigue as the limiting factor for blade and disk life.

If a critical resonance cannot be avoided, one current practice to overcome these problems is to avoid operation at the resonant frequency by changing the speed rapidly when a resonance is encountered, thereby minimizing the number of fatigue cycles that a blade accumulates during operation. If the number of vibration cycles is minimized, then blade failure is controlled by mechanisms other than downstream wakes, acoustic pulsations, flow inhomogenities, or vortex shedding. However, this practice places undesirable limits on operation of turbomachinery.

Another current approach is to reduce the spatial variations in the flow field by directly injecting fluid into low-velocity wakes behind obstructions (<NPL>). This approach requires the use of either fluid from the compressor or from an additional external fluid source in relatively large quantities. Use of compressor fluid has a detrimental impact on performance. The addition of a separate fluid supply adds weight and requires additional power. Both methods have detrimental impacts on performance of the turbomachinery. Also, wake filling does not address modal excitation due to bow waves from downstream flow obstructions.

In recent years, it has been discovered that besides non-uniform flow excitation, such as from stator wakes, acoustic pressure pulsation can be a concern at least for high pressure centrifugal compressor impellers. This has been termed "triple coincidence" and explains rare failures and likely a reason, at least partially, for some previous undocumented failures. Bladed disk interaction resonance can be avoided for centrifugal impellers as needed, depending on vibratory mode involved, available damping, and potential excitation level. Especially for stages having vanes in the diffuser near impeller tips, concern for high cycle fatigue is very high as certain numbers of vanes combined with a number of rotating blades can give a correct phase to excite a highly responding mode. A similar but more complex interaction is with transverse acoustic modes having a specific number of nodal diameters. In this case, acoustic gas modes in cavities at sides of impellers, matching rotating acoustic pulsations at impeller blade passing frequency termed Tyler/Sofrin modes, and a matching structural impeller mode give the triple coincidence causing higher resonant response of the impeller, besides increased noise. The concern for this coincidence is often difficult to evaluate and to correct unless there is a known failure to modify number of vanes or blades. This coincidence can add to the direct response from either upstream wakes or downstream diffuser vane interacting flow pulsations. Dimensions of impeller side cavities are axisymmetric and are set by aerodynamics, so that outer and inner radii define transverse modes with small radial dimensional changes available. Often a minor aerodynamic performance compromise can be used to change designs to avoid serious resonances, e.g. numbers of vanes and blades, changing the response of a matching diameter mode or have a different less responsive mode to alleviate concern. Besides turbomachinery, e.g. compressors and pumps, the methods as described could be utilized for any cavity that has diametrical mode shapes, or possibly other patterns of pressure pulsation frequencies. These modification(s) can alleviate if not eliminate concern for any mechanism having structural vibration and/or environmental noise issues. <CIT> discloses a volute for a radial turbomachine. The turbomachine includes a radial compressor or a radial turbine. The volute has a substantially annular cavity which is delimited at least by a first radial side surface. At least one substantially annularly circumferential groove is formed in the side surface. <CIT> discloses an acoustic liner for attenuating noise in rotating machinery. The acoustic liner may include a plurality of cells coupled together to form an annular cell matrix, the plurality of cells being made of a non-metallic material, for example, plastics, polymers, thermoplastics, or thermosets. Each cell of the acoustic liner may be hexagonally-shaped such that the annular cell matrix forms a honeycomb structure. <CIT> discloses a radial compressor with a bladed diffusor connected downstream of the impeller. In the radial compressor the compression shocks occurring in front of the blades on a radial guide vane ring are to be stabilised. To achieve this, the blades of the radial guide vane ring are manufactured with two different alternating lengths and slots, which are connected to a rear reservoir chamber, which are so disposed that the slots are positioned at least approximately at right angles in front of the short blades and extend as far as the side of the leading edges of the nearest long blades.

The claimed solution is specified by an arrangement according to claim <NUM>, and by a cavity as claimed in claim <NUM>. Dependent claim specify embodiments thereof. In one aspect of the disclosure, an arrangement for intentionally mistuning a cavity formed adjacent an impeller hub and another cavity formed when there is an impeller cover on a turbomachine having "n"-diameter acoustic modes includes at least two bladed elements defined within a perimeter of a casing wall on a hub side of the impeller, wherein the hub side of the impeller is positioned opposite a cover side of the impeller. The bladed elements are positioned to mistune the cavities to minimize acoustic pulsations in the cavities. The at least two bladed elements are positioned adjacent the impeller.

In another aspect, two times "n" bladed elements may be defined in the casing wall, wherein "n" is the n-diameter cavity mode that is to be mistuned. The bladed elements may be grooves or ridges defined in the casing wall. For example, of an n-diameter mode where "n"=<NUM>, at least <NUM> bladed elements may be defined in the casing wall. Ten bladed elements may have a depth greater than the other ten adjacent bladed elements. Ten bladed elements may have a width greater than the other ten adjacent bladed elements. The bladed elements may be spaced equidistant from one another. The bladed elements may be also defined in the casing wall on the cover side of the impeller.

In another aspect, a turbomachine includes a casing having an inlet end opposite an outlet end along a longitudinal axis of the casing, a shaft assembly provided within the casing, the shaft assembly extending from the inlet end to the outlet end, a rotor having a plurality of rotating impellers extending radially outward from the shaft assembly, and at least two bladed elements defined within a perimeter of the casing adjacent one of the impellers. The bladed elements are positioned to mistune at least one cavity adjacent to the impeller to minimize acoustic pulsations in the cavity for an n-diameter mode, wherein the bladed elements are positioned adjacent to the impeller.

In another aspect, the bladed elements may be grooves or ridges defined in the casing wall. At least ten bladed elements may be defined in the casing wall. Five bladed elements may have a depth greater than the other five bladed elements. Five bladed elements may have a width greater than the other five bladed elements. The bladed elements may be spaced equidistant from one another. The bladed elements may be defined in the casing wall on a hub side of the impeller. The bladed elements may be defined in the casing wall on a cover side of the impeller. At least sixteen bladed elements may be defined in the casing wall with a specific harmonic mistuning pattern.

In another aspect, up to one less than four times "n" bladed elements may be defined in the casing wall, where "n" is the n-diameter cavity mode that is to be mistuned. The bladed elements may be defined in the casing wall on a hub side of the impeller. The bladed elements may be defined in the casing wall on a cover side of the impeller. The bladed elements may vary in a linear pattern from minimum to maximum size around the circumference to mistune the cavities to minimize acoustic pulsations in the cavity. In another aspect, the number of bladed elements can be selected from two up to one less than <NUM> x "n". For example, if the mode is a <NUM>-diameter mode, then (2n-<NUM>) or nine bladed elements can be selected to mitigate or eliminate the <NUM>-diameter mode within the area containing the bladed elements. If needed, the nine vanes can still be mistuned to give a <NUM> x "n" harmonic or other mistuning pattern to reduce noise and/or impeller vibration.

These and other features and characteristics of the turbomachine, as well as the methods of operation and functions of the related elements of structures and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. As used in the specification and the claims, the singular form of "a", "an", and "the" include plural referents unless the context clearly dictates otherwise.

For purposes of the description hereinafter, the terms "upper", "lower", "right", "left", "vertical", "horizontal", "top", "bottom", "lateral", "longitudinal", and derivatives thereof shall relate to the claimed invention as it is oriented in the drawing figures.

As described above, rotating blades or impellers <NUM> in a conventional turbomachine <NUM> are excited into resonant vibrations by a) upstream stator strut and/or vane wakes and potential flow interaction with downstream struts and vanes, b) other inhomogeneities in the flow stream formed by non-uniform circumferential pressure distribution, c) acoustic pulsations either at rotating blade passing frequency, multiples of blade passing frequency, and/or from vortex shedding from struts or vanes, in turn causing coincident acoustic resonance of the gas within the casing. Rotating blades or impellers <NUM> can be excited to a large amplitude when a blade modal frequency corresponds to the shaft rotational frequency multiplied by the harmonic number of the flow inhomogeneity seen by the rotating blade or impeller <NUM>.

The present disclosure is directed to a method of reducing cavity noise to both reduce noise signature, as well as potential excitation of adjacent bladed disks or impellers from acoustic pressure pulsations. The method considers cavities adjacent the impeller as equivalent gas-filled disks with boundary conditions at the sides and ends of the cavities. Acoustic modes within cavities are affected by the swirling flow of the gas within the cavities, differently on the cover side versus the hub side of the turbomachine. By adding blade-like elements to replace the basic smooth boundaries of the cavities, an effective disk with blades can be modified with intentional mistuning to greatly reduce the response of vibratory modes of the gas in the cavities.

With reference to <FIG>, an impeller <NUM> is housed within a casing <NUM> within a turbomachine, such as the turbomachine <NUM> shown in <FIG> and <FIG>. <FIG> illustrates a single stage of a turbomachine; however, one of ordinary skill in the art will understand that specific components illustrated in <FIG> can be easily adapted for use in multi-stage turbomachines, such as a multi-stage, centrifugal-flow compressor. A plurality of impellers <NUM> may be spaced apart in multiple stages along the axial length of the shaft. The impeller <NUM> is configured to rotate about the shaft during operation of the turbomachine. In one aspect, the impeller <NUM> is fixedly coupled to the shaft such that the impeller <NUM> rotates with the rotation of the shaft. The casing <NUM> may extend around a cover side <NUM> of the impeller <NUM> and a hub side <NUM> of the impeller <NUM>. It is also contemplated that a cover may not be provided with the turbomachine, thereby providing an "open" impeller configuration. A first cavity <NUM> is defined on the cover side <NUM> of the impeller <NUM> and a second cavity <NUM> is defined on the hub side <NUM> of the impeller <NUM>. In one aspect, the cavities <NUM>, <NUM> are considered to be equivalent to gas-filled disks provided on the sides of the impeller <NUM>. In one aspect, at least one cavity <NUM>, <NUM> is modified so that the surface boundaries of the gas contained within the cavity <NUM>, <NUM> has blade-like elements <NUM>, <NUM>. In another aspect, both cavities <NUM>, <NUM> are modified so that the surface boundaries of the contained gas within the cavities <NUM>, <NUM> have blade-like elements <NUM>, <NUM> (also referred to as "bladed elements"). In one aspect, the blade-like elements <NUM>, <NUM> are non-axisymmetric blade-like elements.

With reference to <FIG>, in one aspect, the blade-like elements <NUM>, <NUM> are grooves and/or ridges spaced around the circumference of the cavities <NUM>, <NUM>. In another aspect, the blade-like elements <NUM>, <NUM> may be ribs that extend into the cavity. In another aspect, a machining process produces a wave-like length (also referred to as a scallop shape) that is spaced and subsequently modified to give a desired number of effective mistuned elements in the cavities <NUM>, <NUM> to greatly reduce the response of vibratory modes of the gas in the cavities <NUM>, <NUM>. The number of blade-like elements <NUM>, <NUM> can be chosen in order to apply desired variations to reduce a response of a particular diameter mode or modes. When referring to a diameter mode, it to be understood to mean a configuration of high acoustic pressure pulsations separated by areas having low acoustic pressure pulsations. For example, a <NUM>-diameter acoustic mode would have five areas with high acoustic pressure pulsations alternating with five areas having low acoustic pressure pulsations. Therefore, with reference to <FIG>, five of the blade-like elements <NUM> may have a high acoustic pressure pulsation and the adjacent blade-like elements <NUM> may have low acoustic pressure pulsations. For example, a <NUM>-diameter acoustic mode of the gases within the cavities <NUM>, <NUM> could be resonant with <NUM> times the operating speed, the impeller rotating blade passing frequency. The spinning modes for blade passing frequency could have five lobes due to <NUM> impellers interacting with <NUM> inlet vanes <NUM> or <NUM> diffuser vanes <NUM>. With a spinning acoustic mode at frequency ω with "n" patterns, there may be a variation of pressure in the circumferential direction that is continuous and repeats every <NUM>π radians, as defined by the cylindrical geometry. This pressure distribution rotates at (ω / n) and generates, at every stationary point, a fluctuating pressure at frequency ω. The pattern sweeps the cavity annulus walls at a velocity (r × ω / n), in which r is the radius of the cavity. One or both of the cavities <NUM>, <NUM> on the cover side <NUM> and the hub side <NUM> could have <NUM> equally-distant blade-like elements <NUM>, <NUM>, such as grooves and/or ridges, machined in the vertical walls of the casing <NUM>. It is also contemplated that alternative degrees of separation between the blade-like elements <NUM>, <NUM> may be used. Intentional mistuning can then be selected to give, for example, a <NUM>-diameter mismatch pattern to greatly reduce the response of vibratory modes of the gas in the cavities <NUM>, <NUM>. In one aspect, to provide the intentional mistuning, a pattern with <NUM> blade-like elements <NUM>, <NUM>, such as grooves and/or ridges, would give a <NUM>-diameter pattern or, alternatively, could have every second groove in the pattern twice as deep and/or wide as the adjacent groove, as shown in <FIG>. It is to be understood that the configuration of the blade-like elements <NUM> shown in <FIG> can also be used for the blade-like elements <NUM> on the hub side <NUM> of the impeller <NUM>. In another aspect, the configuration of the blade-like elements <NUM> on the cover side <NUM> of the impeller <NUM> could be different from the configuration of the blade-like elements <NUM> on the hub side <NUM> of the impeller <NUM>. In another aspect, a pattern could have <NUM> blade-like elements <NUM>, <NUM>, such as grooves, ridges, and/or scallop shapes. Using these patterns of blade-like elements <NUM>, <NUM>, acoustic pulsations are reduced and impeller response at <NUM> times speed is minimized.

If needed, the number of blade-like elements <NUM>, <NUM> to mistune may be chosen depending on whether the harmonic of the stationary vanes is greater than or less than the harmonic of the rotating blades. In an example where the harmonic of vanes is greater than the harmonic of the blades passing by with <NUM> x "n" blade-like elements <NUM>, <NUM>, the spinning modes due to interaction with the rotating blade passing frequency will rotate in an opposite direction to that from the stationary vanes and cancel some of the pulsations. For example, if the <NUM>-diameter mode is due to the difference of <NUM> stationary vanes and <NUM> rotating blades, then <NUM> blade-like elements <NUM>, <NUM> would be used so that spinning modes would be in an opposite direction from those for the <NUM> stationary vanes. The acoustic pulsations due to the blade-like elements <NUM>, <NUM> would counter those caused by the vanes. In an example where the number of stationary vanes is less than the number of rotating blades, four times "n" blade-like elements <NUM>, <NUM> would be used. For example, if the <NUM>-diameter mode is due to the difference of <NUM> stationary vanes and <NUM> rotating blades, <NUM> blade-like elements <NUM>, <NUM> would be used so that spinning modes would rotate in opposite directions from those for the ten stationary vanes. The acoustic pulsations due to blade-like elements <NUM>, <NUM> would counter those caused by the vanes. As an example of the present disclosure, shown in <FIG>, the blade-like elements <NUM> may be milled into the upstream and downstream diaphragms on the surface of the cavity between the diaphragm and the impeller. In this example, ten blade-like elements <NUM> are spaced equally around the circumference. In this example, five larger blade-like elements <NUM> are spaced <NUM> degrees apart from one another. Smaller blade-like elements <NUM> are spaced equally between each of the larger blade-like elements <NUM>. Each blade-like element <NUM> may be <NUM>,<NUM> (<NUM> inches) in length located at an inner radius of <NUM>,<NUM> (<NUM> inches). The larger blade-like elements <NUM> may be <NUM>,<NUM> (<NUM> inches) wide and <NUM>,<NUM> (<NUM> inches) deep. The smaller blade-like elements <NUM> may be <NUM>,<NUM> (<NUM> inches) wide and <NUM>,<NUM> (<NUM> inches) in depth. <FIG> a similar arrangement of blade-like elements <NUM>. However, this arrangement includes twenty blade-like elements <NUM>, including ten larger blade-like elements <NUM> and ten smaller blade-like elements <NUM>. It is also to be understood that the blade-like elements <NUM> may be arranged in similar arrangements as described above.

In another aspect, by including the blade-like elements <NUM>, <NUM> within one of the cavities <NUM>, <NUM>, concern for circular modes are eliminated by having a different number of blade-like elements <NUM>, <NUM> than the rotating blades in the adjacent disk. Even <NUM>-diameter modes that are of less concern structurally could have blade-like elements <NUM>, <NUM> to reduce acoustic pulsations. In other aspects, the blade passing frequency pulsations at the sides of the disks excite plate modes with high motion near the outer diameter of the disk. In this aspect, the blade-like elements <NUM>, <NUM> within the cavities <NUM>, <NUM> are spaced and modified depending on the relative phase angles of the acoustic pulsations versus the plate mode.

Modifications to one or both of the cavities <NUM>, <NUM> can be separate to reduce response of disks to one or more gas modes, but the modifications can also be in addition to structural bladed disk mistuning as is used in the prior art for even greater reliability. In particular, along with the blade-like elements <NUM>, <NUM> provided on the casing <NUM>, similar blade-like elements, such as grooves, ridges, and/or scallop-shaped protrusions, could be provided on the disk or cover <NUM> of the impeller <NUM> facing the cavities <NUM>, <NUM> to further reduce response. Alternatively, these could serve as the blade-like elements that mistune the adjacent fluid-filled cavities. It is to be understood that the term "fluid" used throughout this description encompasses gases, liquids, and gas/liquid mixtures. Acoustic modes would thus be affected by changes in the swirling flow of the gas within the cavities, differently on the cover side versus the hub side of the turbomachine. Cavity modifications could use direct machining and/or welding to form the blade-like element <NUM>, <NUM> on the casing <NUM> or utilize inserts that can be installed and replaced in the cavities <NUM>, <NUM> if needed. The blade-like elements <NUM>, <NUM> may be worn down during use of the turbomachine and may need to be reformed or redefined. It is also contemplated that further blade-like elements could be added to the cavities <NUM>, <NUM> to have other function within the cavities <NUM>, <NUM>, such as reducing swirl of flow into seals in the turbomachine, reducing thrust load, or reducing flow point when the compressor or turbomachine stall begins at the impeller tip or diffuser entrance.

In other aspects, beside compressor impellers, the blade-like element arrangement may be used to mistune any cavity or annulus that has diametrical mode shapes or other pattern pressure pulsation frequency, excited by various sources, alleviating concern for structures, including rotating and stationary components, and/or environmental noise issues. Similar liquid-handling pumps, axial compressors, fans, as well as steam or gas turbines, are turbomachines that could utilize this disclosed method. Other potential applications for the mistuned blade-like element arrangement to reduce response of acoustic modes include fluid-handling including air mechanisms, such as engines, machinery, fuel cells, piping, ducts, diffusers, nozzles, valves, silencers, mufflers, seals, heat exchangers, airframes, tires and wheels, rockets, combustion chambers, vehicles, speakers, and double-pane windows.

With reference to <FIG>, blade-like elements <NUM> are defined in a diaphragm wall <NUM> adjacent to a cover disk <NUM> on the cover side of the impeller <NUM> in the turbomachine. In one aspect, two diaphragms 122a, 122b define cavities to receive the impeller <NUM>. The blade-like elements <NUM> may be defined in the diaphragm wall <NUM> of one or both of the diaphragms 122a, 122b. One or more blade-like elements <NUM> are circumferentially spaced around the diaphragm wall <NUM>. In another aspect, <NUM> blade-like elements <NUM> are defined in each diaphragm 122a, 122b at the sides of the impeller <NUM>. In one aspect, the blade-like elements <NUM> are separated <NUM> degrees apart from one another. As some acoustic modes are coupled to one another, both cavities and/or the impeller sides could have blade-like elements that are located out-of-phase with each other to provide additional mistuning.

Claim 1:
An arrangement for intentionally mistuning a cavity formed adjacent an impeller hub and another cavity formed when an impeller cover is provided in a turbomachine having "n"-diameter acoustic modes, the arrangement being characterised by
at least two bladed elements (<NUM>) defined within a perimeter of a casing wall on a hub side (<NUM>) of the impeller (<NUM>), wherein the hub side (<NUM>) of the impeller (<NUM>) is opposite a cover side (<NUM>) of the impeller (<NUM>),
wherein the bladed elements (<NUM>) are positioned to mistune the cavities (<NUM>, <NUM>) to minimize acoustic pulsations in the cavities (<NUM>, <NUM>),
wherein the at least two bladed elements (<NUM>) are positioned adjacent the impeller (<NUM>).