A Review of Purge Air Designs for Aeroengine-Based Optical Pyrometers

Abstract

With the advent of “power by the hour” type agreements within the civil aeroengine market, the application of engine monitoring system data has reached the level of strategic use for informed decision making in not only the aftermarket but increasingly in the contract negotiation stage. One of the key cost drivers in these dollar-per-hour contracts for the OEMs to analyze is the life and maintenance requirements of the turbine blades leading ultimately to blade life management. Such life management of key components is of critical importance to ensure that the economic and technical risks to both service provider and customer are minimized. The optical pyrometer, through providing a direct temperature measurement of the turbine blades, is a primary input for providing a more realistic assessment of the component’s operating history associated with the use of life usage/remaining algorithms. However, the greatest concern with the in-service use of pyrometry is the issue of fouling since the pyrometer’s lens is exposed to the turbine environment. The level of optical contamination is usually minimized by introducing purge air, bled from the compressor, down the sight tube to prevent both the build-up of contaminants on the exposed system optics and particles in the gas stream from coming in contact with the lens. This paper provides a review of purge air designs and the key methodologies for engine designers to be acquainted with when seeking to integrate the use of optical pyrometry systems in new engine concepts.