The measurement of high frequency alternating pressures in gas turbines

Abstract

The measurement of alternating pressures in gas turbines could not be achieved by existing techniques. The pressures consisted of small-amplitude alternating pressures superimposed on pressures up to 100 lb per sq in. and at temperatures up to 250 deg C in compressors and up to 850 deg C in turbines. The frequencies of the predominant harmonic components varied from 100 to 17,000 c.p.s, and those for the smaller components up to 40,000 c.p.s. Improvements were required in the recording techniques which are reported elsewhere but the changes in amplifiers are described. The development of a capacity pressure element to record the alternating pressures at a point in a casing is described. A small diameter diaphragm was used to facilitate installation and to obtain the pressure over as small an area as possible. The diaphragm was arranged near the gas stream and the effect of temperature changes was eliminated by applying a filtered balance air supply behind the diaphragm to permit calibration during the investigation. The balance air supply permits equalisation of pressure across the diaphragm so that higher sensitivities can be used. The alternating pressures decrease sharply with distance from the source and if the alternating pressure is required at a point other than in the casing a special approach will be required.