Dual-beam laser Doppler

The current status of prediction and modelling in the area of fuel spray combustion requires, among other parameters, the measurement of droplet or solid particle size distribution and the relative velocity between the fuel and the surrounding gas. Many optical techniques, based on laser light scattering, have been investigated to this purpose (Refs.1,2,2,]+,, 6 and j), but the only system able to simultaneously determine the size and the velocity is the dual-beam laser Doppler velocimeter shown in Figure 1. 

The dual beam configuration of LDA is most widely used today, where the Doppler difference frequency is directly measured and the receiving optics may be placed at an arbitrary position with respect to the transmitting beams. Laser-Doppler anemometry has been first applied to measurements of mean velocities and turbulence properties in single phase flows. In this case small particles, which follow the flow and the turbulent fluctuations, need to be present in the flow or must be added to it (i.e. seeding the flow with a tracer). The principles of LDA are, for example, described in detail by Durrani and Greated (1977), Durst et al. (1981), and Durst et al. (1987). 

The application of laser Doppler velocimetry (LDV) to measure the electrophoretic mobility n of charged colloidal particles is known as laser Doppler electrophoresis (LDE). In a typical LDE experiment, an applied electric field drives the collective motion of charged colloidal particles. The particles pass through an interference pattern created by a dual-beam experimental setup (Section III.A.2). The collective electrophoretic velocity of the particles is then determined via intensity- or spectrum-based analysis of the scattered light, and the electrophoretic mobility n is calculated by dividing the velocity by the applied electric field strength. 

Scattered light from one laser beam interferes with scattered light from another beam (dual beam or differential Doppler or fringe anemometer). 

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