Method and device for determining an indicator for a prediction of an instability in a compressor and use thereof

Abstract

The invention relates to a method for determining an indicator for a prediction of an instability in a compressor, which is designed as an axial or radial compressor, having the following steps: operating a compressor designed as an axial or radial compressor in operating states, which differ by different values of a characteristic parameter for a flow mass flux of the compressor, wherein the operating states are run through at decreasing flow mass fluxes; determining the values of the characteristic value for the flow mass flux for the operating states; detecting time-resolved pressure measurement values when running through the operating states by means of a pressure sensor, which is arranged in a housing of the compressor, upstream adjacent to an entrance plane of a rotor stage determining the skew for the operating states and determining an indicator for an instability of the compressor, if an algebraic sign change of the curve rise is determined for a curve profile of the skew over the characteristic parameter for the flow mass flux for the operating states. The invention further relates to the use of the method and a device for determining an indicator for a prediction of an instability in an compressor.

Claims

1. A method for determining an indicator for a prediction of an instability in a compressor, which is designed as an axial or radial compressor, with the following steps: operation of the compressor designed as an axial or radial compressor in operating states which differ by different values of a characteristic parameter for a flow mass flux of the compressor, wherein the operating states are hereby run through at decreasing flow mass fluxes; determination of the values of the characteristic parameter for the flow mass flux for the operating states; acquisition of time-resolved pressure measurement values when the operating states are run through by means of a pressure sensor, which is arranged in a housing of the compressor, upstream, adjacent to an entrance plane of a rotor stage; determination of the skew for the operating states; and determination of the indicator of an instability of the compressor, if an algebraic sign change of a curve rise is determined for a curve profile of the skew over the characteristic parameter for the flow mass flux for the operating states.

2. The method as claimed in claim 1, wherein the pressure sensor is arranged in the housing of the compressor on an inner wall of the housing.

3. The method as claimed in claim 1, wherein the pressure sensor is arranged in the housing of the compressor over blade tips of blades of the rotor stage.

4. The method as claimed in claim 1, wherein during the acquisition of the time-resolved pressure measurement values when the operating states are run through, pressure fluctuations are acquired in a time-resolved manner by means of the pressure sensor.

5. The method as claimed in claim 1, wherein the algebraic sign change of the curve rise indicates a local maximum being run through.

6. The method as claimed in claim 1, wherein a further indicator for the instability of the compressor is determined, if a further algebraic sign change of the curve rise is determined for the curve profile of the skew over the characteristic parameter for the flow mass flux towards lower flow mass fluxes.

7. The method as claimed in claim 6, wherein the algebraic sign change of the curve rise indicates a local minimum being run through.

8. The method as claimed in claim 1, wherein at least one of the flow coefficient or the reduced mass flux for the operating states are determined as the characteristic parameter for the flow mass flux.

9. The method as claimed in claim 1, wherein proceeding from the determination of the indicator, a warning signal is generated as an early warning for compressor instability and is outputted via an output device.

10. The method as claimed in claim 1, wherein the compressor is operated in operating states which lie below a surge limit of the compressor.

11. The use of a method as claimed in claim 1 in the: determination of an operating limit of the compressor designed as an axial or radial compressor on a test bench or monitoring of an engine with the compressor designed as an axial or radial compressor in operation.

12. A device for determining an indicator for a prediction of an instability in a compressor, which is designed as an axial or radial compressor, with: the compressor which is designed as an axial or radial compressor; a measuring device, which is set up, to determine values of a characteristic parameter for a flow mass flux of the compressor in operating states during the operation of the compressor, wherein the operating states differ by different values of the characteristic parameter for the flow mass flux of the compressor and the operating states are hereby run through at decreasing flow mass fluxes, and to acquire time-resolved pressure measurement values by means of a pressure sensor when the operating states are run through, which pressure sensor is arranged in a housing of the compressor, upstream, adjacent to an entrance plane of a rotor stage, and an evaluation unit configured to determine the skew for the operating states; and to determine an indicator for an instability of the compressor, in response to an algebraic sign change of a curve rise being determined for a curve profile of the skew over the characteristic parameter for the flow mass flux for the operating states.

Description

DESCRIPTION OF EXAMPLES OF EMBODIMENT

(1) Further examples of embodiment are explained below, reference being made to the figures of a drawing. In the figures:

(2) FIG. 1 shows a diagrammatic representation of an arrangement for a test bench for testing an axial compressor;

(3) FIG. 2 shows a diagrammatic representation of an axial compressor in cross-section;

(4) FIG. 3 shows a diagrammatic representation of a radial compressor in cross-section;

(5) FIG. 4 shows a graphic representation of the curve profile for operating states of a compressor, wherein the skew is plotted over the flow coefficient;

(6) FIG. 5 shows a graphic representation for operating states at a speed of 5500 revolutions per minute, wherein the skew is plotted over the flow coefficient; and

(7) FIG. 6 shows a graphic representation for operating states at a speed of 9000 revolutions per minute, wherein the skew is plotted over the flow coefficient.

(8) FIG. 1 shows a diagrammatic representation of an arrangement for a test bench for measuring or determining an axial compressor. A rotor 2 with blades 3 and a drive device 4 for rotating rotor 2 are arranged in a flow tube 1. Stator blades are installed downstream of rotor 2. FIG. 1 moreover shows a front view.

(9) For the measurement of characteristic parameters, a Prandtl tube 5 as well as a pressure sensor 6 are provided, which is arranged on a tube wall 7, in such a way that pressure measurement values can be acquired in a time-resolved manner in respect of an entrance plane of rotor 2 upstream adjacent to the entrance plane on the inner side of tube wall 7. Prandtl tube 5 is used to measure the dynamic pressure in flow tube 1.

(10) Pressure sensor 6 is used to measure the static unsteady pressure. The pressure measurement is carried out time-resolved, wherein for example pressure fluctuations can be measured with a high time resolution in a frequency range from approximately 10 kHz to approximately 50 kHz.

(11) In the embodiment in FIG. 1, a further pressure sensor 6a is provided, with which pressure measurements comparable to the measurement with pressure sensor 6 can be acquired in a time-resolved manner and which can alternatively be omitted.

(12) Furthermore, a pressure measurement device 9 is provided in order to measure the static pressure at a compressor exit. In combination with the pressure measurement data from Prandtl tube 5, a pressure ratio generated by the compressor can thus be determined.

(13) FIG. 2 shows a diagrammatic representation of an axial compressor 20, wherein for example a plurality of stage packs 20.1, . . . , 20.5 are arranged behind one another and each comprise a blade rotor and a blade stator, which are arranged in compressor housing 21. Pressure sensor 6 is arranged, comparable to the representation in FIG. 1, adjacent to the entrance plane of the first stage pack 20.1. Alternatively, pressure sensor 6 can also be arranged adjacent to the entrance plane of one of the subsequent stage packs 20.2, . . . , 20.5, in order to acquire the measurement values for the time-resolved pressure measurement.

(14) FIG. 3 shows a diagrammatic representation of a radial compressor 30 with rotor 31 and stator 32, wherein the pressure sensor is arranged in a comparable position.

(15) With the aid of the arrangement represented in FIG. 1, different operating states can be adjusted for the compressor, for example with the speed of rotor 2 kept constant. In the case of throttling of the compressor when running through the operating states, the latter are characterized by an increasingly smaller flow mass flux. When the operating states are run through, the flow mass flux for the respective operating state and assigned pressure measurement values acquired time-resolved are measured with the aid of pressure sensor 6. The skew (third statistical moment) can be determined as an integral parameter, as it is known as such, from the measurement values for the static unsteady pressure.

(16) The acquired measurement values can be evaluated with the aid of an evaluation device not shown, for example by means of a computer, which comprises a processor and a memory. The evaluation device can be connected to the various elements of the measurement device in order to exchange electronic data and signals. An output for outputting optical and/or acoustic signals, in particular for outputting one or more warning signals, can be connected to the evaluation device.

(17) FIG. 4 shows a diagrammatic representation for a curve 40, which results when running through the various operating states with a diminishing flow mass flux, when the skew is plotted over a characteristic parameter for the flow mass flux, wherein flow coefficient φ is indicated specifically in FIG. 4.

(18) If the course of the curve 40 is considered from greater flow coefficients to smaller ones, it emerges that a local minimum 41 is first run through before a local maximum 43 is run through, before surge limit 42 is reached. When local extrema 41, 43 are run through, an algebraic sign change for the rise of curve 40 takes place, which can be determined in each case as an indicator of running towards surge limit 42. Local maximum 43 and local minimum 41 each form here indicators of differing quality, because, with respect to flow coefficient φ, they are at “different distances” from surge limit 42.

(19) FIGS. 5 and 6 show graphic representations for experimental values with speeds of 5500 and 9000 revolutions per minute, wherein the skew is plotted over flow coefficient φ. The characteristic curve profile can be seen, as was explained for FIG. 4.

(20) Further aspects for the determination of the instability indicator or indicators are explained below.

(21) If the axial compressor is on a test bench (see FIG. 1), all the possible operating points can be approached in a targeted manner. The mass flux which flows through the compressor and the pressure which the compressor builds up are controlled separately by means of a throttle mechanism. It is explained below how the determination of the operating limit of the compressor can be proceeded with.

(22) By means of drive device 4, the compressor is operated at a specified speed. Whereas the speed remains constant, the exit opening of the compressor is successively reduced in size, as a result of which the mass flux diminishes and the built-up pressure increases. The so-called throttling of the compressor can be carried out only until the operating limit is reached. That is to say that, at each speed, there is a maximum possible pressure build-up, after which a collapse of the stable aerodynamics in the interior of the compressor occurs—the compressor enters into so-called “surging”.

(23) To construct the curve profile according to FIG. 3, the following parameters are recorded or calculated in the course of the progressive throttling. The characteristic flow parameter plotted on the x-axis represents a similarity parameter for the comparison of different compressor mass fluxes and is ascertained during the test. As an alternative to the characteristic flow parameter, the “reduced mass flux” can also be determined at each operating point. The selection between the two similarity parameters has no effect on the evaluation. For the parameter plotted on the y-axis, a pressure fluctuation is measured with a high time resolution at each operating point on the blade tips. The pressure signal with an arbitrary length can be reduced to an integral parameter, the third statistical moment—the skew. The pair of values, consisting of the flow coefficient (reduced mass flux) and the skew, is transferred to the diagram in FIG. 3. The procedure is repeated for all the following operating points.

(24) The proposed method can use pairs of values for two successive operating points in each case in the various embodiments for the early detection of compressor surging, in order to determine a local curve rise. With the aid of the simple difference quotient, the gradient of the graphic course (rise of the curve) can be determined sequentially between individual operating points. As soon as an algebraic sign change of the difference quotient takes place for the first time during the throttling process (see local minimum 41 in FIG. 3), this result is interpreted as a preliminary stage to the compressor surging. If a further algebraic sign change subsequently takes place (see local maximum 43 in FIG. 3), the last adjusted operating point characterizes the last stable operating point before surge limit 42 is reached. The method provides at this point for the outputting of a corresponding recommendation to discontinue the throttling process in order to prevent the surge limit being exceeded.

(25) The features disclosed in the above description, the claims and the drawing may be of importance both individually as well as in an arbitrary combination for the implementation of the various embodiments.