Thermographic 3D particle tracking velocimetry for turbulent gas flows

Turbulent flows are characterized by diverse and unsteady three-dimensional (3D) features that require 3D measurements to study. In case of non-isothermal flows, combined 3D measurements of temperature and velocity are necessary. In this paper, a thermographic 3D particle tracking velocimetry (thermographic 3D-PTV) concept is introduced for simultaneous 3D temperature and velocity measurements in turbulent gas flows. It is based on sub-micron thermographic phosphor particles seeded into the flow as flow tracers with low response times of a few microseconds. To obtain each tracer's position and velocity, the measurement region is illuminated volumetrically using a double-pulse green laser and Mie-scattered light is imaged by four double-frame cameras. Following the pinhole model-based calibration of all cameras, 3D particle positions are computed for both laser pulse-times using a fast minimum line of sight reconstruction code. Three-component velocities are derived from tracking individual particles between these time steps. For simultaneous 3D thermometry, temperature dependent luminescence emissions from the same phosphor particles are exploited. These emissions are excited using a UV laser synchronized with the first green laser pulse and imaged using two cameras equipped with spectral filters for ratiometric phosphor thermometry. As a result, instantaneous 3D fields of discrete temperature and velocity measurements are obtained throughout the volume. The concept is demonstrated in a turbulent heated gas jet emerging from a circular nozzle at a particle image concentration of 0.005 particle per pixel, where the symmetry of the velocity and temperature distributions about the jet axis is successfully reconstructed.