Flow modulation method and apparatus

1. Apparatus for inducing periodic pressure waves within a gas turbine combustor assembly to model the acoustic process within the gas turbine combustor assembly, comprising:

a gas turbine combustor assembly having an inlet and an exit, the combustor is arranged to permit a through-flow of fluid flowing generally from the inlet to the exit during use;

wave inducing means located at the exit, the wave inducing means is constructed to continuously rotate about an axis that is perpendicular to an axis of the combustor that extends between the inlet and an outlet at the area of the exit, and also constructed such that when the wave inducing means is continuously rotated unidirectionally to vary the area of the exit, periodic pressure waves having a repeatedly known magnitude and directionality within the combustor are produced; and

at least one sensor constructed to detect interactions between the pressure waves and the combustion process within the combustor assembly to determine the effect of the periodic pressure waves on the combustion process during use.

2. Apparatus according to claim 1 , wherein the wave inducing means is elongate and has a polygonal cross-section.

3. Apparatus according to claim 2 , wherein the wave inducing means has a square cross-section.

4. Apparatus according to claim 2 , wherein the wave inducing means has a hexagonal cross-section.

5. Apparatus according to claim 1 , wherein the wave inducing means is located at the exit such that during use a flow of fluid through the combustor assembly may diverge at the wave inducing means, flow around the wave inducing means and re-converge downstream of the wave inducing means.

6. Apparatus according to claim 1 , wherein the combustor assembly exit is defined between at least two walls, at least one wall having a cavity, the wave inducing means being partially sheltered within the cavity.

7. Apparatus according to claim 1 , wherein the wave inducing means is capable of rotation by the flow of air past the wave inducing means during use.

8. Apparatus according to claim 1 , wherein the wave inducing means is functionally mounted to a drive motor adapted to rotate the wave inducing means.

9. Apparatus according to claim 1 , wherein the wave inducing means is adapted to be cooled.

10. Apparatus according to claim 9 , wherein the wave inducing means comprises at least one cooling passage for the provision of cooling fluid.

11. Apparatus according to claim 1 , wherein the exit is a transition tube mounted to a combustor.

12. Apparatus according to claim 11 , wherein the combustor assembly comprises a fuel injector for injecting a fuel into the through-flow of fluid.

13. A method of inducing periodic pressure waves within a gas turbine engine combustor having an inlet and an exit to model the acoustic process within the gas turbine combustor assembly comprising the steps of:

providing wave inducing means at the exit of the combustor that is operable to vary the area of the exit of the combustor, the wave inducing means is constructed to continuously rotate about an axis that is perpendicular to an axis of the combustor that extends between the inlet and an outlet at the area of the exit;

flowing air through the combustor between the inlet and the outlet;

rotating the wave inducing means unidirectionally to continuously vary the area of the exit of the combustor thereby inducing periodic pressure waves having a repeatedly known magnitude and directionality within the combustor; and

recording the effect of the pressure waves within the combustor.

14. A method according to claim 13 , wherein the wave inducing means has a hollow cavity which is provided with a flow of cooling air.

15. A method according to claim 13 , wherein the wave inducing means is induced to rotate by the air flowing through the combustor.