Development of a Thermographic Imaging Technique for Simultaneous Interfacial Temperature and Velocity Measurements

An experimental technique, hereby referred to as ‘thermographic particle velocimetry’ (TPV) and capable of recovering twodimensional (2-D) surface temperature and velocity measurements at the interface of multiphase flows is presented. The proposed technique employs a single infrared (IR) imager and highly reflective, silver-coated particles, which when suspended near or at the interface, can be distinguished from the surrounding fluid due to their different emissivity. The development of TPV builds upon our previous IR imaging studies of heated liquid-film flows; yet, the same measurement principle can be applied for the recovery of 2-D temperatureand velocity-field information at the interface of any flow with a significant density gradient between two fluid phases. The image processing steps used to recover the temperature and velocity distributions from raw IR frames are demonstrated by application of TPV in a heated and stirred flow in an open container, and include the decomposition of each raw frame into separate thermal and particle frames, the application of perspective distortion corrections and spatial calibration, and the implementation of standard particle image velocimetry algorithms. Validation experiments dedicated to the measurement of interfacial temperature and velocity were also conducted, with deviations between the results generated from TPV and those from accompanying conventional techniques not exceeding the errors associated with the latter. Finally, the capabilities of the proposed technique are demonstrated by conducting temperature and velocity measurements at the gas-liquid interface of a wavy film flow downstream of a localised heater.