Uncertainty of the PIV testing results for gas flow field caused by tracer particles and seeding method

The quality of flow visualization is very important for PIV. A lot of testing results indicate that the slip between tracer particles and continuous phase is very severe for high velocity gradient flow field and yield uncertain testing results. In addition, improper seeding method of tracer particles often results in false results. The systematical investigation on the above-mentioned problem has been done based on the theory of dilute phase gas-particle flow. The results indicate that tracer particles must e have the slip with the fluid as long as the velocity gradient presents, even the particle is sufficiently accelerated, and it has severely influence on the reliability of PIV testing results for the flow field with high velocity gradient or (and) larger tracer particles size. The relative error caused by the slip of particles and fluid is in proportion to the particle Stokes time and the particle velocity gradient, but it is in inverse proportion to the correction factor for Stokes-drag force. The above conclusions can be used to judge and correct the measurement error or to select proper tracer particles for the non-uniform flow field. During the study on seeding method of tracer particles, the Reynolds number ratio of dragging gas flow for tracer particles and main gas flow Re/Re is considered as the disturbance intensity scale of dragging gas flow on main gas flow. The attenuation process and attenuation distance of disturbance is investigated for different Re/Re. The optimal seeding position can be selected based on the data of the attenuation distance for different Re/Re.