| CPC F02C 7/14 (2013.01) [F02C 7/06 (2013.01); F02C 7/10 (2013.01); F02C 7/224 (2013.01); F02C 7/36 (2013.01); F05D 2220/323 (2013.01); F05D 2260/205 (2013.01); F05D 2260/213 (2013.01); F05D 2260/98 (2013.01)] | 20 Claims |

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1. A method of operating a gas turbine engine, the gas turbine engine comprising:
an engine core comprising a turbine, a compressor, a combustor arranged to combust a fuel, and a core shaft connecting the turbine to the compressor;
a fan located upstream of the engine core;
a recirculating lubrication system arranged to supply oil to lubricate engine components; and
a heat exchange system arranged to transfer heat between the oil and the fuel,
wherein the method comprises, at cruise conditions:
controlling the heat exchange system so as to transfer 200-600 kJ/m3 of heat to the fuel from the oil.
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