Acetone-condensation nano-particle image velocimetry in a supersonic boundary layer

Condensation nanoparticle planar laser light scattering imaging was conducted in a suction type Mach 1.9 supersonic flow. A series of the nanoparticle image pair was obtained by using the same optical components of PIV. The velocity data were obtained by the image pairs using the image-based correlation procedure as same as PIV. Acetone vapor instead of tracer particles in a conventional PIV was added to a mainstream gas in a reservoir. Acceleration process through a Laval nozzle automatically generated condensation particles of the acetone which were uniformly seeded into the entire flowfield. The increase in the additive concentration increased the number density of the particle and enabled a detail visualization of the vortex structures in the boundary layer. The increase in the additive concentration also increased the mean molecular weight of the acetone-seeded air. This decreased the flow speed. However, this is not a big matter because the heat release due to the condensation was negligible and the decrease in the flow speed was easily predicted from the thermodynamic properties of the gas. The particle size was difficult to be measured directly, so the tracer response time was estimated by the oblique shock test. The Stokes diameter of the particle was estimated to be 160 nm. Such a small diameter particle provides high traceability resulting in capturing shock wave with a few vector spacing and also provides high spatial resolution image resulting in capturing sub-mm scale vortices within the boundary layer. The mean and turbulent velocity fields evaluated from such high spatially and temporally resolved image pairs fairly agreed with previous measurement in the boundary layer.