LDA Applications in Mixing

1 Experimental Considerations. There are several important experimental issues specific to the application of LDA to mixing experiments, particularly in stirred tanks. 

Curvature. As the laser beams travel through curved surfaces, they are deflected due to changes in the refractive index. This produces a lensing effect that moves the measuring volume away from its expected location. The problem can be addressed in two ways by refractive index matching of the fluid with the vessel, or by performing beam tracing calculations to correct for the displacement (see Kresta and Wood, 1993). 

Reflections from the Shaft and Impeller. Signal quality close to the shaft or impeller can be improved by minimizing reflections. This can be done with black matte paint or by anodizing the parts after fabrication. 

Impeller Geometry. The impeller geometry must be exactly scaled down from full scale dimensions. Thickness of blades, hub size, hub thickness, and placement of blades can all have significant effects on power draw, velocity profiles, and turbulence characteristics. 

Shaft Encoding. LDV measurements in stirred vessels are typically combined with shaft encoding so that the velocity can be analyzed either as a long-time 

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