Coolant Supply System

A cooling system for a gas turbine engine (10) having a compressor (14), a combustor (16) and a turbine section arranged to receive combustion products from the combustor. The cooling system includes ducting (32,44) defining a flow path from the compressor to a component, such as a turbine blade (26) to be cooled within the turbine section. The ducting (32) bypasses the combustor (16). A heat exchanger (34) may be arranged in the flow path to extract heat from the flow between the compressor and the component. One or more valves (36;64;66) in the flow path are actuated under the control of a controller (48;60) at a predetermined frequency of actuation so as to pulse the flow between the heat exchanger and the component, typically to improve the aerodynamic efficiency of turbine blades in use.