Molecular tagging velocimetry (MTV) is a non intrusive velocimetry technique based on laser spectroscopy. It is particularly adequate in challenging gas flow conditions encountered in thermal hydraulics where particle-based methods such as particle image (or tracking) velocimetry do not perform well. The main principles for designing and operating this diagnostic are presented as well as a set of gases that have been identified as potential seeds. Two gases (H2O and N2O) have been characterized extensively for thermodynamic conditions ranging from standard temperature and pressure to environments encountered in integral effect test (IET) facilities for high temperature gas reactors (HTGR). A flexible, modular, and trans- portable laser system has been designed and demonstrated with H2O and N2O seed gases. The laser system enabled to determine the optimum excitation wavelength, tracer concentration, and timing parameters. Velocity precision and thermodynamic domain of applicability are discussed for both tracers. The spectroscopic nature of the diagnostics enables to perform first principle uncertainty analysis which makes it attractive for validating numerical models. MTV is demonstrated for two flows. First, in blow down tests with H2O seed, the unique laser system enables one of the largest dynamic ranges reported to date for velocimetry: 5,000:1 (74 dB). N2O-MTV is then deployed in-situ in an IET, the high-temperature test facility at Oregon State University during a depressurized condition cooldown (DCC) event. Data enable to gain insights into flow instabilities present during DCC. Thus, MTV shows a strong potential to gain fundamental understanding of gas flows in nuclear thermal hydraulics and to provide validation data for numerical solvers.
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