Conference Agenda

Despite their importance and ubiquity, high void fraction two-phase flows are notoriously difficult to probe optically. With a significant number of dense scattering bubbles, images become rapidly corrupted by intensely scattered photons, resulting in occlusion, loss of definition, and errors in size and motion estimation. Information lost through these detrimental effects can be recovered if scattered photons are excluded entirely. However, this corrupting light appears femtoseconds to picoseconds after initial illumination, depending on bubble sizes. Achieving such short measurement times is not possible by conventional electronic means. Through non-linear optical phenomena, including the Optical Kerr Effect, images can be acquired with picosecond and sub-picosecond exposure times and gated in 33 femtosecond intervals.
Using semiconductor and liquid crystal-mediated Kerr gating, static and dynamic bubbly flow scenes have been temporally decomposed, capturing only useful imaging light. In doing so, the definition of bubbles is preserved, reducing uncertainty in size determination, even in optically thick scenes. Furthermore, as the spatial scattering profile is time-dependent, experimentally capturing the complete scattering history allows bubble size measurement. Applying this technique to a newly built, high void fraction bubble column, proof of concept is shown for simultaneous tracking and measurement of size and deformation. With continued development, ultrafast imaging may significantly improve multiphase imaging and provide insight into ultra-high-speed transient events.

In order to further decarbonize energy uses, numerous compact and small-scale nuclear systems are being developed worldwide. In this context, there is a growing interest in conducting research on two-phase flows in mini-channels, as understanding the behavior of these flows is essential for optimizing the heat transfer processes and enhancing the efficiency of these advanced nuclear systems. Among the thermal-hydraulic issues identified in this field is the frictional pressure drop under two-phase conditions. The present study addresses those issues by means of an experimental investigation of the two-phase frictional pressure drop which was carried out at a millimetric-scale. Those laboratory experiments were conducted using a set of two mini-channels of different length, with an inner diameter of 1.38 mm and arranged horizontally, under controlled conditions to measure the pressure drop as a function of imposed phasic flow rates. Demineralized water was used as the working liquid. In order to simulate an adiabatic two-phase flow, air was injected within the liquid at the inlet of the mini-channels. Phasic mass flow rates were imposed up to 9 g/s and 0.14 g/s, yielding maximum Reynolds numbers of 21,000 and 8,000, respectively for the liquid and gas phases. A dimensionless two-phase pressure drop was calculated from the acquired data and compared with the most recommended model for pressure drop in mini-channels to date. This model has proven incapable of reproducing the experimental data. This highlights the need to improve the predictability of pressure drop models in mini-channels.

This study presents a miniaturized impedance sensor probe designed for two-phase flow measurements, with significant implications for nuclear reactor systems. Positioned between Electrical Resistance Tomography (ERT) and optical/resistive needle probes, the sensor is based on closely spaced parallel needle electrodes to accurately measure local flow parameters, including void fraction, interfacial velocity, and bubble size. Its design allows for high spatial resolution without requiring interaction with the bubble surface, a key advantage over conventional local intrusive probes.

Numerical simulations to assess the electric field distribution across various electrode configurations were employed for two critical purposes. First, assess the impact of bubbles passing outside the primary measurement zone (inter-electrode area), which is essential for understanding how off-axis flow phenomena affect measurement accuracy. Second, the simulations provided insights into the signal processing requirements by generating simulated sensor outputs. These outputs were a key aspect for refining the algorithms used to extract key flow parameters from the sensor data.

Experimental validation, including high-speed imaging and comparisons with resistive probes, confirmed the sensor's capability to detect smaller bubbles and continuously track two-phase flow changes in real-time. The combination of its robust design, high spatial resolution and temporal resolution makes this sensor a promising alternative to existing technologies, suited for applications in nuclear reactor coolant monitoring, where precise control over multiphase flows is essential for ensuring system safety and performance.

Aerosols play a critical role in the transport of radioactive products within nuclear reactors. During severe accident scenarios, high-temperature and high-pressure coolant sprays can lead to complex temperature distributions within the containment, influencing the thermophoretic transport, evaporation, condensation, and coalescence of aerosols. Aerosol measurement technologies have evolved significantly over the years, leading to the development of diverse methodologies, including single-point/full-field, sampling/in-situ, and intrusive/non-intrusive approaches. For instance, laser-based particle visualization has been widely employed to study aerosol particle dynamics. However, temperature field measurements during aerosol transport remain rarely reported in the literature. This study introduces a novel non-intrusive method capable of simultaneously capturing the temperature field during aerosol transport and visualizing the motion of aerosol particles. The Background Oriented Schlieren (BOS) method is employed to obtain the temperature fields of aerosol particle flow through a controlled temperature gradient in a visualized channel. Experimental results demonstrate that this method can accurately obtain the velocity and temperature fields within the measurement domain, with uncertainties less than 5% for the temperature field. Additionally, this study quantitatively analyzed the influence of aerosol introduction on BOS measurements through comparative experiments. The results indicate that light scattering caused by the aerosol particles has no significant effect on the BOS measurement outcomes.

CATHARE is the French thermal-hydraulic code used for nuclear reactors safety analysis. Its reflooding module has been extensively validated for Large Break Loss-Of-Coolant Accident (LB-LOCA) scenarios. However, as Intermediate Break LOCA (IB-LOCA) studies are becoming more and more frequently, the validation of the CATHARE reflooding module needs to be extended to higher pressures than those encountered during LB-LOCA core reflooding.

The PERICLES experimental program at CEA primarily aims to improve the understanding of core thermal-hydraulics during the reflooding phase of a Pressurized Water Reactor (PWR). The test section consists of an insulated stainless-steel shroud, containing 368 full-length electrically heated fuel rod simulators (FRSs) and 25 stainless steel guide tubes arranged in a 17 × 17 geometry. The PERICLES facility has been considerably used for validating the CATHARE code. The experimental program includes 15 high-pressure (10 to 60 bar) reflooding tests which have not previously been used for CATHARE validation.

This paper first introduces the CATHARE code, the reflooding module, and its adaptation to pressures above 6 bar. Then, it presents a comparison between the PERICLES experimental data and the CATHARE computation results. The CATHARE code demonstrates a good prediction of the reflooding tests. Activating the reflooding module of the CATHARE code allows better predictions compared to results without the module's activation.

This paper will conclude with a discussion on the limitations of the presented validation and the need of further experimental data on high-pressure reflooding.