Detecting phase transitions in a CaCl2-H2O system at low temperatures using a fiber-optic Fresnel reflection sensor.

Temperature-induced crystallization events in an aqueous calcium chloride solution in the concentration range of 15-40 mass% are monitored using an optical fiber Fresnel reflection sensor in the temperature range of 30°C to -200°C. The deviation of the phase boundary from equilibrium and the formation of an eutectic mixture followed by its densification during rapid cooling are inferred from the distinct signatures of the optical fiber sensor via the changes in refractive index. During the natural heating at laboratory ambient conditions, the optical signals impart the completion of dissolution of ice and CaCl2·6H2O. The corresponding temperatures have been used in Linke's equations to obtain the salinities, which are in good agreement with the intended solution concentrations. The sensor signal imparts simultaneous melting of the constituents of the eutectic mixture of a 29.7 mass% solution during the natural heating phase. The persistence of the metastable liquid phase at -200°C for tens of minutes followed by solidification is observed at all the concentrations studied. Finally, the feasibility of monitoring phase transitions in a NaCl-CaCl2-H2O system has been demonstrated.

Monitoring phase changes in supercooled aqueous solutions using an optical fiber Fresnel reflection sensor.

We report on a technique for monitoring the crystallization of water and aqueous solution of NaCl at atmospheric pressure, when cooled via liquid nitrogen, using a Fresnel reflection-based optical fiber sensor. The crystallization of distilled water and the associated changes in refractive index inferred from the sensor response comply with the previous reports on physical properties of supercooled water. The phase separation of NaCl.2H2O and the formation of eutectic mixture were inferred from the distinct features of the sensor signal during the cooling of NaCl solution. But the thermocouple did not detect the exothermic heat of crystallization due to rapid cooling. The influence of temperature gradients while interpreting the optical signals during this rapid cooling process and the effects of sensor debonding during the heating phase are discussed. The results demonstrate the potential of Fresnel sensors for monitoring the crystallization-induced phase changes in supercooled salt solutions and offers applications in areas where monitoring and controlling crystallization is important.