Velocimetry and sizing in multi-constituent flows for full-field macro and microfluidic applications

Particle image velocimetry (PIV) is a powerful laser-based tool for the non-intrusive measurement of fluid flow in a wide range of engineering applications. In PIV, a pulsed laser is used to illuminate tracer particles in a flow and a CCD camera takes a 'snapshot' image of the position of the particles for each laser pulse. By analysing the relationship between the positions of the particles in pairs of successive images (correlation analysis) a map of the velocity vectors (speed and direction) of the particles, and hence the fluid flow, is obtained. Some important applications of PIV are particularly challenging, such as the measurement of flow in multi-constituent gas mixtures or very fine liquid sprays (droplet sizes of sub-micrometer to 10's of micrometers) injected into the gas phase. Typical examples of the latter are fuel sprays for combustion in automotive and gas turbine engines, agricultural sprays and metered dose inhalers. To address this challenge, we have developed high-intensity, micrometer-size red and blue fluorescent tracer droplets that can be excited simultaneously by a UV (355 nm) laser pulse and give chromatically separate images in the blue and red channels of a colour camera, thereby achieving discrimination between two gas phase flows. A second green laser pulse (532 nm), from the same twin-cavity Nd:YAG laser, generates scattered light (Mie scatter) from both types of tracer, recorded in the green channel of the camera. Hence, the velocity vector field of each flow constituent can be obtained, using a solid state laser as the excitation source. The application of these tracer droplets in multiphase PIV and in the removal of 'flare', caused when the light used to illuminate the tracers impinges on reflective surfaces within the flow system, will be exploted by our collaborators, Prof D Towers and Dr C Towers, in the jointly funded project at the University of Leeds (GR/S69108/02).
Because the optical properties of micron-sized droplets of highly concentrated fluorescent dye are not the same as those of the bulk solution, we have devised a method to mimic gas-borne tracer droplets by forming an oil-in-water emulsion in which the micrometer-sized droplets are suspended within a non-fluorescent and non-miscible continuous phase. Quantitative spectroscopic measurements could then be made on the droplets in a stable and controlled laboratory environment. This enabled us to develop optimised fluorescent tracer droplets that are small enough (diameter of 1 micrometer or less) to accurately follow gas-phase flow while being sufficiently intense to produce high quality PIV images. We have used the same approach to develop suspensions of fluorescent droplets for use as tracers in micro-PIV measurements of flow in microfluidic, lab-on-a-chip devices.
For measurements on fuel sprays, we have devised a means of attenuating the intensity of Mie scattering of green (532 nm) laser light by the fuel droplets, by dissolving in the fuel a non-fluorescent dye which absorbs strongly at this wavelength. This was envisaged as a means of measuring the size of fuel droplets from the ratio of the intensity of Mie-scattered green and red laser light, but it may also prove useful for adjusting the intensity ratio of Mie scatter and fluorescence to optimise PIV imaging of fuel sprays. These applications will are being pursued by our collaborators, Towers and Towers, in Leeds.