Phase Doppler Measurements on a Pressurised Spray Nozzle Using Scattering Angles of 72° and 150°

Two-dimensional phase Doppler anemometry (PDA) measurements of the size and velocity of individual water droplets produced by a Variflo pressure-swirl atomizer, issuing into ambient air are reported for a pressure range from 7 to 15 bar. Experimental measurements cover a downstream distance of 380 mm from the nozzle exit. Eight horizontal diameters have been considered. All measurements were based upon 10 000 validated droplets or a specified time-out of 60 s. Two PDA scattering angles (72° and 150°) have been considered, together with two different argon-ion lasers with powers of 4 W and 150 mW for 72° and 150° scatter respectively. The two optical geometries employed provided a capability to measure droplets with a diameter up to 402 μm and 395 μm for 72° and 150° scatter respectively. The influence of the scattering angle and laser power are such that significant differences are evident, especially for both the number mean diameter (D₁₀) and the Sauter mean diameter (D₃₂) profiles. The lower D₁₀ and D₃₂ values of the 72° scattering angle measurements are deemed more representative of the true droplet size distribution within the spray, due to less interference being caused by secondary forms of scattering and the enhanced sensitivity to small droplets due to the higher laser power. The results also compare the axial and radial velocity profiles as well as typical droplet diameter/velocity correlations which emphasise the influence of aerodynamic drag particularly upon the smaller droplets. The experimental D₃₂ measurements are compared with the predictions from two empirical equations published by Radcliffe [3] and Jasuja [4]; their equations being based upon experimentally derived data for the Variflo type of pressure-swirl nozzle.