In this study, we suggest a new working fluid: magnetic nanofluid, or magnetite-water nanofluid, which is a colloidal suspension of magnetite nanoparticles in the pure water. By using the nanofluid, we can expect the critical heat flux (CHF) enhancement, and also for the magnetic nanofluid. Since the magnetite nanoparticles can be controlled by an external magnetic field, the magnetic nanofluid is regarded as a controllable nanofluid, and thus, we can expect the advantages of magnetic nanofluid: 1) the nanoparticle suspension in nanofluid can be maintained by applying the alternating magnetic field, 2) the nanofluid concentration can be localized by applying the magnetic field for a region of interest and 3) the magnetite nanoparticles can be removed from magnetic nanofluid easily.

In this study, we focused on the CHF characteristics of magnetic nanofluid in both pool boiling and flow boiling. The first part is for the pool boiling CHF of magnetic nanofluid. At atmospheric pressure, saturated pool boiling CHF experiments were conducted using Ni-Cr wire for magnetic nanofluid and the other nanofluids. Among the various nanofluids, magnetic nanofluid has the highest value of pool boiling CHF, and the enhancement ratio (with respect to the pure water) ranges from 170 to 240 percent. To elucidate the mechanism underlying the pool boiling CHF enhancement, three approaches were introduced: 1) scanning electron microscope (SEM) images were obtained to explain the pool boiling CHF enhancement mechanism due to the deposited nanoparticles, which is related to the surface wettability of the heat transfer surface, 2) ultra-high speed movie were taken and analyzed to observe the bubble dynamics at the heat transfer surface and 3) the strength of electricity-induced magnetic field neat the heat transfer surface were calculated to examine the effect of magnetic field on the pool boiling CHF.

The second part is for the flow boiling CHF of magnetic nanofluid. A series of flow boiling CHF experiments were performed at atmospheric pressure and low mass flux conditions. Based on the experimental data, we conclude that the use of magnetic nanofluid improves the flow boiling CHF characteristics: the flow boiling CHF enhanced for the magnetic nanofluid. This is mainly due to the deposition of magnetite nanoparticles on the heat transfer surface, which results in the improvement of wettability and re-wetting characteristics. And we need enough time to ensure the nanoparticle deposition and the flow boiling CHF enhancement, when a nanofluid is used as a working fluid.

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