Gas turbine performance

1 . A method of operating a gas turbine engine, the gas turbine engine comprising:

a combustor arranged to combust a sustainable aviation fuel or a blend thereof; and

a fuel management system arranged to provide the fuel to the combustor, wherein the fuel management system comprises:

two fuel-oil heat exchangers through which oil and the fuel flow, the two fuel-oil heat exchangers arranged to transfer heat between the oil and the fuel and comprising a primary fuel-oil heat exchanger and a secondary fuel-oil heat exchanger; and

a fuel pump arranged to deliver the fuel to the combustor, wherein the fuel pump is located between the two fuel-oil heat exchangers;

a controller that is configured to control the fuel management system to:

adjust the fuel viscosity to lower than 0.58 mm 2 /s and greater than 0.35 mm 2 /s on entry to the combustor at cruise conditions; and

control a ratio of heat transfer from the oil to the fuel for the primary and secondary fuel-oil heat exchangers such that the ratio is between 70:30 and 90:10,

wherein the method comprises controlling the fuel management system to adjust the fuel viscosity to lower than 0.58 mm 2 /s and greater than 0.35 mm 2 /s on entry to the combustor at cruise conditions, and controlling the fuel management system to raise the fuel temperature to between 150° C. and 170° C. on entry to the combustor at cruise conditions.

2 . A method of operating an aircraft, the aircraft comprising:

a fuel tank; and

a gas turbine engine comprising:

a combustor arranged to combust a sustainable aviation fuel or a blend thereof provided from the fuel tank; and

a fuel management system arranged to provide the fuel to the combustor, wherein the fuel management system comprises:

two fuel-oil heat exchangers through which oil and the fuel flow, the two fuel-oil heat exchangers arranged to transfer heat between the oil and the fuel and comprising a primary fuel-oil heat exchanger and a secondary fuel-oil heat exchanger; and

a fuel pump arranged to deliver the fuel to the combustor, wherein the fuel pump is located between the two fuel-oil heat exchangers; and

a controller that is configured to control the fuel management system to:

adjust the fuel viscosity to lower than 0.58 mm 2 /s and greater than 0.35 mm 2 /s on entry to the combustor at cruise conditions; and

control a ratio of heat transfer from the oil to the fuel for the primary and secondary fuel-oil heat exchangers such that the ratio is between 70:30 and 90:10,

wherein the method comprises;

controlling the fuel management system to adjust the fuel viscosity to lower than 0.58 mm 2 /s and greater than 0.35 mm 2 /s on entry to the combustor at cruise conditions, and wherein the sustainable aviation fuel is a fuel derived from renewable hydrocarbons.

3 . The method of claim 2 , wherein the oil enters the two fuel-oil heat exchangers at a higher temperature than the fuel.

4 . The method of claim 2 , wherein the method comprises controlling the fuel management system to adjust the fuel viscosity to between 0.35 mm 2 /s and 0.53 mm 2 /s on entry to the combustor at cruise conditions.

5 . The method of claim 2 , wherein the method comprises controlling the fuel management system to adjust the fuel viscosity to between 0.40 mm 2 /s and 0.48 mm 2 /s on entry to the combustor at cruise conditions.

6 . The method of claim 2 , wherein:

the fuel management system further comprises:

a recirculation valve located downstream of the primary fuel-oil heat exchanger, the recirculation valve arranged to allow a controlled amount of fuel which has passed through the primary fuel-oil heat exchanger to be returned to an inlet of the primary fuel-oil heat exchanger;

and wherein the controlling the fuel management system comprises controlling the proportion of the fuel returned to the inlet of the primary fuel-oil heat exchanger via the recirculation valve.

7 . The method of claim 2 , wherein:

the fuel management system further comprises:

a bypass pipe arranged to allow a proportion of the fuel to bypass the primary fuel-oil heat exchanger;

and wherein the controlling the fuel management system comprises controlling the proportion of the fuel which passes through the bypass pipe instead of through the primary fuel-oil heat exchanger.

8 . The method of claim 2 , wherein:

the fuel management system further comprises an oil bypass pipe arranged to allow a proportion of the oil to bypass at least one of the primary fuel-oil heat exchanger and the secondary fuel-oil heat exchanger; and

wherein the controlling the fuel management system comprises controlling the proportion of the oil which passes through the oil bypass pipe instead of through the at least one of the primary fuel-oil heat exchanger and the secondary fuel-oil heat exchanger.

9 . The method of claim 6 , wherein the gas turbine engine comprises a fuel metering valve downstream of the recirculation valve arranged to control fuel flow rate therethrough and to provide information on fuel flow therethrough, and wherein the controlling the fuel management system comprises controlling the fuel metering valve and the recirculation valve based on the information provided by the fuel metering valve.

10 . The method of claim 2 , wherein the method comprises controlling the fuel management system to raise the fuel temperature to between 150° C. and 170° C. on entry to the combustor at cruise conditions.

11 . The method of claim 2 , wherein between 10% and 20% of the fuel is delivered to the secondary fuel-oil heat exchanger.

12 . The method of claim 2 , wherein the fuel flows through the primary fuel-oil heat exchanger prior to flowing through the secondary fuel-oil heat exchanger whereas the oil flows through the secondary fuel-oil heat exchanger prior to flowing through the primary fuel-oil heat exchanger.

13 . A gas turbine engine for an aircraft, the gas turbine engine comprising:

a combustor arranged to combust a sustainable aviation fuel or a blend thereof;

a fuel management system arranged to provide the fuel to the combustor, wherein the fuel management system comprises:

two fuel-oil heat exchangers through which oil and the fuel flow, the two fuel-oil heat exchangers arranged to transfer heat between the oil and the fuel and comprising a primary fuel-oil heat exchanger and a secondary fuel-oil heat exchanger; and

a fuel pump arranged to deliver the fuel to the combustor, wherein the fuel pump is located between the two fuel-oil heat exchangers; and

a controller that is configured to control the fuel management system to;

adjust the fuel viscosity to lower than 0.58 mm 2 /s and greater than 0.35 mm 2 /s on entry to the combustor at cruise conditions; and

control a ratio of heat transfer from the oil to the fuel for the primary and secondary fuel-oil heat exchangers such that the ratio is between 70:30 and 90:10,

wherein the sustainable aviation fuel is a fuel derived from renewable hydrocarbons.

14 . The gas turbine engine of claim 13 , wherein the controller is configured to control the fuel management system to adjust the fuel viscosity to between 0.35 mm 2 /s and 0.53 mm 2 /s on entry to the combustor at cruise conditions.

15 . The gas turbine engine of claim 13 , wherein the controller is configured to control the fuel management system to adjust the fuel viscosity to between 0.40 mm 2 /s and 0.48 mm 2 /s on entry to the combustor at cruise conditions.

16 . The gas turbine engine of claim 13 , wherein the fuel management system further comprises a recirculation valve located downstream of the primary fuel-oil heat exchanger, the recirculation valve arranged to allow a controlled amount of fuel which has passed through the primary fuel-oil heat exchanger to be returned to an inlet of the primary fuel-oil heat exchanger.

17 . The gas turbine engine of claim 13 , wherein the fuel management system is arranged to control fuel flow through the two fuel-oil heat exchangers to raise the fuel temperature to between 150° C. and 170° C. on entry to the combustor at cruise conditions.

18 . The gas turbine engine of claim 13 , wherein the fuel management system is arranged such that the fuel flows through the primary fuel-oil heat exchanger prior to flowing through the secondary fuel-oil heat exchanger whereas the oil flows through the secondary fuel-oil heat exchanger prior to flowing through the primary fuel-oil heat exchanger.