Carbon nanotube (CNT) suspensions have shown promise as a heat transfer nanofluid due to their relatively high thermal conductivity and ability to remain in stable suspension for long durations. To assess their potential as a phase change material for thermal management systems, the stability of such suspensions under repeated phase change cycles is investigated. Electrical resistance testing was used to monitor stability of the CNT network during freeze-thaw cycling. With distilled water as the base fluid, the effects of CNT size and type, CNT concentration, surfactant type and concentration, and processing parameters were investigated. Nanofluids tested included laboratory-prepared and commercially supplied samples. Experiments showed breakdown of the nanofluid in less than 12 phase change cycles for all samples tested. Ultrasonication after breakdown was shown to restore resistance values to prebreakdown levels. The results suggest the use of CNT-enhanced water as a phase change material presents a significant operational challenge due to instability of the CNT network during phase change cycling. Should the use of such nanofluids be warranted as a phase change material, electrical resistance testing along with repeated ultrasonication may be considered as a means to control and monitor stability of the nanoparticle suspension in service.
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