Turbofan engine with heat exchanger module having optimized fan to element area parameter

A method of operating a turbofan gas turbine engine, the turbofan gas turbine engine comprising, in axial flow sequence, (i) a heat exchanger module, (ii) an inlet duct, (iii) a fan assembly comprising a plurality of fan blades defining a corresponding fan area (AFAN), (iv) a compressor module, and (v) a turbine module, and the method comprising the steps of:

• (i) providing the fan assembly, the compressor module, and the turbine module;
• (ii) positioning the heat exchanger module in fluid communication with the fan assembly by way of the inlet duct;
• (iii) providing the heat exchanger module with a plurality of radially-extending hollow vanes arranged in a circumferential array, at least one of the vanes comprising a heat transfer element positioned within the vane and extending axially along the vane,
• a swept heat transfer element area (AHTE) being a wetted surface area of all the heat transfer elements in contact with an airflow passing over a surface of each heat transfer element prior to entry of the airflow into an inlet to the fan assembly,
• a Fan to Element Area FEA parameter being defined as:
• F
• EA
• =
• A
• HTE
• A
• FAN
• and the FEA parameter lying in a range of 47 to 132;
• and
• (iv) operating the engine such that the airflow passes through a hollow portion of the vane and transfers heat energy from a first fluid contained within each heat transfer element to the airflow.