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The High Temperature Test Facility (HTTF) is a quarter-scale integral-effect facility designed to study pressurized and depressurized conduction cooldowns (PCC and DCC, respectively) in high temperature, gas-cooled reactors (HTGR). This study focuses on DCC and aims to characterize the gas exchange between the reactor core and the reactor cavity to assess the risk for air ingress and subsequent potential for natural circulation core cooldowns. High-resolution velocity profile measurements are performed at the coolant pipe break location over a long time-scale using molecular tagging velocimetry. This non-intrusive, laser-based technique is applied for the first time in the field of nuclear thermal hydraulics, providing new insights for understanding the underlying phenomena at play during a DCC event. Such data are also valuable for validation of numerical simulations such as Reactor Excursion and Leak Analysis Program (RELAP). Results are reported here for various gas mixtures of helium (the coolant) and nitrogen (surrogate for air) to study the effect of density ratio. The short transient (30 s) lock-exchange phase is well captured with helium flow velocity between 0.8 and 2 m/s. The flow is shown to persist over much longer time-scales (hours) at a velocity of about 0.2 m/s, a likely consequence of gas mixing in the hot leg and in the lower plenum of the reactor. Flow visualization at the exit of the hot leg shows shear instabilities, which supports the hypothesis of significant mixing.

Nanoparticle tracking velocimetry (NPTV), a novel particle tracking velocimetry method using nano-sized tracer particles, was investigated as a hybrid tool for the determination of the flow velocity in a microscale area. In contrast to the common optical microscopic velocimetry techniques with submicron/micron sized tracer particles, the NPTV method is based on the observation of the motion of bright spots attributed to the light scattering from individual nano-sized tracer particles. We observed light scattering from individual nano-sized tracers induced by high-intensity laser light, which are difficult to observe by direct observation of tracers using optical microscope in common flow velocimetry systems. Although the PTV/PIV method using fluorescent nano-tracer such as quantum dots make it possible to use nano-tracer in the flow velocity assessment, our light scattering observable optical system can be applied to nano-tracers of any kinds of materials. Because the determination of the flow velocity in the NPTV method is affected by the random Brownian motion of the nano-sized tracers, we demonstrated that reliable flow velocity values could be successfully obtained by ensemble averaging the displacement of each particle with an accurate correction of the effect of the Brownian motion of the nano-sized tracers. The NPTV method developed in this study can be utilized for various liquid samples owing to non-limited variety of nano-sized tracer materials. In addition, because the velocimetry by nano-sized tracers enables the determination of the flow velocity in a narrower area than that studied with submicron/micron tracers, this novel method could play an important role in the flow velocity measurement in the design of nanoscale/microscale reaction/mixing channel processes.

Femtosecond laser electronic excitation tagging (FLEET) is a new non-seed molecular tagging velocity measurement method. Fluorescent light excited by femtosecond laser is used as tagging filament, and will disturb local temperature which may affect the flow field measurement precision. In this paper, the Rayleigh scattered of nanosecond laser is used to measure the ambient air temperature changing. The feasibility of this measurement method is proved. The key data processing of low signal to noise ratio (SNR) images including the filament segmentation, noise filtering, skeleton extraction of filament position is tested. The preliminary test on the heating process between 1us and 800us is demonstrated. The limitations of this measurement method due to the gas media condition changing are discussed briefly.

Molecular tagging velocimetry is a little-intrusive technique based on the properties of specific molecules able to emit luminescence once properly excited. Several variants of this technique have been successfully developed for analyzing external gas flows or internal gas flows in large systems. There is, however, very few experimental data on molecular tagging velocimetry for gas flows in mini or microsystems, and these data are strongly affected by the molecular diffusion of the tracer molecules. In the present paper, it is demonstrated that the velocity field in gas microflows cannot be directly deduced from the measured displacement field without taking into account Taylor dispersion effects. For that purpose, the benchmark case of a Poiseuille gas flow through a rectangular channel 960 μm in depth is experimentally investigated by micro molecular tagging velocimetry using acetone vapor as a molecular tracer. An appropriate reconstruction method based on the advection diffusion equation is used to process the data and to correctly extract the velocity profiles. The comparison of measured velocities with flowrate data and with theoretical velocity profiles shows a good agreement, whereas when the reconstruction method is not implemented, the extracted velocity field exhibits qualitative and quantitative anomalies, such as a non-physical slip at the walls. The robustness of the reconstruction method is demonstrated on flows with light and heavy molecular species, namely helium and argon, at atmospheric as well as at lower pressure conditions, for which diffusion effects are stronger. The obtained results represent an encouraging step for future analysis of rarefied gas microflows.

Superconducting radio-frequency (SRF) cavities cooled by superfluid 4He (He II) are building blocks of many modern particle accelerators due to their high quality factor. However, Joule heating from sub-millimeter surface defects on cavities can lead to cavity quenching, which limits the maximum acceleration gradient of the accelerators. Developing a non-contacting detection technology to accurately locate these surface hot spots is the key to improve the performance of SRF cavities and hence the accelerators. In a recent proof-of-concept experiment (Phys. Rev. Applied, 11, 044003 (2019)), we demonstrated that a molecular tagging velocimetry (MTV) technique based on the tracking of a He 2 * molecular tracer line created nearby a surface hot spot in He II can be utilized to locate the hot spot. In order to make this technique practically useful, here we describe our further development of a stereoscopic MTV setup for tracking the tracer line’s motion in three-dimensional (3D) space. We simulate a quench spot by applying a transient voltage pulse to a small heater mounted on a substrate plate. Images of the drifted tracer line, taken with two cameras from orthogonal directions, are used to reconstruct the line profile in 3D space. A new algorithm for analyzing the 3D line profile is developed, which incorporates the finite size effect of the heater. We show that the center location of the heater can be reproduced on the substrate surface with an uncertainty of only a few hundred microns, thereby proving the practicability of this method.

Since 2000 there has been a revolution in diagnostics of high-speed air flows. The foundations for this revolution were laid over the past few decades, but with the development of new short pulse and pulse burst laser technologies, higher laser powers and higher pulse energies, new high-speed cameras, better laser control and improved detection and laser delivery methodologies, many very effective new capabilities have been developed. Newly developed methods for molecular tagging velocimetry provide high fidelity visualization of transport properties and may be extended to simultaneous temperature measurements. Rapid field imaging with frequency tunable pulse burst lasers shows instantaneous flow structure and complex boundary and mixing interactions. Extending these pulse burst concepts to swept volumetric imaging is very promising for full volumetric data collection. Fast wavelength modulation spectroscopy follows real-time flow variation, and three-dimensional particle imaging extends particle imaging velocimetry to volumetric data acquisition.

The use of polychromatic sequential light pulses in particle streaking and tracking velocimetry is proposed. The main advantage of this technique is the easy determination of the velocity direction, even in recycling flows. The method has been validated on a hydrodynamic flow, in the wake of a conventional cylindrical model.

A new flow tagging technique was developed which visualizes small-scale flow structures by writing a spatial line of NO into an air flow homogeneously seeded with NO2. The NO-line was generated by photodissociation of NO2 at 308 nm using a XeCl excimer laser and was imaged by planar laser-induced fluorescence at various delays after its formation. With seeding levels of 600 ppm, signal-to-noise levels are shown to be sufficient for detecting the shifted structure at delays of up to 20 ms. The technique was used to resolve small scale turbulent flow structures within a quartz cell.

We report a precise velocity measurement of a fast atomic lithium beam using a calibrated solid etalon. This technique is important to our program of precision atomic lifetime measurements which includes the 2p states of neutral 7Li. For our lifetime measurements, we use a dye laser crossed with a fast lithium beam to selectively excite the 2 p 2 P 1 2 , 3 2 states from the 2 s 2 S 1 2 ground state, and photon-counting detectors record the decay in flight of the fluorescence. With the laser aligned antiparallel to the atomic beam direction, we use a calibrated solid etalon to measure the Doppler shift of the 2 s 2 S 1 2 → 2 p 2 P 3 2 resonance to be 1742.67(53) GHz. The resulting velocity converts our position scale to a time scale with a precision of 0.03%.