Gas turbine engine having a heat exchanger located in an annular duct

A gas turbine engine defining a centerline, a radial direction, and a circumferential direction, the gas turbine engine comprising:

• a thermal circuit comprising a thermal transport bus with a thermal fluid flowing therethrough, the thermal transport bus comprising a first bus segment and a second bus segment, the first bus segment defining a first operational parameter, the second bus segment defining a second operational parameter different than the first operational parameter;
• and
• a first heat exchanger in thermal communication with one of the first bus segment or the second bus segment for rejecting heat to engine fuel of the gas turbine engine;
• a second heat exchanger in thermal communication with one of the first bus segment or the second bus segment, wherein the thermal fluid is split between each of the first and second bus segments such that only a portion of the thermal fluid flows through the first heat exchanger;
• a turbomachine comprising a compressor section, a combustion section, and a turbine section arranged in serial flow order;
• and
• a rotor assembly driven by or incorporated into the turbomachine and operable at a blade passing frequency (f) greater than or equal to 300 hertz and less than or equal to 12,500 hertz during an operating condition of the gas turbine engine, the gas turbine engine comprising a substantially annular duct relative to the centerline, the substantially annular duct defining a flowpath and a duct height along the radial direction;
• wherein the second heat exchanger is positioned at a location within the substantially annular duct and extends substantially continuously along the circumferential direction, the second heat exchanger defining a heat exchanger height equal to at least 10% of the duct height, wherein an effective transmission loss (ETL) for the second heat exchanger positioned within the substantially annular duct is between 5 decibels and 1 decibels for the operating condition, wherein the second heat exchanger comprises a heat transfer section defining an acoustic length (Li), and wherein an Operational Acoustic Reduction Ratio (OARR) is greater than or equal to 0.75 to achieve the ETL at the operating condition, the OARR equal to:
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• wherein a is representative of a speed of sound through the location of the gas turbine engine at the operating condition and is greater than or equal to 11,600 inches per second and less than or equal to 30,924 inches per second during the operating condition.