Abstract Fiber optic sensors offer a new and unique way to detect and analyze phase transitions, due to their small thermal mass and inert material. This paper presents and demonstrates a dual-sensor system to detect and analyze phase transitions in pure water and aqueous ethanol mixtures. With a multi-mode interferometer based on a thin-core fiber, and a fiber Bragg grating sensor, it is possible to differentiate between refractive index, temperature and strain in the sample system. The three parameters supply important information during a phase transition, but also in the characterization of the liquid and solid phases. Binary mixtures at non-eutectic concentrations are expected to separate into a solid phase consisting of only one constituent, and the sensors are demonstrated to be able to estimate the concentration in the remaining liquid phase. For pure water and low ethanol concentrations, the progression of the phase transition was found to be limited by heat transfer, whereas for higher concentrations the process becomes mass transfer limited. In pure water, strain due to thermal expansion of the ice hinderes temperature measurements in the solid phase. The reflection-based geometry enables insertion probes that measure the properties inside the samples, with little or no disturbance of the system. By interpreting the sensor response in a known system, the sensing capabilities in unknown substances can be evaluated. The sensor system is able to capture the dynamics of the phase transition, which can be difficult to predict theoretically due to the multitude of contributing effects. Analysis of the combined signal from the two sensors enables the determination of the ethanol mixture melting points in agreement with the literature, within the uncertainty of the system (0.25 K).

中文翻译：

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使用光纤传感器深入了解纯流体和混合物中的液固相变

摘要 光纤传感器提供了一种新的、独特的方法来检测和分析相变，因为它们的热质量和惰性材料小。本文介绍并演示了一种双传感器系统，用于检测和分析纯水和乙醇水溶液混合物中的相变。使用基于细芯光纤的多模干涉仪和光纤布拉格光栅传感器，可以区分样品系统中的折射率、温度和应变。这三个参数在相变期间提供重要信息，而且在液相和固相的表征中也提供了重要信息。预计非共晶浓度的二元混合物会分离成仅由一种成分组成的固相，并且传感器被证明能够估计剩余液相中的浓度。对于纯水和低乙醇浓度，发现相变的进展受到传热的限制，而对于较高浓度的过程，传质受到限制。在纯水中，由于冰的热膨胀引起的应变阻碍了固相中的温度测量。基于反射的几何结构使插入探针能够测量样品内部的特性，而对系统几乎没有干扰。通过解释已知系统中的传感器响应，可以评估未知物质的传感能力。传感器系统能够捕捉相变的动态，由于多种影响，这在理论上很难预测。