Method of operating a clearance control system

1. A method of operating a clearance control system for a gas turbine engine, comprising:

estimating, in a control module, a first thermal expansion value of a set of turbine blades in the gas turbine engine based at least partially on an exhaust gas temperature of the gas turbine engine and a first thermal expansion lag factor of the set of turbine blades;

estimating, in the control module, a second thermal expansion value of a casing concentric to the set of turbine blades based at least partially on the exhaust gas temperature of the gas turbine engine and a second thermal expansion lag factor of the casing;

determining, in the control module, an estimated clearance value based on the first and second thermal expansion values;

determining, in the control module, a first airflow value for adjusting a temperature of the casing based on a comparison of the estimated clearance value with a target clearance value;

determining, in the control module, a second airflow value for adjusting the temperature of the casing based on a comparison of the second thermal expansion value of the casing with a casing limit value; and

ducting airflow for adjusting the temperature of the casing according to the greater of the first airflow value or the second airflow value.

2. The method of claim 1 wherein estimating at least one of the first thermal expansion value or the second thermal expansion value is at least partially based on an operating phase of the gas turbine engine.

3. The method of claim 2 wherein estimating at least one of the first thermal expansion value or the second thermal expansion value is at least partially based on a start-up engine phase, a steady state engine phase, or a cool down engine phase.

4. The method of claim 1 wherein the determining of the second airflow value includes determining a current temperature of the casing.

5. The method of claim 1 wherein the determining of the first airflow value includes determining a current temperature of the set of turbine blades or a corresponding set of discs.

6. The method of claim 1 wherein the ducting includes controlling a controllable valve to regulate the greater of the first airflow value or the second airflow value.

7. The method of claim 6 wherein the ducting includes sensing at least one of temperature or pressure upstream of the controllable valve, and the controlling the controllable valve regulates at least one of a temperature or pressure downstream of the controllable valve based on at least one of the determined first or second airflow values.

8. The method of claim 6 wherein the ducting includes extracting an airflow from a compressor stage upstream of the controllable valve.

9. The method of claim 1 wherein the determining the second airflow value is based on a comparison of the second thermal expansion value of the casing with at least one of a casing material thermal capabilities value, a casing thermal cracking value, a casing thermal deformation value, a casing thermal deflection value, or a casing thermal failure value.

10. The method of claim 1 further comprising:

estimating, in the control module, a first mechanical deflection value of a blade in the set of turbine blades due to centrifugal and pressure loading;

estimating, in the control module, a second mechanical deflection value of the casing due to pressure loads in the gas turbine engine; and

determining, in the control module, the estimated clearance value based on the first and second thermal expansion values and the first and second mechanical deflection values.

11. A method of operating a clearance control system for a gas turbine engine, comprising:

in a control module, estimating a clearance profile between a set of turbine blades in the gas turbine engine and a casing concentric to the set of turbine blades, while simultaneously estimating a thermal expansion profile of the casing;

determining, in the control module, a first airflow profile for adjusting a temperature of the casing based on a comparison of the estimated clearance profile with a target clearance profile;

determining, in the control module, a second airflow profile for adjusting the temperature of the casing based on a comparison of the estimated thermal expansion profile of the casing and a predetermined thermal expansion profile limit of the casing;

selecting, in the control module, at least one of the first or the second airflow profiles based on which of the at least first or second airflow profiles results in greater adjusting the temperature of the casing; and

controlling an airflow for cooling the casing according to the selected at least one of the first or second airflow profiles.

12. The method of claim 11 wherein estimating the thermal expansion profile is at least partially based on an operating phase of the gas turbine engine.