Anti-surge regulation for a charging compressor with which an auxiliary power unit is equipped

Abstract

An aircraft auxiliary power unit is equipped with a charging compressor. A method determines a surge parameter indicative of a risk that the charging compressor will display the phenomenon known as surge. A method and a system control a relief valve of this charging compressor. The method for determining the surge parameter includes calculating this surge parameter Ppomp as being the sum of a first term T1 and of a second term T2, the first term T1 being calculated on the basis of a first pressure P1 measured downstream of a diffuser of the charging compressor, and of a second pressure P2 measured upstream of the diffuser, the second term T2 being calculated on the basis of a third pressure P3 measured upstream of the diffuser and of an ambient pressure Psamb indicative of a pressure of an ambient environment.

Claims

1. A method for determining a surge parameter indicative of a risk of the surge phenomenon arising in a charging compressor with which an auxiliary power unit for an aircraft is equipped, the charging compressor comprising a diffuser and a casing disposed downstream of the diffuser, the method comprising calculating the surge parameter P.sub.surge surge as being the sum of a first term T.sub.1 and of a second term T.sub.2, the first term T.sub.1 being calculated from a first pressure P.sub.1 measured downstream of the diffuser and a second pressure P.sub.2 measured upstream of the diffuser, the second term T.sub.2 being calculated from a third pressure P.sub.3 measured upstream of the diffuser and from an ambient pressure Ps.sub.amb indicative of a pressure of an ambient environment surrounding the auxiliary power unit.

2. The method according to claim 1, wherein the first pressure P.sub.1 is a total pressure PT.sub.1300 upstream of the casing, a total pressure Pt.sub.1800 downstream of the casing, a static pressure Ps.sub.1300 upstream of the casing or a static pressure Ps.sub.1800 downstream of the casing.

3. The method according to claim 1, wherein the second pressure P.sub.2 and/or the third pressure P.sub.3 is a static pressure Ps.sub.1270i upstream of the diffuser measured between blades of the diffuser at the leading edge thereof.

4. The method according to claim 1, wherein the ambient pressure Ps.sub.amb is a static pressure of the ambient environment of the auxiliary power unit.

5. The method according to claim 1, wherein the first term T.sub.1 is calculated using the following equation: $T_1=\frac{P_1-P_2}{P_1}.$

6. The method according to claim 1, wherein the second term T.sub.2 is calculated using the following equation: $T_2=\frac{1}{1+\exp \left(10.Math.\left(P_3-P{s}_{amb}\right)\right)}.$

7. The method for controlling a relief valve for a charging compressor with which an auxiliary power unit for an aircraft is equipped, the charging compressor comprising a diffuser and a casing disposed downstream of the diffuser, the relief valve being disposed downstream of the casing, the control method comprising: calculating the surge parameter P.sub.surge in accordance with the determination method according to claim 1, comparing the surge parameter P.sub.surge determined by the determination method with a predetermined pressure relief threshold Th.sub.rel and opening the relief valve when the surge parameter P.sub.surge is below the pressure relief threshold Th.sub.rel or when the surge parameter P.sub.surge is above the pressure relief threshold Th.sub.rel.

8. A device for controlling a relief valve for a charging compressor with which an auxiliary power unit for an aircraft is equipped, the charging compressor comprising a diffuser and a casing disposed downstream of the diffuser, the relief valve (24) being disposed downstream of the casing, the control device comprising a processing unit arranged to determine the surge parameter P.sub.surge in accordance with the determination method according to claim 1, to compare said surge parameter P.sub.surge with a predetermined pressure relief threshold Th.sub.rel and to trigger the opening of the relief valve when the surge parameter P.sub.surge is below the pressure relief threshold Th.sub.rel or when the surge parameter P.sub.surge is above the pressure relief threshold Th.sub.rel.

9. An auxiliary power unit for an aircraft, the auxiliary power unit comprising a charging compressor, a relief valve and a relief valve control device according to claim 8, the charging compressor comprising a diffuser and a casing disposed downstream of the diffuser, and the relief valve being disposed downstream of the casing.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Other features, details and advantages of the invention will appear after reading the following description, which is provided for illustration purposes only, given with reference to the accompanying drawings, for which:

(2) FIG. 1 shows an example of an auxiliary power unit comprising a relief valve for a charging compressor and a processing unit arranged to control the relief valve in accordance with the control method according to the invention;

(3) FIG. 2 diagrammatically shows a charging compressor of the auxiliary power unit in FIG. 1;

(4) FIG. 3 shows an example method for controlling the relief valve according to the invention.

DETAILED DESCRIPTION

(5) FIG. 1 diagrammatically shows an example of an auxiliary power unit 1 with which an aircraft can be equipped. The auxiliary power unit 1 (APU) comprises a gas generator 10 and a compressed air supply system 20. The gas generator 10 includes a main compressor 11, a combustion chamber 12, a turbine 13, a nozzle 14, a power shaft 15, an air supply line 16, a compressed air line 17, a flue gas line 18 and an exhaust line 19. The main compressor 11 is supplied with air via the air supply line 16 and supplies compressed air to the combustion chamber 12 via the compressed air line 17. The compressed air is mixed with fuel in the combustion chamber 12. The combustion of this mixture generates high-energy gases which are conveyed into the turbine 13 via the flue gas line 18. The passage of these gases through the turbine 13 causes it to rotate. The power shaft 15 mechanically connects the turbine 13 to the main compressor 11, such that the main compressor 11 is also driven in rotation. After passing through the turbine 13, the exhaust gases are exhausted from the auxiliary power unit 1 through the exhaust line 19 and the nozzle 14.

(6) The compressed air supply system 20 comprises a charging compressor 21, inlet guide vanes 22, a processing unit 23, a relief valve 24, an air intake line 25, an air output line 26, a first pressure sensor 27, a second pressure sensor 28 and a third pressure sensor 29. FIG. 2 diagrammatically shows the charging compressor 21. The charging compressor 21 is a centrifugal compressor. It is supplied with air from the air supply line 16 via the air intake line 25. The inlet guide vanes 22 are positioned on the air intake line 25, so as to regulate the air flow to the inlet of the charging compressor 21 without interfering with the air flow to the main compressor 11. The inlet guide vanes 22 are also referred to as “IGVs”. The charging compressor 21 comprises an impeller 211, a diffuser 212 and a casing 213. The impeller 211 is mechanically connected to the power shaft 15 in order to be driven in rotation with the main compressor 11 and the turbine 13. The casing 213 is disposed downstream of the diffuser 212 and connected to an inlet of the air output line 26. The outlet of the air output line 26 is connected to the relief valve 24 and to an environmental control system 30. In the example embodiment shown in FIG. 1, the first pressure sensor 27 measures the total pressure Pt.sub.1300 at the interface between the diffuser 212 and the casing 213 of the charging compressor 21. The second pressure sensor 28 measures the static pressure Ps.sub.1270i upstream of the diffuser 212. The third pressure sensor 29 measures a static pressure Ps.sub.amb of the ambient environment of the auxiliary power unit 1. According to other embodiments, the pressure sensors 27, 28, 29 can measure other pressures. In particular, the pressure sensor 27 could measure the static pressure Ps.sub.1300 at the interface between the diffuser 212 and the casing 213 of the charging compressor 21, or the total pressure Pt.sub.1800 or static pressure Ps.sub.1800 downstream of the casing 213. The pressure sensor 29 could measure the total pressure Pt.sub.amb of the ambient environment. The processing unit 23 comprises, for example, a processor. It is arranged to receive the measurements from the pressure sensors 27, 28 and 29, to determine a surge parameter P.sub.surge from these measurements, and to trigger the opening of the relief valve 24 as a function of this surge parameter P.sub.surge and as a function of a predetermined pressure relief threshold Th.sub.rel, as described hereinbelow. The processing unit 23 can further be arranged to control the inlet guide vanes 22, for example as a function of one or more measurements from the pressure sensors 27, 28, 29, and/or as a function of other measurements.

(7) The environmental control system 30 is arranged to regulate the air pressure inside the aircraft cabin. It in particular comprises a valve 31, referred to as the ECS valve, for regulating the air flow to the environmental control system. In particular, the ECS valve 31 can be in the closed position when compressed air is being supplied to the environmental control system 30 by the main engines.

(8) FIG. 3 shows an example method for controlling the relief valve 24. The control method 100 comprises a step 101 of measuring three control pressures, i.e. the total pressure Pt.sub.1300 upstream of the casing 213, the static pressure Ps.sub.1270i upstream of the diffuser 212 and the static pressure Ps.sub.amb of the ambient environment of the auxiliary power unit. In a step 102, the surge parameter P.sub.surge is calculated from these control pressures:

(9) $P_{surge}=\frac{P{t}_{1300}-P{s}_{1270i}}{P{t}_{1300}}+\frac{1}{1+\exp \left(10.Math.\left({\mathrm{Ps}}_{1270i}-P{s}_{amb}\right)\right)}$

(10) The method then comprises a step 103 of comparing this surge parameter P.sub.surge with a predetermined pressure relief threshold Th.sub.rel. This pressure relief threshold Th.sub.rel can be constant, regardless of the speed of the gas generator 10 and the opening position of the IGVs. If the surge parameter P.sub.surge is below or equal to the pressure relief is threshold Th.sub.rel the method 100 is resumed at step 101 of measuring the control pressures in order to perform a monitoring loop. If, on the other hand, the surge parameter calculated in step 102 is above the pressure relief threshold Th.sub.rel, the method proceeds to step 104 of opening the relief valve 24. During this step 104, the relief valve 24 is opened in order to reduce the pressure at the outlet of the charging compressor 21 and thus prevent the surge phenomenon. The relief valve 24 can be completely or partially opened.

(11) The method for controlling the relief valve according to the invention thus makes it possible to prevent the charging compressor from exhibiting a surge phenomenon by controlling this surge risk using a limited number of measured parameters.