Simultaneous Temperature and Velocity measurements by LIF and PIV in a complex geometry
Résumé
Advanced Sodium Technological Reactor for Industrial Demonstration ASTRID is a project of construction of a 4th generation reactor cooled by sodium. There are thermal-hydraulic issues of upper plenum of ASTRID which can’t be studied from past reactors feedback and numerical simulations since the calculation codes do not allow to model these problems with sufficient confidence .Thus in order to validate thenumerical approaches and systems concept of ASTRID, it is important to measure temperature and velocity simultaneously using optical measurement techniques. For these reasons, a prototype is developed to study the thermal hydraulic behavior of ASTRID. Sodium experiments are very complicated to perform since sodium is opaque and reacts violently with water. Thus it is more practical to perform measurements on a water model since both have similar physical properties concerning viscosity and density. From what preceded, the MICAS mockup was designed . It is a water model of the ASTRID, made up of PMMA foroptical measurements since laser methods will be implemented to measure both temperature and velocity. The challenge in implementing non-intrusive optical techniques to simultaneously measure temperature and velocity on the MICAS mock up resides in designing the experiments so that they are carried out under ambient optical conditions with accuracy and precision attained as much as possible during the execution. Laser Induced Fluorescence LIF and Particle Image Velocimetry PIV are used to measure temperature and velocity respectively. In the LIF technique, a dye absorbs a portion of the excitation energy and spon-taneously reemits a portion of the absorbed energy as fluorescence. Calibration experiments using onecolor LIF were carried out to choose the right dyes. The fluorescence response of three dyes: FL27, RhWTand Rh6G was studied as function of three parameters: dye concentration, laser power and temperature variation from 20°C till 60°C. After that, two color LIF technique will be carried out using two dyes, one temperature dependent and the other insensitive using a pulsed 532 nm Nd:YAG laser .The temperature isdetermined from the ratio of the signal of two dyes, which have highly different temperature sensitivities. When pulsed lasers are used for the excitation of the fluorescence, their irradiance usually exceeds thesaturation intensity of common fluorescent dyes. Therefore, the fluorescence signal loses its linear depen-dence on the laser irradiance. Studies have shown that this loss of linearity is not necessarily an importan tsource of error for the ratio metric methods. Experiments are followed by coupling between PIV and LIF ona simple aquarium and then on the complex geometry MICAS. The same optical approach will be appliedon another experiment which is in progress of preparation. It is a representative prototype and a dimen-sional analysis of the thermal hydraulics of the core in the MICAS mock up. The mixture of the two jets of different temperatures must be studied. The first jet is being hot and the second cold. The prototype aimsto study the interaction between two jets at different temperatures
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