Patent Description:
In centrifugal compressors, the rotor accelerates a gas flow in a circumferential direction around an axis in order to compress it centrifugally. Stator blades are usually placed downstream of one or more impellers of the rotor to straighten the gas flow following compression, in particular to correct the radial circumferential component of the velocity. <CIT> discloses a centrifugal compressor. <CIT> discloses a compressor for a turbocharger. <CIT> discloses a centrifugal compressor. <CIT> discloses a diffuser. <CIT> discloses a centrifugal compressor. <CIT> discloses a compressor assembly.

For example, in multi-stage compressors having one or more impellers, stator blades are placed in return channels between two consecutive impellers in order to receive a gas flow from the first rotor and direct it to the second rotor and straighten it in the process.

The shape of the stator blade interacts with the fluid differently depending on the flow conditions which depend on the operative condition of the compressor.

Typically, the stator blades design is optimized to cause a smooth flow around the blade at the design operational speed of the compressor. However, these blades may create losses when the compressor does not operate at its operational design speed, for example during start-up or shut-down or in operational conditions that require a continuous change of the compressor speed.

In these conditions, prior-art stator blades optimized for just one operative condition tend to cause local or even total flow separations, which cause stalls or recirculation areas and impact the performance of the compressor.

Therefore, it would be desirable to provide a stator blade which could operate over a wider range of operative conditions avoiding flow separations or at least reducing flow separations.

According to one aspect, the subject-matter disclosed herein relates to a stator blade for a centrifugal compressor as defined in claim <NUM>.

According to another aspect, the subject-matter disclosed herein relates to a centrifugal compressor comprising at least one stator blade as defined in claim <NUM>.

The subject matter herein disclosed relates to a stator blade to be positioned in a compressor, downstream of an impeller of the rotor, in order to straighten the gas flow coming from the impeller. The stator blade has a front portion configured to generate one or more streamwise vortices in the gas flow which follow the stream of the flow and remain attached to an upper surface (also known as the "suction surface") of the stator blades.

A streamwise vortex is a vortex which extends parallel to the direction of the flow and defines a "vortex tube" in which the flow moves with a substantially helical trajectory. The streamwise vortices shuffle the boundary layer of the flow on the upper surface of the stator blade, re-energizing the boundary layer in order to prevent or delay the detachment of the flow from the surface, therefore delaying and/or reducing the entity of a stall of the stator blade. "Streamwise vorticity" and its generation are known as such from textbooks, for example from the book "<NPL>.

More in detail, the vortices are generate by at least two pointed protrusions located in the front portion of the stator blade and are carried downstream by the gas flow along the upper surface of the blade.

Reference now will be made in detail to embodiments of the disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the disclosure, not limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope of the disclosure. Reference throughout the specification to "one embodiment" or "an embodiment" or "some embodiments" means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase "in one embodiment" or "in an embodiment" or "in some embodiments" in various places throughout the specification is not necessarily referring to the same embodiment(s).

When introducing elements of various embodiments the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements. The terms "comprising", "including", and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

<FIG> discloses a stator blade <NUM> for a centrifugal compressor, in particular for a multi-stage centrifugal compressor to be used in a plant for processing gasses such as methane, ethane, propane, ethylene, carbon dioxide, helium, argon, hydrogen, refrigerant gasses or a mixture of these substances.

<FIG> each shows a different embodiment of a stage <NUM> of a centrifugal compressor, each comprising a different embodiment of a stator blade <NUM> according to the subject-matter disclosed herein installed downstream of an impeller <NUM> of the compressor. The centrifugal compressor may be employed in a variety of different oil and gas applications, including: production, transportation, refinery, petrochemical and chemical industries, handling a very large variety of gases and mixtures of gases in a wide range of operating conditions (pressure and temperature).

More in detail, <FIG> shows an embodiment in which the stator blade <NUM> is installed in a return channel <NUM> of the compressor to act as a return channel blade of the compressor itself. <FIG> shows an embodiment in which the stator blade <NUM> is installed in a diffuser <NUM> to act as a diffuser blade of the compressor itself. <FIG> shows an embodiment in which the stator blade <NUM> extends at least partially through both the diffuser <NUM> and the return channel <NUM> of the compressor and through a <NUM>° turn between the diffuser <NUM> and the return channel <NUM>.

It is to be noted that, according to a variant of the embodiment of <FIG>, there are blades according to the subject-matter disclosed herein both in the diffuser and in the return channel.

The gas flow coming from e.g. the impeller <NUM> has a velocity comprising a radial component and a circumferential component with respect to the longitudinal axis "Z" of the compressor stage <NUM>. The stator blade <NUM> is either fixed to compressor stage <NUM> or configured to be arranged in the compressor stage <NUM>, in the diffuser <NUM> and/or the return channel <NUM> in a predetermined position.

The position of the stator blade <NUM> in the compressor stage <NUM> is arranged and positioned in order for the stator blade <NUM> to be invested by the above-mentioned gas flow and to deviate it to lower or cancel its circumferential velocity component. The stator blade <NUM> is arranged and positioned in order to maintain the gas flow attached to its external surface, limiting or avoiding flow separations, from the leading edge <NUM> to the trailing edge <NUM> at least when the compressor is operated at its design operational speed.

More in detail, the stator blade <NUM> has a curved external surface configured to guide the gas flow from the leading edge <NUM> to the trailing edge <NUM> of the stator blade <NUM> itself. The external surface of the stator blade <NUM> comprises a pressure surface <NUM>, that extends between the leading edge <NUM> and the trailing <NUM> on the "lower side" of the stator blade <NUM>, and a suction surface <NUM> that extends between the leading edge <NUM> and the trailing <NUM> on the "upper side" of the stator blade <NUM>. The pressure surface <NUM> identifies the portion of the external surface of the stator blade <NUM> which is subject to a pressure higher than the pressure of the surrounding the gas flow under normal operational conditions. The suction surface <NUM> identifies the portion of the external surface which is subject to a pressure lower than the pressure of the surrounding the gas flow under normal operational conditions. In general, the suction surface <NUM> is convex and the pressure surface <NUM> is concave or has a lower convexity than the suction surface <NUM>.

The stator blade <NUM> comprises a front portion <NUM> arranged to receive a gas flow coming from e.g. the impeller <NUM> and to split it in a pressure-side gas flow adjacent to the pressure surface <NUM> and in a suction-side gas flow adjacent to the suction surface <NUM>.

The front portion <NUM> is configured to generate at least one streamwise vortex "V" in the gas flow adjacent to the external surface of the stator blade <NUM>. According to the invention the front portion <NUM> is configured to generate at least a couple of counter-rotating streamwise vortices "V", as shown in <FIG> and <FIG>. In a possible embodiment shown in <FIG>, the front portion <NUM> is configured to generate a plurality of couples of counter-rotating streamwise vortices "V".

Preferably, the streamwise vortices "V" have a diameter comprised between a minimum value and a maximum value.

In particular, the front portion <NUM> is configured to generate streamwise vortices "V" which have a diameter proportional to the median spanwise dimension of the stator blade <NUM> (and thus of the width of the channel in which the stator blade <NUM> is installed) and inversely proportional to the total number of streamwise vortices "V".

Preferably, the leading edge <NUM> of the stator blade <NUM> has at least four oblique stretch <NUM>, configured to generate a streamwise vortex "V" of the above-mentioned type. In particular, the oblique stretches <NUM> define an angle of attack with the incoming gas flow and cause the streamwise vortices "V" by lowering the pressure of the incoming gas flow. The oblique layout of the oblique stretches <NUM> determine an uneven distribution of pressure in the spanwise direction of the stator blade <NUM>; this causes the establishment of a spanwise velocity component in the flow which leads to the formation of one of the streamwise vortices "V" shown in <FIG>.

Preferably, the leading edge <NUM> has a plurality of oblique stretches <NUM>. <FIG>, <FIG> and <FIG> show stator blades <NUM> in which the leading edge <NUM> has two oblique stretches <NUM>. <FIG> and <FIG> show stator blades <NUM> in which the leading edge <NUM> has four oblique stretches <NUM>. <FIG> and <FIG> show stator blades <NUM> in which the leading edge <NUM> has eight oblique stretches <NUM>. <FIG> and <FIG> show stator blades <NUM> in which the leading edge <NUM> has six oblique stretches <NUM>. In a non-illustrated stator blade <NUM> not forming part of the invention, the leading edge <NUM> has only one oblique stretch <NUM>.

In particular, the oblique stretches <NUM> are oblique with respect to a spanwise direction of the stator blade <NUM> and can be either straight (as shown in <FIG>, <FIG>), or curved (as shown in <FIG>). Advantageously, the oblique stretches <NUM> configured as described above are also oblique with respect to the direction of the gas flow investing the front portion <NUM>.

The front portion <NUM> comprises at least one pointed protrusion <NUM> having a vertex <NUM>. <FIG>, <FIG> and <FIG> show stator blades <NUM> in which the front portion <NUM> comprises a single pointed protrusion <NUM>. <FIG> and <FIG> show stator blades <NUM> in which the front portion <NUM> comprises two pointed protrusions <NUM>. <FIG> and <FIG> show stator blades <NUM> in which the front portion <NUM> comprises four pointed protrusions <NUM>.

Each pointed protrusion <NUM> defines at least two of the oblique stretches <NUM> of the leading edge <NUM> described above. In particular, the two oblique stretches <NUM> are positioned at opposite sides of the vertex <NUM> and are configured to generate two counter-rotating streamwise vortices "V".

In <FIG>, <FIG>, the pointed protrusions <NUM> have a triangular shape in a median plane of the stator blade <NUM> and defines the straight oblique stretches <NUM>. In <FIG>, the pointed protrusions <NUM> have a cusp shape in a median plane of the stator blade <NUM> and define the curved oblique stretches <NUM>.

Preferably, the pointed protrusions <NUM> project in a forward direction which defines an angle between <NUM>° and -<NUM>° with respect to a line tangent at a front end of a stretch of the mean camber line of the stator blade <NUM> located at the front portion <NUM>; this stretch starts from the leading edge of the blade (excluding the pointed protrusion) and may amount for example to <NUM>-<NUM>% of the total length of the mean camber line. In <FIG>, the forward direction in which the pointed protrusion <NUM> project is substantially tangent to the above-mentioned stretch of the mean camber line located at the front portion <NUM>.

In a possible non-illustrated embodiment, the stator blade <NUM> comprises a plurality of pointed protrusions <NUM> projecting in different forward directions; a forward direction defines an angle between <NUM>° and -<NUM>° with respect to a line tangent at a front end of a stretch of said mean camber line; this stretch is located at the front portion <NUM> of the stator blade <NUM> and starts from the leading edge of the stator blade <NUM> (excluding the pointed protrusion).

Preferably, each pointed protrusion <NUM> is symmetrical with respect to a longitudinal plane of the stator blade <NUM>. According to a possible alternative non-illustrated embodiment of the stator blade <NUM> comprises one or more asymmetrical pointed protrusions <NUM> in which the oblique stretches <NUM> relative to the pointed protrusion <NUM> define different angles with respect to the spanwise direction of the stator blade <NUM>.

Preferably, the pointed protrusions <NUM> have a longitudinal extension along the forward direction comprised between a minimum value and a maximum value. The minimum value is given by the formula <NUM>b/M and the maximum value of the extension is given by the formula <NUM>. <NUM>b/M, wherein b is the median spanwise dimension of the stator blade <NUM> and M is the number of pointed protrusions <NUM> in the front portion <NUM>.

Preferably, the leading edge <NUM> has a vertex angle of less than <NUM>° at the vertex <NUM> of the pointed protrusions <NUM>, more preferably less than <NUM>°. The vertex angle is to be intended as the angle between the two oblique stretches <NUM> adjacent to the same vertex <NUM>. More in detail, the vertex angle should be measured in a camber plain of the stator blade <NUM>.

Preferably, the stator blade <NUM> has a spanwise variable airfoil, wherein the airfoil changes gradually between a vertex airfoil located at the pointed protrusion <NUM> and a trough airfoil located next to (at some distance from) the pointed protrusion <NUM> or in a trough <NUM> between two pointed protrusions <NUM>. More in detail, the vertex airfoil has a sharp leading edge 102a and the trough airfoil has either a sharp or a rounded leading edge.

<FIG> shows a sharp leading edge airfoil which can be employed as the vertex airfoil. In this figure, the pointed protrusion <NUM> projects in a forward direction D; in general, the forward direction defines an angle between <NUM>° and -<NUM>° with respect to a line tangent at a front end of a stretch <NUM> of the mean camber line; stretch <NUM> is located at front portion <NUM> of stator blade <NUM> and starts (see point <NUM>) from the leading edge of stator blade <NUM> (excluding the pointed protrusion); in the embodiment of <FIG>, forward direction D coincides with the tangent line, i.e. the angle is <NUM>°.

<FIG> shows a round leading edge airfoil which can be employed as the trough airfoil.

<FIG> shows an embodiment of the stator blade <NUM> having a sharp leading edge 102a extending along the whole spanwise dimension. <FIG> shows an embodiment of the stator blade <NUM> having a sharp leading edge 120a at the vertex <NUM> extending spanwise for a portion of the spanwise dimension and then changing to a rounded leading <NUM> on the sides of the stator blade <NUM>. <FIG> shows an embodiment of the stator blade <NUM> having a sharp leading edge 102a only at the vertex <NUM> of the pointed protrusion <NUM>, immediately changing to a round leading edge 102b at the sides of the vertex <NUM>.

According to another aspect and with reference to <FIG> and <FIG>, the subject-matter disclosed herein provides a centrifugal compressor <NUM>, preferably of multi-stage type, which comprises a plurality of stator blades <NUM> of the type described above. In particular, the centrifugal compressor comprises a plurality of compressor stages <NUM>, each having an impeller <NUM>, a diffuser <NUM> and a return channel <NUM>, and each compressor stage <NUM> comprises a plurality of stator blades <NUM> arranged in a circular array in the diffuser <NUM> and/or in the return channel <NUM>. More in detail, the circular arrays of stator blades <NUM> extends around the longitudinal axis "Z" in order to receive an incoming flow from the impeller <NUM> of the compressor stage <NUM> having a circumferential component of the velocity around the longitudinal axis "Z" and to change the direction of the flow in order to lower or cancel the circumferential component of the velocity and to deliver a clean, straight flow to the rotor of the following compressor stage.

Claim 1:
A stator blade (<NUM>) for a centrifugal compressor, said stator blade (<NUM>) comprising a front portion (<NUM>) arranged to receive a gas flow and having an external surface configured to guide said gas flow adjacent to said external surface, characterized in that said front portion (<NUM>) has a leading edge (<NUM>) configured to generate at least one streamwise vortex (V) in said gas flow, the axis of the vortex (V) being substantially parallel to the direction of the gas flow,
the leading edge (<NUM>) having at least two pointed protrusions (<NUM>) each having a vertex (<NUM>) with a cusp shape defining two oblique stretches (<NUM>) that are curved; and
adjacent oblique stretches (<NUM>) of the pair of protrusions (<NUM>) together forming a rounded trough (<NUM>) therebetween.