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When considering critical heat flux (CHF), a difficult compromise has to be made between economy and safety in many types of thermal systems such as nuclear power plants. Much research work has been carried out to increase CHF values using nano-fluids and to change the characteristics of the heating surface. In this work, the pool boiling experiments were carried out with and without ZnO nano-fluid (0.01 vol.%) on a SS-304 sample in atmospheric and saturated water conditions. The experimental test loop provides the evolution of the heat flux and wall temperature from nucleate boiling to film regime, without breaking and/or damaging the sample. Thus, it was possible to carry out several consecutive tests with the same sample in the same or different fluid conditions. Three different types of test conditions were used: (a) tests with de-ionized water (DIW), (b) tests with nano-fluid (NF) and (c) tests in DIW with a sample with an initial ZnO nano-particle coating (NFC). It was found that during the tests with nano-fluid, a very sizeable layer of nano-particle deposit was formed (∼70 μm thick after five tests). The nature of the deposit phenomenon is highly transitory, and its performance changes during each test. On the other hand, the increase in CHF is probably due to the characteristics of the heating sample surface (porosity, capillary wicking, roughness and wettability) resulting from the deposit of nano-particles and not to the nano-fluids themselves. Indeed, we obtain the same level of CHF for the tests with an initial nano-particle deposit on the sample and the nano-fluid tests. Finally, for the tests with NF and NFC we found that the heat flux in the film regime is much higher than that in DIW conditions, of the order of up to 10 times due to the surface characteristics with nano-particles deposited. |
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