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 turbomachine comprising a compressor section, a combustion section, and a turbine section arranged in serial flow order;
• a thermal load;
• a thermal management system comprising a turbine and a fluid circuit providing a flow of bleed air from the compressor section to the turbine and from the turbine to the thermal load;
• 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 of the gas turbine engine, the substantially annular duct defining a flowpath and a duct height along the radial direction, the rotor assembly being separate from the turbomachine or integrated into the turbomachine;
• and
• a heat exchanger positioned at a location within the substantially annular duct and extending substantially continuously along the circumferential direction, the heat exchanger defining a heat exchanger height equal to at least 10% of the duct height and being in thermal communication with the fluid circuit of the thermal management system at a location downstream of the turbine;
• wherein an effective transmission loss (ETL) for the heat exchanger positioned within the substantially annular duct is between 5 decibels and 1 decibels for the operating condition;
• wherein the 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:
• (
• sin
• ⁡
• (
• 2
• ×
• π
• ×
• f
• a
• ×
• L
• i
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• 2
• 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.