A heat exchanger

A turbofan gas turbine engine 200 has, in axial flow sequence, a heat exchanger module 210, a fan assembly 130, a compressor module 140, a turbine module 150, and an exhaust module 156. The fan assembly contains a number of fan blades 132 defining a fan with diameter D 136. The heat exchanger module is in fluid communication with the fan via an inlet duct 260 and has radially-extending hollow vanes (fig.5,120) arranged circumferentially, with a channel (fig.5,124), extending axially between each pair of adjacent vanes, each vane accommodating a number of elements (fig.7,112) extending axially along the vane, for transferring heat from a first fluid within each element to an airflow passing through the hollow vane and over a surface of each element. Each of the vanes has a flow modulator (fig.8C,127E;fig.9B,120A) actively regulating airflow as a proportion of total airflow entering the heat exchanger module in response to user requirement. In use the flow modulator regulates a sum of each vanes mass flow rates FlowVaneTot as a proportion of a total air mass flow entering the heat exchanger; alternatively, it may regulate a ratio between FlowVaneTot and the sum of the channel flow rates FlowChanTot.