Frequency response of a cold-wire in a flow seeded with oil particles

Experiments have been carried out to investigate the effects of aging on the frequency response of a fine cold-wire located in a flow seeded with oil particles used for laser Doppler anemometry. Transfer functions are shown to depend strongly on the running time. Photo-micrographs show that the contamination effect is due to discrete droplets, the diameter of which increases with running time. Reduction of frequency response is shown to affect at first the high-frequency range and then an intermediate range of frequencies that are larger than the inverse of the oil droplets time constant. Good agreement is found between experimental results and predictions derived from a simple model of the contaminated wire. This contamination process requires testing and cleaning the wire steadily in order to keep a significant frequency response and, if needed, to correct temperature fluctuations.