Stereoscopic particle image velocimetry in inhomogeneous refractive index fields of combustion flows.

Particle image velocimetry (PIV) measurements in reactive flows are disturbed by inhomogeneous refractive index fields, which cause measurement deviations in particle positions due to light refraction. The resulting measurement errors are known for standard PIV, but the measurement errors for stereoscopic PIV are still unknown. Therefore, for comparison, the velocity errors for standard and stereoscopic PIV are analyzed in premixed propane flames with different Reynolds numbers. For this purpose, ray-tracing simulations based on the time-averaged inhomogeneous refractive index fields of the studied non-swirled flame flows measured by the background-oriented Schlieren technique are performed to quantify the resulting position errors of the particles. In addition, the performance of the volumetric self-calibration relevant to tomographic PIV is analyzed with respect to the remaining position errors of the particles within the flames. The position errors cause significant standard PIV errors of 2% for the velocity component radial to the burner symmetry axis. Stereoscopic PIV measurements result in measurement errors of up to 3% radial to the burner axis and 13% for the velocity component perpendicular to the measurement plane. Due to the lower refractive index gradients in the axial direction, no significant velocity errors are observed for the axial velocity component. For the investigated flame configurations, the position errors and velocity errors increase with the Reynolds numbers. However, this dependence needs to be verified for other flame configurations such as swirled flame flows.