Molecular tagging techniques and their applications to the study of complex thermal flow phenomena

This review article reports the recent progress in the development of a new group of molecule-based flow diagnostic techniques, which include molecular taggingvelocimetry (MTV) andmolecular tagging thermometry (MTT), for both qualitative flow visualization of thermally induced flow structures and quantitative whole-field measurements of flowvelocity and temperature distributions. The MTV and MTT techniques can also be easily combined to result in a so-called molecular tagging velocimetry and thermometry (MTV&T) technique, which is capble of achieving simultaneousmeasurements of flowvelocity and temperature distribution in fluid flows. Instead of using tiny particles, the molecular tagging techniques (MTV, MTT, and MTV&T) use phosphorescent molecules, which can be turned into long-lasting glowing marks upon excitation by photons of appropriate wavelength, as the tracers for the flow velocity and temperature measurements. The unique attraction and implementation of the molecular tagging techniques are demonstrated by three application examples, which include: (1) to quantify the unsteady heat transfer process from a heated cylinder to the surroundingfluidflow inorder to examine the thermal effects on the wake instabilities behind the heated cylinder operating in mixed and forced heat convection regimes, (2) to reveal the time evolution of unsteady heat transfer and phase changing process insidemicro-sized, icing water droplets in order to elucidate the underlying physics pertinent to aircraft icing phenomena, and (3) to achieve B Hui Hu [email protected] 1 Department of Aerospace Engineering, Iowa State University, Ames, IA 50011, USA 2 School of Aeronautics & Astronautics, Shanghai Jiao Tong University, Shanghai 200240, China simultaneous droplet size, velocity and temperaturemeasurements of “in-flight” droplets to characterize the dynamic and thermodynamic behaviors of flying droplets in spray flows.