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Acoustic Emission (AE) is a potential technique to study the release of elastic energy originated from a localized source in the form of acoustic signatures during various physical processes such as plastic deformation, crack growth, rupturing, phase transition and crystallization processes. The phase transition processes in water, between its different phases, is a phenomenon that could be better understood in terms of the AEs produced during different phase transformation processes. In this investigation an attempt has been made to detect the acoustic emissions during phase-transitions between the snow, ice (solid phases) and water (liquid phase). Snow is a porous composite material composed of ice crystals, air and water vapors. To study these processes the AE experimental work was carried out both for freezing and melting processes of water and the energy released was detected with the help of AE sensors coupled to specially designed waveguides. The two different AE resonant sensors in the frequency range 40 kHz to 100 kHz and 125 kHz to 450 kHz, were coupled to the aluminum waveguide using the couplant silicon grease. The AE waveguide (wing – shaped) of length 248 mm and end diameter 65 mm was coupled to the AE Sensors. This AE waveguide - sensor coupled arrangement was submerged in the liquid water (as well as into snow) and covered by a lid of foam material to prevent the surrounding (external) noise components. The freezing of distilled water (capacity 200 ml) was carried out inside a cold chamber controlled at a temperature -12°C. The temperature was continuously measured with the help of a thermo NPT measurement setup. The whole process was continuously monitored in terms of various AE parameters acquired with help of an AE data acquisition system. The AE sensors with the central frequencies of 55 kHz and 300 kHz detected the complete AE activity in terms of AE Counts, Energy, Amplitude, Signal Strength, etc. and spectral features including Initiation Frequency, Peak Frequency and Frequency Centroid. The AE data analysis has revealed an abrupt increase in terms of AE counts, Energy and amplitude during phase transition from water to ice as well as from ice to water. During the freezing process, a sudden increase in the AE characteristics were observed near the temperature close to 0°C and a dense band in terms of AE characteristic counts was observed. Further, the same sample was melted out and during the process of melting a sudden dip in the trend of AE counts was recorded after the band formation near 0°C. During such band formation of AE counts, the maximum values of AE amplitude was recorded up to 99 dB. The maximum values of AE absolute energy corresponding to melting and freezing were recorded in the order of 1.3 x 108 aJ and 0.18 x 108 aJ respectively for both the processes. The spectral response of phase transition corresponding to the freezing was observed in a wide frequency range 310 to 599 kHz and 311 to 562 kHz corresponding to two different water samples connected to two different AE sensors (Channel 2 and 3), in terms of the frequency centroid, however for melting, the frequency centroid was observed in the range 599 to 335 kHz and 564 to 326 kHz corresponding to Channel 2 and 3 respectively. This investigation has opened a new frontiers of research work and also some insight about the phase transition process in water through non-invasive method of acoustic emission and that could be of wide interests to many scientific communities.
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