Development of a Novel Flow Control Device for Limiting the Efflux of Air Through a Failed Pipe

Abstract

The secondary air-systems of a gas turbine engine frequently incorporate pipes and ducts to transport air for duties such as cooling and sealing of the turbine components, pressurisation of the aircraft cabin and component de-icing. The engine must be capable of operating safely in the event of failure of a pipe or duct. The ducts typically pass through the low pressure ventilation zones outboard of the core engine and failure will result in a large mass flow of relatively high temperature air escaping from the secondary air system; the design of the engine must accommodate this potential escape so that no component is over-pressurised or over-heated as a result. A novel device is presented that will limit the flow that escapes in the event of a pipe failure. This device has been developed from a number of flow elements from Fluidic technology applications. It has no moving parts and is thus suitable for use as a high-reliability failure protection device. The device consists of a Coanda diverter that can switch the flow through a vortex throttle so that the device has high and low resistance states. The diverter is conditioned to default to the low resistance state unless a control flow extracted from the device exceeds a critical value whereupon it will switch the device to a high resistance state. The level of the control flow is determined by the pressure ratio acting across the device. This is achieved by contrasting the flow characteristics of a metering orifice that determines the control flow with that of a diffuser fitted to the device outlet. The device has been shown to half the flow that escapes from a failed duct compared with an unrestricted duct of the same flow capacity. Experimental and numerical results are presented that show that the device is effective at the high pressure ratios pertaining to gas turbine operation. With suitable modification the device could be adapted to fulfill a number of other functions within a secondary air-system that require variation of flow resistance in response to a change in pressure ratio combined with the high reliability and robustness of a no-moving-parts device.