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 comprises a plurality of radially-extending hollow vanes arranged in a circumferential array with a channel extending axially between each pair of adjacent hollow vanes. An airflow entering the heat exchanger module is divided between a set of vane airflows through each of the hollow vanes and a set of channel airflows through each of the channels. Each vane airflow having a vane mass flow rate FlowVane, and each channel air flow has a channel mass flow rate FlowChan. At least one 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 the or each corresponding vane airflow passing over a surface of the or each heat transfer element.In use, a Vane Airflow Ratio parameter VAR is defined as: VAR=FlowVaneTotFlowChanTot where:FlowVaneTot = total mass flow rate of the vane mass flow rates, FlowVane; andFlowChanTot = total mass flow rate of the channel mass flow rates, FlowChan; and the VAR parameter is in the range of 0.05 to 3.0.