Particle image velocimetry in refractive index fields of combustion flows

Optical measurements inside reacting flows are often disturbed by refractive index fields, e.g., due to the strong density gradients in flames. Although occurring measurement errors due to light refraction are a known problem for certain particle image velocimetry (PIV) applications, only a qualitative analysis of the resulting measurement uncertainty inside flame flows has been carried out to date. As an important step forward, a measurement approach is proposed, which enables a quantification of the resulting measurement uncertainties due to light refraction. As an example, the measurement approach is applied to a premixed propane flame. The uncertainty analysis is based on the determination of occurring particle position errors due to light refraction inside the flame. For three different measurement planes, the velocity field is measured with PIV and the particle position errors are experimentally measured and verified by ray-tracing simulation based on the measured refractive index field, which is determined by the background-oriented Schlieren method. In the examined flow, maximal position errors amount up to 14 μm and yield significant systematic velocity errors of up to 4% and random velocity errors of up to 6%. In contrast to the systematic velocity error, the random velocity error varies significantly for the analyzed measurement planes inside the flame flow.