Method of starting a gas turbine engine including a cooling phase

1. A method of starting a gas turbine engine having a rotor comprising at least a shaft mounted compressor and turbine, with a casing surrounding the rotor, the method comprising:

initiating a first acceleration phase where the rotational speed of the rotor is increased toward a combustion speed to force air through the gas turbine engine;

sensing vibrations during the first acceleration phase while the rotational speed of the rotor is equal to or less than 500 rpm;

sensing a first bowed rotor condition during the first acceleration phase;

initiating a first bowed-rotor cooling phase during the first acceleration phase, where the rotational speed of the rotor is maintained below a first bowed-rotor threshold speed until a first non-bowed rotor condition is satisfied, wherein the air forced through the gas turbine engine cools the rotor;

sensing the first non-bowed rotor condition;

thereafter, initiating a second acceleration phase, wherein the rotor speed is increased from at least 500 rpm toward the combustion speed;

sensing a second bowed rotor condition during the second acceleration phase;

initiating a second bowed-rotor cooling phase during the second acceleration phase, where the rotational speed of the rotor is decelerated below a second bowed-rotor threshold speed and above the first bowed-rotor threshold speed until a second non-bowed rotor condition is satisfied, wherein the air forced through the gas turbine engine cools the rotor;

sensing the second non-bowed rotor condition; and

thereafter, initiating a combustion phase, where upon reaching the combustion speed during the combustion phase fuel is supplied to the gas turbine engine and ignition is turned on.

2. The method of claim 1 , wherein the rotor is accelerated during the first bowed rotor cooling phase at an acceleration rate that is slower than during the first acceleration phase prior to sensing the first bowed rotor condition.

3. The method of claim 1 , wherein the rotor is accelerated during the second bowed rotor cooling phase at an acceleration rate that is slower than during the second acceleration phase prior to sensing the second bowed rotor condition.

4. The method of claim 1 , wherein the first acceleration phase and the second acceleration phase comprise externally rotating the rotor.

5. The method of claim 1 , wherein the first bowed-rotor threshold speed comprises a non-contact speed below which the rotor does not contact the casing.

6. The method of claim 5 , wherein the non-contact speed is less than a natural frequency of the turbine engine.

7. The method of claim 1 , wherein the first non-bowed-rotor condition comprises a rotational imbalance of the rotor.

8. The method of claim 1 , wherein sensing the first non-bowed rotor condition comprises:

sensing a rotational imbalance of the rotor during the first bowed-rotor cooling phase; and

determining that the rotational imbalance is less than a rotational imbalance threshold to satisfy the first non-bowed-rotor condition.

9. The method of claim 1 , wherein sensing the first non-bowed rotor condition comprises:

monitoring a temperature at multiple portions of the rotor; and

determining when the temperatures monitored are within a predetermined range of each other to satisfy the first non-bowed rotor condition.

10. The method of claim 1 , wherein the second non-bowed-rotor condition comprises a rotational imbalance of the rotor.

11. The method of claim 1 , wherein sensing the second non-bowed rotor condition comprises:

sensing the rotational imbalance of the rotor during the second bowed-rotor cooling phase; and

determining that the rotational imbalance is less than a rotational imbalance threshold to satisfy the second non-bowed-rotor condition.

12. The method of claim 1 , wherein sensing the second non-bowed rotor condition comprises:

monitoring a temperature at multiple portions of the rotor; and

determining when the temperatures monitored are within a predetermined range of each other to satisfy the second non-bowed rotor condition.

13. The method of claim 1 , wherein maintaining the rotational speed of the rotor below the first bowed-rotor threshold speed comprises maintaining the rotational speed of the rotor at a substantially constant speed.

14. The method of claim 13 , wherein the constant speed is less than a natural frequency of the turbine engine.

15. A method of starting a gas turbine engine having a rotor comprising at least a shaft mounted compressor and turbine, with a casing surrounding the rotor, the method comprising:

accelerating the rotor during an acceleration phase, where the rotational speed of the rotor is increased toward a combustion speed to force air through the gas turbine engine;

actively sensing vibrations during a condition monitoring phase during the acceleration phase, wherein vibrations are actively monitored while the rotational speed of the rotor is equal to or less than 500 rpm;

actively sensing vibrations during a bowed-rotor condition monitoring phase in the acceleration phase, where the rotor is monitored for a bowed-rotor condition;

maintaining a first rotational speed of the rotor, during a first bowed-rotor cooling phase during the acceleration phase, below a non-contact speed below which the rotor does not contact the casing upon the presence of the bowed-rotor condition; and

maintaining a second rotational speed of the rotor during a second bowed-rotor cooling phase following the first bowed-rotor cooling phase, whereupon in the presence of the bowed-rotor condition the rotational speed of the rotor continues to be maintained below the non-contact speed below which the rotor does not contact the casing, and wherein the second rotational speed of the rotor is greater than the first rotational speed,

wherein at least a portion of the bowed-rotor condition monitoring phase occurs after initiating the first bowed-rotor cooling phase.