Pilot/main fuel shifting in an axial staged combustor for a gas turbine engine

1. A method of controlling a fuel flow to an annular combustor of a gas turbine engine comprising:

shifting a fuel split between a first percentage of a total fuel flow to an axial fuel injection system arranged along an axis of the annular combustor to a forward combustion zone and a second percentage of the total fuel flow to a radial fuel injection system transverse to the axis to a downstream combustion zone in response to an engine operating parameter to shift the heat release away from the forward combustion zone and toward the downstream combustion zone which alters the axial heat release distribution and associated convective time delays to mitigate combustor instabilities associated with each zone to de-tune away from the combustor instabilities;

wherein mitigating combustor instabilities comprises de-tuning the forward combustion zone and the downstream combustion zone;

for the engine operating parameter during idle power, the first percentage of fuel to the axial fuel injection system is between 80% to 100% of the total fuel flow;

for the engine operating parameter during approach power, the first percentage of fuel to the axial fuel injection system is between 40% to 100% of the total fuel flow; and

for the engine operating parameter during cruise, climb, and take-off power, the first percentage of fuel to the axial fuel injection system is between 15% to 50% of the total fuel flow;

to alter the heat release distribution and convective time delays associated with each zone and enable de-tuning away from instabilities.

2. The method as recited in claim 1 , wherein mitigating combustor instabilities further comprises controlling a combustor metric by shifting the fuel split.

3. The method as recited in claim 2 , wherein the combustor metric comprises emissions.

4. The method as recited in claim 2 , wherein the combustor metric comprises combustor efficiency.

5. The method as recited in claim 2 , wherein the combustor metric comprises lean blow-out margin.

6. The method as recited in claim 2 , wherein the combustor metric comprises altitude re-light capability.

7. A method of controlling a fuel flow to an annular combustor of a gas turbine engine comprising:

determining a percent of total power at which a gas turbine engine is to be operated;

shifting a fuel split between a first percentage of a total fuel flow to an axial fuel injection system arranged along an axis of the annular combustor to a forward combustion zone, and a second percentage of the total fuel flow to a radial fuel injection system transverse to the axis to a downstream combustion zone in response to the percent of total power to mitigate combustor instabilities which alters the axial heat release distribution and associated convective time delays to mitigate combustor instabilities associated with each zone to de-tune away from the combustor instabilities, the second percentage of the total fuel flow equivalent to the total fuel flow less the first percentage;

wherein for less than 25 percent of total power for the gas turbine engine, the first percentage of fuel to the axial fuel injection system is between 80%-100% of the total fuel flow;

wherein for between 25-50 percent of total power for the gas turbine engine, the first percentage of fuel to the axial fuel injection system is between 40%-100% of the total fuel flow; and

wherein for greater than 50 percent of total power for the gas turbine engine, the first percentage of fuel to the axial fuel injection system is between 15%-50% of the total fuel flow.

8. The method as recited in claim 7 , wherein for greater than 50 percent of total power for the gas turbine engine, the first percentage of fuel to the axial fuel injection system is 35% of the total fuel flow.