A novel technique for measurement of high frequency temperature fluctuations in unseeded gas flows using molecular Rayleigh scattering is investigated. The spectrum of laser light scattered from molecules in a gas flow is resolved using a Fabry-Perot interferometer. The width of the spectral peak is broadened by thermal motion of the molecules and hence is related to gas temperature. The interference fringe pattern containing spectral information is divided into four concentric regions using a series of mirrors angled with respect to one another. Light from each of these regions is directed towards photomultiplier tubes and sampled at 10 kHz using photon counting electronics. Monitoring the relative change in intensity within each region allows measurement of gas temperature. Independently monitoring the total scattered intensity provides a measure of gas density. This technique also has the potential to simultaneously measure a single component of flow velocity by monitoring the spectral peak location. Measurements of gas temperature and density are demonstrated using a low speed heated air jet surrounded by an unheated air co-flow. Mean values of temperature and density are shown for radial scans across the jet flow at a fixed axial distance from the jet exit plane. Power spectra of temperature and density fluctuations at several locations in the jet are also shown. The instantaneous measurements have fairly high uncertainty; however, long data records provide highly accurate statistically quantities, which include power spectra. Mean temperatures are compared with thermocouple measurements as well as the temperatures derived from independent density measurements. The accuracy for mean temperature measurements was +/- 7 K.
Many industrial and fossil fuel energy processes involve two phase flows for mass transit. In order to improve and better understand these two phase flows, knowledge of both the droplet/particle size and spatial distribution are required. The useful diagnostic tools should be able to make in-situ measurements without disturbing the flow field so that undisturbed flow measurements can be obtained. This suggests the need for a novel non-intrusive optical technique that can provide instantaneous measurements of particle size and velocity at multiple spatial points in planar (2-D) fields. Digital Particle Image Velocimetry (DPIV) is being studied as a candidate technique for making these measurements. A Monte-Carlo simulation has been developed that simulates the PIV optical recording system and the electric field scattered from particles in the flow. The simulation incorporates diffraction and the variation in light sheet intensity across the depth of field of the optical system. The simulation also computes the Mie scattered electric fields and images these onto the CCD detector. For small size particles, diffraction effects of the optical system dominate the recorded particle image light intensity distribution on the CCD array, and precludes our ability to determine their size. However, for larger sized particles other effects become more dominant, such as the glare spots in optically clear particles. The simulation allows us to examine the Mie scattered electric field of the particles recorded on the CCD and use the higher order structure in the recorded images to determine particle size. A technique for sizing is proposed which uses the separation between two glare spots visible on the particle image to estimate the particle diameter. The results show that the input particle size distribution to the simulation can be reasonably reproduced using the proposed sizing technique. Analysis of experimental PIV images also demonstrated that this technique is capable of providing moderate accuracy particle size estimates.