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Molecular tagging velocimetry (MTV) is a nonintrusive velocimetry technique based on laser spectroscopy. It is particularly effective 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 nitrous oxide (N2O)] have been characterized extensively for thermodynamic conditions ranging from standard temperature and pressure to environments encountered in integral effects test (IET) facilities for high-temperature gas reactors. A flexible, modular, and transportable laser system has been designed and demonstrated with H2O and N2O seed gases. The laser system enables determining 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 one to perform first-principle uncertainty analysis, which makes it attractive for validating numerical models. Molecular tagging velocimetry is demonstrated for two flows. First, in blowdown tests with H2O seed, the unique laser system enables one of the largest dynamic ranges reported to date for velocimetry: 5000:1 (74 dB). N2O-MTV is then deployed in situ in an IET facility, i.e., the High-Temperature Test Facility at Oregon State University, during a depressurized conduction cooldown (DCC) event. Data enable researchers to gain insights into flow instabilities present during DCC. Thus, MTV shows a strong potential to gain a fundamental understanding of gas flows in nuclear thermal hydraulics and to provide validation data for numerical solvers. [ABSTRACT FROM AUTHOR]

Laser measurement techniques are becoming more commonly applied to many areas of thermofluids and heat transfer. An area of their application, which presents highly challenging requirements, is in the measurement of aerospace flows. In this case the experimentalist may encounter high mass flowrates, high-speed flows and in some cases high temperatures, and the measurements may also have to be taken with restricted optical access. Such types of flow have already been measured using established laser techniques such as particle image velocimetry, laser Doppler anemometry, laser-induced fluorescence and liquid crystal thermography. Other more recent techniques which have seen application include Doppler global velocimetry and molecular tagging velocimetry for velocity measurement and thermographic phosphor thermometry for surface temperature measurement. This paper reviews the most recent advances in laser techniques and their application to aerospace environments. [ABSTRACT FROM AUTHOR]