Abstract:
Three dimensional complex flows particularly those of turbomachinery present
challenges to current measurement technology in terms of restricted optical access,
measurement accuracy for the on-axis velocity component, the need to resolve flow
turbulence and measurement difficulty from close to surface or intra-channel
measurements in rotating machinery.
A novel non-intrusive in-line fibre-optic laser Doppler velocimeter is presented
specifically for the measurement of the on-axis component of velocity. The
measurement principle is based on a Doppler frequency to intensity transducer in the
form of a fibre-optic Bragg grating based Fabry-Perot interferometric filter. The filters
were fabricated at 514.5 nm but in principle any desired wavelength may be used thus
permitting any laser wavelength source to be used. Filters with appropriate features
were designed with the aid of the theoretical models based on the coupled mode
theory and transfer matrix approach.
The argon-ion laser emission wavelength was locked to a corresponding Doppler
broadened absorption line of molecular iodine vapour while the Fabry-Perot
interferometer phase was controlled in an independent feedback system using digital
lock-in amplifiers. The optical frequency was stabilized to within 10 MHz for at least
one hour while the phase was controlled to an equivalent of (within) ± 3 MHz in
frequency. Both feedback loops utilized custom designed PID electronic circuit
controllers. The bandwidth of the filter was tunable by up to 400 MHz, with a
resolution of between 0.2 ms'1 and 1 ms"1, and a sensitivity range of between 0.5
[GHz]'1 and 1.7 [GHz]'1. In this technique the filter was tuned to the optical
wavelength, rather than tuning the laser wavelength to match the filter. The finished
instrument was applied to the measurement of the on-axis component of velocity, of a
rotating disc, over an available range of up to ± 42 ms'1, limited only by the maximum
velocity of the disc. The detection system was reconfigured for low velocity
measurements at twice the sensitivity over a velocity range of ± 7 ms'1. This technique
demonstrates a unique contribution to fluid dynamics for the measurement of the
traditionally difficult in-line component of velocity.
