Phase‐Doppler Particle Sizing with Off‐Axis Angles in Alexander's Darkband

A promising optical layout for standard phase‐Doppler particle sizing instruments for use in two‐phase flows with extremely limited optical access is presented. When applied to turbine oil of variable temperature and therefore refractive index (1.40< n < 1.46), the layout procedure leads to several suitable off‐axis configurations. From these, a backscatter angle of ϕ = 125° is chosen owing to several geometrical restrictions. Although this angle lies in Alexander's darkband and therefore the intensities are low, the theoretical analysis indicates that this configuration is extremely insensitive to Gaussian‐beam defects, which usually aggravate backscatter applications. Monosized droplet experiments confirmed the application of the configuration and showed an acceptable accuracy of the measurements. Experiments in a bearing chamber test rig gave reliable results and for the first time allowed an insight into the flow phenomena of this highly complex technical two‐phase flow.