A HEAT EXCHANGER

A turbofan gas turbine engine comprises, in axial flow sequence, a heat exchanger module, a fan assembly, a compressor module, a turbine module, and an exhaust module. The fan assembly comprises a plurality of fan blades defining a fan diameter (D). The heat exchanger module is in fluid communication with the fan assembly by an inlet duct, and the heat exchanger module further comprises a plurality of radially-extending hollow vanes arranged in a circumferential array, with a channel extending axially between adjacent hollow vanes. Each of the hollow vanes accommodates at least one heat transfer element for the transfer of heat from a first fluid contained within the or each heat transfer element to a corresponding vane airflow passing through the hollow vane and over a surface of the or each heat transfer element. Each of the hollow vanes further comprises a flow modulator. The flow modulator is configured to actively regulate the vane airflow as a proportion of a total airflow entering the heat exchanger module in response to a user requirement.