Gas turbine engine with heat exchanger assembly

A gas turbine engine defining a radial direction and an axial direction, the gas turbine engine comprising:

• a fan assembly comprising a fan;
• a turbomachine drivingly coupled to the fan and comprising a compressor section, a combustion section, and a turbine section arranged in serial flow order and defining in part a working gas flowpath, the gas turbine engine defining a bypass passage over the turbomachine;
• a core cowl;
• and
• a heat exchanger assembly disposed aft of the fan in the axial direction and forward of the core cowl in the axial direction, the heat exchanger assembly comprising:
• a heat exchanger;
• and
• a heat exchanger cowl defining a cooling air flowpath extending between a flowpath inlet in airflow communication with the bypass passage to receive a cooling airflow from the bypass passage and a flowpath outlet in airflow communication with the bypass passage to exhaust the cooling airflow back to the bypass passage, the heat exchanger positioned within the cooling air flowpath, the cooling air flowpath comprising a diffusion section located between the flowpath inlet and the heat exchanger;
• wherein the turbomachine defines an inlet to the working gas flowpath and defines a circumferential groove for mounting at least a portion of the core cowl, wherein the heat exchanger cowl defines a first length along the axial direction from the flowpath inlet to the flowpath outlet, wherein the turbomachine defines a second length along the axial direction from the inlet to the circumferential groove, and wherein L1 is greater than or equal to 10% of L2 and is less than or equal to 60% of L2.