Refractive index fitting method and measurement device for particle image velocimetry applications in nuclear fuel bundle experiments

Abstract The flow conditions in nuclear reactor cores strongly influence heat transfer and thereby nuclear safety, requiring accurate experimental validation of numerical analysis, like computational fluid dynamics (CFD) models. particle image velocimetry (PIV) combined with the matching index of refraction (MIR) technique is widely used for such measurements; however, accurate refractive index matching remains a major challenge. This paper presents a novel image-based method and a compact measurement device for determining refractive index matching conditions. The method is based on comparing experimentally recorded distorted calibration images with simulated images generated using a ray-tracing optical model. The refractive index is obtained by minimizing the pixel-wise difference between measured and simulated images. Validation measurements were performed using water–glycerol solution and an FEP (Fluorinated ethylene propylene) tube, and the results were compared with a handheld refractometer. The method shows good linearity and achieves an expanded uncertainty of approximately ±2.34 × 10 −4 ( k = 2). The proposed approach determines refractive index matching directly under experimental conditions without requiring precise absolute refractive index measurements, thereby simplifying MIR-based PIV measurements.