Flow measurement for a gas turbine engine

1. A turbomachine comprising:

an air flow measurement system including a plurality of acoustic sensors, the plurality of acoustic sensors being disposed upstream of a compressor of the turbomachine, the plurality of acoustic sensors being configured to transmit acoustic waveforms through an intake airflow to a plurality of further acoustic sensors;

a duct defined by an inner circumferential flow surface and an outer circumferential flow surface, the intake airflow flowing between the inner circumferential flow surface and the outer circumferential flow surface; and

an acoustic liner configured to absorb vibrations generated by rotation of a rotor, the acoustic liner being disposed entirely external to the outer circumferential flow surface and outside of the intake airflow, and the plurality of acoustic sensors being located upstream of the acoustic liner, wherein:

the plurality of further acoustic sensors are arranged entirely recessed in the inner circumferential flow surface, the plurality of further acoustic sensors being disposed outside of the intake airflow,

the plurality of acoustic sensors are located in the outer circumferential flow surface in a single plane,

the plurality of further acoustic sensors are configured to communicate with the plurality of acoustic sensors at a point intercepting the single plane, and

the acoustic liner is located upstream of a fan that is positioned at an airflow intake of the turbomachine.

2. The turbomachine of claim 1 , wherein the plurality of acoustic sensors and the plurality of further acoustic sensors are provided as a circumferential array about the duct.

3. The turbomachine of claim 1 , wherein the acoustic sensors do not protrude into the airflow intake or are mounted behind a gas-washed surface of the airflow intake.

4. The turbomachine of claim 1 , wherein the acoustic sensors are located downstream of an intake throat.

5. The turbomachine of claim 1 , wherein the acoustic sensors are configured to generate tomographic data of airflow through the airflow intake.

6. The turbomachine of claim 1 , wherein the number of the plurality of acoustic sensors is in a range of 2 to 40.

7. The turbomachine of claim 1 , wherein the inner circumferential flow surface rotates about an axial axis and the plurality of further acoustic sensors is configured to rotate with the inner circumferential flow surface.

8. The turbomachine of claim 7 , wherein the inner circumferential flow surface is a surface of a rotor hub or nose cone.

9. The turbomachine of claim 1 , wherein the acoustic sensors are mounted in a nacelle or rotor case of the turbomachine.

10. The turbomachine of claim 1 , further comprising a gas turbine engine for an aircraft comprising:

an engine core including a turbine, the compressor, and a core shaft connecting the turbine to the compressor; wherein the fan is located upstream of the engine core and comprises a plurality of fan blades; and

a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft.

11. The turbomachine of claim 10 , wherein:

the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft;

the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor; and

the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft.

12. The turbomachine of claim 1 , wherein the plurality of further acoustic sensors are located upstream of the fan.

13. A system configured to determine air flow through a turbomachine, the system comprising:

a plurality of acoustic sensors provided upstream of a compressor of the turbomachine, the plurality of acoustic sensors being configured to transmit acoustic waveforms through an intake airflow to a plurality of further acoustic sensors;

a duct of the turbomachine defined by an inner circumferential flow surface and an outer circumferential flow surface, the intake airflow flowing between the inner circumferential flow surface and the outer circumferential flow surface;

an acoustic liner configured to absorb vibrations generated by rotation of a rotor, the acoustic liner being disposed entirely external to the outer circumferential flow surface and outside of the intake airflow, and the plurality of acoustic sensors being located upstream of the acoustic liner, and

a processing system configured to receive signals from the plurality of acoustic sensors and the plurality of further acoustic sensors and determine a flow rate of the intake airflow, wherein:

the plurality of further acoustic sensors are arranged entirely recessed in the inner circumferential flow surface, the plurality of further acoustic sensors being disposed outside of the intake airflow,

the plurality of acoustic sensors are located in the outer circumferential flow surface in a single plane,

the plurality of further acoustic sensors are configured to communicate with the plurality of acoustic sensors at a point intercepting the single plane, and

the acoustic liner is located upstream of a fan that is positioned at an airflow intake of the turbomachine.

14. The system of claim 13 , wherein the processing system is configured to determine a time of flight of at least one of the acoustic waveforms between at least one of the plurality of acoustic sensors and one of the plurality of further acoustic sensors.

15. The system of claim 14 , wherein the processing system is configured to determine an average flow velocity of the intake airflow.

16. The system of claim 14 , wherein the plurality of acoustic sensors and the plurality of further acoustic sensors are configured to determine a flow profile across a cross-section of the airflow intake or to generate tomographic data of the intake airflow.

17. The system of claim 13 , wherein the plurality of further acoustic sensors are located upstream of the fan.

18. A method of determining air flow properties of an intake airflow of a turbomachine, the method comprising steps of:

providing signal communication between a plurality of acoustic sensors and a plurality of further acoustic sensors located in a single plane defined by the plurality of acoustic sensors and the plurality of further acoustic sensors provided upstream of a compressor of the turbomachine;

determining a time of flight of acoustic waveforms between the plurality of acoustic sensors and the plurality of further acoustic sensors; and

using a time of flight between the plurality of acoustic sensors and the plurality of further acoustic sensors to determine an average flow velocity of the intake airflow, wherein

the turbomachine includes:

a duct defined by an inner circumferential flow surface and an outer circumferential flow surface, the intake airflow flowing between the inner circumferential flow surface and the outer circumferential flow surface, and

an acoustic liner configured to absorb vibrations generated by rotation of a rotor, the acoustic liner being disposed entirely external to the outer circumferential flow surface and outside of the intake airflow, and the plurality of acoustic sensors being located upstream of the acoustic liner,

the plurality of further acoustic sensors are arranged entirely recessed in the inner circumferential flow surface, the plurality of further acoustic sensors being disposed outside of the intake airflow, and

the acoustic liner is located upstream of a fan that is positioned at an airflow intake of the turbomachine.

19. The method of claim 18 , wherein the time of flight between the plurality of acoustic sensors and the plurality of further acoustic sensors is used to determine an average mass flow of the intake airflow.

20. The method of claim 18 , wherein the plurality of further acoustic sensors are located upstream of the fan.